A
MINI PROJECT REPORT
ON
OPERATION AND MAINTENANCE OF 220/132KV
SUBSTATION
Submitted in partial fulfillment for the award of the Degree of
Bachelor of Technology in Electrical and Electronics Engineering
Submitted By
V.RAVALIKA (08281A0212)
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
KAMALA INSTITUTE OF TECHNOLOGY AND SCIENCE
(Affiliated to J.N.T.U, Hyderabad)
SINGAPUR, KARIMNAGAR -505468
(2008-2012)
ABSTRACT
A Substation receives electrical power from generating station via incoming
transmission line and delivers electrical power through feeders and this is used for
controlling the power on different routes. Substations are integral part of a power
system and form important part of transmission and distribution network of electrical
power system.
Their main functions are to receive energy transmitted at high voltage from
the generating stations, reduce the voltage to a value appropriate for local distribution
and provide facilities for switching some sub-station are simply switching stations
different connections between various transmission lines are made, others are
converting sub-stations which either convert AC into DC or vice-versa or convert
frequency from higher to lower or vice-versa.
The various circuits are joined together through these components to a bus-bar
at substation. Basically, Sub-station consists of power transformers, circuit breakers,
relays, isolators, earthing switches, current transformers, voltage transformers,
synchronous condensers/ Capacitor banks etc.
This mini project covers the important equipments & their function in a SubStation. And also an attempt is made to cover the general maintenance of Substation
and Checks the observations to be made by Shift Engineer. As a part of case study we
are going to visit a 220/132Kv TRANSCO substation in Warangal.
CONTENTS
Chapter No
1
2
3
4
TITLE
Page no.
List of Abbreviations
iii
List of Symbols
iv
List of Figures
v
List of Tables
vi
INTRODUCTION
1
1.1
Introduction
1
1.2
Construction of a substation
1
CLASSIFICATION OF SUBSTATIONS
3
2.1
According to the requirement
3
2.2
According to the constructional features
4
SINGLE LINE DIAGRAM
6
3.1
Feeder Circuit
6
3.2
Transformer Circuit
6
3.3
Auxiliary supply
7
BRIEF DISCRIPTION OF INSTRUMENTS IN THE
SUBSTATION
8
4.1
Lightening Arrestors
8
4.2
Earthing
12
4.3
Capacitor Voltage Transformer
13
4.4
Wave Trap
15
4.5
Isolator with ES (Earth Switches)
16
4.6
Instrument Transformers
17
4.7
Circuit Breakers
26
i
Chapter No
TITLE
Page no.
4
5
6
7
4.8
Bus
31
4.9
Transformers
31
4.10
Capacitor Bank attached to the bus
35
TYPES OF CONTROL
37
5.1
Capacitors
38
5.2
Bus bar systems
38
5.3
Station battery
38
5.4
Insulators
40
PROTECTION FOR VARIOUS EQUIPMENTS
43
6.1
Transformer
43
6.2
Feeder
43
220/132KV SUBSTATION AT WARANGAL
44
7.1
Substation at Warangal
45
7.2
Salient Features of 220/132KV Substation
46
7.3
Important points to be kept in view while
48
laying out the substation
8
CONCLUSION
50
REFERENCES
51
ii
LIST OF ABBREVIATIONS
EHV–Extra high voltage
SLD– Single line diagram
PT – Potential transformer
CT – Current transformer
HVCT- High voltage CT
LVCT – Low voltage CT
CVT – Capacitor voltage transformer
LA – Lightening arrestors
ES - Earth switches
CB – Circuit breaker
HV side – High voltage side
LV side – Low voltage side
PLCC - Power Line Carrier Communication
OLTC – On load tap changer
HG Fuse - -Horn gap fuse
OTI – Oil temperature indicator
WTI – Winding temperature indicator
IDMT Characteristics – Inverse definite minimum time characteristics
iii
LIST OF SYMBOLS
X0 – Zero sequence reactance
X1- Positive sequence reactance
R0- Zero sequence resistance
Ip – Primary current
Np – Primary Winding Turns
Is – Secondary Current
Ns – Secondary Winding Turns
Vp – Primary voltage
Vs – Secondary voltage
Zs – Impedance attached at the secondary side coil
iv
LIST OF FIGURES
Fig No
Title
Page No.
2.1
Construction of the substation
5
3.1
Single line dig of a 220/132kv substation
7
4.1(i)
Surge diverter
9
4.1(ii)
Characteristics of Non linear resistor
9
4.2
Lightening arrestors
9
4.3.1
Circuit diagram of CVT
14
4.3.2
Capacitor voltage transformer
14
4.4.1
Wave trap
15
4.5.1
Isolator with earth switch
17
4.6.1
Line diagram of CT
19
4.6.2.1
Line diagram of VT
24
4.6.2.2
Potential transformer
25
4.7.1
SF6 Circuit breaker
29
4.9.1.1
Electrical transformer
31
4.9.1.2
Ideal transformer
32
4.9.1.3
Mutual induction
33
4.9.3
Three phase 100MVA Auto transformer
34
4.10.1
Capacitor bank in the Distribution system
35
4.10.2
Reactive Losses
36
5.1
Types of control
37
5.3
Station Batteries
41
5.4
Ball and socket type Disc insulator
43
v
LIST OF TABLES
Table No.
Title
Page No
4.1.3
LA voltage rating
11
4.1.4
The limits of LA and Transformers
11
4.6.1.5
The specifications of HVCT
22
4.6.1.6
The specifications of HVCT
23
5.4
Insulators
42
vi
CHAPTER 1
INTRODUCTION
1.1 INTRODUCTION :
The present-day electrical power system is A.C. i.e. electric power is
generated, transmitted and distributed in the form of alternating current. The electric
power is produced at the power stations which are located at favourable places,
generally quite away from the consumers. It is delivered to the consumers through a
large network of transmission and distribution. At many places in the line of the
power system, it may be desirable and necessary to change some characteristic (e.g.
voltage, A.C. to D.C., frequency, Power factor etc.) of electric supply.
This is accomplished by suitable apparatus called sub-station. For example,
generation voltage (11KV or 6.6KV) at the power station is stepped up to high
voltage (say 220KV or 132KV) for transmission of electric power. The assembly of
apparatus (e.g. transformer etc.) used for this purpose is the sub-station. Similarly,
near the consumer’s localities, the voltage may have to be stepped down to utilization
level. This job is again accomplished by a suitable apparatus called ‘substation.
1.2 CONSTRUCTION OF A SUBSTATION
At the time of constructing a substation, we have to consider some factors
which affect the substation efficiency like selection of site.
1
1.2.1 SELECTION OF SITE:
Main points to be considered while selecting the site for EHV Sub-Station are
as follows:
i) The site chosen should be as near to the load centre as possible.
ii) It should be easily approachable by road or rail for transportation of equipments.
iii) Land should be fairly levelled to minimize development cost.
iv) The source of water should be as near to the site as possible. This is because water
is required for various construction activities;
(Especially civil works,), earthing and for drinking purposes etc.
v) The sub-station site should be as near to the town / city but should be clear of
public places, aerodromes, and Military / police installations.
vi) The land should be have sufficient ground area to accommodate substation
equipments, buildings, staff quarters, space for storage of material, such as store yards
and store sheds etc. with roads and space for future expansion.
vii) Set back distances from various roads such as National Highways, State
Highways should be observed as per the regulations in force.
viii) While selecting the land for the substation preference to be given to the Govt.
land over Private land.
ix) The land should not have water logging problem.
x) The site should permit easy and safe approach to outlets for EHV lines.
2
CHAPTER 2
CLASSIFICATION OF SUBSTATIONS
There are several ways of classifying sub-stations. However, the two most
important ways of classifying them are according to (1) service requirement and (2)
constructional features.
2.1 ACCORDING TO THE REQUIREMENT:
A sub-station may be called upon to change voltage level or improve power
factor or convert A.C. power into D.C. power etc. According to the service
requirement, sub-stations may be classified into:
(i)
Transformer sub-stations: Those sub-stations which change the voltage
level of electric supply are called transformer sub-stations. These sub-stations receive
power at some voltage and deliver it at some other voltage. Obviously, transformer
will be the main component in such sub-stations. Most of the sub-stations in the
power system are of this type.
(ii)
Switching sub-stations: These sub-stations do not change the voltage level
i.e. incoming and outgoing lines have the same voltage. However, they simply
perform the switching operations of power lines.
(iii)
Power factor correction sub-stations: Those sub-stations which improve
the power factor of the system are called power factor correction sub-stations. Such
sub-stations are generally located at the receiving end of transmission lines. These
sub-stations generally use synchronous condensers as the power factor improvement
equipment.
3
(iv)
Frequency changer sub-stations: Those sub-stations which change the
supply frequency are known as frequency changer sub-stations. Such a frequency
change may be required for industrial utilization.
(v)
Converting sub-stations: Those sub-stations which change A.C. power into
D.C. power are called converting sub-stations. These sub-stations receive A.C. power
and convert it into D.C. power with suitable apparatus (e.g. ignitron) to supply for
such purposes as traction, electroplating, electric welding etc.
(vi) Industrial sub-stations:- Those sub-stations which supply power to individual
industrial concerns are known as industrial sub-stations.
2.2 ACCORDING TO THE CONSTRUCTIONAL FEATURES:
A sub-station has many components (e.g. circuit breakers, switches, fuses,
instruments etc.) which must be housed properly to ensure continuous and reliable
service. According to constructional features, the sub-stations are classified as:


Indoor sub-station

Underground sub-station

(i)
Outdoor sub-station
Pole-mounted sub-station
Indoor sub-stations:- For voltages up to 11KV, the equipment of the sub-
station is installed indoor because of economic considerations. However, when the
atmosphere is contaminated with impurities, these sub-stations can be erected for
voltages up to 66 KV.
4
(ii)
Outdoor sub-stations:- For voltages beyond 66KV, equipment is invariably
installed out-door.
It is because for such voltages, the clearances between conductors and the
space required for switches, circuit breakers and other equipment becomes so great
that it is not economical to install the equipment indoor.
(iii)
Underground sub-stations:- In thickly populated areas, the space available
for equipment and building is limited and the cost of land is high. Under such
situations, the sub-station is created underground.
(iv)
Pole-Mounted sub-stations:- This is an outdoor sub-station with equipment
installed over-head on H-pole or 4-pole structure. It is the cheapest form of substation for voltages not exceeding 11KV (or 33 KV in some cases). Electric power is
almost distributed in localities through such sub-station.
Fig: 2.1 CONSTRUCTION OF THE SUBSTATION.
5
CHAPTER 3
SINGLE LINE DIAGRAM (SLD)
A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of
the concerned Electrical System which includes all the required electrical equipment
connection sequence wise from the point of entrance of Power up to the end of the
scope of the mentioned Work. As in the case of 132KV Substation, the SLD shall
show Lightening Arrestor, C.T/P.T Unit, Isolators, Protection and Metering P.T &
C.T. Circuit Breakers, again Isolators and circuit Breakers, Main Power Transformer,
all protective devices/relays and other special equipment like CVT, GUARD RINGS,
etc as per design criteria. And the symbols are shown below. There are several
feeders enter into the substation and carrying out the power. As these feeders enter
the station they are to pass through various instruments.
3.1 FEEDER CERCUIT:
1. Lightening arrestors; 2. CVT; 3. Wave trap; 4. Isolators with earth switch
5. Current transformer; 6. Circuit breaker; 7. Feeder Bus isolator
8. BUS; 9. Potential transformer in the bus with a bus isolator
3.2 TRANSFORMER CIRCUIT:
i) HV side:
1. Transformer bus Isolator
3. Current transformer
2. Circuit breaker
4. Lightning Arrestors
5. Auto Transformer 100MVA (220/132KV)
ii) LV side:
1. Lightening arrestors
5. Bus
2. Current transformer
6. Potential transformer with a bus isolator
3. Circuit breaker
7. A capacitor bank attached to the bus
4. Bus Isolator.
6
3.3 AUXILIARY SUPPLY:
220V.Battery system: To control and protect the substation equipment the
220 volts DC battery system is necessary. It is provided in the main control room. It
will be discussed below.
Fig: 3.1 SINGLE LINE DIAGRAM OF A 220/132KV SUBSTATION
WARANGAL.
7
CHAPTER 4
BRIEF DISCRIPTION OF INSTRUMENTS IN THE
SUBSTATION
4.1 LIGHTENING ARRESTORS:
4.1.1 Lightening Arrestors:
Lightening arrestors are the instruments that are used in the incoming feeders
so that to prevent the high voltage entering the main station. This high voltage is very
dangerous to the instruments used in the substation. Even the instruments are very
costly, so to prevent any damage lightening arrestors are used. The lightening
arrestors do not let the lightening to fall on the station. If some lightening occurs the
arrestors pull the lightening and ground it to the earth. In any substation the main
important is of protection which is firstly done by these lightening arrestors. The
lightening arrestors are grounded to the earth so that it can pull the lightening to the
ground.
These are located at the entrance of the transmission line in to the substation
and as near as possible to the transformer terminals.

LA will be provided on the support insulators to facilitate leakage current
measurement and to count the no of surges discharged through the LA.

LA bottom flange will be earthed via leakage ammeter and surge counter.
Leakage current is to be recorded periodically. If the leakage current enters into the
red range from the green range, the LA is prone for failure. Hence, it is to be
replaced.

There should be independent earth pit for LA in each phase so as to facilitate
fast discharging and to raise the earth potential.
8
The lightning arresters or surge diverters provide protection against such
surges. A lightning arrester or a surge diverter is a protective device, which conducts
the high voltage surges on the power system to the ground.
Fig.4.1 (i) Surge diverter
(ii)Characteristics of the non linear resister

Fig 4(i) shows the basic form of a surge diverter. It consists of a spark gap in series
with a non-linear resistor. One end of the diverter is connected to the terminal of the
equipment to be protected and the other end is effectively grounded. The length of the
gap is so set that normal voltage is not enough to cause an arc but a dangerously high
voltage will break down the air insulation and form an arc. The property of the nonlinear resistance is that its resistance increases as the voltage (or current) increases
and vice-versa. This is clear from the volt/amp characteristic of the resistor shown in
Fig 4 (ii).
Fig: 4.2 LIGHTENING ARRESTORS.
9
4.1.2. The action of the Lightning Arrester or surge diverter is as
under:
(i) Under normal operation, the lightning arrester is off the line i.e. it conducts
no current to earth or the gap is non-conducting.
(ii) On the occurrence of over voltage, the air insulation across the gap breaks
down and an arc is formed providing a low resistance path for the surge to the
ground. In this way, the excess charge on the line due to the surge is harmlessly
conducted through the arrester to the ground instead of being sent back over the line.
(iii) It is worthwhile to mention the function of non-linear resistor in the
operation of arrester. As the gap sparks over due to over voltage, the arc would be a
short circuit on the power system and may cause power-follow current in the arrester.
Since the characteristic of the resistor is to offer low resistance to high voltage (or
current), it gives the effect of short circuit. After the surge is over, the resistor offers
high resistance to make the gap non conducting.
4.1.3. Guide for selection of LA:
(i) Before selecting the LA it should be ascertained whether the system is
effectively earthed, non-effectively earthed or having isolated neutral.
(ii) The system neutrals are considered to be effectively earthed when the coefficient of earthing does not exceed 80%.
In this case, the reactance ratio X0/ X1 (zero sequence reactance/positive
sequence reactance) is positive and less than 3 and at the same time the resistance
ratio RO/X1 (zero sequence resistance/positive sequence reactance) is less than 1 at
any point on the system. For this system the arrestor rating will be 80% of the highest
phase to phase system voltage.
10
(iii)The LA voltage rating corresponding to the system voltages normal are
indicated below :
Rated system
Highest system
Arrester rating in KV
Voltage (KV)
Voltage (KV)
11
12
9
33
36
30
66
72.5
60
132
145
120/132 (latex)
220
245
198/216 (latex)
400
420
336
Effectively earthed systems
Table: 4.1.3 LA voltage rating
4.1.4 LOCATION OF LIGHTING ARRESTORS:
The LAs employed for protecting transformers should be installed as close as
possible to the transformer. The electrical circuit length between LA and the
transformer bushing terminal should not exceed the limits given below:
Rated system
BIL
Max. distance between L.A and
Voltage
KV
Transformer bushing terminal
KV
Peak
(inclusive of lead length) (in metres)
Effectively earthed
11
75
12.0
33
200
18.0
66
325
24.0
132
550
35.0
650
43.0
900
Closes to
1050
Transformer
220
400
1425
1550
Table: 4.1.4 The limits of LA and Transformers
11
4.2 EARTHING:
The earthing practice adopted at generating stations, sub-stations and lines
should be in such a manner as to provide:
a) Safety to personnel
b) Minimum damage to equipment as a result of flow of heavy fault currents
c) Improve reliability of power supply
4.2.1 The primary requirements are:
The impedance to ground (Resistance of the earthing system) should be as low as possible
and should not exceed,
Large sub-stations -1 ohm
Small sub-stations -2 ohms
Power stations -0.5 ohms
Distribution transformer stations- 5 ohms
4.2.1.1 All exposed steel earthing conductors should be protected with bituminous
paint.
4.2.1.2 PLATE EARTHING:
i) EHT Substation - 1.3 M x 13 M.Ms cast iron plates 25mm thick Plates are
to be buried vertically in pits and surrounded by finely divided coke, crushed coal or
char coal at least 155 mm all round the plates. Plates should not be less than 15 m
apart and should be buried to sufficient depth to ensure that they are always
surrounded by moist earth.
4.2.1.3 PIPE EARTHING:
a) EHT substations Cast iron pipes 125 mm in diameter 2.75 m long and not less than
9.5 mm thick pipes 50.8mm in dia and 3.05m long. Pipes are to be placed vertically at
intervals of not less than 12.2 m in large stations surrounded by finely broken coke
crushed coal and charcoal at least 150 mm around the pipe on the extra depth.
a) Peripheral or main earth mat-
100 x 16 m MS flat
b) Internal earth mat-
50 x 8m MS flat to be placed at 5m apart
c) Branch connections-
Cross section not less than 64.5 square meters
12
Joints are to be kept down to the minimum number. All joints and
connections in earth grid are to be brazed, riveted, sweated, bolted or welded. For rust
protection the welds should be treated with barium chromate. Welded surfaces should
be painted with red lead and aluminium paint in turn and afterwards coated with
bitumen. Joints in the earthing conductor between the switch gear units and the cable
sheaths, which may require to subsequently broken should be bolted and the joint
faces tinned. All joints in steel earthing system should be made by welding except the
points for separating the earthing mat for testing purposes which should be bolted.
These points should be accessible and frequently supervised.
4.2.1.4 In all sub-stations there shall be provision for earthing the following:
a) The neutral point of earth separate system should have an independent
earth, which in turn should be interconnected with the station grounding mat
b) Equipment frame work and other non-current carrying parts (two
connections)
c) All extraneous metallic frame work not associated with equipment (two
connections)
d) Lightning arrestors should have independent earths which should in turn be
connected to the station grounding grid.
e) Over head lightning screen shall also be connected to the main ground mat.
4.2.1.5 The earth conductor of the mat could be buried under earth to economical
depth of burial of the mat 0.5 meters.
4.3 CAPACITOR VOLTAGE TRANSFORMER (CVT):
A capacitor voltage transformer (CVT) is a transformer used in power systems
to step-down extra high voltage signals and provide low voltage signals either for
measurement or to operate a protective relay.
13
These are high pass Filters (carrier frequency 50KHZ to 500 KHZ) pass
carrier frequency to carrier panels and power frequency parameters to switch yard. In
its most basic form the device consists of three parts: two capacitors across which the
voltage signal is split, an inductive element used to tune the device and a transformer
used to isolate and further step-down the voltage.
Fig: 4.3.1 CIRCUIT DIAGRAM OF CVT.
The device has at least four terminals, a high-voltage terminal for connection
to the high voltage signal, a ground terminal and at least one set of secondary
terminals for connection to the instrumentation or protective relay. CVTs are typically
single-phase devices used for measuring voltages in excess of one hundred KV where
the use of voltage transformers would be uneconomical. In practice the first capacitor,
C1, is often replaced by a stack of capacitors connected in series. This results in a
large voltage drop across the stack of capacitors, that replaced the first capacitor and a
comparatively small voltage drop across the second capacitor, C2, and hence the
secondary terminals.
Fig: 4.3.2 CAPACITOR VOLTAGE TRANSFORMER.
14
4.3.1 Specifications of CVT:
CVT type
: CVEB/245/1050
Weight
: 665 kg
Total output simultaneous
: 250 VA
Output maximum
: 750 VA at 50O C
Rated voltage
: A-N, 220/√3
Highest system voltage
: A-N, 245/√3
Insulation level
: 460/1050 KV
Rated frequency
: 50Hz
Nominal intermediate voltage
: A1-N, 20/√3 KV
Voltage factor
: 1.2Cont. 1.5/30 sec
‘HF’ capacitance
: 4400pF +10% -5%
Primary capacitance C1
: 4840pF +10% -5%
Secondary capacitance C2
: 48400 pF +10%-5%
Voltage ratio
: 220000/√3/ 110/√3/110-110/√3
Voltage
: 110/√3
110-110/√3
Burden
: 150
100
Class
: 0.5
3P
4.4 WAVE TRAP:
Wave trap is an instrument using for trapping of the wave. The function of
this wave trap is that it traps the unwanted waves. Its shape is like a drum. It is
connected to the main incoming feeder so that it can trap the
waves which may be dangerous to the instruments in the
substation. Generally it is used to exclude unwanted frequency
components, such as noise or other interference, of a wave.
Note: Traps are usually unable to permit selection of unwanted
or interfering signals.
Fig: 4.4.1 WAVE TRAP.
15
Line trap also is known as Wave trap. What it does is trapping the high
frequency communication signals sent on the line from the remote substation and
diverting them to the telecom/tele protection panel in the substation control room
through coupling capacitor.
This is relevant in Power Line Carrier Communication (PLCC) systems for
communication among various substations without dependence on the telecom
company network. The signals are primarily tele protection signals and in addition,
voice and data communication signals. The Line trap offers high impedance to the
high frequency communication signals thus obstructs the flow of these signals in to
the substation bus bars. If these are not present in the substation, then signal loss is
more and communication will be ineffective/probably impossible.
4.5. ISOLATOR WITH EARTH SWITCHES (ES):
Isolators are the no load switches and used to isolate the equipment. (Either
line equipment, power transformer equipment or power transformer). With the
isolators, we are able to see the isolation of the equipment with our naked eye. The
line isolators are used to isolate the high voltage from flow through the line into the
bus. This isolator prevents the instruments to get damaged. It also allows the only
needed voltage and rest is earthed by itself.
Isolator is a type of switching device. It has non control devices. Isolator are
operated after the circuit breaker is opened. While closing the circuit, first close the
isolator and after the circuit breaker is closed. Strictly speaking Isolators are operated
under no current condition. In the following cases it is permissible to use isolator for
making and breaking of the circuits.
16
Fig: 4.5.1 ISOLATOR WITH EARTH SWITCH.
Air break isolators or disconnecting switches are not intended to break load
though these are meant for transfer of load from one bus to another and also to isolate
equipment for maintenance. These are available mainly in two types vertical break
type and horizontal break type. The later type requires larger width. However the
space requirement can be reduced in the horizontal break isolators by having double
break with a centre rotating pillar.
Pantograph and semi-pantograph disconnects involve vertical movements of
contact arm and therefore require less separation between phases and thereby require
less separation between phases and thereby help in reducing the sub-station area to a
larger extent. The isolators could be operated mechanically or hydraulically or
pneumatically or by electric motor. Earthing facility shall be provided wherever
required.
4.6. INSTRUMENT TRANSFORMERS:
“Instrument Transformers are defined as the instruments in which the
secondary current or voltage is substantially proportional to the primary current or
voltage and differs in phase from it by an angle which is approximately zero for an
appropriate direction of connection”.
Basic Function of Instrument Transformers:
17
Direct measurement of current or voltage in high voltage system is not
possible because of high values and insulation problems of measuring instruments
they cannot be directly used for protection purposes.
Therefore an instrument transformer serves the purpose and performs the
following function:
 Converts the higher line voltages or line currents into proportionally reduced
values by means of electromagnetic circuit and thus these values can be
measured easily.
 Protects measuring instruments and distribution systems by sensing the
abnormalities and signals to the protective relay to isolate the faulty system.
Types of Instrument Transformers:
Instrument transformers are of two types:
 Current Transformers
 Voltage Transformers
4.6.1 Current transformers:
Current transformer is a current measuring device used to measure
the currents in high voltage lines directly by stepping down the currents to
measurable values by means of electromagnetic circuit.
4.6.1.1 Basic Design Principle of Current Transformers:
The basic principle induced in designing of current transformers is
Primary ampere turns = Secondary ampere turns
Ip  Np = Is  Ns
Where, Ip - Primary current
Np - Primary Winding Turns
Is - Secondary Current; Ns - Secondary Winding Turns
18



Ampere turns plays very important role in designing current transformers.
Current transformers must be connected in series only.
Current transformer has less no of turns in primary and more no of turns
in secondary.


The secondary current is directly proportional to primary current.
The standards applicable to CT's are IEC-60044-1 and IS – 2705.
4.6.1.2 Simple Line Diagram of Current Transformer:
The line diagram of a current transformer contains different components:
P
R
S
E
Fig: 4.6.1 LINE DIAGRAM OF CT.
 Primary Winding: It is the winding which is connected in series with the
circuit, the current of which is to be transformed.
These are of two types:
1. Single turn primary winding 2. Multi-turn primary winding
 Magnetic Core: Performance of any current transformer depends on its
accuracy of transformation and characteristics of the core material used.
Design of a current transformer depends on the frequency of excitation.
 Secondary Winding: The winding which supplies the current to the measuring
instruments, meters, relays, etc.
 Burden: The relay, instrument or other device connected to the secondary
winding is termed as 'burden' of a current transformer.
Ex. Burden for Metering is CT – 20 VA, 15 VA.
19
4.6.1.3 Tests generally to be conducted on CT:

Insulation resistance values (IR values): Primary to earth, primary to
secondary core1, primary to secondary core2, core1 to earth, core2 to earth and core1
to core2. Primary to earth and primary to secondary cores are to be checked with 5KV
motor operated insulation tester (megger) and secondary to earth values are to be
checked with 1000V insulation tester or preferably with 500V insulation tester.



Ratio test: Primary injection test is to be conducted for this purpose
TAN-DELTA test: on 132KV CTs and above
Polarity test at the time of commissioning (at least on the CTs connected to
revenue meters)


Excitation (saturation) characteristic check

Secondary injection check

Secondary and lead resistance check
Primary injection check
4.6.1.4 The accuracy of a CT is directly related to a number of factors including:
* Burden
* Burden class/saturation class
* Rating factor
* Load
* External electromagnetic fields
* Temperature and
* Physical configuration.
* The selected tap, for multi-ratio CTs
Number of secondary cores in the current transformer is based on its usage.
CTs used for 11KV and 33KV feeder will have 2 secondary cores. Core 1 is generally
for Over current and earth fault protection. Core 2 is for metering. Usage of core is
decided by the accuracy class of the CT .Core material decides the accuracy class
Core with accuracy class 1.0, 0.5 and latest is 2.0 is used for metering.
Allowable errors are +/-1.0% in case of 1.0 accuracy class CTs.
20
CT secondary current is proportionate upto120% of the rated primary current
with +/-1% error in case of 1.0 accuracy class CTs. This indicates that 0.2 accuracy
class CTs are expensive than 0.5 and 1.0 accuracy class CTs. Beyond 120% of the
rated primary current, the metering core get saturated.
Core with accuracy class 5P10, 5P15 and 5P20 is used for o/c & e/f
protection. In 5p10, the 5 denotes allowable errors i.e. +/-5%, P denotes protection
and 10 denotes accuracy limit factor. CT secondary current is proportionate upto10
times the rated primary current with +/-5% errors in case of 5P10 accuracy class CTs.
This indicates that 5P20 accuracy class CTs are expensive than 5P15 and 5P10
accuracy class CTs. CT with 2cores (protection core and metering) is used for
11KV& 33KV feeders and capacitor bank protection. CT with 3cores (protection,
special protection and metering) is used for 132/11, 132/33KV ptrs&132KV feeders
protection 220/132KV PTR LV CT is also having 3 cores.
CT with 4 cores (protection, special protection, special protection and
metering) is used for 220KV Bus couplers for the twin bus substations. CT with 5
cores (4 cores for special protection, and metering) is used for 220KV feeder
protection, In all the above cases, protection means O/L &E/L protection, special
protection means differential protection and REF protection in case of power
transformers, bus bar protection (bus differential protection) in case of bus, and
distance protection in case of feeders. At the rate of 220KV level we should use 1:5
cores Current transformer.
4.6.1.5 Specifications of HVCT:
Type
: IT-245
Frequency
: 50 Hz
H.S.V
: 245 KV
BIL
: 460/1050KV
Oil weight
: 360kgs
Total weight
: 1250kgs
Lth
: 40/1 KA/sec
21
RATIO
800-600-400/1-1-1-1-1
2
3
4
CORE NUMBER
RATED PRIMARY
CURRENT (A)
RATED
SECONDARY
CURRENT(A)
1
1
1
OUTPUT(VA)
--------
----------
ACCURACY CLASS
I.S.F/A.L.F
PS
----
PS
---
800
TURN RATIO
RCT at 75 C AT 800/1
(ohms)
5
1
-----------PS
---
2/1600
6
1
1
------- -
30
PS
---
1200
800
6
6
0.5
<=5
---
Table: 4.6.1.5 Specifications of HVCT.
At the rate of LV (132KV) side we can use 1:3 core CT. The specifications of LVCT
are given below:
4.6.1.6 Specifications of LVCT:
Type
: IT-145
Frequency
: 50 Hz
HSV/NSV
: 145/132 KV
BIL
: 650/275 KV
Oil weight
: 75Kg
Total weight
: 550Kg
Lth
: 31.5/1 kA/sec.
Ldyn
: 78.75kAp
22
500/1-1, 0.66-1
RATIO
CORE NUMBER
1
2
RATED PRIMARY
500
CURRENT (A)
PRIMARY &
3
500/1
500/1
500/0.66
500/1
1s1-1s2
2s1-2s2
2s1-2s3
3s1-3s2
1
1
0.66
1
OUTPUT(VA)
20
-------
------------
20
ACCURACY CLASS
5p
I.S.F/A.L.F
20
--------
---------
<=5
Rct at 75o C (Ohms)
--------
<=5
---------
-------------
SECONDARY
CONNECTION
RATED SECONDARY
CURRENT(A)
PS
0.2
Table: 4.6.1.6 Specifications of LVCT
Important:
a)
CT secondary circuit and PT primary should never be open circuited. It is
vulnerable to the CT/PT
b)
CT primary circuit and PT secondary should never be short circuited.
NOTE:-Loose connections should not be allowed in the electrical circuit. It increases
the contact resistance which in turn the rises the temperature in that area due to load
current. It damages the oil seals in CTs and transformers bushings causing oil leak
and in term entry of moisture in to the equipment causing falling of IR values and
damages ‘O’ rings in circuit breaker causing SF6 gas leakage. Entry of moisture in to
the VCB insulator chamber cause vacuum interrupter failure and pull rod failure due
to electrical break down. Hence loose connections should not be allowed.
23
4.6.2 Potential Transformers (PT):
An instrument transformer in which the secondary voltage, in normal
conditions of use, is substantially proportional to the primary voltage and
differs in phase from it by an angle which is approximately zero for an
appropriate direction of the connections.
Basic Functions of Voltage Transformers are:

To reduce the line voltage to a value which is suitable for standard measuring
instruments relays etc.

To isolate the measuring instruments, meters, relays etc. from high voltage
side an installation.

To sense abnormalities in voltage and give signals to protective relays to
isolate the defective system.
4.6.2.1 Simple Line Diagram of Voltage Transformer:
Fig: 4.6.2.1 LINE DIAGRAM OF VT.
Basic Design Principle Involved in Voltage Transformer’s:
The basic principle involved in the designing of Voltage Transformer is
Voltage Ratio = Turns Ratio
VP / VS = NP / NS
Thus NS  VP = NP  VS
24
As heavy primary voltages will be reduced to low secondary voltages, it will have
more turns in the primary & less turns in the secondary. It must always be connected
in parallel only. Even if we connect it directly from high voltage to earth, it is not
going to be a short circuit as its primary winding has very high resistance. Its core is a
set of assembled laminations. It operates at constant flux density. The standards are
IEC – 600044 – 2 and IS – 3156.
Fig: 4.6.2.2 POTENTIAL TRANSFORMER.
4.6.2.2 Tests generally to be conducted on the PTs:

Insulation resistance values (IR values): primary to earth, primary to
secondary core-1, primary to secondary core-2, core1 to earth, core 2 to earth and
core-1 to core-2. These values are to be checked with 1000V insulation tester
(megger) or preferably with 500V insulation tester.

Ratio Test: By applying single phase voltage across primary the voltage
induced in the secondary winding is to be measure. This is approximately equal to
voltage applied in the primary winding or voltage ratio of the PT.

Polarity test at the time of commissioning (at least on the PTs connected to
revenue meters)


PT secondary injection check
PT combined primary and secondary injection check
4.6.2.3 General checks for PT:


Mechanical alignment for PT power jaws
PT primary winding star earthing
25



Tightness of all connections
Primary/secondary fuse ratings
PT specifications
In PTs no of secondary cores is 1 or more than 1 based on the requirement.
Generally in 11KV or 33KV bus PTs, there is one secondary winding which is used
both for protection and metering and in 132KV and above, there are two secondary
cores. First core is of metering core with 1.0 or 0.5 or 0.2 accuracy classes. This will
be used metering, directional over current protection and distance protection.
The second core is protection core with 3P accuracy class. This will be used
for the directional earth fault protection (open delta voltage) of power transformers
and 132KV feeders.
Accuracy class 0.5 means +/- 0.5% errors are allowable and 3P means +/- 3%
errors are allowable and P denotes protection.
Permissible load to be connected on PT secondary winding is decided by the
burden of the PT secondary winding. It is expressed in volt-amperes (VA). If more
than rated burden is connected then error will be increased.
4.7. CIRCUIT BREAKER:
The circuit breakers are used to break the circuit if any fault occurs in any of
the instrument. These circuit breaker breaks for a fault which can damage other
instrument in the station. For any unwanted fault over the station we need to break the
line current. This is only done automatically by the circuit breaker.
These are load switches. It is able to make or break the normal load current as
well as the fault currents. The basic construction of any circuit breaker requires the
separation of contacts in an insulating fluid, which serves two functions. It
extinguishes the arc drawn between contacts when the CB opens and it provides
adequate insulation between the contacts and from each contact to earth. For
successful operation of the circuit breaker, two functions are to be performed.
a) Operating mechanism function, b) Arc quenching function.
There are
26
 various operating mechanisms:
Spring charge mechanism, Pneumatic mechanism, Hydraulic Mechanism

 Arc quenching medium:

Bulk oil (called bulk oil circuit breakers-BOCB)

Natural air (called air circuit breakers-ACB) (415v)

Vacuum (called vacuum circuit breaker-VCB)

Minimum oil (called minimum oil circuit breakers-MOCB)

Forced air (called air blast circuit breaker-ABCB)
SF6 gas (called Sulphur Hexafluoride-SF6 gas CB)
The present trend is up to 33KV, VCBs are preferred and beyond 33KV, SF6 gas
circuit breakers are preferred.

VCB is to be meggered periodically to know the healthiness of the vacuum
interrupter and the insulating pull rod. Vacuum integrity test is the correct test to
know the healthiness of the vacuum interrupter.

SF6 gas pressure is to be noted in log sheets at least twice in a day. If it is
reaching to SF6 gas pressure low alarm stage, it is to be brought to the notice of the
maintenance personnel.SF6 gas circuit breaker goes to lockout conditions after falling
to lockout pressure close and trip circuits will be blocked and circuit breaker
operation can’t be performed N<0 contacts of 63GLX were used in close and trip
circuits of the CB and 63GLX contactor is in picked up conditions when the gas
pressure is sufficient. Some of the SF6 gas circuit breaker automatically trips while
going to lockout stage N<C contacts of 63GLX contactor were used in close and trip
circuits and 63GLX is in drop off condition when the gas pressure is sufficient.
Oil condition in the air compressor is to be checked periodically. And it is
to be replaced based on condition of oil.
There are mainly two types of circuit breakers used for any substations. They are
(a) SF6 circuit breakers;
(b) Vacuum circuit breakers.
27
4.7.1 SF6 circuit breakers:
Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and
arc extinguishing properties. The dielectric strength of the gas increases with pressure
and is more than the dielectric strength of oil at 3 kg/cm2. SF6 is now being widely
used in electrical equipment like high voltage metal enclosed cables; high voltage
metal clad switchgear, capacitors, circuit breakers, current transformers, bushings,
etc. The gas is liquefied at certain low temperature, liquidification temperature
increases with the pressure.
Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur
in the fluorine gas, in a steel box, provided with staggered horizontal shelves, each
bearing about 4 kg of sulphur. The steel box is made gas tight.
The use of SF6 circuit breaker is mainly in the substations which are having
high input KV, say above 220KV and more. The gas is put inside the circuit breaker
by force i.e. under high pressure. When if the gas gets decreases there is a motor
connected to the circuit breaker. The motor starts operating if the gas went lower than
20.8 bar. There is a meter connected to the breaker so that it can be manually seen if
the gas goes low. The circuit breaker uses the SF6 gas to reduce the torque produce in
it due to any fault in the line. The circuit breaker has a direct link with the instruments
in the station, when any fault occur alarm bell rings.
Some of the properties of SF6 are,


Very high dielectric strength

Superior arc extinguishing capability

High thermal and chemical inertia
Low decomposition by arcing
28
Fig: 4.7.1 SF6 CIRCUIT BREAKERS.
29
4.7.2 Vacuum circuit breakers:
Vacuum type of circuit breakers is used for small KV rated stations below
33KV. They are only used in low distribution side.
4.7.3 Control Circuit of Circuit Breakers:
In closing circuit of the Circuit Breaker there are no. of series inter locks we can say
that it is an AND Gate and tripping circuit there are no.of parallel paths it is an OR

Gate.
For ‘closing’ the Circuit Breaker following conditions are to be met.
a) Local/Remote selector shall be in ‘Remote’ for closing the CB from
remote and it shall be in ‘Local’ for closing the CB from Local.
b) Spring is in charged condition in spring type CBs, Air pressure shall
be sufficient in kinematic CBs and Hydraulic Pressure is sufficient in
Aero shell fluid CBs.
c) SF6 Gas pressure is sufficient.

d) Master Trip is resettled.
For tripping the circuit breaker,
a) Local/Remote selector Switch shall be in ‘Remote’ for tripping the CB
from Remote and it shall be in ‘Local’ for tripping the CB from Local.
b) SF6 Gas pressure is sufficient.
c) Air Pressure is sufficient/Hydraulic Pressure is sufficient.

d) Protection trip bypasses the local/Remote selector switch.
Trip circuit healthiness is to be ensured immediately after closing the circuit breaker.
It is to be ensured at regular intervals at least once shift, as there is no trip circuit
supervision relay and annunciation relay for 33KV feeders and in case of old panels
of 132KV feeders If any deviation is found it is to be brought to the notice of
maintenance personnel.
30
4.8 BUS:
The bus is a line in which the incoming feeders come into and get into the
instruments for further step up or step down. The first bus is used for putting the
incoming feeders in la single line. There may be double line in the bus so that if any
fault occurs in the one the other can still have the current and the supply will not stop.
The two lines in the bus are separated by a little distance by a conductor having a
connector between them. This is so that one can work at a time and the other works
only if the first is having any fault.
4.9 TRANSFORMERS:
Transformers come in a range of sizes from a thumbnail-sized coupling
transformer hidden inside a stage microphone to huge units weighing hundreds of
tons used to interconnect portions of national power grids. All operate with the same
basic principles, although the range of designsis wide. While new technologies have
eliminated the need for transformers in some electronic circuits, transformers are still
found in nearly all electronic devices designed for household ("mains") voltage.
Transformers are essential for high voltage power transmission, which makes long
distance transmission economically practical.
Fig: 4.9.1.1 ELECTRICAL TRANSFORMER.
31
4.9.1 Basic Principle:
The transformer is based on two principles: firstly, that an electric current can
produce a magnetic field (electromagnetism) and secondly that a changing magnetic
field within a coil of wire induces a voltage across the ends of the coil
(electromagnetic induction).
Changing the current in the primary coil changes the magnetic flux that is
developed. The changing magnetic flux induces a voltage in the secondary coil.
Fig: 4.9.1.2 IDEAL TRANSFORMER.
An ideal transformer is shown in the adjacent figure; Current passing through
the primary coil creates a magnetic field. The primary and secondary coils are
wrapped around a core of very high magnetic permeability, such as iron, so that most
of the magnetic flux passes through both primary and secondary coils.
4.9.1.1 Induction law:
The voltage induced across the secondary coil may be calculated from
Faraday's law of induction, which states that, where VS is the instantaneous voltage,
NS is the number of turns in the secondary coil and Φ equals the magnetic flux
through one turn of the coil.
32
If the turns of the coil are oriented perpendicular to the magnetic field lines, the
flux is the product of the magnetic field strength and the area A through which it cuts.
The area is constant, being equal to the cross-sectional area of the transformer core,
whereas the magnetic field varies with time according to the excitation of the
primary.
Fig: 4.9.1.3 MUTUAL INDUCTION.
Since the same magnetic flux passes through both the primary and secondary
coils in an ideal transformer, the instantaneous voltage across the primary winding
equals Taking the ratio of the two equations for VS and VP gives the basic equation
for stepping up or stepping down the voltage Ideal power equation The ideal
transformer as a circuit element.
If the secondary coil is attached to a load that allows current to flow, electrical
power is transmitted from the primary circuit to the secondary circuit. Ideally, the
transformer is perfectly efficient; all the incoming energy is transformed from the
primary circuit to the magnetic field and into the secondary circuit. If this condition is
met, the incoming electric power must equal the outgoing power.
Giving the ideal transformer equation Transformers are efficient so this
formula is a reasonable approximation. If the voltage is increased, then the current is
decreased by the same factor. If an impedance ZS is attached across the terminals of
the secondary coil, it appears to the primary circuit to have an impedance of ZS =
(VS/IS).
33
4.9.2 Detailed operation:
The simplified description above neglects several practical factors, in
particular the primary current required to establish a magnetic field in the core, and
the contribution to the field due to current in the secondary circuit.
Models of an ideal transformer typically assume a core of negligible
reluctance with two windings of zero resistance. When voltage is applied to the
primary winding, small current flows, driving flux around the magnetic circuit of the
core. The current required to create the flux is termed the magnetizing current; since
the ideal core has been assumed to have near-zero reluctance, the magnetizing current
is negligible, although still required to create the magnetic field.
The changing magnetic field induces an electromotive force (EMF) across
each winding. Since the ideal windings have no impedance, they have no associated
voltage drop, and so the voltages VP and VS measured at the terminals of the
transformer, are equal to the corresponding EMFs. The primary EMF, acting as it
does in opposition to the primary voltage, is sometimes termed the "back EMF". This
is due to Lenz's law which states that the induction of EMF would always be such
that it will oppose development of any such change in magnetic field.
There are three transformers in the incoming feeders so that the three lines are
step down at the same time. In case of a 220KV or more KV line station auto
transformers are used. While in case of lower KV line such as less than 132KV line
double winding transformers are used.
Fig: 4.9.3 THREE PHASE 100MVA AUTO TRANSFORMER.
34
4.9.3 Specifications of 220/132KV Auto transformer:
Rated MVA: 100MVA
Frequency: 50HZ
No of phases: 3
Insulation level:
HV
HVN
IV
LV
LI 900 AC 395
LI 95 AC 38
LI 550 AC 230
LI 170 AC 70
Type of cooling:
Rated MVA :
Rated KV at no load: HV
IV
LV
ONAN DNAF
75
100
220KV
-132KV
-11KV
--
Line Amperes :
196.8
328.0
1299.0
HV
IV
LV
262.4
437.4
1732.1
Temperature Rise oC: Top oil
- 50oC
O
Avg.WDG - 55 C
Impedance volts
HV-IV
7.667
Normal Tap conditions)
HV-LV 24.55
IV-LV
17.69
10.222
32.72
23.59
4.10 CAPACITOR BANK ATTACHED TO THE BUS:
The capacitor banks are used across the bus so that the voltage does not get
down till at the require place. A capacitor bank is used in the outgoing bus so that it
can maintain the voltage level same in the outgoing feeder.
Fig: 4.10.1 CAPACITOR BANK IN THE DISTRIBUTION SYSTEM.
35
Capacitor Control is usually done to achieve the following goals:
Reduce losses due to reactive load current; Reduce KVA demand, decrease customer
energy consumption, Improve voltage profile, and increase revenue.
Indirectly capacitor control also results in longer equipment lifetimes because of
reduced equipment stresses.
Experience shows that switched feeder capacitors produce some of the fastest
returns on equipment investment Sources of Energy Loss. Energy losses in
transmission lines and transformers are of two kinds: resistive and reactive. The
former are caused by resistive component of the load and cannot be avoided. The
latter, coming from reactive component of the load, can be avoided. Reactive losses
come from circuit In the case of concentrated industrial loads, there should be a bank,
sized to almost equal the reactive load current, located as close to each load as
possible (Fig. 5.10).
Fig: 4.10.2 REACTIVE LOSSES.
36
CHAPTER 5
TYPES OF CONTROL
VAR control is the natural means to control capacitors because the latter adds
a fixed amount of leading VARs to the line regard less of other conditions, and loss
reduction depends only on reactive current. Since reactive current at any point along a
feeder is affected by downstream capacitor banks, this kind of control is susceptible
to interaction with downstream banks. Consequently, in multiple capacitor feeders,
the furthest downstream banks should go on-line first and off-line last. VAR controls
require current sensors.
Current control is not as efficient as VAR control because it responds to total line
current, and assumptions must be made about the load power factor. Current controls
require current sensors. Voltage control is used to regulate voltage profiles; however
it may actually increase losses and cause instability from highly leading currents.
Voltage control requires no current sensors.
Fig: 5.1 TYPES OF CONTROL
37
Temperature control is based on assumptions about load characteristics.
Control effectiveness depends on how well load characteristics are known. Not useful
in cases where those characteristics change often. Temperature control does not
require any current sensors. Time control is based on assumptions about load
characteristics. Control effectiveness depends on how well load characteristics are
known. Not useful in cases where those characteristics change often. Time control
does not require any current sensors.
Power factor control is not the best way to control capacitor banks because
power factor by itself is not a measure of reactive current. Current sensors are needed.
Combination control using various above methods is usually the best choice.
If enough current, and/or other sensors are available, a centrally managed
computerized capacitor control system taking into account the variety of available
input parameters can be most effective, though expensive to implement.
5.1 CAPACITORS:
a) Before commissioning a capacitor bank, capacitance of each capacitor shall be
measured with a capacitance meter. These shall be compared with the value
obtained by calculation using the formula,
C = KVAR x 109 Micro Farads
2 ∏f (V) 2
Where V is the rated voltage of capacitor and KVAR is the rated KVAR of capacitor.
As per IS the tolerance in the capacitance value for a capacitor unit is + 10% to – 5%.
b) In the event of failure of one capacitor unit (say in R-phase) it is observed that
balancing is done by removing one capacitor each from Y and B-phases.
c) It is therefore necessary that number of capacitor units connected in parallel in
each series group in all the three phases on one star bank shall be same.
38
5.2 BUS BAR SYSTEM:
5.2.1 Mesh (Ring) bus bar system:
Merits: 1. Busbars gave some operational flexibility
Demerits: 1. If fault occurs during bus maintenance, ring gets separated into two
sections.
2. Auto-reclosing and protection complex.
3. Requires VT’s on all circuits because there is no definite voltage reference point.
These VT’s may be required in all cases for synchronizing live line or voltage
indication.
4. Breaker failure: During fault on one circuit causes loss of additional circuit
because of breaker failure.
Remarks1.Most widely used for very large power stations having large no. of
incoming and outgoing lines and high power transfer.
5.2.2 Bus bar Isolator:
These can be used for the protection of the instruments in the substation by isolating
the buses at the required instant.
5.3 STATION BATTERY:
“Observe me every day” is a slogan mentioned on the batteries provided for
the vehicles. This holds good to battery at substations also. Battery is the heart of the
substation at control and protection side and this is the uninterrupted power source to
operate the switchgear and protection.
5.3.1 Periodical works on Batteries:

Pilot voltages and specific gravities are to be recorded by the shift personnel
in the morning shift by switching off the battery charger.
39
While switching off the battery charger, one should observe the battery for the
sparks if any due to loose connections. Once charger is switched off, entire DC load
off the station is to be met by the battery. Voltage of all the cells and their specific
gravities, are to be recorded once in a month by the maintenance personnel. If any
deviation is found either in cell voltages or specific gravities, the battery may be kept
in boost mode duly topping up the electrolyte levels with the distilled water and
keeping the cell caps in open position. Specific gravity of the healthy cell is 1200+/20 i.e. it ranges from 1180 to 1220 and the voltage is about 2.1v.
5.3.2 During Boost Charging:
a.
Boost charger can be switched on duly keeping the coarse and fine selector
switches in position-1 to maintain the boost charger output voltage at minimum so
that boost charger current is minimum during starting. Later, coarse and fine selector
switches are to be adjusted as per the requirement,
b.
Boost charging current should not exceed 1/10th of the battery Ampere Hour
capacity i.e. 8 Amperes for 80 AH battery. Cell temperature should not exceed 50
Deg.Cen. Boost charger voltage should not exceed 297V (i.e.2.7v/cell).
c.
Float charger shall also be kept in service otherwise load will be connected
across first 84 cells and boost charger will be connected across 110 cells leading
under charging of first 84 cells or over charging of 85th cell to 110th cell causing
damage to the cells. Once float charger is switched on, load will automatically
connects across the float charger as float charger output voltage is generally more
than the first 84 nos. cells voltage.
d.
At the end of the boost charging, all the cells shall be thoroughly cleaned, caps
shall be kept back and petroleum jelly is to be applied at the cell terminals to avoid
exposing of electrodes direct to atmosphere which will cause formation of sulphation
on the terminals due to oxidation. Cell terminals shall be tightened periodically duly
keeping brass bolts & nuts as spares to meet the requirement. Battery shall be
discharged yearly once. It increases the battery life period. Earth leakage is to be
avoided as far as possible to discharging of 50% of the cells.
40
Fig: 5.3 STATION BATTERIES.
5.3.3 Specifications of VRLA (Valve Regulated Led Acid) batteries:-
System details:
Make
: AMARA RAJA
System voltage
: 220v
Capacity at 27oc
: 200AH
Cell type
: 2V
No. of cells
: 110
Charging Requirements:
Float voltage
: 2.45-3V
Boost voltage
: 2.53-3V
Maximum charging
: 40A
Maximum allowable ripple: 2.1 rms
Current in each cell
: 2A
5.4 INSULATORS:
Ball and socket type disc insulators are assembled to the 132 KV, 220 KV and
400 KV suspension and tension hardware, certain important design aspects and other
details are indicated below: in next page:
41
Sl.
Description
132 KV lines
220 KV lines
No.
400 KV
lines
1. Type of insulators
Ball and socket
Ball and socket
Ball and
type disc insulator type disc insulator socket type
disc
insulator
2. Dimensions
of
insulators
of 255mm x 145mm 280mm x 145mm
suspensions string
3. Dimensions
of
280mm x
145mm
insulators
for 280mm x 145mm 280mm x 145mm
tension string
4. Number of insulator disc per single
280mm x
170mm
9 nos.
13 nos.
23 nos.
2 x 9 nos.
2 x 13 nos.
2 x 23 nos.
10 nos.
14 nos.
24 nos.
2 x 10 nos.
2 x 14 nos.
2 x 24 nos.
7000 Kgs.
7000 Kgs.
11,500 Kgs.
11,500 Kgs.
11,500 Kgs.
16,500 Kgs.
280 mm
280 mm
315 mm
280 mm
280 mm
330 mm
110 KV
110 KV
120 KV
suspension string
5. Number of insulator disc per double
suspension string
6. Number of insulator discs per
single tension string
7. Number of insulator discs per each
double tension string
8. Electro Mechanical strength for
tension string insulator
9. Electro Mechanical strength for
suspension string insulator
10. Total creapage distance of each disc
insulator for suspension strings
11. Total creapage distance of each disc
insulator for tension string
12. Minimum impulse dry withstand
voltage (wave of 1 x 50 Micro
second) for each disc insulator
42
(I.E.C standard)
13. One
minute
power
frequency
70 KV (dry)
70 KV (dry)
70 KV
withstand voltage for each disc
40 KV (wet)
40 KV (wet)
(dry)
insulator
40 KV
(wet)
14. Power frequency puncture voltage
per each disc insulator
110 KV
110 KV
140 KV
(Suspension
(Suspension
(Suspension
strings)
strings)
strings)
140 KV
140 KV
140 KV
(Tension Strings)
(Tension Strings)
(Tension
Strings)
15. Size and designation of bal pin
16 mm
16 mm
20 mm
20 mm
20 mm
20 mm
shank for suspension string discs
16. Size and designation of bal pin
shank for tension string discs
17. Maximum Radio Influence Voltage 50 Micro Volts at 50 Micro Volts at
at 10 KV (RMS) for each disc
1 MHz
1 MHz
50 Micro
Volts at 1
MHz
insulator
18. Corona
extinction
complete
(RMS)
voltage
string
for
-
both
(RMS)
suspension and tension strings
Table: 5.4 Insulators
Fig: 5.4 BALL AND SOCKET TYPE DISC INSULATOR.
43
320 KV
CHAPTER 6
PROTECTION FOR VARIOUS EQUIPMENTS
6.1 TRANSFORMER PROTECTION:
a) Station Transformer: HG Fuse protection on HV side and fuse protection on LV side
and Vent pipe.
b) Power transformers up to 7.5MVA:
HV side: O/L & Directional E/L protection with highest element in O/L relays.
LV side: O/L & E/L protection Buchholz Relay OLTC Buchholz Relay OTI and
WTI.
c) Power transformers from 8.0MVA and above: HV side O/L & Directional E/L
protection with high set element in O/L relays. LV side O/L & E/L protection:
differential protection Buchholz Relay OLTC Buchholz Relay OTI, WTI and PRV.
d) Power transformers from 31.5MVA and above: Over flux protection & LV WTI in
addition to protection.
e) 220/132KV power transformers: Over flux protection on both HV & LV sides LBB
protection on HV side OLTC Buchholz phase wise in addition to protection.
6.2 FEEDER PROTECTION:
a) 33KV feeders: Non directional O/L & E/L protection with highest and IDMT
characteristics.
b) 132KV feeders: Main protection: Distance protection.
Backup protection: Directional O/L & E/L protection.
c) 220KV feeders: Main-1 protection: Distance protection
Main-2protection: Distance protection, LBB protection, pole discrepancy
Relay.
44
CHAPTER 7
220/132KV SUBSTATION AT WARANGAL
The present-day electrical power system is A.C. i.e. electric power is
generated, transmitted and distributed in the form of alternating current. The electric
power is produced at the power stations which are located at favourable places,
generally quite away from the consumers. It is delivered to the consumers through a
large network of transmission and distribution. At many places in the line of the
power system, it may be desirable and necessary to change some characteristic (e.g.
voltage, A.C. to D.C., frequency, P.f. etc.) of electric supply.
This is accomplished by suitable apparatus called sub-station. For example,
generation voltage (11KV or 6·6KV) at the power station is stepped up to high
voltage (say 220KV or 132KV) for transmission of electric power. The assembly of
apparatus (e.g. transformer etc.) used for this purpose is the sub-station. Similarly,
near the consumer’s localities, the voltage may have to be stepped down to utilization
level. This job is again accomplished by a suitable apparatus called ‘substation’.
7.1 SUBSTATION AT WARANGAL:
The substation in Mulugu cross road, Warangal (dist.), Andhra Pradesh was
completed by the year 1969, under APTRANSCO; it is one of the largest substations
in the state of Andhra Pradesh.
This substation has the carrying capacity of 300MW at different voltage levels
of 220KV and can step down to 132KV and again step down to 33KV, using four
input lines through the incoming feeders.
45
7.2 SAILENT FEATURES OF 220/132/33KV SS WARANGAL
The 220/132/33KV Substation Warangal has the following equipment and
feeder bays
1) 220KV Feeders – 4 Nos.
2) 220/132KV 100MVA PTRs – 3 Nos.
3) 132KV Feeders – 6 Nos.
4) 132/33KV 50MVA PTR – 1 Nos.
5) 132/33KV 80MVA PTR – 1 Nos.
6) 33KV Feeders – 10 Nos.
7) 33KV Capacitor Bank – 2 Nos.


10MVAR
7.2MVAR
The 220KV supply is fed from either 220KV Nagaram –I&II or 220KV
Budidampadu ckt I&II.
7.2.1 220KV Features:
20KV Bus Twin Zebra Bus
3 No’s 220/132KV PTRs Namely
i) 100MVA PTR-I
Make-TELK
ii) 100MVA PTR-II
Make-GEC ALSTHOM
iii)
100MVA PTR-III
Make-TELK
7.2.2 132KV Features:
132KV Bus Twin Zebra Bus
6 No’s 132KV Feeders Namely
i) 132KV Waddepally
ii) 132KV Jangaon
iii) 132KV Jammikunta
iv) 132KV RTS-B
v) 132KV Narsampet
vi) 132KV Nekkonda
46
7.2.3 132/33KV PTRs:
2 Nos 132/33KV PTRs
i) 80MVA PTR-I
Make-BBL
ii) 50MVA PTR-II
Make-BBL
7.2.4 33KV Features:
Tubular Copper Bus with Bus Coupler
i. 33KV Gorrekunta
ii. 33KV Machapur
iii. 33KV Atmakur
iv. 33KV Pothana
v. 33KV A.J.Mills
vi. 33KV Parkal
vii. 33KV Kamalapur
viii. 33KV Chintagattu
ix. 33KV KUC33KV
x. 33KVBalasamudram
xi. 10MVAR Capacitor Bank
Make-NGEF
xii. 7.2MVAR Capacitor Bank
Make-UNISTAR
7.2.5 DC SYSTEM
i.Battery Bank-A:
Make: STAR Batteries 220V DC, 200AH, Lead Batteries
Connected Battery Charger:
Make: HEE 220V Dc, 200AH
Float Current: 8 Amps, Boost Current: 16 Amps
ii.Battery Bank-B:
Make: AMARARAJA, 245V DC, 200AH, VLRA Batteries
Connected Battery Charger:
Make: HEE 220V DC, 200AH
Float Current: 8 Amps, Boost Current: 16 Amps
47
7.2.5.1 Chargers:
Main Input AC supply is fed from AC Distribution Board-I
33KV/415V, 250KVA Station Transformer on Bus-II side and it is connected to AC
Distribution Board-I.
Any Substation mainly consists of Transformers. These transformers are like
the heart of substation. These transformers are step down to the required voltage
levels. And the different types of equipments are used in the substation.
The assembly of apparatus used to measure and protect the require parameters
of the power transformer like (e.g. voltage, AC to DC, frequency, P.F. etc.) of electric
supply is called a substation.
EHV (Extra High Voltage) Sub-Station forms an important link between
Transmission network and Distribution network. It has a vital influence of reliability
of service. Apart from ensuring efficient transmission and Distribution of power, the
sub-station configuration should be such that it enables easy maintenance of
equipment and minimum interruptions in power supply.
Flexibility for future expansion in terms of number of circuits and
transformer MVA Capacity also needs to be considered while choosing the actual
configuration of the substation.
EHV Substation is constructed as near as possible to the load centre. The
voltage level of power transmission is decided on the quantum of power to be
transmitted to the load centre. Generally, the relation between EHV Voltage level and
the power to be transmitted is as follows:
48
7.2.4 Power to be transmitted Voltage level:
1) Up to 80MVA to 132KV.
2) From 100MVA to 300MVA 2KV.
3) 300 MVA to 1000 MVA 400 KV.
7.3 IMPORTANT POINTS TO BE KEPT IN VIEW WHILE LAYING OUT
THE SUBSTATION:
Substations are important part of power system. The continuity of supply
depends to a considerable extent upon the successful operation of sub-stations. It is,
therefore, essential to exercise utmost care while designing and building a substation.
The following are the important points which must be kept in view while
laying out a substation:
(i)
It should be located at a proper site. As far as possible, it should be located at
the centre of gravity of load.
(ii)
It should provide safe and reliable arrangement. For safety, consideration must
be given to the maintenance of regulation clearances, facilities for carrying out repairs
and maintenance, abnormal occurrences such as possibility of explosion or fire etc.
For reliability, consideration must be given for good design and construction, the
provision of suitable protective gear etc.
(iii)
It should be easily operated and maintained.
(iv)
It should involve minimum capital cost.
49
CHAPTER 8
CONCLUSION
Transmission and distribution stations exist at various scales throughout a
power system. In general, they represent an interface between different levels or
sections of the power system, with the capability to switch or reconfigure the
connections among various transmission and distribution lines.
The major stations include a control room from which operations are
coordinated. Smaller distribution substations follow the same principle of receiving
power at higher voltage on one side and sending out a number of distribution feeders
at lower voltage on the other, but they serve a more limited local area and are
generally unstaffed. The central component of the substation is the transformer, as it
provides the effective in enface between the high- and low-voltage parts of the
system. Other crucial components are circuit breakers and switches. Breakers serve as
protective devices that open automatically in the event of a fault, that is, when a
protective relay indicates excessive current due to some abnormal condition. Switches
are control devices that can be opened or closed deliberately to establish or break a
connection. An important difference between circuit breakers and switches is that
breakers are designed to interrupt abnormally high currents (as they occur only in
those very situations for which circuit protection is needed), whereas regular
switches are designed to be operable under normal currents. Breakers are placed on
both the high- and low-voltage side of transformers. Finally, substations may also
include capacitor banks to provide voltage support.
50
REFERENCES
[1] Principles of Power Systems by V.K. Mehtha
[2] Electrical Power Systems by C.L. Wadhwa
[3] Power System Engineering by ML. Soni
[4]www.littelfuse.com/.../Littelfuse-Protection-Relay-Transformer- Protection
[5]www.osha.gov/SLTC/etools/electric_power/.../substation.html.
[6]http://www.scribd.com/doc/13595703/Substation-Construction-andCommissioning.
[7]http://www.authorstream.com/Presentation/marufdilse-881803-electriealpower-trasmission/
[8]http://skindustrialcorp.tradeindia.com/Exporters_Suppliers/Exporter17825.
277078/66-KV-Disc-Insulator-Ball-Socket-Type.html.
[9]http://en.wikipedia.org/wiki/Electrical_substation.
51