HVDC BY SCAD

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ENGINEERING COLLEGES

2016 – 17 Even Semester

IMPORTANT QUESTIONS AND ANSWERS

Department of EEE

SUBJECT CODE: EE 6010

SUBJECT NAME:HIGH VOLTAGE DIRECT CURRENT TRANSMISSION

Regulation: 2013

Prepared by

Year and Semester: IV/VIII

4

5

Sl. No. Name of the Faculty Designation Affiliating College

1

2

P.BALAJI

A.RAJESH

AP

AP

SCADCET

SCADCET

3 A.FERMINUS RAJ AP

SCADCET

SUMATHY

SELVAKUMAR

AP

AP

MTEC

FXEC

Verified by DLI, CLI and Approved by the Centralized Monitoring Team dated

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SYLLABUS

EE6010 HIGH VOLTAGE DIRECT CURRENT TRANSMISSION L T P C

3 0 0 3

OBJECTIVES:

• To understand the concept, planning of DC power transmission and comparison with AC Power transmission.

• To analyze HVDC converters.

• To study about the HVDC system control.

• To analyze harmonics and design of filters.

• To model and analysis the DC system under study state.

UNIT I INTRODUCTION 9

DC Power transmission technology

– Comparison of AC and DC transmission –

Application of DC transmission

– Description of DC transmission system –

Planning for HVDC transmission

– Modern trends in HVDC technology – DC breakers

–

Operating problems

–HVDC transmission based on

VSC

– Types and applications of MTDC systems.

UNIT II ANALYSIS OF HVDC CONVERTERS 9

Line commutated converter

– Analysis of Graetz circuit with and without overlap –

Pulse number – Choice of converter configuration – Converter bridge characteristics – Analysis of a 12 pulse converters – Analysis of VSC topologies and firing schemes.

UNIT III CONVERTER AND HVDC SYSTEM CONTROL 9

Principles of DC link control – Converter control characteristics – System control hierarchy – Firing angle control – Current and extinction angle control – Starting and stopping of DC link – Power control – Higher level controllers – Control of

VSC based HVDC link.

UNIT IV REACTIVE POWER AND HARMONICS CONTROL 9

Reactive power requirements in steady state – Sources of reactive power – SVC and STATCOM

– Generation of harmonics – Design of AC and DC filters – Active filters.

UNIT V POWER FLOW ANALYSIS IN AC/DC SYSTEMS 9

Per unit system for DC quantities

– DC system model – Inclusion of constraints –

Power flow analysis

– case study.

TOTAL: 45 PERIODS

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TEXT BOOKS:

1. Padiyar, K. R., ―HVDC power transmission system‖, New Age International (P)

Ltd., New Delhi, Second Edition, 2010.

2. Edward Wilson Kimbark, ―Direct Current Transmission‖, Vol. I, Wiley interscience, New York, London, Sydney, 1971.

REFERENCES:

1. Kundur P., ―Power System Stability and Control‖, McGraw-Hill, 1993.

2. Colin Adamson and Hingorani N G, ―High Voltage Direct Current Power

Transmission‖, Garraway Limited, London, 1960.

3. Arrillaga, J., ―High Voltage Direct Current Transmission‖, Peter Pregrinus,

London,1983.

4. S. Kamakshaiah, V. Kamaraju, ‗HVDC Transmission‘, Tata McGraw Hill

Education Private Limited, 2011.

AIM



To provide clear knowledge of importance and the neccessity of HVDC transmission system .



Elaborate discussion on converter control strategy and Recent trends.

IMPORTANCE AND NEED OF SUBJECT



To understand the basic principles of HVDC transmission .



It is essential to know, cost effective solution for power transmission.



To understand the concept, planning of DC power transmission

COURSE OUTCOME (CO)



Ability to understand power system operation.



Ability to analyze, stability, control and protection.



Ability to design a small HVDC model.



Ability choose a feasible converter for DC transmission .

LATEST DEVELOPMENTS

 HVDC back-to-back station – eagle pan (USA) - eagle pan (USA)(Texas)

±15.9kv,36mw,

CONTENT BEYOND SYLLABUS



DSTATCOM



DUAL CONVERTER

INDUSTRIAL CONNECTIVITY



ABB private Ltd. AREVA. private Ltd (control and protection unit)

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DETAILED LESSON PLAN

Name of the subject and code: EE6010 HIGH VOLTAGE DC TRANSMISSION

TEXT BOOKS:

1. Padiyar, K. R., ―HVDC power transmission system‖, New Age International (P)

Ltd., New Delhi, Second Edition, 2010.

2. Edward Wilson Kimbark, ―Direct Current Transmission‖, Vol. I, Wiley interscience, New York, London, Sydney, 1971.

REFERENCES:

1. Kundur P., ―Power System Stability and Control‖, McGraw-Hill, 1993.

2. Colin Adamson and Hingorani N G, ―High Voltage Direct Current Power

Transmission‖, Garraway Limited, London, 1960.

3. Arrillaga, J., ―High Voltage Direct Current Transmission‖, Peter Pregrinus,

London, 1983.

4. S. Kamakshaiah, V. Kamaraju, ‗HVDC Transmission‘, Tata McGraw Hill

Unit

No.

Education Private Limited, 2011.

Topic

DC Power transmission technology

Comparison of AC and DC transmission Application of HVDC

Description of DC transmission system

Hours

Required/Planned

1

2

1

Planning for HVDC transmission 1

Cumulative

T or R

Hrs

1 T1

3

4

5

T1

T3

T1

T3

T1

T2

Modern trends in HVDC technology DC breakers

Operating problems HVDC transmission based on VSC

Types and applications of MTDC systems.

Line commutated converter

Analysis of Graetz circuit with overlap

Analysis of Graetz circuit without overlap

Pulse number

– Choice of converter configuration

1

2

1

1

1

1

1

6

8

9

10

11

12

13

T1

T1

T1

T1

T1

Converter bridge characteristics

1 14

15

T1

T2

T1

T2

T1 Analysis of a 12 pulse converters 1

4



Analysis of VSC topologies and

Firing schemes.

Principles of DC link control

Converter control characteristics

System control hierarchy

Firing angle control

Current and extinction angle control

Starting and stopping of DC link

Power control

Higher level controllers

Control of VSC based HVDC link

Reactive power requirements in steady state

Sources of reactive power

SVC

STATCOM

Generation of harmonics

Design of AC filter

DC filters

Active filters

Per unit system for DC quantities

DC system model

Inclusion of constraints

Power flow analysis AC

Power flow analysis DC

Case studies.

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Sl No Unit No

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TABLE OF CONTENTS

Topic/Title

PART - A

Comparison of ac and dc transmission

Applications of HVDC

Components of HVDC system

HVDC technology and their types

Dc circuit breakers used in HVDC

Vsc based HVDC system

PART - A

Characteristics of 12 pulse converter

Selection of converters for HVDC

Graetz converter with overlap

6 pulse converter bridge with filters.

Graetz circuit without overlap

PART - A

Starting and stopping of HVDC link

Control characteristics of converter

Control of vsc based HVDC link

Equidistant pulse firing scheme

Characteristics of current controller

Current and extinction angle control schemes used in HVDC systems

PART - A

Sources of reactive power and reactive power control

SVC operation and characteristics

STATCOM operation & characteristics

Designing procedure of ac filters

Designing procedure of dc filters

Active filters

PART - A

Per unit quantities

Power flow analysis of ac system

Power flow analysis of dc system

Case study - IEEE 30-busbar system

Case study - IEEE 14-busbar system

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UNIT 1 INTRODUCTION

PART A

1 . List out two merits of AC and DC transmission.

DC Transmission.

 It requires only two conductors as compared to three for a.c transmission

 There is no skin effect in a d.c system.

 A d.c line has less corona loss and reduced interference.

AC Transmission .

 The power can be generated at high voltages

 The maintenance of a.c sub-station is easy and cheaper

2 . What are the types of DC link?



Monopolar link



Bipolar link



Homopolar link

3 . Draw the block diagram of bipolar link

4 . List the types of power devices for HVDC transmission

 Thyristor



Insulated fiats bipolar transistor



GTO-gate turn-off thyristor



LTT- Light hissered thyrisor



Mos-controlled thyristo(MCT)

5 . Write the advantages and disadvantages of HVDC transmission?

Advantages



Full control over power transmitted

 The ability to enhance transient and dynamic stability in

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 associated AC networks

 Fast control to limit fault current in DC lines

 Reduced transmission lines.

 Interconnection of systems operating at different frequencies

Disadvantages

 Inability to use transformer to change voltage levels

 High cost of converter equipment

 Generation of harmonics which requires AC and DC filters,

 adding to the cost of converters station

 Complexity of control

6 . Mention the some of HVDC projects from abroad?

1. Gotland 1

– 98km, 200kv, 20mw, 1954

2. HVDC Gotland 2

– vastervik (Sweden) to yipne (Sweden) 92.9km, 150kv, 130mw,

3. Nelson river bipole 2

– sundance(Canada) to rosser (Canada) 937km, ±500kv,

1800Mw, 1985

4. HVDC Tjaereborg - Tjaereborg (Denmark)

– Tjaereborg 4.3km, ±9kv,

7.2mw,2000(interconnection of wind power station)

5. HVDC back-to-back station

– eagle pan (USA) - eagle pan (USA)(Texas)

±15.9kv,36mw,2000

7. What are the factors to be considered for planning HVDC transmission?

The system planner must consider the factors are,

 Cost

 Technical performance

 Reliability

8. What is the principal of control in DC link?

The control of power in a DC link can be achieved through the control of current of voltage. From minimization of loss considerations, it is important to maintain constant voltage in the link and adjust the current to meet the required power.

9. List some of the converters in HVDC systems.

Line commutated converter



Six pulse converter

 12-pulse converter

Voltage source converter

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 Basic two level converter

 Three level voltage source converter

10. Distinguish between AC & DC transmission.

S.no AC Transmission DC Transmission

1

2

It requires three conductors for transmission

Skin effect is present in AC

It requires two conductors for transmission

There is no skin effect in DC

Transmission

3

4

More corona loss

Stability problem occurs

Less corona loss

No stability problem

PART B

1. Write short notes on Comparison of a.c and d.c transmission

More power can be transmitted per conductor per circuit.

The capabilities of power transmission of an a.c. link and a D.C link are different. For the same insulation, the direct voltage Vd is equal to the peak value

(√2 x rms value) of the alternating voltage Vd.

Vd = √2 Va

For the same conductor size, the same current can transmitted with both D.C and a.c. if skin effect is not considered.

Id = Ia

Thus the corresponding power transmission using 2 conductors with D.C and a.c. are as follows. d c power per conductor Pd = Vd Id a c power per conductor Pa = Va Ia cos φ

The greater power transmission with D.C over a.c. is given by the ratio of powers

In practice, a.c. transmission is carried out using either single circuit or double circuit 3 phase transmission using 3 or 6 conductors. In such a case the above ratio for power must be multiplied by 2/3 or by 4/3 . In general

The result has been calculated at unity power factor and at 0.8 lag to illustrate the effect of power factor on the ratio. It is seen that only one-half the

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amount of copper is required for the same power transmission at unity power factor, and less than one-third is required at the power factor of 0.8 lag.

(b) Use of Ground Return Possible

In the case of HVDC transmission, ground return (especially submarine crossing) may be used, as in the case of a Monopolar D.C link. Also the single circuit bipolar D.C link is more reliable, than the corresponding a.c. link, as in the event of a fault on one conductor; the other conductor can continue to operate at reduced power with ground return. For the same length of transmission, the impedance of the ground path is much less for D.C than for the corresponding a.c. because D.C spreads over a much larger width and depth.

In fact, in the case of D.C the ground path resistance is almost entirely dependent on the earth electrode resistance at the two ends of the line, rather than on the line length.

(c) Smaller Tower Size

The D.C insulation level for the same power transmission is likely to be lower than the corresponding a.c. level. Also the D.C line will only need two conductors whereas three conductors (if not six to obtain the same reliability) are required for a.c. Thus both electrical and mechanical considerations dictate a smaller tower.

(d) Higher Capacity available for cables

In contrast to the overhead line, in the cable breakdown occurs by puncture and not by external flashover. Mainly due to the absence of ionic motion, the working stress of the D.C cable insulation may be 3 to 4 times higher than under a.c. Also, the absence of continuous charging current in a D.C cable permits higher active power transfer, especially over long lengths. (Charging current of the order of 6 A/km for 132 kV). Critical length at 132 kV ≈ 80 km for a.c cable.

Beyond the critical length no power can be transmitted without series compensation in a.c. lines. Thus derating which is required in a.c. cables, thus does not limit the length of transmission in D.C A comparison made between D.C and a.c. for the transmission of about 1550 MVA is as follows. Six number a.c.

275 kV cables, in two groups of 3 cables in horizontal formation, require a total trench width of 5.2 m, whereas for two number D.C ±500 kV cables with the same capacity require only a trench width of about 0.7 m.

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(e) No skin effect

Under a.c. conditions, the current is not uniformly distributed over the cross section of the conductor. The current density is higher in the outer region (skin effect) and result in under utilization of the conductor cross-section. Skin effect under conditions of smooth D.C is completely absent and hence there is a uniform current in the conductor, and the conductor metal is better utilized.

(f) Less corona and radio interference

Since corona loss increases with frequency (in fact it is known to be proportional to f+25 ), for a given conductor diameter and applied voltage, there is much lower corona loss and hence more importantly less radio interference with

D.C Due to this bundle conductors become unnecessary and hence give a substantial saving in line costs. [Tests have also shown that bundle conductors would anyway not offer a significant advantage for d.c as the lower reactance effect so beneficial for a.c is not applicable for D.C]

(g) No Stability Problem

The D.C link is an asynchronous link and hence any a.c. supplied through converters or D.C generation do not have to be synchronized with the link. Hence the length of D.C link is not governed by stability. In a.c. links the phase angle between sending end and receiving end should not exceed 30o at full-load for transient stability (maximum theoretical steady state limit is 90o).

(h) Asynchronous interconnection possible

With a.c. links, interconnections between power systems must be synchronous. Thus different frequency systems cannot be interconnected. Such systems can be easily interconnected through HVDC links. For different frequency interconnections both convertors can be confined to the same station. In addition, different power authorities may need to maintain different tolerances on their supplies, even though nominally of the same frequency. This option is not available with a.c. With D.C there is no such problem.

(i) Lower short circuit fault levels

When an a.c. transmission system is extended, the fault level of the whole system goes up, sometimes necessitating the expensive replacement of circuit breakers with those of higher fault levels. This problem can be overcome with

HVDC as it does not contribute current to the a.c. short circuit beyond its rated current.

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In fact it is possible to operate a D.C link in "parallel" with an a.c. link to limit the fault level on an expansion. In the event of a fault on the d.c line, after a momentary transient due to the discharge of the line capacitance, the current is limited by automatic grid control. Also the D.C line does not draw excessive current from the a.c. system.

(j) Tie line power is easily controlled

In the case of an a.c. tie line, the power cannot be easily controlled between the two systems. With D.C tie lines, the control is easily accomplished through grid control. In fact even the reversal of the power flow is just as easy.

2. What are the applications of HVDC?

The first application for HVDC converters was to provide point to point electrical power interconnections between asynchronous a.c. power networks.

There are other applications which can be met by HVDC converter transmission which include:

 Interconnections between asynchronous systems. Some continental electric power systems consist of asynchronous networks such as the East, West, Texas and Quebec networks in North America and island loads such as the Island of

Gotland in the Baltic Sea make good use of HVDC interconnections.

 Deliver energy from remote energy sources. Where generation has been developed at remote sites of available energy, HVDC transmission has been an economical means to bring the electricity to load centers. Gas fired thermal generation can be located close to load centers and may delay development of isolated energy sources in the near term.

 Import electric energy into congested load areas. In areas where new generation is impossible to bring into service to meet load growth or replace inefficient or decommissioned plant, underground d.c. cable transmission is a viable means to import electricity.

 Increasing the capacity of existing a.c. transmission by conversion to d.c. transmission. New transmission rights-of-way may be impossible to obtain.

Existing overhead a.c. transmission lines if upgraded to or overbuilt with d.c. transmission can substantially increase the power transfer capability on the existing right-of-way.

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 Power flow control. A.c. networks do not easily accommodate desired power flow control. Power marketers and system operators may require the power flow control capability provided by HVDC transmission.

HVDC in the new Electricity Industry

The question is often asked as to when HVDC transmission should be chosen over an AC system. In the past, conventions were that HVDC was chosen when :

 Large amounts of power (>500 MW) needed to be transmitted over long distances(>500 km);

 Transmitting power under water;

 Interconnecting two AC networks in an asynchronous manner.

HVDC systems remain the best economical and environmentally friendly option for the above conventional applications. However, three different dynamics

- technology development, deregulation of electricity industry around the world, and a quantum leap in efforts to conserve the environment - are demanding a change in thinking that could make HVDC systems the preferred alternative to high voltage AC systems in many other situations as well. To elaborate:

 New technologies, such as the VSC based HVDC systems, and the new extruded polyethylene DC cables, have made it possible for HVDC to become economic at lower power levels (up to 200 MW) and over a transmission distance of just 60 km.

 Liberalization has brought other demands on the power infrastructure overall.

Transmission is now a contracted service, and there is very little room for deviation from contracted technical and economic norms. HVDC provides much better control of the power link and is therefore a better way for providing contractual transmission services.

 Liberalization has brought on the phenomenon of trading to the electricity sector, which would mean bi-directional power transfers, depending on market conditions.

HVDC systems enable the bi-directional power flows, which is not possible with

AC systems (two parallel systems would be required).16



In the past, when the transmission service was part of a government owned, vertically integrated utility, the land acquisition and obtaining rights-of-way was relatively easier, and very often was done under the principle of ―Eminent Domain‖ of the State. With liberalization, transmission service provision is by and large in the domain of corporatized, sometimes privatized, entities. Land acquisition and/or

13



obtaining rights-of-way is now a significant portion of the project‘s costs. Once these costs are included in their entirety in the economical analysis of HVDC versus AC alternatives, it would be seen that HVDC is much more economical in this regard, since it requires much less land/right-of-way for a given level of power.

 In an environmentally sensitive areas, such as national parks and protected sanctuaries, the lower foot print of HVDC transmission systems becomes the only feasible way to build a power link.

So how should power system planners, investors in power infrastructure (both public and private), and financiers of such infrastructure be guided with respect to choosing between an HVDC and an high voltage AC alternative? The answer is to let the ―market‖ decide. In other words:



The planners, investors and financiers should issue functional specifications for the transmission system to qualified contractors, as opposed to the practice of issuing technical specifications, which are often inflexible, and many times include older technologies and techniques) while inviting bids for a transmission system.



The functional specifications could lay down the power capacity, distance, availability and reliability requirements; and last but not least, the environmental conditions.

 The bidders should be allowed to bid either an HVDC solution or an AC solution; and the best option chosen.

It is quite conceivable that with changed circumstances in the electricity industry, the technological developments, and environmental considerations,

HVDC would be the preferred alternative in many more transmission projects.

14



Fig - HVDC applications

3. Explain the components of an HVDC transmission system in detail.

To assist the designers of transmission systems, the components that comprise the HVDC system, and the options available in these components, are presented and discussed. The three main elements of an HVDC system are: the converter station at the transmission and receiving ends, the transmission medium, and the electrodes. The converter station: The converter stations at each end are replica‘s of each other and therefore consists of all the needed equipment for going from AC to DC or vice versa. The main components of a converter station are:

 Thyristor valves,

 VSC valves,



Transformers,

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 AC Filters and Capacitor Banks,

 DC filters.

Fig - Typical HVDC System components

Thyristor valves :

The thyristor valves can be build-up in different ways depending on the application and manufacturer. However, the most common way of arranging the thyristor valves is in a twelve-pulse group with three quadruple valves. Each single thyristor valve consists of a certain amount of series connected thyristors with their auxiliary circuits. All communication between the control equipment at earth potential and each thyristor at high potential is done with fiber optics.

VSC valves :

The VSC converter consists of two level or multilevel converter, phasereactors and AC filters. Each single valve in the converter bridge is built up with a certain number of series connected IGBTs together with their auxiliary electronics.

VSC valves, control equipment and cooling equipment would be in enclosures

(such as standard shipping containers) which make transport and installation very easy. All modern HVDC valves are water-cooled and air insulated.

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Transformers :

The converter transformers adapt the AC voltage level to the DC voltage level and they contribute to the commutation reactance. Usually they are of the single phase three winding type, but depending on the transportation requirements and the rated power, they can be arranged in other ways

AC Filters and Capacitor Banks :

On the AC side of a 12-pulse HVDC converter, current harmonics of the order of 11, 13, 23, 25 and higher are generated. Filters are installed in order to limit the amount of harmonics to the level required by the network.. In the conversion process the converter consumes reactive power which is compensated in part by the filter banks and the rest by capacitor banks. In the case of the CCC the reactive power is compensated by the series capacitors installed in series between the converter valves and the converter transformer.

The elimination of switched reactive power compensation equipment simplify the AC switchyard and minimize the number of circuit-breakers needed, which will reduce the area required for an HVDC station built with CCC. With VSC converters there is no need to compensate any reactive power consumed by the converter itself and the current harmonics on the AC side are related directly to the PWM frequency. Therefore the amount of filters in this type of converters is reduced dramatically compared with natural commutated converters.

DC filters :

HVDC converters create harmonics in all operational modes. Such harmonics can create disturbances in telecommunication systems. Therefore, specially designed DC filters are used in order to reduce the disturbances. Usually no filters are needed for pure cable transmissions as well as for the Back-to-Back

HVDC stations. However, it is necessary to install DC filters if an OH line is used in part or all the transmission system the filters needed to take care of the harmonics generated on the DC end, are usually considerably smaller and less expensive than the filters on the AC side.

The modern DC filters are the Active DC filters. In these filters the passive part is reduced to a minimum and modern power electronics is used to measure, invert and re-inject the harmonics, thus rendering the filtering very effective.

Transmission medium for bulk power transmission over land, the most frequent transmission medium used is the overhead line. This overhead line is normally

17



bipolar, i.e. two conductors with different polarity. HVDC cables are normally used for submarine transmission. The most common types of cables are the solid and the oil-filled ones. The solid type is in many cases the most economic one.

Its insulation consists of paper tapes impregnated with high viscosity oil.

No length limitation exists for this type and designs are today available for depths of about 1000 m. The self –contained oil-filled cable is completely filled with a low viscosity oil and always works under pressure. The maximum length for this cable type seems to be around 60 km. The development of new power cable technologies has accelerated in recent years and today a new HVDC cable is available for HVDC underground or submarine power transmissions.

This new HVDC cable is made of extruded polyethylene, and is used in

VSC based HVDC systems. Design, Construction, Operation and Maintenance considerations In general, the basic parameters such as power to be transmitted, distance of transmission, voltage levels, temporary and continuous overload, status of the network on the receiving end, environmental requirements etc. are required to initiate a design of an HVDC system. For tendering purposes a conceptual design is done following a technical specification or in close collaboration between the manufacturer and the customer. The final design and specifications are in fact the result of the tendering and negotiations with the manufactures/suppliers.

It is recommended that a turnkey approach be chosen to contract execution, which is the practice even in developed countries. In terms of construction, it can take from three years for thyristor-based large HVDC systems, to just one year for VSC based HVDC systems to go from contract date to commissioning. The following table shows the experience for the different HVDC technologies:

4.a. Write short notes on HVDC technology and explain their types.

The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to DC (rectifier) at the transmitting end and from DC to AC (inverter) at the receiving end. There are three ways of achieving conversion:



Natural Commutated Converters. Natural commutated converters are most used in the HVDC systems as of today. The component that enables this conversion process is the thyristor, which is a controllable semiconductor that can carry very

18



high currents (4000 A) and is able to block very high voltages (up to 10 kV). By means of connecting the thyristors in series it is possible to build up a thyristor valve, which is able to operate at very high voltages (several hundred of kV).The thyristor valve is operated at net frequency (50 hz or 60 hz) and by means of a control angle it is possible to change the DC voltage level of the bridge. This ability is the way by which the transmitted power is controlled rapidly and efficiently.

 Capacitor Commutated Converters (CCC). An improvement in the thyristor-based commutation, the CCC concept is characterized by the use of commutation capacitors inserted in series between the converter transformers and the thyristor valves. The commutation capacitors improve the commutation failure performance of the converters when connected to weak networks.



Forced Commutated Converters. This type of converters introduces a spectrum of advantages, e.g. feed of passive networks (without generation), independent control of active and reactive power, power quality. The valves of these converters are built up with semiconductors with the ability not only to turn-on but also to turnoff. They are known as VSC (Voltage Source Converters). Two types of semiconductors are normally used in the voltage source converters: the GTO

(Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Both of them have been in frequent use in industrial applications since early eighties.

 The VSC commutates with high frequency (not with the net frequency). The operation of the converter is achieved by Pulse Width Modulation (PWM). With

PWM it is possible to create any phase angle and/or amplitude (up to a certain limit) by changing the PWM pattern, which can be done almost instantaneously.

Thus, PWM offers the possibility to control both active and reactive power independently. This makes the PWM Voltage Source Converter a close to ideal component in the transmission network. From a transmission network viewpoint, it acts as a motor or generator without mass that can control active and reactive power almost instantaneously. Rationale for Choosing HVDC There is many different reasons as to why HVDC was chosen in the above projects. A few of the reasons in selected projects are:



In Itaipu, Brazil, HVDC was chosen to supply 50Hz power into a 60 Hz system; and to economically transmit large amount of hydro power (6300 MW) over large distances (800 km)

19



 In Leyte-Luzon Project in Philippines, HVDC was chosen to enable supply of bulk geothermal power across an island interconnection, and to improve stability to the

Manila AC network

 In Rihand-Delhi Project in India, HVDC was chosen to transmit bulk (thermal) power (1500 MW) to Delhi, to ensure: minimum losses, least amount right-of-way, and better stability and control.

 In Garabi, an independent transmission project (ITP) transferring power from

Argentina to Brazil, HVDC back-to-back system was chosen to ensure supply of

50 Hz bulk (1000MW) power to a 60 Hz system under a 20-year power supply contract.



In Gotland, Sweden, HVDC was chosen to connect a newly developed wind power site to the main city of Visby, in consideration of the environmental sensitivity of the project area (an archaeological and tourist area) and improve power quality.

 In Queensland, Australia, HVDC was chosen in an ITP to interconnect two independent grids (of New South Wales and Queensland) to: enable electricity trading between the two systems (including change of direction of power flow); ensure very low environmental impact and reduce construction time. Details about the above projects are provided elsewhere (under Details of Selected HVDC

Applications).

Different types of HVDC links

In the previous topic, we learn about the HVDC transmission, which is economical for long distance power transmission, and for the interconnection of two or more networks that has different frequencies or voltages. For connecting two networks or system, various types of HVDC links are used. HVDC links are classified into three types. These links are explained below;

Monopolar link

It has a single conductor of negative polarity and uses earth or sea for the return path of current. Sometimes the metallic return is also used. In Monopolar link, two converters are placed at the end of each pole. Earthing of poles is done by earth electrodes placed about 15 to 55 km away from the respective terminal stations. But this link has several disadvantages because it uses earth as a return path. Monopolar link is not much in use nowadays.

20



Bipolar link

A bipolar link has two conductors, one positive and the other negative with respect to earth. The midpoints of converters at each terminal station are earthed via electrode lines. The voltage between the conductors is equal to two times the voltages between either of the two conductors and ground. Since one conductor is at the positive polarity with respect to earth and other is at negative polarity with respect to earth. In bipolar link when one pole goes out of operation, the system may be changed to the monopolar mode with the ground return. Thus, the system continues to supply the half rated power.Bipolar links are most commonly used in all high power HVDC systems.

21



Homopolar link

It has two conductors of the same polarity usually negative polarity, and always operates with earth or metallic return. In the homopolar link, poles are operated in parallel, which reduces the insulation cost. This system is not used presently.

4. b. Explain the DC circuit breakers used in HVDC

HVDC circuit breaker

Circuit breakers will be positioned on DC grids and act when a fault occurs.

Breakers would have to fulfill some basic requirements. Current zero crossing should be created to interrupt the current once a fault occurs. At the same time the energy that is stored in the system‘s inductance should be dissipated and the breaker should withstand the voltage response of the network.

There are two types of HVDC circuit breakers:

Electromechanical

Solid-state.

Electromechanical can be grouped into three categories: inverse voltage generating method, divergent current oscillating method, and inverse current injecting method. Only the inverse current injecting method can be used in high voltage and current ratings. In this type of breaker, current zero can be created by superimposing an inverse current (of high frequency) on the input current by discharging a capacitor (that was pre-charged) through an inductor. (Explained on next section) The cost of components required for an electromechanical DC circuit breaker would not be significantly higher than that of an AC circuit breaker.

Electromechanical HVDC circuit breakers are available up to 500 kV, 5 kA and

22



have a fault-clearing time of the order of 100 ms. Solid-state circuit breakers are the second type of HVDC breakers.

These breakers can interrupt current much faster (which is required in some cases) than electromechanical circuit breakers, having an interruption time of a few milliseconds. Current flows through the IGCT and in order to interrupt, the

IGCT is turned off. Once that happens, voltage quickly increases until a varistor

(that is in parallel to the thyristor) starts to conduct. The varistor is designed to block voltages above the voltage level of the system. The main disadvantages of these types of circuit breakers are the high on-state losses and the capital costs.

Typical ratings of solid-state circuit breakers in operation are 4 kV, 2 kA, although in ratings of up to 150 kV, 2 kA were considered.

Electromechanical Circuit Breakers

On the figure we can see a basic electromechanical circuit breaker. The breaker consists of three parts:

Fig - Electromechanical Circuit Breaker

 The nominal current path is where DC current passes through and the switch is closed during normal operation

 The commutation path consists of a switch and a resonant circuit with an inductor and a capacitor and is used to create the inverse current

 The energy absorption path consists of a switch and a varistor

The commutation path has a series resonance. When interruption is required, current oscillation can occur between the nominal and the commutation path at the natural frequency (1/LC). If the amplitude of the oscillating current is larger than that of the input current then zero crossing occurs and the switch can

23



interrupt the current in the nominal path. Current (Io) will continue to flow and will charge the capacitor. If the capacitor voltage exceeds a given value, which is chosen to be the voltage capability of the circuit breaker, the energy absorption path will act causing the current to decrease.

This is a basic circuit that would need further implementations to be efficient in high voltages. Reduction in cost and better use of the costly components (varistor, capacitor) will be required. Also, the optimum capacitance value would minimize the breaker‘s interruption time and improve the whole interruption performance. Furthermore, current oscillations grow when the arc resistance (dU/dt) of the switch on the nominal path is negative. Growing oscillations can lead to faster current interruption. At the same time a large C/L ratio can help maximize the breaker‘s interruption performance.

Solid State Circuit Breakers

The second type of circuit breaker we will be analyzing is the solid-state circuit breaker. In the following figure we can see that a solid-state circuit breaker uses gatecommuted thyristors instead of integrated gate-commuted thyristors for semiconductor devices, this is due to the fact that in this topology our immediate concern is lowering the on-state losses. When there is no circuit failure detected current flows through the GCTs.

Once it is detected, the semiconductors are switched-off. This leads to the rapid increase

24



of the voltage until the varistor begins to conduct. Any voltage higher than the grid voltage is blocked due to the design of the varistor. This in turn, leads to the demagnetization of the line inductance. In this topology, parallel connections are unnecessary; hence we have a total of fourteen devices.

5. With neat sketch explain the concept of VSC based HVDC system

Conventional HVDC transmission employs line-commutated, current-source converters with thyristor valves. These converters require a relatively strong synchronous voltage source in order to commutate. The conversion process demands reactive power from filters, shunt banks, or series capacitors, which are an integral part of the converter station.

Any surplus or deficit in reactive power must be accommodated by the ac system .

This difference in reactive power needs to be kept within a given band to keep the ac voltage within the desired tolerance. The weaker the system or the further away from generation, the tighter the reactive power exchange must be to stay within the desired voltage tolerance.

Fig - Configuration

These VSC-based systems are force-commutated with insulated-gate bipolar transistor (IGBT) valves and solid-dielectric, extruded HVDC cables HVDC transmission and reactive power compensation with VSC technology has certain attributes which can be beneficial to overall system performance. VSC converter technology can rapidly control both active and reactive power independently of one another. Reactive power can also be controlled at each terminal independent of the dc transmission voltage level.

This control capability gives total flexibility to place converters anywhere in the ac network since there is no restriction on minimum network short-circuits capacity.

1. Physical Structure

The main function of the VSC-HVDC is to transmit constant DC power from the rectifier to the inverter. As shown in Figure.1, it consists of dc-link capacitors

25



Cdc, two converters, passive high-pass filters, phase reactors, transformers and dc cable.

2. Converters

The converters are VSCs employing IGBT power semiconductors, one operating as a rectifier and the other as an inverter. The two converters are connected either back-to-back or through a dc cable, depending on the application.

3. Transformers

Normally, the converters are connected to the ac system via transformers. The most important function of the transformers is to transform the voltage of the ac system to a value suitable to the converter. It can use simple connection (twowinding instead of three to eight-winding transformers used for other schemes).

The leakage inductance of the transformers is usually in the range 0.1-0.2p.u.

4. Phase Reactors

The phase reactors are used for controlling both the active and the reactive power flow by regulating currents through them. The reactors also function as ac filters to reduce the high frequency harmonic contents of the ac currents which are caused by the switching operation of the VSCs. The reactors are essential for both active and reactive power flow, since these properties are determined by the power frequency voltage across the reactors. The reactors are usually about

0.15p.u. Impedance.

5. AC Flters

The ac voltage output contains harmonic components, derived from the switching of the IGBTs. These harmonics have to be taken care of preventing them from being emitted into the ac system and causing malfunctioning of ac system equipment or radio and telecommunication disturbances. High-pass filter branches are installed to take care of these high order harmonics. With VSC converters there is no need to compensate any reactive power consumed by the converter itself and the current harmonics on the ac side are related directly to the

PWM frequency. The amount of low-order harmonics in the current is small.

6. Dc Capacitors

On the dc side there are two capacitor stacks of the same size. The size of these capacitors depends on the required dc voltage. The objective for the dc capacitor is primarily to provide a low inductive path for the turned-off current and

26



energy storage to be able to control the power flow. The capacitor also reduces the voltage ripple on the dc side.

7. Dc Cables

The cable used in VSC-HVDC applications is a new developed type, where the insulation is made of an extruded polymer that is particularly resistant to dc voltage. Polymeric cables are the preferred choice for HVDC, mainly because of their mechanical strength, flexibility, and low weight.

8 IGBT Valves

The insulated gate bipolar transistor (IGBT) valves used in VSC converters are comprised of series-connected IGBT positions. The IGBT is a hybrid device exhibiting the low forward drop of a bipolar transistor as a conducting device. A complete IGBT position consists of an IGBT, an anti parallel diode, a gate unit, a voltage divider, and a water-cooled heat sink. Each gate unit includes gate-driving circuits, surveillance circuits, and optical interface. The gate-driving electronics control the gate voltage and current at turn-on and turn-off, to achieve optimal turn-on and turn-off processes of the IGBT. To be able to switch voltages higher than the rated voltage of one IGBT, many positions are connected in series in each valve similar to thyristors in conventional HVDC valves.

9 AC Grid

Usually a grid model can be developed by using the Thevenin equivalent circuit. However, for simplicity, the grid was modeled as an ideal symmetrical three-phase voltage source. Converter Topology

The converters so far employed in actual transmission applications are composed of a number of elementary converters, that is, of three-phase, two-

27



level, six-pulse bridges, as shown in Figure. or three phase, three-level, 12-pulse bridges, as shown in Figure

Fig – Two level VSC and Three level VSC

The two-level bridge is the most simple circuit configuration that can be used for building up a three-phase forced commutated VSC bridge. It has been widely used in many applications at a wide range of power levels. As shown in Figure 2, the two-level converter is capable of generating the twovoltage levels −0.5·VdcN and +0.5·VdcN . The two-level bridge consists of six valves and each valve consists of an IGBT and an anti-parallel diode. In order to use the two-level bridge in high power applications series connection of devices may be necessary and then each valve will be built up of a number of series connected turn-off devices and anti-parallel diodes. The number of devices required is determined by the rated power of the bridge and the power handling capability of the switching devices.

With a present technology of IGBTs a voltage rating of 2.5kV has recently become available in the market and soon higher voltages are expected. The

IGBTs can be switched on and off with a constant frequency of about 2 kHz. The

IGBT valves can block up to 150kV. A VSC equipped with these valves can carry up to 800A (rms) ac line current. This results in a power rating of approximately

140MVA of one VSC and a±150kV bipolar transmission system for power ratings up to 200MW.

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UNIT-2

Part A

1. What is Graetz circuit?

This is a six pulse converter and the 12 pulse converter is composed of two bridges in series supplied from two different (three phase) transformers with voltages differing in phase by 30

0

2What is pulse number of a converter?

The pulse number of a converter is defined as the number of pulsations (cycles of ripple) of direct voltage per cycle of alternating voltage

3. State the effect of source reactance on a twelve pulse converter without AC filters.

When the source reactance is not zero and no AC filters are provided the operation of either bridge is affected by the commutation process taking place in the other bridge.

There could be two additional modes (i)5 valve conduction (ii) 6-7-8-7 valve conduction

4. What is a twelve pulse converter?

A 12 pulse converter is obtained by the series connection of two bridges. The 30

0 phase displacement between the two sets of source voltages is achieved by the transformer connections, Y/Y for feeding one bridge and Y/∆ for feeding the second bridge

5. Draw the output voltage Vs firing angle characteristic of a three phase rectifier.

6. Why is 3-phase bridge connection used invariably in converter and inverter circuits in HVDC systems?

Three phase circuits are preferable when large power is involved. The controlled rectifier can provide controllable output dc voltage in a single unit. Control over the output dc voltage is obtained by controlling the conduction interval of each thyristor. Since thyristors can block voltage in both directions it is possible to reverse the polarity of the output dc voltage and hence feed power back to the ac supply from the dc side.

7. State the constraints that can be applied for determining the boundary conditions

29



in the detailed analysis of converters.

 Magnetic fluxes and electric charges must be continuous functions of time.

 The current in the outgoing valve is zero at t=t

1

8. Define valve rating.(Valve utilization factor)

The valve voltage rating is specified in terms of peak inverse voltage (PIV) it has to withstand. The ratio of PIV to the average dc voltage is an index of the valve utilization.

Valve utilization factor = for q even

for q odd

9. What are the advantages of higher pulse number of a converter?

With a higher pulse number filtering requirements can be reduced

10. What is meant by compounding a converter?

Compounding a converter is a system of 2 converters, connected by a HVDC link. Both converters are provided with CEA and CC control so that either can work as a rectifier or an inverter

PART B

1.Explain the characteristics of a twelve pulse converter. (May/Jun 2013)

[A] Rectifier

The rectifier in general has three modes.

 First mode: Two and three valve conduction mode (u<60 o

)

 Second mode: Three valve conduction 0≤ α ≤ 30 o

; u=60 o

 Third mode: Three and four valve conduction mode α > 30 o

; (60 o < u ≤120 o

)

As the DC current continues to increase, the converter operation changes over from mode 1 to 2 and finally to mode3. The DC voltage continues to decrease

30



until it reaches zero. For, α ≥ 30 o

mode 2 is bypassed. for modes (1) and (3) we have respectively

The voltage and current characteristics are linear (with different slopes) in these cases. For mode (2), u=60o. For u= constant, the characteristics are elliptical and the equation is given by

Inverter

The inverter characteristics are similar to the rectifier characteristics.

However, the operation as an inverter requires a minimum commutation angle during which the voltage across the valve is negative. Hence the operating region of an inverter is different from that for a rectifier. The commutation margin angle

( 𝜉 ) is equal to the extinction angle ( 𝛾 ) only for values of 𝛽 ≤ 0 o

. The voltage across the valve has a positive dent D because of the successive commutation. This dent normally occurs (for 𝛽 ≤ 60 o

) after the sinusoidal voltage (on which it is superposed) has become positive.

31



Relationship between inverter angles

Hence under this condition 𝜉 = 𝛾 . With increased overlap and consequently earlier ignition of the valve, the dent encroaches on the period in which the valve voltage would otherwise be negative and this makes 𝜉 < 𝛾 . After the front of the dent becomes entirely negative, further advance of the dent does not decrease commutation margin further. Thus the margin angle ( 𝜉 ) has different relationship to 𝛾 depending on the range of operation which is summarized below.

In the inverter operation, it is necessary to maintain a certain minimum angle 𝜉 0. This results in 3 sub-modes of the first mode.

Characteristics of a 12 pulse converter

As long as the AC voltages at the converter bus remain sinusoidal (with effective filtering), the operation of one bridge is unaffected by the operation of the other bridge connected in series. In this case, the converter characteristics are as shown in the following figure. With the assumption that the AC voltages at the converter bus remain constant. The region of rectifier operation can be divided into five modes as follows:

Mode 1 : 4 and 5 valve conduction

0<u<30 o


30 o

<u<60 o

Mode 3 : 6 valve conduction

0<u<30 o

; u = 60 o


60 o

<u<90 o


90 o

<u<120 o

32



It is to be noted that the second mode is a continuation of the first and similarly fifth is a continuation of the fourth. Five valve and seven valve conduction modes are just points on the boundaries corresponding to their previous and following modes. The region corresponding to mode 3 shrinks to a point when α exceeds 30 o

. The characteristics of the inverter are similar except that a submode can exist for mode 1 or 2 depending on the value of margin angle chosen

(usually for mode 2). The characteristics of this sub-mode are elliptical as explained in the previous section. With no AC filters are provided and the source reactance is not zero the operation of either bridge is affected by the commutation process taking place in the other bridge. In this case, the operation of the twelve pulse converter is quite complex and there could be additional modes- (i) 5 valve conduction and (ii) 6-7-- 8-7 valve conduction. Also, there could be new mode of

5-6

—7-6 valve conduction (instead of 6 valve conduction), depending on the value of coupling factor K, defined by

𝐾 = 𝑋𝑆 /( 𝑋𝑆 + 𝑋𝑇 )

Where XS is the source reactance and is the XT converter transformer leakage reactance.

2. Explain the selection of converters for HVDC system.

Graetz bridge is a six pulse converter for which the lowest DC voltage harmonic is sixth. Correspondingly, lowest AC current harmonics are 5th and 7th.

To reduce the harmonic content in the AC current and DC voltage, it is desirable to use higher pulse numbers. In general, it can be stated that the characteristic harmonics (under ideal conditions) are of the order hdc= np, hac=np±1 where ‗n‘ is an integer, ‗p‘ is the pulse number. There are several configurations for a converter of a specified pulse number. For p=6, we have in addition to the

Graetz bridge, six-phase diametric connection, cascade of three single phase full wave converters, cascade of two three phase converter, parallel connection with interphase transformer etc. It is convenient to consider a ‗p‘ pulse converter made up of series and parallel connections of a basic valve (commutation) group of ‗q‘ valves or switches as shown in the below figure. Here, the switch is a thyristor valve whose firing can be delayed (from the instant when the valve voltage becomes positive). The voltage sources are actually obtained from the

33



converter transformer windings. Neglecting overlap, only one valve conducts in a commutation group of ‗q‘ valves.

If the converter is made up of a matrix of‗s‘ valve groups in series and ‗r‘ valve groups in parallel, then,

P=QRS

Converter made up of series and parallel connection of communication groups

See the above figure for the converter configuration. In general, there are ‗p‘ transformer windings. It will be shown later that sometimes windings can be combined (in particular for q=3, r=1, s=2).

Valve rating

The valve voltage rating is specified in terms of peak inverse voltage (PIV) it has to withstand. The ratio of PIV to the average dc voltage is an index of the valve utilization. The average maximum of dc voltage across the converter is given by

34



The peak inverse voltage (PIV) across a valve can be obtained as follows: If ‗q‘ is even, then the maximum inverse voltage occurs when the valve with a phase displacement of 𝜋 radian (180o) is conducting and this is given by

PIV = 2Em

If ‗q‘ is odd, maximum inverse voltage occurs when the valve with a phase shift of 𝜋 ± 𝜋 / 𝑞 is conducting. In this case,

PIV = 2 Emcos ( 𝜋 /2 𝑞 )

The valve utilization factor is given by,

Transformer rating

The current rating of a valve (as well as transformer winding supplying it) is given by

Where Id is the DC current which is assumed to be a constant. The transformer rating on the valve side (in volt amperes) is given by

The transformer utilization factor

𝑆𝑡𝑣 / 𝑉𝑑𝑜 𝐼𝑑 =1.481 is only a function of ‗q‘. The optimum valve of q which results in maximum utilization is equal to 3. It is a fortunate coincidence that the AC power supply is 3 phase and

35



the commutation group of 3 valves is easily arranged. For q = 3, The transformer utilization can be improved further if two valve groups can share a single transformer winding. In this case, the current (rms) rating of the winding can be increased by a factor of √2 while decreasing the number of windings by a factor of

2. For this case,

𝑆𝑡𝑣 / 𝑉𝑑𝑜 𝐼𝑑 = 1.047

For a 6 pulse converter, this can be easily arranged. It is shown that both from valve and transformer utilization considerations, Graetz circuit is the best circuit for a six pulse converter.

3. Explain the complete analysis of six pulse graetz converter circuit with overlap for two valve conduction. (Nov/Dec 2013)

Due to the leakage inductance of the converter transformer and the impedance in the supply network, the current in a valve cannot change suddenly and thus commutation from one valve to the next cannot be instantaneous. For example, when valve 3 is fired, the current transfer from valve 1 to valve 3 takes a finite period during which both valves are conducting. This is called overlap and its duration is measured by the overlap (commutation) angle 'u'. Each interval of the period of supply can be divided into two subintervals. In the first interval, three valves are conducting and in the second subinterval, two valves are conducting.

This is based on the assumption that the overlap angle is less than 60 o

. As the overlap angle increases to 60o, there is no instant when only two valves are conducting. As the overlap angle increases beyond 60o, there is finite period during an interval, when four valves conduct and the rest of the interval during

36



which three valves conduct. Thus there are three modes of the converter as follows:

 Mode 1- Two and three valve conduction (u<60 o

)

 Mode 2-Three valve conduction (u=60 o

)

 Mode 3-Three and four valve conduction (u>60 o

)

(i) Analysis of two and three valve conduction mode

For the interval considered, the bridge circuit can be reduced to that shown in figure.

For this circuit,

Equivalent circuit for 3 valve conduction

The L.H.S. in the above equation is called commutating emf whose value is given by which is also the voltage across valve 3 before it starts conducting. Since we get,

Solving the above equation, we get

37



Note that the solution is obtained from the initial condition

Voltage and current waveform during firing of a valve (a) rectifier; (b) inverter

The waveforms of direct voltage and the valve currents during commutation for a rectifier and an inverter are shown in fig (a) and (b) respectively. It is to be noted that during commutation, the instantaneous dc voltage is

Average direct voltage:-

The average direct voltage can be obtained as

38



Rc is called equivalent commutation resistance. It is something analogous to armature reaction in DC machines in the sense that it only represents a voltage drop and not a power loss. The equivalent circuit of the bridge rectifier is shown in the above figure.

4. Explain the methods for obtaining the steady state solution of equations for a six pulse HVDC converter bridge with filters.

Here the assumptions of constant DC current and constant, sinusoidal voltages at the AC bus are relaxed. The analysis takes into account impedances in both AC and DC networks. The steady-state solution of HVDC converter equation is periodic. With linear network elements, the equations are also piecewise linear. A method for obtaining fast steady-state solution of the system equations for the general case in described next.

Outline of the method:

The method is based on the following assumptions:

 The system is described by sets of linear differential equations and each set is applicable for particular conduction pattern of the valves in bridge.

 AC system is symmetrical and source voltages are balanced.

39



 Firing pulses are generated at equal intervals of time.

Given the initial conditions and forcing functions, the solution can be obtained analyticallycover each interval or sub-interval. The solution is periodic in steadystate, each period can be divided into p intervals, where p is the pulse number of the converter. Each interval, in general can be divided into two sub-intervals as follows:

1. 0<t<t1, corresponding to the conduction of (m+1) valves.

2. t1<t< T1 corresponding to the conduction of m valves.

(T1=T/P, T is the period of the AC voltage).

For example, in a 6-pulse converter, the normal mode consists of 3 and 2 valve conduction with m=2. The derivation of the steady-state solution proceeds from first computing t1 from non-linear equation of the form f(t1) = 0

Once t1 is obtained, the initial conditions can be calculated from sets of linear algebraic equations. These equations are derived from boundary conditions and symmetry considerations. In general, the following constraints apply in determining the boundary conditions.

1. Magnetic fluxes and electric charges must be continuous functions of time.

2. The current in the outgoing valve is zero at t= t1

Development of the method for fast steady-state analysis

It is already mentioned that there are two sub-intervals to be considered. Let the system be described by the following state equations.

The orders of the state vectors x1 and x2 are n+1 and n respectively. Since the outgoing valve current becomes zero at t=t1, one state variable is eliminated in the second sub-interval. The solutions of the above equations are: where xsi(t) is the forced response in the sub-interval i. There is no loss of generality in assuming the current in the outgoing valve as 𝑥 1 𝑛 +1, superscript indicates the

(n+1)th variable in the vector x1. Thus, we have

40



where C=[0 1] is a row vector. The vector x2 (t) is the subset of the vector x1 (t) with the last variable removed from x1 (t). From the consideration of continuity in the state variables, we get x2 (t1)=[In : 0] x1 (t1)= [K] x1 (t1) where In is the identity matrix of nth order. From considerations of symmetry, we have x1 (0)=[S] x2 (T1) where S is a constant matrix of dimension (n+1)*n.

5. With a neat diagram and waveforms explain the 6 pulse Graetz circuit.without overlap (16) (Nov/Dec 2012)

Line commutated converter

Without Overlap:

At any instant, two valves are conducting in the bridge, one from the upper commutation group and the second from the lower commutation group. The firing of the next valve in a particular group results in the turning off of the valve that is already conducting. This assumption- that there is no overlap between the two valves in a group is incorrect. The valves are numbered in the sequence in which they are fired. Thus valve 2 is fired 60 o

after the firing of valve 1 and the valve 3 is fired 60 o

after the firing of the second valve. Each valve conducts for 120 o

and the interval between consecutive firing pulse is in 60 o steady state.

The following assumptions are made to simplify the analysis

 The dc current is constant

 The valves can be modelled as ideal switches with zero impedance when on

(conducting) and with infinite impedance when off (not conducting)

 The AC voltages at the converter bus are sinusoidal and remain constant.

41



Timing diagram

Table: DC and Valve Voltages

One period of the AC supply voltage can be divided into 6 intervals

– each corresponding to the conduction of a pair of valves. The dc voltage waveform repeats for each interval. Thus, for the calculation of the average dc voltage, it is necessary to consider only one interval (say the interval corresponding to the conduction of valves 2 and 3). Assuming the firing of valve 3 is delayed by an angle α, the instantaneous dc voltage vd during the interval is given by

DC Voltage waveform

The dc voltage waveform contains a ripple whose fundamental frequency is six times the supply frequency. This can be analysed in Fourier series and contains harmonics of the order. h=np where p is the pulse number and n is an integer.

The rms value of the hth order harmonic in dc voltage is given by

42



The waveforms of the direct voltage for different values of α are shown in the figure below. The waveforms of the valve voltage are also shown in the same figure.

AC Current Waveform

It is assumed that the direct current has no ripple (or harmonics). This is normally valid because of the smoothing reactor provided in series with the bridge circuit. The AC currents flowing through the valve (secondary) and primary windings of the converter transformer contain harmonics. The waveform of the current in a valve winding is shown in the figure below.

The rms value of the fundamental component of the current is given by

Whereas the rms value of the current is

43



The harmonics contained in the current waveform are of the order given by where n is an integer, p is the pulse number. For a 6 pulse bridge converter, the order of AC harmonics are 5, 7,11,13 and higher order. These are filtered out by using tuned filters or each one of the first four harmonics and a high pass filter for the rest. The rms value of 11th harmonic is given by

𝑰𝒉 = 𝑰𝟏 / 𝒉

The Power Factor

The AC power supplied to the converter is given by where cos Ф is the power factor. The DC power must match the AC power ignoring the losses in the converter. Thus we get,

Substituting for Vdc and I1 in the above equation we obtain cos Ф= cos α

The reactive power requirements are increased as α from zero (or reduced from

180 o ). When α = 90 o

, the power factor is zero and only reactive power is consumed.

44



UNIT III

CONVERTER AND HVDC SYSTEM CONTROL

PART A

1. Define extinction advance angle.

Extinction advance angle: γ is needed for the valves to recover their ability to withstand positive voltage after conducting current. The extinction angle γ is related to the turn-off time t q of the thyristors. A typical value of γ is 15°.

2.

What is the significance of using light triggered thyristors in DC transmission?(Apr2014)

The LTT has an operating current of 4 KA and a blocking voltage of 8 kV and a light source may trigger and control its operation. Development of such thyristors with a big capacity facilitates convenient control and improves the system's reliability. So, it is highly suited for use in HVDC applications.

3. What is the need for constant current regulation in HVDC converter valves?

(May/June2013)

In a d.c. link it is common practice to operate the link at constant current rather than at constant voltage. In constant current control, the power is varied by varying the voltage. There is an allowed range of current settings within which the current varies. In this practice the short circuit currents are ideally limited to the value of load current and in real time upto twice the rated current.

4. Define firing angle control

It is the concept of control of rectifier side of a HVDC link by means of controlling the triggering angle of the converter switches in order to achieve voltage control.

5. What is equidistant pulse control?

In this scheme no direct synchronization of the control pulse to the ac system is applied. The principle is the production of single pulse spacing at equal intervals of 1/pf through a ring counter where p is pulse number and f is frequency.

6. Explain the necessity of power control in a DC link.

Current is sensitive to change in voltage resulting in large fluctuations which can damage the thyristors. Hence control of DC systems current and power is a must.

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7. State the important circuit parameters which control power in a HVDC link.

 The control angle of the rectifier α

 The control angle of the inverter β or γ

 The rectifier transformer secondary voltage winding by tap changer.

 The inverter transformer secondary voltage winding by tap changer.

8. What is the principal of control in DC link?

The control of power in a DC link can be achieved through the control of current of voltage. From minimization of loss considerations, it is important to maintain constant voltage in the link and adjust the current to meet the required power.

9. What is firing angle?

The angle at which thyristor is triggered it is defined as the angle between the zero crossing of the input voltage and the instant the thyristor is fired.

10. What is meant by current and extinction control?

 The current controller is invariably of feedback type the controller which Is PI type.



The extinction angle controller can be of predictive type or feedback type With

EPC control. The predictive controller is considered to be less Prone to commutation failure.

PART B

1. Explain the Starting and stopping of HVDC link in detail

Basic principles of control of HVDC transmission

At the rectifier a closed-loop current control is provided, which adjusts firing

46



angle & in response to the difference between measured d.c. current Id, measured by means of a d.c. current transformer, and a current order signal Io, assumed fixed for the present. At the inverter, closed-loop &control is provided, operating similarly but from measured , with a fixed reference demanding 1 of typically 15 to 18 . A current control loop is also provided, similar to that at the rectifier, supplied with the same current order, but with a `current margin' signal Im subtracted from it. Im is typically 0.1 o rated d.c. current, Idl. Tap changers on each converter transformer are often used. These do not have any major control functions; their duty is to optimize working conditions for each converter. The inverter tap changer is usually arranged to effectively move FCE up or down to obtain rated d.c. voltage; whereas the rectifier tap changer adjusts AB up or down so that measured &lies within a range of about 5 to 20. Changes of a.c. voltages experienced by the converters are effectively corrected in the long term (many seconds) by the tapchangers, but can temporarily shift the characteristics. The only important case is that in which the rectifier a.c. voltage falls significantly (or inverter a.c. voltage rises), sothat for example AB moves down to HJK. In the absence of the current loop at the inverter this would cause a complete loss ofd.c. current. When this loop exists, the working point moves only to point J, at a d.c. current lower than Idl by the current margin Im.

STARTING

HVDC converters can be started and stopped very rapidly if required.

However, in normal operation this is done relatively slowly to avoid shocks to the a.c. systems. The normal starting procedure is to first de-block (i.e. initiate firing pulses) at the inverter, with a firing angle of about 160; as the d.c. voltage is zero, this causes no current. The rectifier is then de-blocked, initially at a similar firing angle, which is then slowly reduced over a few hundred milliseconds, raising d.c. voltage until the inverter current rises and the system settles at normal firing angles, with a low current order (0.1 per-unit (p.u.) or less). Current (or power) is then increased slowly over, say, 10 seconds to 10 minutes to the desired final value.

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STOPPING

Whilst in a.c. practice a circuit is invariably taken out of service by opening a switch, on the d.c. side of a converter station the technique for shutting down a two-terminal scheme is normally to reduce power by control action over a period to suit the needs of the a.c. system, and then to block all valves. When a converter is shut down, a bypass path is often provided on the d.c. side. For example, normal firing pulses may be blocked and a pair of series connected valves fired to provide a bypass path. This collapses the d.c. voltage and the change in voltage can be detected at the other station and used to initiate its shut down sequence. If converters at both ends of a link are bypassed whilst high current is flowing in the link, because the resistance of the line circuit is low and the inductance of the d.c. reactors is large, current may take a long time to decay.

The d.c. line can be discharged faster by ensuring that one or both of the converter stations remains in inverter mode until current has stopped. In an emergency, stopping is achieved much more rapidly. A typical method is to separate fault signals according to their origin, into non-urgent stop (from relays detecting persistent commutation failure, asymmetry or misfire, under voltage, abnormal firing angle, etc.) or emergency stop (from relays detecting over current, or flash-overs from differential measurement). Non-urgent stop signals are usually allowed to persist for about 300 ms, and at a rectifier cause forced-retard, i.e. firing angle is forced into inversion at, say, 150 , which will normally stop d.c. current in about 10 ms, at an inverter bypass operation is caused by blocking normal firing pulses and instead firing a pair of series connected valves in each bridge. The latter does not directly stop d.c. current, but causes zero d.c. voltage, which is detected by the rectifier which then stops current by forced retard after

300 ms. For an emergency stop signal, full blocking, i.e. suppression of all firing signals is applied within about 2 ms, and

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the converter group circuit breaker is tripped. Zero a.c. side current usually occurs in less than 10 ms, except for some types of flashover within the station, which the circuit breakers will clear in, say, three cycles.

2. With neat sketch explain control characteristics of converter used in HVDC.

HVDC Classic control characteristics:

HVDC control overview

Normally, HVDC system operates in constant power control mode. Power order is given by the user. Current order (Iorder) derived from the power controller, which is send to the VDCOL (voltage dependent current order limiter) and into the current control amplifier (CCA). The alpha order from the CCA is send to the converter firing control which determines the firing instant of valves (shown in figure). The function of VDCOL and CCA will be explained later in this report.

The primary function of HVDC controls are:

Fast and flexible power control between the terminals under steady state and transient operation.

Better stability of ac system. Fast protection of ac and dc system faults. i) it minimizes over voltage across the valves ii) it reduces the short circuit current through the valves and lines/cables iii) it reduces the reactive power consumption iv) avoids repetitive commutation failures

These above advantages are achieved by varying exact firing instant of valves. The converter firing control which determines the firing instants for each valve to determines the rated DC voltage. The input for the firing control system could be the output of current control, voltage control, minimum alpha control, and minimum commutation margin control mode or alpha max control

49



. Usually, dc transmission controlling and co-operation between rectifier and inverter has been explained based on Ud/Id characteristics (figure).

Traditionally rectifier controls the Current and inverter operates with constant commutation margin under normal operation. Under steady state, typically rectifier would be act as constant current source i.e. constant current control and inverter will operate as constant counter voltage source i.e. constant extinction angle. The current order at the rectifier is determined by the manipulation of power order and inverter dc voltage. To maintain stability at rectifier, it is necessary to have less

(Idref – Id) deviation in dc current and also deviations should be keep as low as possible for inverter stability. The intersection of two modes gives normal operation point. Under steady state, typically rectifier would be act as constant current source i.e. constant current control and inverter will operate as constant counter voltage source i.e. constant extinction angle. The current order at the rectifier is determined by the manipulation of power order and inverter dc voltage.

To maintain stability at rectifier, it is necessary to have less (Idref – Id) deviation in

50



dc current and also deviations should be keep as low as possible for inverter stability. The intersection of two modes gives normal operation point.

Constant current characteristics at rectifier:

This characteristic could also be explained by the same equation, by assuming current as constant and alpha as variable. It can be seen from the figure that higher dc voltage at minimum alpha and increasing of alpha decreases the dc voltage. The direct current is determined based on the current order, which could be selected between minimum current capability and the rated current of valves.

The maximum current carrying capacity of valves would be determined for a transient time period to limit valve stress.

Constant extinction angle characteristics:

Inverter is normally operating as alpha-max or constant commutation margin mode in order to have certain extinction angle to commutate the valves without fail. Under normal operation, inverter operates at 50Hz, it is not recommended to increase or decrease to limit reactive power consumption and avoid commutation failure. At steady state, inverter operates normally as constant dc voltage control mode. Assuming gamma constant and Idc as variable gives negative slope characteristics. This slope would be even more negative if the ac system is weaker.

Alpha minimum at inverter:

The power reversal could be obtained by increase the current order of the inverter higher than rectifier. In case of dc line fault, it is recommended that both converters should operate as inverter to make the fault current in dc line to zero as fast as possible. If there is no minimum alpha limit at inverter, it could also operate as rectifier by reduced alpha cause feeding of dc fault. Therefore, always minimum alpha at the inverter is limited to 1100. However, rectifier could be

51



operating as inverter for reason explained above. Also because of one more reason, inverter should have minimum counter voltage to start current flow after the fault clearance.

Current margin:

To de activate the inverter current controller at normal operation, current order at inverter is subtracted from rectifier by 10% .such as called current margin.

The solution with a current controller also at the inverter, but normally deactivated by the current margin, avoids the current to become zero during disturbances at rectifier. When there is sudden voltage drop at the rectifier ac system cause hit the minimum alpha limit, if there is no current controller at inverter, reversed potential difference between inverter and rectifier will force the current to zero since the unidirectional current device has been used. Alpha at inverter could increase up to when it hits minimum extinction angle.

Modified inverter characteristics:

3. Explain the concept of Control of VSC based HVDC link.

VSC-HVDC between two grids and to isolated loads To analyze the designed control system, the system shown in Figure is simulated and the control

52



system is implemented using the electromagnetic transient simulation program

PSCAD/EMTDC. Simulation results are presented in this chapter. The, load changes and disturbances in supplying network and supplying the passive loads.

As shown in Figure all simulations have been performed with two two-level converters. The converter bridge valves are represented as a turn-on IGBT and an anti-parallel diode with ideal switches in PSCAD/EMTDC models. State losses and switching losses are neglected. The ac system voltages at both sides are

33kV and 150kV, respectively. The rated dc voltage is 160kV, the set reference value of the dc voltage is 160kV, the rated power is 60MW, the reactors are

0.15p.u., the switch frequency used in the VSC is 2000Hz, the fundamental frequency of the ac systems is 50Hz. Two dc capacitors (2Cdc = 37:6¹F) corresponding to the time constant of 4ms are used on the dc side of the converter. The outer control loop implemented will depend on the application. If the load is an established ac system, then the VSC-HVDC can control ac voltage, reactive power and active power Here, Two different control strategies are implemented to evaluate their performances:

Strategy 1:



converter 1 controls dc voltage and ac voltage.

 converter 2 controls the active power and ac voltage

Strategy 2:



converter 1 controls dc voltage and reactive power.

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 converter 2 controls the active power and reactive power.

On the other hand, if the load is a passive system, then VSC-HVDC can control frequency and ac voltage. Here, the same control scheme is used, that is, the dc voltage controller and the ac voltage controller are used at converter 1, and the frequency controller and ac voltage controller are used at converter 2, when isolated loads are connected at the converter 2 side Dc link control between two grids by using strategy 1 three phase ac voltages and currents are obtained at both sides. The dc voltage is a constant equal to the set reference value. In fact dc voltage includes § 0:5% ripple at steady state due to the use of small capacitors on the dc side. The reference voltage es and the carrier waves at both sides are also illustrated. The high-frequency ripple on the ac voltages is due to the switching of the converter valves. This ripple is relatively high in the simulation for two reasons: the harmonic ¯lters on grid-side of the converter reactors have not been optimized. - the supplying grid was modelled in insu±cient detail to get a correct response for the harmonic frequencies involved. Both capacitance and resistance of the system have not been included, leading most likely to an overestimation of the voltage distortion. Especially the various contributions to the damping are hard to model correctly. The limitation and correct modelling of harmonic distortion due to voltage-source converters are beyond the scope of this thesis. Ac voltage controller In order to test the operation of the VSC-HVDC as an ac voltage controller, a test case has been studied. The setting of the ac voltage controller for converter 2 is instantaneously increased from 0.95 p.u. to 1.05 p.u..

The set active power °ow is 0.3 p.u., which is transmitted from converter 1 to converter 2 and is not changed when the step is applied.

It can be seen that a step of the ac voltage reference value causes a change of VSC-HVDC operating point from reactive power absorption to generation.

From the simulation results, it can be concluded that when the ac voltage reference at converter 2 is equal to 0.95p.u., the converter 2 operates on the active and reactive power absorption states that absorb active power 0.3 p.u. from the dc link and reactive power 0.7 p.u. from the ac system. As soon as the step change in the reference voltage is applied around 120ms, the ac voltage is increased to the ac voltage reference value 1.05 p.u. after approximately 2 cycles.

From the phase voltages at both sides, it can be seen that the step change does not a®ect the phase voltages at converter 1 side, but a®ects the phase voltages

54



at converter 2 side at the beginning of the application of the step. The phase currents at converter 2 are displaced by 180 degree after the step change is applied and have an overcurrent duration of about 0.25 cycle. The phase currents at converter 1 have some oscillations. It should be noted that the response of the dc voltage is fast due to using the small capacitors and the dc voltage can be maintained to the set reference value except some variations about 10ms during the step change of the ac voltage. If a more constant dc voltage is required, the size of the capacitors should be increased.

4 a. Explain equidistant pulse firing scheme with a neat sketch with their variations (16) (Apr/May 2014)

The difficulties encountered with the original scheme encouraged the development of an alternative control philosophy which could get away from the voltage waveform dependence. A new principle, initially referred to as the phaselocked oscillator, appeared in the late 1960s3 and largely achieved the target. The basis of this control system, illustrated in Figure 5.3, is a voltage controlled oscillator which delivers a train of pulses at a frequency directly proportional to a

DC control voltage, Vc. The train of pulses is fed to a six-stage ring counter in which only one stage is on at a time; the ON stage is stepped cyclically from positions 1 to6 by the oscillator pulses. As each ring-counter stage turns on, it produces a short pulse at the output (once per cycle). Therefore the complete set of six output pulses normally occurs at successive intervals of 60°. The STOP pulses are also obtained from the ring counter but two stages later (e.g. the

START pulse for valve 1 is from stage 1 and the STOP pulse for valve 1 is from stage 3, normally 120° later). One oscillator and one ring counter per bridge constitute the basic control hardware. The various control modes only differ in the type of control loop which provides the oscillator control voltage, Vc.

The phase of each firing pulse will have some arbitrary value relative tothe

AC-line voltage, i.e. an arbitrary value of converter firing angle a. However, when the three-phase AC-line voltages are symmetrical fundamental sine waves, a is the same for each valve. In practice the simple independent oscillator would drift in frequency and phase relative to the AC system; hence some method of phase locking the oscillator to the AC system is required.

55



This is normally achieved by connecting Vc in a conventional negativefeedback loop for constant current or constant extinction angle, as described in the following two sections.

4.b Draw and explain the combined rectifier and inverter characteristics with current regulation from both sides constant voltage. [Of course,constant current means that current is held nearly constant and not exactly constant].In constant current control, the power is varied by varying the voltage. There is an allowed range of current settings within which the current varies.

In ad. c. link it is common practice to operate the link at constant current rather than at constant voltage. [Of course,constant current means that current is held nearly constant and not exactly constant].In constant current control, the power is varied by varying the voltage. There is an allowed range of current settings within which the current varies.

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5.With necessary sketches, describe the constant current and constant extinction angle

Control schemes used in HVDC systems.

Constant Current Control(CC)

Ind.c.linkitiscommonpracticetooperatethelinkatconstantcurrentratherthanatconstan

tvoltage.[Of course, constant current means that current is held nearly constant and not exactly constant]. In constant current control, the power is varied by varying the voltage. There is an allowed range of current settings within which the current varies.

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Inverter Extinction angle control

For the extinction angle control for the inverter, a technique similar to the current controller at the rectifier is employed. However, the approach is complicated due to the measurement of gamma. For the measurement of the gamma, a direct method would be to measure the valve voltage VV, and the gamma value would correspond to the period that the VV is negative. However, direct measurement of the VV is not always practically nor economically feasible, and alternative or indirect techniques to either measure or predict gamma are used. Furthermore, since there are 6 (or 12) valves in a converter, it is necessary to obtain the minimum value of the gamma of all the valves.

Measurement of Gamma - Approach 1

One method uses the moment of the firing of the out-going valve and the detection of current zero in that valve to determine the value of the overlap angle

(Figures 4-9 and 4-10). The ac commutation voltage zero cross-over point, with the voltage going positive, then provides the end of the gamma angle Hence, the ignition angle can be calculated from a knowledge of the period from the moment of firing of the out-going valve to the moment of the commutation voltage reversal, going positive i.e.

Prediction of Gamma - Approach 2

In this method, a prediction of the remaining commutation voltage-time area after commutation is made, and it is maintained to be larger than a specified minimum necessary for successful commutation. The prediction is approximate, but to increase its precision, a feedback loop is employed which measures the error and feeds it back. The choice of the voltage-time area is justified since commutation of a valve is a function of the remaining commutation voltage-time area rather than just the remaining time period alone. The predictor continuously calculates (by a triangular approximation) the total remaining voltage-time area if firing would occur at that instant. Since the predictor is common to all the valves in one 6-pulse converter, it operates for a period of 60 degrees per valve.

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59



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UNIT IV - REACTIVE POWER AND HARMONICS CONTROL

PART A

1. What is Reactive power and what is the need for reactive power?

The reactive power flows from load to source . The average value for reactive power is zero. It does not result in any active power consumption. Unit: Volt Ampere

Reactive (VAR)

The reactive power is essential for the operation of electromagnetic energy devices ; it provides required coupling fields for energy devices.

2. What is meant by STATCOM?

The static synchronous compensator (STATCOM or SSC) is a shunt connected reactive power compensation device that is capable of generating and/or absorbing reactive power and in which the output can be varied to control the specific parameters of an electric power system. It is capable of generating or absorbing independently controllable real and reactive power at its output terminals when it is fed from dc energy source or energy storage device at its input terminals.

3. What are the functions of STATCOM in the improvement of power system performance area?

 It provides dynamic voltage control in transmission and distribution system

 It provides damping against the oscillation in power system.

 It provides better transient stability

 It has voltage flicker control (it withstands sudden changes)



It controls both real and reactive power

4. How voltage is controlled by SVC?

The transmission line voltage is maintained by connecting static var compensator ( SVC) in the receiving end side. The comparator will measure the actual and reference values of transmission line voltage; depends on the comparator output the reactive power is injected into the transmission line, and the transmission line voltage will be controlled.

5. What is harmonics and how it is generated?

A harmonic is a signal or wave whose frequency is an integral (whole-number) multiple of the frequency of some reference signal or wave. The term can also refer to the ratio of the frequency of such a signal or wave to the frequency of the reference signal or wave.

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In power systems, Harmonics are multiples of the fundamental wavelengths.

Thus, the third order harmonic is the third multiple of the fundamental wavelength.

This type of harmonics is generated in non-linear loads. Nonlinear loads create disturbances in the fundamental harmonic, which produce all types of harmonics.

6. Define ac filter

A filter is an AC circuit that separates some frequencies from others within mixedfrequency signals. Audio equalizers and crossover networks are two well-known applications of filter circuits. A Bode plot is a graph plotting waveform amplitude or phase on one axis and frequency on the other.

7. Define dc filter

To remove the AC components or filter them out in a rectifier circuit, a filter circuit is used. A filter circuit is a device to remove the A.C components of the rectified output, but allows the D.C components to reach the load.

8. Define THD

The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.

9. Mention the performance criteria for selection of harmonic filter

 Harmonic distortion

 Telephone influence factor

 Telephone Harmonic form factor

 IT product

10. Mention the Types of filters

There are basically two types of filters

 Passive filters ---- tuned filters and damped filter; single and double tuned , high pass filters

 Active filters

11. State the ill effects of harmonics injected into the AC line?



Telephone interference



Extra power looses & consequent heating in machines



Over voltages due to resonances



Instability of converter controls

 Interference with ripple control system used in load management.

12.

What are the sources of harmonics?

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 Magnetization nonlinearities of transformer

 Rotating machines and Adjustable speed drives.

 Arcing devices and Electronic and medical test equipment

 PCs and office machines , Induction Heaters

 Semiconductor based power supply system

 Inverter fed A.C. drives , Thyristor controlled reactors

 Phase controllers and A.C. regulators

PART B

1. What are the sources of Reactive power? Why reactive power is to be controlled in power systems?

CONCEPT OF FACTS

A Flexible Alternating Current Transmission System (FACTS) is a system composed of static equipment used for the AC transmission of electrical energy and it is meant to enhance controllability and increase power transfer capability of the network and it is generally a power electronics-based system.

A FACT is defined by the IEEE as ―power electronics based system other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability‖.

REACTIVE POWER CONTROL

To make transmission networks operate within desired voltage limits and methods of making up or taking away reactive power is called reactive-power con trol‖. The AC networks and the devices connected to them create associated time-varying electrical fields related to the applied voltage and as well as magnetic fields dependent on the current flow and they build up these fields store energy that is relea sed when they collapse‖.



Apart from the energy dissipation in resistive components, all energy-coupling devices (e.g.: motors and generators) operate based on their capacity to store and release energy.



While the major means of control of reactive power and voltage is via the excitation systems of synchronous generators and devices may be deployed in a transmission network to maintain a good voltage profile in the system.



The shunt connected devices like shunt capacitors or inductors or synchronous inductors may be fixed or switched (using circuit breaker).



The Vernier or smooth control of reactive power is also possible by varying

63



effective susceptance characteristics by use of power electronic devices.

Example: Static Var Compensator (SVC)‖ and a Thyristor Controlled Reactor

(TCR).

SOURCES OF REACTIVE POWER:

Most equipment connected to the electricity system will generate or absorb reactive power, but not all can be used economically to control voltage. Principally synchronous generators and specialised compensation equipment are used to set the voltage at particular points in the system, which elsewhere is determined by the reactive power flows.

Synchronous generators:

Synchronous machines can be made to generate or absorb reactive power depending upon the excitation (a form of generator control) applied. The output of synchronous machines is continuously variable over the operating range and automatic voltage regulators can be used to control the output so as to maintain a constant system voltage.

Synchronous compensators:

Certain smaller generators, once run up to speed and synchronised to the system, can be declutched from their turbine and provide reactive power without producing real power. This mode of operation is called Synchronous Compensation.

Capacitive and inductive compensators:

These are devices that can be connected to the system to adjust voltage levels. A capacitive compensator produces an electric field thereby generating reactive power whilst an inductive compensator produces a magnetic field to absorb reactive power.

Compensation devices are available as either capacitive or inductive alone or as a hybrid to provide both generation and absorption of reactive power .

Transformers:

Transformers produce magnetic fields and therefore absorb reactive power. The heavier the current loading the higher the absorption.

Consumer Loads - a typical load bus supplied by a power system is composed of a large number of devices. The composition changes depending on the day, season and weather conditions. The composite characteristics are normally such that a load bus absorbs reactive power. Both active and reactive powers of the composite loads

Concepts Of Reactive Power Control And Voltage Stability Methods In Power

System Network vary due to voltage magnitudes. Loads at low-lagging power

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factors cause excessive voltage drops in the transmission network. Industrial consumers are charged for reactive power and this convinces them to improve the load power factor.

Underground cables - they are always loaded below their natural loads, and hence generate reactive power under all operating conditions

Overhead lines - depending on the load current either absorb or supply reactive power.

At loads below the natural load, the lines produce net reactive power; on the contrary, at loads above natural load lines absorb reactive power.

2. Explain the construction and operation of SVC and Discuss in detail about the static and dynamic V-I characteristics of SVC

Static Var Compensator

A SVC is an electrical device for providing fast acting reactive power on high-voltage electricity transmission networks. SVCs are part of the FACTS device family and regulating voltage and stabilizing the system. Unlike a synchronous condenser which is a rotating electrical machine a SVC has no significant moving parts and prior to the invention of the SVC power factor Compensation was the preserve of large rotating machines such as synchronous condensers or switched capacitor banks. The SVC is an automated impedance matching device designed to bring the system closer to unity power factor.

SVCs are used in two main situations:

 Connected to the power system, to regulate the transmission voltage.

 Connected near large industrial loads, to improve power quality.

In transmission applications the SVC is used to regulate the grid voltage. If the power system‘s reactive load is capacitive (leading) the SVC will use thyristor controlled reactors to consume vars from the system lowering the system voltage. Under inductive (lagging) conditions the capacitor banks are automatically switched on thus providing a higher system voltage and by connecting the thyristor-controlled reactor which is continuously variable along with a capacitor bank step and the net result is continuously-variable leading or lagging power. In industrial applications

SVCs are typically placed near high and rapidly varying loads such as arc furnaces where they can smooth flicker voltage.

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Fig - Svc single line diagram

Description:

The elements which may be used to make an SVC typically include:

 Thyristor Controlled Reactor (TCR) where the reactor may be air or iron cored.

 Thyristor Switched Capacitor (TSC).

 Harmonic filter(s).

 Mechanically switched capacitors or reactors.

Connection:

This reduces the size and number of components needed in the SVC although the conductors must be very large to handle high currents associated with the lower voltage.

The dynamic nature of the SVC lies in the use of thyristors connected in series and inverseparallel forming ―thyristor valves‖ and the disc-shaped semiconductors usually several inches in diameter are usually located indoors in a ―valve house‖.

Prevention of Voltage Stability

Voltage instability is caused by the inadequacy of the power system to supply the reactive-power demand of certain loads, such as induction motors. A drop in the load voltage leads to an increased demand for reactive power that, if not met by the power system, leads to a further decline in the bus voltage. This decline eventually leads to a progressive yet rapid decline of voltage at that location, which may have a cascading effect on neighboring regions that causes a system voltage collapse.

Principle of SVC Control

The voltage at a load bus supplied by a transmission line is dependent on the magnitude of the load, the load-power factor, and the impedance of the transmission line.

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Consider an SVC connected to a load bus, as shown in Fig. The load has a varying power factor and is fed by a lossless radial transmission line. The voltage profile at the load bus, which is situated at the receiver end of the transmission line, is depicted in Fig.

For a given load-power factor, as the transmitted power is gradually increased, a maximum power limit is reached beyond which the voltage collapse takes place.

An SVC connected at the load bus by a radial transmission line supplying a load .

Voltage profile at the receiving end of a loaded line with a varying power factor load

Advantages:



Near instantaneous response to changes in the system voltage. For this reason they are often operated at close to their zero-point in order to maximize the reactive power correction they can rapidly provide when required.



In general, cheaper, higher-capacity, faster and more reliable than dynamic compensation schemes such as synchronous condensers.

V-I Characteristics of SVC

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The steady-state and dynamic characteristics of SVCs describe the variation of SVC bus voltage with SVC current or reactive power.

(a) The voltage

–current characteristic of the SVC

(b) The voltage –reactive-power characteristic of the SVC

Dynamic Characteristics

Reference Voltage ,

V ref :

This is the voltage at the terminals of the SVC during the floating condition, that is, when the SVC is neither absorbing nor generating any reactive power. The reference voltage can be varied between the maximum and minimum limits — V ref max and V ref

min —either by the SVC control system, in case of thyristor-controlled

68



compensators, or by the taps of the coupling transformer, in the case of saturated reactor compensators. Typical values of V ref

max and V ref

min are 1.05 pu and

0.95 pu, respectively.

Linear Range of SVC Control:

This is the control range over which SVC terminal voltage varies linearly with SVC current or reactive power, as the latter is varied over its entire capacitive-toinductive range.

3. Explain the basic construction, principle of operation of STATCOM. And its

V-I characteristics.



The STATCOM (or SSC) is a shunt-connected reactive-power compensation device that is capable of generating and/ or absorbing reactive power and in which the output can be varied to control the specific parameters of an electric power system.



It is in general a solid-state switching converter capable of generating or absorbing independently controllable real and reactive power at its output terminals when it is fed from an energy source or energy-storage device at its input terminals.



Specifically, the STATCOM considered is a voltage-source converter that, from a given input of dc voltage, produces a set of 3-phase ac-output voltages, each in phase with and coupled to the corresponding ac system voltage through a relatively small reactance (which is provided by either an interface reactor or the leakage inductance of a coupling transformer).



The dc voltage is provided by an energy-storage capacitor and a STATCOM can improve power-system performance in such areas as the following:

1. The dynamic voltage control in transmission and distribution systems;

2. The power-oscillation damping in power-transmission systems;

3. The transient stability;

4. The voltage flicker control; and

5. The control of not only reactive power but also (if needed) active power in the connected line, requiring a dc energy source.

Advantages of STATCOM

1. It occupies a small footprint, for it replaces passive banks of circuit elements by compact electronic converters;

69



2. It uses encapsulated electronic converters, thereby minimizing its environmental impact.

PRINCIPLE OF OPERATION



A STATCOM is a controlled reactive-power source. It provides the desired reactive-power generation and absorption entirely by means of electronic processing of the voltage and current waveforms in a voltage-source converter

(VSC).



A single-line STATCOM power circuit is shown in Fig.(a),where a VSC is connected to a utility bus through magnetic coupling.



The exchange of reactive power between the converter and the ac system can be controlled by varying the amplitude of the 3-phase output voltage, Es , of the converter, as illustrated in Fig. (c).



If the amplitude of the output voltage is increased above that of the utility bus voltage, Et , then a current flows through the reactance from the converter to the ac system and the converter generates capacitive-reactive power for the ac system. If the amplitude of the output voltage is decreased below

Principle diagram: (a) a power circuit ;(b) an equivalent circuit ;(c) a power exchange



the utility bus voltage, then the current flows from the ac system to the converter and the converter absorbs inductive-reactive power from the ac system.



If the output voltage equals the ac system voltage, the reactive-power exchange

70



becomes zero, in which case the STATCOM is said to be in a floating state.



Adjusting the phase shift between the converter-output voltage and the ac system voltage can similarly control real-power exchange between the converter and the ac system. In other words, the converter can supply real power to the ac system from its dc energy storage if the converter-output voltage is made to lead the acsystem voltage.



On the other hand, it can absorb real power from the ac system for the dc system if its voltage lags behind the ac-system voltage.



A STATCOM provides the desired reactive power by exchanging the instantaneous reactive power among the phases of the ac system.



The mechanism by which the converter internally generates and/ or absorbs the reactive power can be understood by considering the relationship between the output and input powers of the converter. The converter switches connect the dcinput circuit directly to the ac-output circuit. Thus the net instantaneous power at the acoutput terminals must always be equal to the net instantaneous power at the dc-input terminals (neglecting losses).



Assume that the converter is operated to supply reactive-output power. In this case, the real power provided by the dc source as input to the converter must be zero.



Furthermore, because the reactive power at zero frequency (dc) is by definition zero, the dc source supplies no reactive ower as input to the converter and thus clearly plays no part in the generation of reactive-output power by the converter.



Although reactive power is generated internally by the action of converter switches, a dc capacitor must still be connected across the input terminals of the converter.



The primary need for the capacitor is to provide a circulating-current path as well as a voltage source.



The magnitude of the capacitor is chosen so that the dc voltage across its terminals remains fairly constant to prevent it from contributing to the ripples in the dc current. The VSC-output voltage is in the form of a staircase wave into which smooth sinusoidal current from the ac system is drawn, resulting in slight fluctuations in the output power of the converter.



However, to not violate the instantaneous power-equality constraint at its input and output terminals, the converter must draw a fluctuating current from its dc

71



source.



Depending on the converter configuration employed, it is possible to calculate the minimum capacitance required to meet the system requirements, such as ripple limits on the dc voltage and the rated-reactive power support needed by the ac system.



The VSC has the same rated-current capability when it operates with the capacitive- or inductive-reactive current.



The VSC may be a 2- level or 3-level type, depending on the required output power and voltage. A number of VSCs are combined in a multi-pulse connection to form the STATCOM.



In the steady state, the VSCs operate with fundamental-frequency switching to minimize converter losses. However, during transient conditions caused by line faults, a pulse width

–modulated (PWM) mode is used to prevent the fault current from entering the VSCs. In this way, the STATCOM is able to withstand transients on the ac side without blocking.

VI characteristics of STATCOM



A typical V I characteristic of a STATCOM is depicted in Fig.



The STATCOM can supply both the capacitive and the inductive compensation and is able to independently control its output current over the rated maximum capacitive or inductive range irrespective of the amount of ac-system voltage.



The STATCOM can provide full capacitive-reactive power at any system voltage — even as low as 0.15 pu.



The characteristic of a STATCOM reveals strength of this technology: that it is capable of yielding the full output of capacitive generation almost independently of the system voltage (constant-current output at lower voltages). This capability is particularly useful for situations in which the STATCOM is needed to support the

72



system voltage during and after faults where voltage collapse would otherwise be a limiting factor.



Figure illustrates that the STATCOM has an increased transient rating in both the capacitive- and the inductive-operating regions.



The maximum attainable transient over current in the capacitive region is determined by the maximum current turn-off capability of the converter switches.



In the inductive region, the converter switches are naturally commutated; therefore, the transient-current rating of the STATCOM is limited by the maximum allowable junction temperature of the converter switches.



In practice, the semiconductor switches of the converter are not lossless, so the energy stored in the dc capacitor is eventually used to meet the internal losses of the converter, and the dc capacitor voltage diminishes.



However, when the STATCOM is used for reactive-power generation, the converter itself can keep the capacitor charged to the required voltage level. This task is accomplished by making the output voltages of the converter lag behind the ac-system voltages by a small angle (usually in the 0.18

–0.28 range).



In this way, the converter absorbs a small amount of real power from the ac system to meet its internal losses and keep the capacitor voltage at the desired level.



The same mechanism can be used to increase or decrease the capacitor voltage and thus, the amplitude of the converter-output voltage to control the var generation or absorption.



The reactive- and real-power exchange between the STATCOM and the ac system can be controlled independently of each other.

The power exchange between the STATCOMe ac system

1.

Control coordination for obviating such interactions may be necessary if the FACTS and HVDC controllers are located within a distance of about three major buses.

Instabilities of harmonics (those ranging from the 2nd to the 5th) are likely to occur in power systems because of the amplification of harmonics in FACTS controller loops.

2.

Harmonic instabilities may also occur from synchronization or voltage-measurement systems, transformer energization, or transformer saturation caused by geomagnetic ally induced currents (GICs).

4. a. Explain the Designing procedure of AC filters

Design criteria

73



An ideal in filter design is the elimination of all the detrimental effects caused by waveform distortion, and particularly telephone interference. However, this ideal criterion is unrealistic because of the difficulty of estimating in advance the harmonic flow throughout the AC system. It is also uneconomic and, in the case of telephone interference, the problem can action in the telephone system itself. A more practical solution is the reduction of harmonic voltage to an acceptable level at the converter terminals. The flow of harmonic current causes no special problem provided that the system harmonic impedance is small and therefore a criterion based on harmonic voltage rather than current is more convenient for filter design. Typical specified factors to be taken into account in filter design are the voltage distortion caused by individual harmonics

The TIF gives an approximation to the effect of the distorted voltage or current waveform of a power line on telephone noise, without considering the geometrical aspects of coupling. The harmonic frequencies that are sensitive to the ear are given high weighting factors, since even if the harmonic magnitudes are small, these harmonics may result in unacceptable telephone noise.

Design factors

Two basic concepts in filter design are filter size and quality. The size of a filter is defined as the reactive power that the filter supplies at fundamental frequency, which is substantially equal to the fundamental reactive power supplied by the capacitors. The total size of all the branches of a filter is determined by the reactive-power requirements of the converter and by how much this requirement can be more economically supplied by the AC generators, extra shunt capacitors, synchronous condensers or static VAR systems (SVS). The quality of a filter (Q) expresses the sharpness of tuning and is therefore defined differently for tuned and high-pass filters The high Q or tuned, filter is sharply tuned to one or two of the lower harmonic frequencies such as the fifth and seventh. The low Q or damped, filter provides a low impedance over a broad band of frequencies and is often used to eliminate the higher-order harmonics, e.g. 17th up. It is normally referred to as a high-pass filter.

HVDC Transmission

74



Figure shows typical circuit diagrams and characteristics of the two types and Figure

3.4 illustrates their incorporation within the conventional six-pulse HVDC converter configuration. The diagram in Figure indicates that the harmonic current generated by the converter divides between the shunt filters and the AC network.

The key to good filter design is a clear understanding of the two components of the equivalent circuit, i.e.:

(a) The harmonic source (discussed in Chapter 2).

(b) The impedance of the AC network at harmonic frequencies.

Type C damped filters

With the ratings of some HVDC links being of the same order as the system short-circuit level, there is an increased probability of low-order harmonic resonance between the system impedance and the filter capacitance. Series or parallel resonance will result depending on whether the low-harmonic source is within the AC system or converter stations, respectively. By way of example, a high probability of third-harmonic resonance had been expected on the British side of the 2000 MW Cross-Channel link. To overcome the problem it was decided to design half the filters for minimum impedance at around the third harmonic frequency. However, the use of damped filters for low-order harmonics involves large fundamental power loss in the damping resistor. The power loss of conventional damped filters can be reduced by a Type C filter,11 illustrated, where the resistor is bypassed by a fundamental-frequency tuned arm (C2 - L). This circuit is more susceptible to frequency variations because of the fundamentalfrequency tuning, but exhibits much lower losses.

75



Simplified filtering for 12-pulse converters

Conventional filter design, based on the use of separate tuned filters of the series resonant type for the 11th and 13th harmonics and a high-pass filter for the higher-order harmonics, provides a more effective reduction of harmonics than is normally required. In conventional design the minimum size of the filters is usually determined by the available economic size of capacitor units and by the minimum amount of reactive power compensation required at the converter's terminal.

Therefore, the filter design can be simplified, either by replacing the tuned filters for harmonics 11 and 13 by a single filter of the damped type, or by replacing all the individual filters by a single damped filter. In the first case, the damped filter replacing the two tuned filters should be tuned to about the 12th harmonic with a fairly high (2(20 to 50) and the damped filter used for the higher harmonics has a much smaller Q (two to four). In the second case, the single damped filter is also tuned to about the 12th harmonic but a fairly low Q has to be chosen (two to six) to achieve a sufficiently low impedance at higher harmonics.

The advantages of the damped filter are:

(a) The performance and loading are less sensitive to temperature, system-frequency deviations and component tolerances.

(b) Because a wide spectrum of harmonic frequencies is filtered, the considerable cost of subdividing the filter into several separate arms is avoided; this also leads to a reduced site area.

(c) Maintenance is reduced.

76



(d) Uncharacteristic harmonics are also absorbed, subject to the filter Q and centre frequency.

(e) The need to carry out tuning on site is reduced or eliminated.

(f) It is easier and cheaper to split the filter into smaller subgroups for reactive power control;

Sharing the harmonic current between these subgroups presents no problem. On the other hand, damped filters need to be bigger in terms of fundamental MVAR to achieve the same level of filtering performance as do tuned resonant filters. The harmonic losses in tuned resonant filters are usually lower than those in damped filters, although the opposite is true for the fundamental-frequency losses.

4.b Explain the designing procedure of DC-side filters

On the DC side of HVDC converters the voltage harmonics generate harmonic currents with amplitudes which depend on known elements such as the delay and extinction angles, the overlap angle and the impedance of DC circuits

(i.e. smoothing reactors, damping circuits, surge capacitors and the line itself). In contrast to the AC filters discussed above, the DC filters:

 do not carry fundamental-frequency power and therefore have substantially lower losses;

 do not need to provide reactive power, their only function being harmonic mitigation;

 have, in their main capacitor, to withstand the full pole to neutral DC voltage. Also, the harmonic impedances do not change with the operating conditions and it is therefore possible to use tuned filters with higher Q factors and thus smaller capacitors and reactors. The criteria to be met by the filters relate mainly to telephone interference from the DC line.

 the induced voltage measured at subscribers' sets, for safety reasons, should not normally exceed 60 volts r.m.s.;

 the psophometric e.m.f induced should not exceed 1 mV, if psophometric weightings are used for noise measurements, or 20 dBrnc of noise when using the

C message weighting. In the first method, all harmonic currents on the DC line are reduced to an equivalent disturbing current (Ieq) at a single frequency, under the assumption that this current should cause the same interference effect on the telephone lines,

77



where n = harmonic order: the upper limit usually considered ranges between 2.5 and 5 kHz

Cn = normalised weighting factor at harmonic n, referred to 800 Hz or 1000 Hz for the psophometric and C message weights, respectively

Hn = frequency-dependent factor, taking into account variations (if significant) of the mutual impedance among the HVDC line and the telephone lines, of shielding and of telephone circuit Balance In the second method, the performance requirements are specified as a longitudinal induced voltage, properly weighted as indicated above, on a hypothetical sample telephone line, which is assumed to be parallel to the main line, 1 km long and 1 km far away; the ground resistivity to be used in the calculation is to be specified. Comprehensive studies must be carried out at the planning stage, in order to decide whether to use filters or reroute parts of the transmission lines away from telephone systems.

The assessment of interference levels requires detailed information about the harmonic voltage and current profiles along the HVDC line; electromagnetic induction from harmonic currents is normally the main problem. Moreover, since both ends of the link contribute to the disturbance, it is necessary to obtain the profiles from each end and add their effects.

At each harmonic a profile along a single equivalent conductor is obtained.

This is determined from the vector addition of the harmonic current values from

78



each of the DC conductors and the DC overhead ground wire. The equivalent conductor is assumed to be located along the centre of the DC lines.

FILTERING CIRCUIT FOR VARIOUS HVDC LINES

79



5. Write short notes on Active filters

AC-side active cancellation

The complexity and high cost of conventional filters, together with their inability to correctly reduce resonances at noncharacteristic harmonics, has stimulated the development of power-electronic compensation techniques, generally described as active filters. The prospective application of active filtering to HVDC converters was suggested as early as 19711S for the elimination of harmonic currents by magnetic-flux compensation, as illustrated in Figure . A current transformer is used to detect the harmonic components coming from the nonlinear load. These are fed, through an amplifier, into the tertiary winding of a transformer in such a manner as to cause cancellation of the harmonic currents concerned. The main area of concern with this system is the coupling of the output of the amplifier to the tertiary winding in such a way that the fundamental current flow does not damage the amplifier. A quaternary winding and filter are used, as shown in Figure. To reduce the fundamental current in the amplifier output. Another important difficulty is the transfer of the amplified compensating current waveform from the low to high-voltage side of the transformer. A smallscale prototype was developed and discussed in Reference 13; its extension to a

300 MW converter was alleged to require a 750 kW amplifier. Thus the replacement of the lower-order characteristic harmonics does not appear to be a viable proposition at the moment.

DC-side active cancellation

As indicated above, the complete replacement of present passive-filter schemes is not envisaged. However, some of the main difficulties encountered on the AC side

80



do not apply on the DC side and an interesting hybrid solution has already been implemented.14'15 the principle of the passive/active filter concept is that the harmonics in the DC-line current are measured, and a controller reproduces a current in counter phase with the disturbance. This signal is then amplified in a high-power amplifier and finally fed into the neutral bus end of the DC filter by a transformer.The main components of the active filter are: a harmonic current transformer (HCT), a computerized controller, a pulse-width modulated (PWM) power amplifier, and a high-frequency transformer together with a transient overvoltage protection. These are shown in Figure. The harmonic current transformers consist of a Rogowski coil and electronic circuits which convert the measurement to optical information that is fed to ground potential by an optical fiber. The controller is realised in the same hardware and software environment as the control equipment for the HVDC pole. A digital signal processor handles the fast controller mathematics in conjunction with a single board computer. The power amplifier consists of a large number of digitally-modulated transistors (using the pulse width modulator principle) working in parallel in switched bridges.

The output ranges up to 4 kHz at a peak voltage of about 300 V and a power of

100 kVA. The high-frequency transformer is of a dry isolated type and has a ratio

1:20, with the low-voltage side connected to the amplifier. The transient overvoltage protection is released by two anti parallel thyristors of the same type as the thyristors used in the HVDC valve. The protection is triggered by overvoltage or over current.

81



UNIT V

PART A

1.What is meant by per unit system in power system?

The per unit value of any quantity is defined as the ratio of actual value in any unit and the base or reference value in the same unit.

Quantity(per unit) =

2.Mension the importance of per unit.



The per-unit values for transformer impedance, voltage and current are identical when referred to the primary and secondary (no need to reflect impedances from one side of the transformer to the other, the transformer is a single impedance).



The per-unit values for various components lie within a narrow range regardless of the equipment rating.



The per-unit values clearly represent the relative values of the circuit quantities. Many of the ubiquitous scaling constants are eliminated.

 Ideal for computer simulations

3. Need for load flow analysis.

Load flow studies determine if system voltages remain within specified limits under normal or emergency operating conditions, and whether equipment such as transformers and conductors are overloaded

 Optimize component or circuit loading

 Develop practical bus voltage profiles

 Identify real and reactive power flow

 Minimize kW and kVar losses

 Develop equipment specification guidelines

 Identify proper transformer tap settings

4. List out some load flow analysis methods.

 Gauss-Seidel



Newton-Raphson



Stott's algorithm

82



5. What are the Classifications of Load flow Power System Busses?

Classification Know ns

Unkno wn s

PQ (Load Bus) P, Q V,



Q,



PV (Generator

Bus)

P, V

V



(Swing Bus) V,



P, Q

6. Write down the solution procedure for Gauss-seidel method.

The solution procedure is to:

 Initialize the bus voltages. For load busses, use V = 1 + j 0. For generator busses (including the swing bus), use V



V spec

 j 0 .

 One-by-one, update the individual bus voltages using

V i



1 y i , i







P i spec 

V i

* jQ i spec

 j

N





1 , j

 y i , i j

V j







.

For PV busses, update the voltage angle, while holding the voltage magnitude constant at the specified value. Do not update the swing bus.

 Check the mismatch P and Q at each bus. If all are within tolerance (typical tolerance is 0.00001 pu), a solution has been found. Otherwise, return to

Step 2.

7. Write down the solution procedure for Newton rapson method.

The solution procedure for the Newton-Raphsonloadflow proceeds with:

 Initialize the bus voltages. For load busses, use V = 1 + j 0. For generator busses (including the swing bus), use V



V spec

 j 0 .

 Form the Jacobian matrix, and update all bus voltage magnitudes and phase angles, except for those at the swing bus, and except for the voltage magnitudes at PV busses.

 Check the mismatch P and Q at each bus. If all are within tolerance (typical tolerance is 0.00001 pu), a solution has been found. Otherwise, return to

Step 2.

83



8. Mention the type of buses.

Four variables are associated with each node

 Bus voltage magnitude (V )

 Voltage angle (δ)

 Real power (P)

 Reactive power (Q)

9. What is meant by DC per unit base?

Computational simplicity can be achieved by using common power and voltage base parameters on both sides of the converter. But to maintain consistency of power in p er unit the direct current base is √ 3 times more than the

AC current base.

10. What is PV bus?

It is also known as Generator Bus. Slack bus

– to balance the active and reactive power in the system. It is also known as the Reference Bus or the Swing Bus.

PART B

1.Explain about Per unit quantities.

Single-Phase Per Unit System

Advantages of the per unit system:

1. Transformers can be replaced by their equivalent series impedances.

2. Equipment impedances can be easily estimated since their per unit impedances lie within a relatively narrow range.

Four quantities in per unit of their respective base values:

V pu



V

V base volts volts

, I pu



I

I base amps amps

,

S pu



S

S base voltamps voltamps

, Z pu



Z

Z base ohms ohms

.

The relationships among the bases are

S base



V base



I base

, and Z base



V base

I base

.

Once two base variables are specified, the other two base variables may be calculated. A convenient relation, derived from the two above equations, is

S base



V base



I base



V base



V base

Z base



V

2 base

Z base

.

84



Consider a transformer with an N

1

: N

2

turns ratio and series impedance, reflected on side 1, equal Z

L 1

. Z

L 1

can be reflected to side 2 using

Z

L 2







N

2

N

1





2

Z

L 1

.

Let side 1 and side 2 have base values designated by subscripts S1 and S2. Then

Z base 1



V

2 base 1

S base 1

, Z base 2



V

2 base 2

S base 2

,

Expressing the transformer impedance on the two respective bases yields

Z

L 1 PU



Z

L 1

S base 1

V

2 base 1

, Z

L 2 PU



Z

L 2

S base 2

V

2 base 2

.

If S

B 1



S

B 2

, the two above equations may be combined so that

Z

L 2 PU







N

2

N

1





2 

V base 1

 V base 2





2

Z

L 1 PU

.

Substituting the relation between Z

L 1

and Z

L 2

yields

Z

L 2 PU







N

2

N

1





2 

V base 1

 V base 2





2

Z

L 1 PU

.

Therefore, if

V base 2

V base 1



N

2

, then

N

1

Z

L 2 PU



Z

L 1 PU

.

Hence, if a common voltampere base is chosen on both sides of the transformer, and if the voltage bases are chosen so that they vary according to the transformer turns ratio, then the per unit series impedance of the transformer is the same value on both sides.When analyzing a circuit with many transformers, a common voltampere base should be chosen throughout the circuit, and a voltage base should be chosen at one location. The voltage base must vary across the circuit according to the transformer turns ratios. When analyzing a circuit in per unit, if the bases are chosen according to the above rules, transformers can be replaced by their equivalent per unit series impedances, and their turns can be ignored.

A manufacturer usually provides the impedance of a transformer on the transformer's rated voltage and power bases. However, when solving a power

85



network circuit, the power and voltage bases must vary according to the above rules, and they may not equal the manufacturer-specified bases. Per unit impedances, specified on one base, may be converted to a new base as follows:

Given Z

PU , old



Z

Z base , old

, on bases V base , old

and S base , old

, Z

PU , new

on new bases

V base , new

and S base , new

is

Z

PU , new



Z ohms

Z base , new



Z

PU , old

Z base , old

Z base , new



Z

PU , old







V

2 base , old

S base , old













S base ,



V

2 base , new new







.

Three-Phase Per Unit System

The same advantages apply to a three-phase system if the following rules are obeyed:

1. A common three-phase voltampere base is used throughout the system, where

S base , 3





3 S base , 1

 .

2. Once selected at a point in the network, the three-phase voltage base must vary according to the line-to-line transformer turns ratios.

Convenient formulas relating single-phase to three-phase bases are given below.

S base , 1





V base , line

 neutral



I base

,

S base , 3





3 S base , 1

 ,

Z base



V

2 base , line

 neutral

S base , 1







V base , line

 line

/

S base , 3

 / 3

3



2



V

2 base , line

 line

S base , 3



.

2. Explain about power flow analysis in AC power system.

Formulation of the load flow problem. Gauss-Seidel, Newton-Raphson, and Stott's algorithm. Calculation of line flows, system losses, and area interchange.

Formulation of the Problem

The loadflow problem is one of the classic power system engineering problems. During the early days of digital computers, many advances in techniques for solving large sets of equations were brought about specifically to

86



help solve the loadflow problem. In most electrical circuit analyses, the network consists of known impedances, voltage sources, and current sources. However, in the loadflow problem, active and reactive powers, rather than shunt impedances, are specified at most network busses, because most loads behave, on average, as constant power loads (active and reactive power), as long as their applied voltage remains within reasonable ranges. Consider, for example, the air conditioning load of a building. A certain amount of energy is required to maintain

 T between inside and outside temperatures. Even though the air conditioner cycles on-and-off, and the voltage may change slightly, the air conditioning load appears, on the average, as a fixed power load, rather than as a fixed impedance load. Power system loads are closely monitored at substations, at large customers, and for total electric utility companies. Loads tend to have predictable daily, weekly, and seasonal patterns. Annual peak demands and energies for electric utilities are forecasted for generation and planning purposes. Most load flows model three-phase balanced systems. Positive-sequence values, usually in per unit, represent R, L, C, P, Q, S, Ω, V, and I.

The purpose of the load flow program is to compute bus voltages and line/transformer/cable power flows once network topology, impedances, loads, and generators have been specified. Ideally, the computed bus voltages for the study system should remain within acceptable ranges, and line/transformer/cable power flows should be below their rated values, for a reasonable set of outage contingencies.

From a load flow perspective, there are four parameters at every bus - voltage magnitude V , voltage angle



, active power P , and reactive power Q .

Two may be specified, and the other two calculated. For most busses, P and Q are specified, and V and



are calculated. Obviously, P and Q cannot be specified at all busses because that would imply that system losses are known a priori. Therefore, the load flow problem must include one "swing bus" at which the

P can assume any value so that it "makes up" system losses. The swing bus is usually a centrally-located large generator whose voltage magnitude and phase angle (usually



= zero) are specified. Although any two of the four parameters can be specified, the usual way in which power system busses are classified is given in Table 1.

87



Classification Knowns Unknowns

PQ (Load Bus)

PV (Generator Bus)

P, Q

P, V

V,



Q,



V



(Swing Bus) V,



P, Q

Table 1. Loadflow Classification of Power System Busses

The loadflow program solves for the set of unknowns that produces power balance at all busses, or as illustrated for bus i in Figure 1,

P i spec  jQ i spec 

P i calc  jQ i calc

, where

P i calc  jQ i calc 

V i

I i

*

.

In other words, the power specified at each bus must equal the power flowing into the system. Note in Figure 1 that specified power is drawn as positive generation, to be consistent with KCL equation YV=I .

Total Current Flowing From Bus i into the System is

IB1

Branch

Currents

Into

System

IB2

IB3

Bus i

| |  i spec

P + jQ i spec i i

*

Figure - Power Balance for Bus i

Since there are two unknowns at every bus, the size of the load flow problem is 2 N , where N is the number of busses. Obviously, to solve the problem, there must be two equations for every bus. These come from KCL, which for any bus i have the form

88




P i calc  jQ i calc 

V i

I i

* 

V i





 j

N





1 y i , j

V j







*

.

Separating into real and imaginary components yields two equations for bus i ,

P i spec  j

N





1

V i y i , j

V j cos



 i

  j

  i , j



,

Q i spec  j

N





1

V i y i , j

V j sin



 i

  j

  i , j



, where V i



V i

  i

, V j



V j

  j

, y i , j

 y i , j

  i , j

.

The problem now is now to find the set of bus voltages that satisfies the above 2 N equations.

Gauss-Seidel Method

Gauss-Seidel is an early formulation of the load flow problem that requires little memory and it is easily programmed. However, it is usually slower than other methods. It is based upon the idea of expanding the complex form of the power balance equation as follows:


V i

I i

* 

V i





 j

N





1 y i , j

V j







*



V i





 y i , i

V i

 j

N





1 , j

 y i , i j

V j







*

, or

P i spec  jQ i spec 

V i

* y i , i

V i



V i

* j

N





1 , j

 y i i

, j

V j

, so that

V i



1 y i , i







P i spec 

V i

* jQ i spec

 j

N





1 , j

 y i , i j

V j







.

The solution procedure is to:

1. Initialize the bus voltages. For load busses, use V = 1 + j 0. For generator busses

(including the swing bus), use V



V spec

 j 0 .

89



2. One-by-one, update the individual

V i



1 y i , i







P i spec 

V i

* jQ i spec

 j

N





1 , j

 y i , i j

V j







. bus voltages using

For PV busses, update the voltage angle, while holding the voltage magnitude constant at the specified value. Do not update the swing bus.

3. Check the mismatch P and Q at each bus. If all are within tolerance (typical tolerance is 0.00001 pu), a solution has been found. Otherwise, return to Step 2.

Convergence is usually faster if an acceleration factor is used. For example, assume that the voltage at bus i at iteration m is V i m

, and that the updating equation in

Step 2 computes V j new

. Instead of using V j new

directly, accelerate the update with

V i m



1 

V i m  



V i new 

V i m



, where acceleration factor



is in the range of 1.2 to 1.6.

Newton-Raphson Method

The Newton-Raphson method is a very powerful loadflow solution technique that incorporates first-derivative information when computing voltage updates. Normally, only 3 to 5 iterations are required to solve the loadflow problem, regardless of system size. Newton-Raphson is the most commonly used loadflow solution technique. An easy way to illustrate the Newton-Raphson technique is to solve a simple equation whose answers are already known. For example, consider

 x



1

 x



99





0 , which when expanded becomes x

2 

100 x



99



0 .

The objective is to find x so that f ( x )

 x

2 

100 x



99



0 .

Of course, in this case, the two solutions are known a priori as x = 1, and x = 99.

The Newton-Raphson procedure is based on Taylor's expansion, truncated past the first derivative, which gives

90



f ( x

  x )

 f ( x )



 f ( x )

 x x

.

Clearly, the above equation gives a straight-line approximation for f ( x

  x ) .

The objective is to find

 x so that f ( x

  x ) is the desired value (which in this example is zero). Solving for

 x yields

 x

 f ( x

  x )

 f ( x )

 x

 x f ( x )

, which for this example is

 x



0

 f

 f ( x )

 x

( x ) x



 f ( x )

 f ( x )

 x x

.

The update equation for iteration ( m + 1 ) is then x

( m



1 )  x

( m )   x

 x

( m ) 

 f f

( x )

 x

( x

( m )

) x

( m )

, where in this example

 f ( x )

 x



2 x



100 .

If a starting point of x = 2 is chosen, then the solution proceeds as follows:

Iteration - m

0

1

2 x

2

0.9896

0.9999 f(x)

-97

1.0193

0.0098

 f ( x )

 x

-96

-98.02

-98.00

91



Additional iterations can be performed if tighter solution tolerance is needed. Note that if a starting point of x = 50 had been chosen, the partial derivative would have been zero, and the method would have failed. If x = 80 is the starting point, then the process yields the following:

Iteration - m x f(x)

 f ( x )

 x

0 80 -1501 60

1

2

3

105.02

99.33

99.00

626.2

32.45

0

110.04

98.66

98.00

Therefore, the starting point greatly affects the ability of a Newton-Raphson method to converge, and the answer to which it converges. Fortunately, in the loadflow problem, most voltages are near 1.0 pu in magnitude and 0.0 degrees, so that we are able to accurately estimate starting values. For the load flow problem, the Newton-Raphson method is expanded in matrix form. For example, consider a set of N nonlinear equations and N unknowns, f

1 f

2



 x

1 x

1

,

, x

2

,  , x

2

,  , x

N x

N



 f

N



 x

1

, x

2

,  , x

N









 y

1 y

2



. y

N

The task is to find the set of unknown x

1

, x

2

,  , x

N

, given the known set y

1

, y

2

,  , y

N

, and given a starting point x

1

( 0 )

, x

( 0 )

2

,  , x

( 0 )

N

.

Applying Taylor's theorem as before, truncated after the first derivative, yields for

Row i y i

 f i

 x

1

( 0 )   x

1

, x

( 0 )

2

  x

2

,  , x

( 0 )

N

  x

N



 f i

 x

1

( 0 )

, x

( 0 )

2

,  , x

( 0 )

N





+

 x

1

 f i

 x

1 x ( 0 )

  x

2

 f i

 x

2 x ( 0 )

    x

N

 f i

 x

N x ( 0 )

,

92



where x(0) represents the starting estimate set x

1

( 0 )

, x

( 0 )

2

,  , x

( 0 )

N

.

N similar equations in matrix form are









 y y



1

2 y

N













 f

1

 x

1

 f

2

 x

1



 f

N

 x

1



















 f

1



 f x

2

 x

2

2

 f



 x

N

2







 f

1



 x f

N

2

 x



N

 f

 x

N

N

















 







  x

 x

1

 x

2



N























 f f f

1

2

N



 x x

( 0 )

1

(

1

0 )

 x

1

( 0 )

,

,

, x x

( 0 )

2

( 0 )

2

 x

( 0 )

2

,  ,

, 

, 

,

, x x x

( 0 )

N

( 0 )

N

N

( 0 )





















, or











 y y y

N

1

2





 f f f

1

2



 x

1

( 0 ) x

1

( 0 )

,

,

( 0 ) x

2 x

( 0 )

2

, 

N

 x

1

( 0 )



, x

( 0 )

2

,

,  ,

,  , x x

N x

(

N

(

( 0 )

0 )

N

0 )





















 















  x x

 x

1

2

N











, where J is an N x N matrix of partial derivatives, known as the Jacobian matrix.

Therefore, in an iterative procedure, the above equation is used to update the X vector according to

X

( m



1 ) 

X

( m )  

X

( m ) 

X

( m ) 

J m )



1

Y spec



Y

( m



)

, where

Y spec













 y y



1

2 y

N













, Y

( m ) 











 f f f

1

2

N





 x

1

( m ) x

1

( m )

,

, x

1

( m )

, x x

(

2 m )

( m

2



)

( x

2 m )

,  ,

, 

, 

,

, x x x

(

(

N m )

( m

N

N

) m )





















.

In the loadflow problem, the matrix update equation is symbolically written in mixed rectangular-polar form as

93























P

P

P

Q

1 spec

2 spec

Nspec



1 spec









Q

 Q

2 spec

Nspec



















P

P

P

Ncalc



( m )

Q

Q

( m )

1 calc

( m )

2 calc

( m )

1 calc

( m )

2 calc

Q

( m )

Ncalc









































J

J

1

3







P





Q



J

2

J

4







P



V



Q



V







































 

 

 m



1



 m



1







 

 

 m



1







V

1

V

2

 m





1









 m



1





V

1

V

2

V

N

 m



1







V

N



























 or, in abbreviated form,







P



Q











J

1

J

3

J

2

J

4















V



 .

The dimension of the above problem is actually



2 N





Number of PV busses





2



since

V updates at PV busses are not required, and since V and



updates at the swing bus are not required.

In highly inductive power systems, P is closely related to voltage angles, and Q is closely related to voltage magnitudes. Therefore, in the above mixed rectangularpolar formulation, the terms in J

1 and J

4

tend to have larger magnitudes than those in J

2 and J

3

. This feature makes the Jacobian matrix more diagonally dominant, which improves robustness This modification is







P



Q

















1

V

1

V



P





Q





P



V



Q



V

















V





V

 





.

The partial derivatives are derived from

P i calc  j

N





1

V i y i , j

V j cos



 i

  j

  i , j



,

Q i calc  j

N





1

V i y i , j

V j sin



 i

  j

  i , j



,

94



and have the following form

For J

1

,



P i

 i





P i k

  j

N





1 , j

V i

 i



V i y i , k

V k y i , j

V j sin



 i sin





  k i

  j

  i , j



  i , k



, k

 i .

For J

2

,





P i

V i





P i

V k

 j

N





1 , j

 y i i

, j



V i y i , k

V j cos



 i cos



 i

  k

  j

  i , j





2 V i

  i , k



, k

 i .

y i , i cos



  i , i



For J

3

,



Q i

 i



Q i

 k

 j

N





1 , j

V i

 i

 

V i y i , j

V j cos



 i

  j

  i , j

 y i , k

V k cos



 i

  k

  i , k



, k

 i .

For J

4

,





Q i

V i





Q i

V k

 j

N





1 , j

 y i i

, j



V i y i , k

V j sin



 i sin



 i

  k

  j

  i , j





2 V i

  i , k



, k

 i .

y i , i sin



  i , i



Note the symmetry in the J terms. If V



is used as an updating parameter rather than



, then the expressions for J

1

are

V i



P i

 i

V k



P i

 k





 j

N 



1 , j

 i y i ,

V i y i , k j

V j sin



 i sin



 i

  k

  j

  i , j

  i , k



, k



 i .

95



and for J

3

,



V i

Q i

 i

V i



Q i

 k

 j

N





1 , j

 y i i

,

  y i , k

V k j

V j cos

 cos



 i

 i

  j

  k

  i , j



  i , k



, k

 i .

The diagonal dominance of J

1

and J

4

can be observed by the examining the partial derivatives as follows: the difference between voltage angles at adjacent busses is usually small, so that the



 i

  j



terms are small. The angles found in the admittance matrix are usually large (i.e. near 90 o

) because most power systems are reactive. Therefore, the sine terms in the matrix update equation tend to be near unity, while the cosine terms tend to be near zero.

Decoupled load flow programs use only J

1

and J

4

, treating the P and Q problems separately. So that the benefits of optimal bus ordering can be fully exploited, nondecoupled load flow Jacobian matrices are usually formulated in the following alternating-row form, rather than that described symbolically above:



























P



Q

P



Q

1

1

2

2



P

N



Q

N























 





















V

1



V

2





V

1



V

2





1

2



V

N





V

N



N





















.

The solution procedure for the Newton-Raphson loadflow proceeds with:

 Initialize the bus voltages. For load busses, use V = 1 + j 0. For generator busses

(including the swing bus), use V



V spec

 j 0 .

 Form the Jacobian matrix, and update all bus voltage magnitudes and phase angles, except for those at the swing bus, and except for the voltage magnitudes at PV busses.

 Check the mismatch P and Q at each bus. If all are within tolerance (typical tolerance is 0.00001 pu), a solution has been found. Otherwise, return to Step 2.

96



3 .

Explain about power flow analysis in DC power system.

Newton-Raphson Solution Algorithm

HVDC Load Flow Solution Method

High Voltage Direct Current (HVDC) transmission is important for long distance, underground, and submarine transmission. Due to the increasing strains on existing systems, it is necessary to develop a better method for performing the load flow analysis of an integrated HVDC power system. However, the power flow has to be substantially enhanced to be capable of modeling the operating state of the combined AC and DC systems, and this must be done fast and efficiently under the specified conditions of load generation and DC system control strategies. The development of an enhanced HVDC-load flow system based on the Newton - Raphson method is the focus of this chapter. The variation of the DC link chosen for the problem formulation are: (1) the converter, terminal DC voltage; (2) the real and imaginary components of the transformer secondary

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current; (3) converting transformer tap ratios; (4) the firing angle of the rectifier; and (5) the current in the DC link.

The equations relating these five variables and their solution strategy are discussed. As the model developed is independent of a particular control mode of the DC link, the AC and DC link equations are solved separately and thus the integration into a standard load flow program is possible without significant modifications of the AC load flow algorithm. In the AC system iterations, each converter is designed as a complex power load at the AC terminal bus bar, and the DC link equations are solved using the most recent value of the AC bus bar voltage. The AC and DC system equations are solved simultaneously, taking

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Single-Phase Equivalent Circuit for Basic Converters (Angles Referred to

DC. Reference) The DC voltage at converter station k is:

G is a conductance matrix of MTDC system.

All the angles are calculated by degree.

For the AC line current of the inverter transformers with converter station k, we can write the relation as

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K=1 means the number of converters present. The normal AC power flow equations are valid, except that mismatches power flow equations at the converter station

AC buses are modified. Therefore, the mismatches power flow equations may be

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summarized


as follows:

Where V is the vector of the voltage magnitudes at all AC system buses and

� is vector of the voltage angles at all AC system buses, except that a once slack bus, which is assumed to be equal to zero. In the power flow equations R of

AC-MTDC system, the only modification required to the usual real and reactive mismatches equations occurs for those equations, which relate to the converter terminal AC buses.

4. Explain the power flow analysis of IEEE 30-Busbar System

The standard bus system has been modified into two and three -terminal

DC link systems in order to solve the AC, DC and HVDC systems independently.

In terms of accuracy and computational speed, this enhanced method is superior to any conventional Newton-Raphson (NR) load flow and performs effectively under various types of network conditions where other methods cannot give satisfactory results. Simulation studies have also been conducted under different control specifications. It is not possible to compare results of various research papers because each one uses a different test system.

The test systems are based on the Institute of Electrical and Electronics

Engineers‘ (IEEE) 30-bus system case studies for power system networks. The system base MVA and the base voltage are 100 MVA and 100 KV, respectively.

The tolerance for convergence checking is 0.00001, the details of which are provided here: Case I: AC line between bus bars 2 and 5 is replaced by two DC links. Case II: A three-terminal DC link is formed between bus bars 2 and 7 by placing an AC line connecting bus bars 2 and 5 by a two-terminal DC link and introducing a two-terminal DC link between bus bars 2 and 7 and bus bars 5 and 7

Test Results

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The test results of the method are compared with those corresponding to the Newton- Raphson power flow method. A number of sample systems are studied through the proposed method and its HVDC power flow performance. In order to investigate the feasibility of the proposed technique, a large number of power systems of varying sizes have been modified under different system conditions in two- and three-terminal DC link configurations in order to solve

HVDC power flow problems.

Flowchart of Unified AC-DC Load Flow

.

Flowchart of Unified AC-DC Load Flow

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5. Explain the power flow analysis of IEEE 14-Busbar System

The test systems are based on the Institute of Electrical and Electronics

Engineers‘ (IEEE) 14- bus system case studies for power system networks. The system base MVA and the base voltage are 100 MVA and 100 KV, respectively.

The tolerance for convergence checking is 0.00001, the details of which are provided here Case I: The AC line between bus bars 4 and 5 is replaced by two

DC links. Case II: The AC lines between bus bars 2-4, 2-5, and 4-5 are replaced by a DC link; a three-terminal DC link is obtained between bus bars 2, 4 and 5.

14-Bus system with two terminal DC converters

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Analysis of Results for 14-Bus with two DC converter system

For14 bus systems with two converters we concluded that the overall active power is improved after using the converters as well as the overall reactive power for mode 1. For mode 2 to mode 7 the active and reactive power is improved for some buses but became worse for others. For example the active power at bus 4 without using the converters was 47.8 MW and became 50.57 MW after using the converters for mode 1 but it is changed to be 49.12 MW for mode 2 , 53.14MW for mode 3, 60.15MW for mode 4, 55.50MW for mode 5, 52.10MW for mode 6, and

56.21MW for mode 7. Also the reactive power at bus 4 without using the converters was 3.9 MVAR and became 8.50 MVAR for mode 1 but it is changed to be 4.96 MVAR for mode 2, 10.72MVAR for mode 3, 4.21MVAR for mode4,

12.03MVAR for mode 5, 6.01MVAR for mode 6, 11.00MVAR for mode 7. For bus

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12 without using the converters was 6.1 MW and became 10.68 MW after using the converters for mode 1 but it is changed to be 12.78 MW for mode 2, 8.40MW for mode 3, 7.75MW for mode 4, 17.73MW for mode 5, 15.10MW for mode 6, and

9.78MW for mode 7. Also the reactive power at bus 12 without using the converters was 1.6 MVAR and became 4.23 MVAR for mode 1 but it is changed to be 3.50 MVAR for mode 2 , 7.73 MVAR for mode 3, 4.90 MVAR for mode 4,

2.87 MVAR for mode 5, 3.61 MVAR for mode 6, 3.35 MVAR for mode 7.

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HVDC BY SCAD

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