PERSPECTIVES OF HVDC AND FACTS
FOR SYSTEM INTERCONNECTION
AND GRID ENHANCEMENT
Wilfried Breuer, Mário Lemes, Dietmar Retzmann
Siemens Power Transmission
and Distribution
Presentation in two Parts:
¾ DC and AC Technology Issues for Bulk Power
EHV and UHV Transmission
¾ Power System Expansion with Advanced
Technologies - Solutions for a "Smart Grid"
1/20
0. Introduction
The growth and extension of AC systems and consequently the introduction of higher voltage
levels have been driven by a fast growth of power demand over decades. Power systems have
been extended by applying interconnections to the neighboring systems in order to achieve
technical and economical advantages. Large systems came into existence, covering parts of or
even whole continents, to gain the well known advantages, e.g. the possibility to use larger
and more economical power plants, reduction of reserve capacity in the systems, utilization of
the most efficient energy resources, as well as to achieve an increase in system reliability. In
the future of liberalized power markets, these advantages will become even more important:
pooling of large power generation stations, sharing of spinning reserve and use of most
economic energy resources, and also taking into account ecological constraints such as
nuclear and hydro-power stations at suitable locations, solar energy from desert areas and
embedding of large off-shore wind farms.
The interconnected systems are becoming extremely large and innovative solutions will be
essential to avoid congestion and to improve the system stability. Examples of large AC
interconnections are systems in Brazil and Asia, as well as in North America, Europe and
Russia. However, there are technical and economical limitations in the interconnections if the
energy has to be transmitted over extremely long distances through the interconnected
synchronous AC systems. In future, the loading of existing power systems will strongly
increase, leading to bottlenecks and reliability problems. System enhancement will be
essential to balance the load flow and to get more power out of the existing grid in total. Large
blackouts in America and Europe confirmed clearly that the favorable close electrical
coupling of the neighboring systems might also include the risk of uncontrollable cascading
effects in large and heavily loaded synchronous systems.
HVDC (High Voltage Direct Current) transmission and FACTS (Flexible AC Transmission
Systems) have developed to a mature technology with high power ratings. There are now
ways of transmitting 3 - 4 GW over large distances with only one bipolar DC transmission
system. For some countries, UHV solutions with AC voltages of 1000 kV and DC systems
with 800 kV are in the planning stage. This will increase the transmission capacity for AC
links up to 10_GW and for DC systems up to 5 - 6 GW.
In the paper, benefits of bulk power transmission solutions with HVDC and FACTS for
system enhancement and grid interconnection are depicted, and UHV applications for AC and
DC are presented. Study and project examples using HVDC and FACTS are given and
prospects of VSC (Voltage-Sourced Converters) applications are discussed.
1. Development of Power Systems
The development of electric power supply began more than one hundred years ago.
Residential areas and neighboring establishments were at first supplied by DC via short lines.
At the end of the 19th century, however, AC transmission was introduced, utilizing higher
voltages to transmit power from remote power stations to the consumers.
The growth and extension of AC systems and consequently the introduction of higher voltage
levels have been driven by a fast growth of power demand over decades. Global studies show
that power consumption in the world follows closely the increase of population. In the next 20
years, power consumption in developing and emerging countries is expected to increase by
220 %, in industrialized countries, however, only by 37 %.
2/20
In Europe, 400 kV became the highest voltage level, in Far-East countries mostly 550 kV, and
in America 550 kV and 765 kV. The 1150 kV voltage level was anticipated in the past in
some countries, and also some test lines were already built. Fig. 1 and 2 depict these
developments and perspectives.
However, some Countries
1600
kV
will finally “go” ≥ 1 GV
1400
6
1200
1000
5
800
600
400
2
1
200
4
3
EHV: 800 kV as “realistic” Standard
The “Initial” Statement
0
1900
1910
1920
1
2
3
4
5
6
1930
1940
1950
1960
1970
1980
1990
2000
Year
2010
110 kV Lauchhammer – Riesa / Germany (1911)
220 kV Brauweiler – Hoheneck / Germany (1929)
287 kV Boulder Dam – Los Angeles / USA (1932)
380 kV Harspranget – Halsberg / Sweden (1952)
735 kV Montreal – Manicouagan / Canada (1965)
1200 kV Ekibastuz – Kokchetav / USSR (1985)
Fig. 1: Development of AC Transmission - Milestones
„
Voltage Levels of 735 kV to 765 kV AC have been
introduced in the following Countries:
Canada, Brazil, Russia (USSR), South Africa,
South Korea, U.S.A. and Venezuela
„
UHV Transmission Lines (1000 kV and above)
have been built in Russia and Japan
1000 kV Line:
SIL = 4 GW
Transmission
of 6-10 GW is
feasible
„
„
Ekibastuz – Kokchetav (500 km)
„
Kokchetav – Kustanay (400 km)
„
Minami – Niigata / Nishi – Gunma (200 km)
„
Kita – Tochigi / Minami – Iwaki (250 km)
However, today these UHV Transmission Lines
are operated at 500 kV
Fig. 2: Development of EHV and UHV AC Transmission - Status and Perspectives
The development of power systems follows the requirements to transmit power from
generation to the consumers. With an increased demand for energy and the construction of
new generation plants, first built close and then at remote locations from the load centers, the
size of power systems has grown. Examples of large interconnected systems are the Western
and Eastern European systems UCTE (installed capacity 530 GW) and IPS/UPS (315 GW),
which are planned to be interconnected in the future.
3/20
With an increasing size of the interconnected systems, the technical and economical
advantages diminish. This is related to problems regarding load flow, power oscillations and
voltage quality. If power is to be transmitted through the interconnected system over longer
distances, transmission needs to be supported. This is, for example, the case in the UCTE
system, where the 400 kV voltage level is in fact too low for large cross-border and inter-area
power exchange. Bottlenecks are already identified, and for an increase of power transfer,
advanced solutions need to be applied.
Such problems are even deepened by the deregulation of the electrical power markets, where
contractual power flows do not follow the design criteria of the existing network
configuration, see Fig. 3.
Globalisation/
Privatisation
Liberalisation
Deregulation - Privatization: Opening of the
Markets, Independent Transmission Companies
ITCs, Regional Transmission Organisations RTOs
Bottlenecks in
Privatisation
Transmission
Problem of uncontrolled Loop Flows
Overloading & Excess of SCC* Levels
System Instabilities & Outages
Investments in
Power Systems
System Enhancement & Interconnections:
Š Higher Voltage Levels **
Š New Transmission Technologies
Š Renewable Energies
**
**Example
ExampleUCTE:
UCTE:400
400kV
kVisisactually
actuallytoo
toolow
low
* SCC = Short-Circuit Current
Fig. 3: Trends in High Voltage Transmission Systems
Large blackouts in America and Europe confirmed clearly, that the favorable close electrical
coupling might also include risk of uncontrollable cascading effects in large and heavily
loaded interconnected systems. Additional problems are expected when renewable energies,
such as large wind farms, have to be integrated into the system, especially when the
connecting AC links are weak and when there is no sufficient reserve capacity in the
neighboring system available. In the future, an increasing part of the installed capacity will,
however, be connected to the distribution levels (dispersed generation), which poses
additional challenges on planning and safe operation of the systems. In such cases, power
electronics can clearly strengthen the power systems and improve their performance.
Based on the global experience with large blackouts, strategies for the development of large
power systems go clearly in the direction of hybrid transmissions, consisting of DC and AC
interconnections, including FACTS. Such hybrid interconnected systems offer significant
advantages, both technical and in terms of reliability.
Fig. 4 shows schematically such a hybrid system using HVDC and FACTS. Power exchange
in the neighboring areas of interconnected systems offering most advantages can be achieved
by AC links, preferably including FACTS for increased transmission capacity and for stability
reasons. The transmission of large power blocks over long distances should, however, be
utilized by the HVDC transmissions directly to the locations of power demand. HVDC can be
implemented as direct coupler – the so-called “Back-to-Back” solution (B2B) or as point-topoint long distance transmission via DC line. The HVDC links can strengthen the AC
4/20
interconnections at the same time, in order to avoid possible dynamic problems which exist in
such huge interconnections.
System
G
System
A
System
B
System
C
System
D
System
E
System
F
Large
LargeSystem
SystemInterconnections,
Interconnections,using
usingHVDC
HVDCand FACTS
HVDC - Long Distance DC Transmission
HVDC B2B - via AC Lines
High Voltage AC Transmission & FACTS
“Countermeasures”
against large
Blackouts
DC – the Stability Booster and
“Firewall” against “Blackout”
Fig. 4: Large Power System Interconnections - Benefits of Hybrid Solutions
Fig. 5 depicts how these ideas of hybrid interconnections are reflected in China's grid
development.
Transmission
Capacity of
DC: 4-6 GW
each Corridor
will be 20 GW Solutions:
800 kV DC &
nx
in 2020 …
North Corridor
1000 kV AC
AC: 6-10 GW
3 x 20 GW
… the installed
Generation
Capacity will
be 900 GW
Central Corridor
Sources:
South Corridor
Fig. 5: Perspectives of Grid Developments in China - AC & DC Bulk Power
Transmission from West to East via three main Corridors
5/20
Focus is on interconnection of 7 large inter-provincial grids of the Northern, Central and
Southern systems via three bulk power corridors which will built up a redundant “backbone”
for the whole grid. Each corridor is planned for a sum of about 20 GW transmission capacity
which shall be realized with both AC and DC transmission lines with ratings of 4 - 10 GW
each (at +/- 800 kV DC and 1000 kV AC, ref. to the figure). Therefore, each corridor will
have a set-up with 2 - 3 systems for redundancy reasons. With these ideas, China envisages a
total amount of about 900 GW installed generation capacity by 2020. For comparison, UCTE
and IPS/UPS together sum up to 850 GW today.
The benefits of such a large hybrid power system interconnection are clear:
•
•
•
•
•
Increase of transmission distance and reduction of losses - using UHV
HVDC serves as stability booster and firewall against large blackouts
Use of the most economical energy resources - far from load centers
Sharing of loads and reserve capacity
Renewable energy sources, e.g. large wind farms and solar fields can much more
easily be integrated
However, using the 1000 kV AC lines, there are also some stability constraints: if for example
such an AC line - with up to 10 GW transmission capacity - is lost during faults, large interarea oscillations might occur. For this reason, additional FACTS controllers for power
oscillation damping and stability support are in discussion.
2. Transmission Solutions with HVDC and FACTS
In the second half of the past century, High Voltage DC Transmission (HVDC) was
introduced, offering new dimensions for long distance transmission. This development started
with the transmission of power of ratings of a few hundred MW. By these developments,
HVDC became a mature and reliable technology. Up to now, over 55 GW HVDC
transmission capacities have been installed worldwide, see Fig. 6.
_
60
GW
50
40
Worldwide installed HVDC
“Capacity”: 55 GW in 2005
This is 1.4 % of the Worldwide
installed Generation Capacity
30
An additional 48 GW are
expected from China
alone until 2020 !
20
10
0
1970
1980
1990
2000
2010
Sources: IEEE T&D Committee 2000 - Cigre WG B4-04 2003
Fig. 6: Development of DC Transmission - Worldwide installed Capacity
6/20
It can be seen that China alone will be contributing significantly to this development because
of its rapidly growing economy (GDP) every year.
Transmission distances over 1000 to 2000 km or even more are possible with DC overhead
lines. In general, for transmission distances above 700 km, DC transmission is more
economical than AC transmission (≥ 1000 MW). With submarine cables, transmission levels
of up to 600 - 800 MW over distances of nearly 300 km have already been attained, and cable
transmission lengths of up to 1,300 km are in the planning stage. Transmission ratings of 3
GW over large distances with just one bipolar DC line are state-of-the-art in many grids
today. As a multi-terminal system, HVDC can also be connected at several points with the
surrounding AC networks.
In Fig. 7 and 8, the transmission grid developments in China and India are depicted, leading to
very large hybrid interconnections with AC and DC solutions, including FACTS.
Initially:
Gezhouba-Shanghai
TianGuang
3G-ECPG I
GuiGuang I
3G-Guangdong
GuiGuang II
NECPG
NCPG
Wangqu Plant
Yangcheng Plant
NWCPG
SPPG
North Power Grid
…
In total: 20 HVDC Interconnections
Russian Power Grid
plus
3 x B2B and
11 x HVDC
Long Distance
Transmissions
2005: 12 GW
2020: 60 GW
CCPG
Three Gorges
CSPG
ECPG
Center Power Grid
Jinshajiang
River
SCPG
Lanchangjiang
River
HPPG
South Power Grid
Tailand Power Grid
Sources: SP China, ICPS - 09/2001; State Grid Corp. China, 2003
Fig. 7: China goes Hybrid - AC plus 20 HVDC Interconnections
Since the 60s Flexible AC Transmission Systems have been being developed to a mature
technology with high power ratings. Excellent operating experiences are available worldwide
and the technology became mature and reliable. FACTS, based on power electronics, have
been developed to improve the performance of weak AC Systems and for long distance AC
transmission.
FACTS can, however, also contribute to solve technical problems in the interconnected power
systems. FACTS are applicable in parallel connection (SVC, Static VAR Compensator STATCOM, Static Synchronous Compensator), in series connection (FSC, Fixed Series
Compensation - TCSC/TPSC, Thyristor Controlled/Protected Series Compensation - S³C,
Solid-State Series Compensator), or in combination of both (UPFC, Unified Power Flow
7/20
Controller - CSC, Convertible Static Compensator) to control load flow and to improve
dynamic conditions. GPFC is a special DC back-to-back link, which is designed for fast
power and voltage control at both terminals. In this manner, GPFC is a “FACTS B2B”, which
is less complex and less expensive than the UPFC. Rating of SVCs is up to 800 MVAr, series
FACTS devices are implemented on 550 and 735 kV levels to increase the line transmission
capacity up to several GW. A large number of different FACTS and HVDC controllers have
been put into operation either as commercial projects or prototypes. Recent developments are
the TPSC (Thyristor Protected Series Compensation) and the Short-Circuit Current Limiter
(SCCL), both innovative solutions using special high power thyristor technology.
D E V E L O P M E N T O F N A T IO N A L G R ID
URI
P H A S E - III
(B y 2 0 1 2 )
W AGOORA
D U LH AS TI
Similar Perspectives
… as in China
R AV I S A TLU J
K IS H E N P U R
JULLAN DH AR
TEHRI
MOGA
B A LLA BG A R H
(D E L H I R IN G )
C H IC K E N N E C K
A 'P U R
BHUTAN
MEERUT
H IS S A R
LU CK NO W
NR
B H IW A D I
S H IR O H I
30
VARANASI
M ALAN PU R
S IN G R A U L I
N A G D A B IN A
PADGHE
T IP A IM U K H
R O U R KELA
R A IP U R
CHANDRAPUR
1000M W
W
M
00
20
AM RAVATI
WR
KARAD
ER
TALC HER
JE YP O RE
Source: Power Grid Corporation of India, 2003
GAZUW AKA
SR
NARENDRA
LEG EN D
X PLAN
E X IS T IN G /
IX P L A N
V IJ A Y A W A D A
XI PLAN
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7 6 5 K V L IN E S
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S IL IG U R I/B IR P A R A
V IN D H Y A CHAL
SATNA
D IH A N G D A M W E
M IS A
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A M R E LI K A W A S
CHEGAON
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BHANDARA
P IP A V A V
B O IS A R
RANGANADI
B O N G A IG A O N
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/B A R H
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MW
W
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M
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M 'B A D
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/U N N A O
AGRA
J A IP U R
TEESTA
ARUN
K R IS H N A P A T N A M
4 0 0 K V L IN E S
H V D C B /B
C H IT T O O R
H V D C B IP O L E
M AN G ALO R E
B A N G A LO R E
HOSUR
KS
HA
K A R A IK U D I
DW
T R IV A N D R U M
KAYATH AR
EE
P
AN AND
NICOBAR
P U G A LU R
KAYAM KU LAM
ANDAM
LA
C O C H IN
SOUTH CHENNAI
S IN G A R P E T
C U D D ALOR E
K O Z H IK O D E
Fig. 8: Grid Extension in India - Hybrid AC plus DC
3. Power Electronics for FACTS and HVDC
FACTS and HVDC use power electronic components and conventional equipment which can
be combined in different configurations for switching or controlling reactive power, and for
active power conversion. Conventional equipment (e.g. breakers, tap-changer transformers)
offer very low losses, but the switching speed is relatively low. Power electronics can provide
high switching frequencies up to several kHz, however, with an increase in losses. A view on
the different kinds of semiconductors is given in Fig. 9. In Fig. 10, the stepwise assembly of
the thyristors in modules and valve groups is shown. Fig. 11 indicates the typical losses
depending on the switching frequency.
8/20
Pellet of
GTO / IGCT
Pellet of
LTT Thyristor
Assembly of
Chips in IGBT
LTT = Light triggered Thyristor
IGCT = Insulated Gate commutated Thyristor
GTO = Gate turn-off Thyristor
IGBT = Insulated Gate bipolar Transistor
Fig. 9: High Power Semiconductors
Valve Group - Example
Indoor for HVDC
Module
Thyristor
Valve Group - Example
Outdoor for FACTS
Module
Thyristor
Fig. 10: HVDC and FACTS - Advanced Power Electronics for High Voltage Systems
From Fig. 11, it can be seen that – due to less losses – the preferred solution for Bulk Power
Transmission is in fact the line-commutated thyristor technology. The today’s losses of high
power VSCs (Voltage-Sourced Converters) with high switching frequencies are within the
range of 4 - 5 %, which is too much for very large transmission projects.
9/20
More Dynamics for better Power Quality:
z
Use of Power Electronic Circuits to control P, V & Q
Parallel and/or Series Connection of Converters
z Fast AC/DC and DC/AC Conversion
z
4-5 %
Transition from “slow” to “fast”
Thyristor
IGBT / IGCT
GTO
1-2 %
Switching
Frequency
> 1000 Hz
< 500 Hz
50/60 Hz
Losses
On-Off Transition 20 - 80 ms
The Solution for Bulk Power Transmission
Fig. 11: Use of Power Electronics for FACTS & HVDC Transient Performance and Losses
4. Projects with HVDC and FACTS for Power System Enhancement
In Fig. 12, the features and cost savings of series compensation for a large transmission
project in Brazil (TCSC Serra da Mesa, on a 1000 km AC line) due to grid enhancement are
summarized. The mentioned SSR (subsynchronous resonances) topic is a critical issue for
large thermal generators with long shafts.
„ Current Control
„ Impedance Control
„ Power Oscillation
Damping (POD)
„ Mitigation of SSR
(Option)
Up to 500 POD
Operations per Day
for saving the
System Stability
A System Outage of
24 hrs would cost
840,000 US$ *
* 25 US$/MWh x 1400 MW x 24 hrs
1999
> + 60 o C
Benefits:
o Increase of Transmission Capacity
o Improvement of System Stability
up to 85 o
Fig. 12: 500 kV TCSC Serra da Mesa, Furnas/Brazil – Essential for Transmission
The flexibility of modern FACTS technologies under extremely harsh environmental
conditions is indicated in the figure: the operating range for the TCSC can reach up to +850 C.
10/20
This is necessary due to the outdoor installation on high voltage potential, with the isolated
platform mounted directly in series with the transmission line.
Figs. 13-14 give an example of system studies for large projects in China, in which both
FACTS and HVDC have been integrated for grid interconnection and point-to-point long
distance transmission in a hybrid way.
Because of the long transmission distances, the system experiences severe power oscillations
after faults, close to the stability limits. In the recordings in Fig. 14 oscillations are depicted.
The first case given is HVDC transmitting power in constant power mode, see curve a. It can
be seen that strong power oscillations occur. If, however, damping control of HVDC GuiGuang is activated (curve b), the oscillations are damped very effectively.
Nayong
Anshun
Conv. Stat.
Guizhou
Guiyang
Guangxi
Huishui
HVDC GuiGuang
Anshun
Anshun
Hydro Power Station
TSQ-I
TSQ-II
Luoping
HVDC Converter
Station
Baise
Hezhou
Liudong
Yantan
Liuzhou
TSQ
Conv. Stat.
Lubuge
TCSC
Guangdong
FSC
Hechi
Thermal Power Station
TCSC & FSC
Pingguo Laibin
Guangzhou
Beijiao
Conv. Stat.
Beijiao
Zhengcheng
Wuzhou
Luodong
Pingguo
FSC
Zhaoqing
HVDC TSQ
Yunnan
Gaomin
Yulin
Guangxi
Zhaoqing
Conv. Stat.
Nanning
Fig. 13: Use of HVDC and FACTS in a hybrid System in China
1500
Power flow in one line
Huishui -Hechi (MVA)
1200
Dynamic Results
a – without Power Modulation
b – with Power Modulation
of HVDC Control
c – further Improvements with
Pingguo TCSC/FSC
900
600
300
b
0
Flow in one Line
b Power
a
-300
c
-600
-900
0
a
5
Huishui-Hechi (MW)
10
Time (s)
15
Time / s
20
Fig. 14: China - Benefits of active Damping with HVDC & FACTS (ref. to Text)
Using series compensation with two TCSCs and two FSCs at Pingguo substation, the stability
of the overall system can be further increased (curve c). Without series compensation and
11/20
without HVDC damping, a power system as large as this one would be unstable in case of
fault contingencies, consequently leading to severe outages (Blackout). Stability studies have
been carried out with the Siemens computer program NETOMAC, followed by intensive
digital Real-Time Simulator tests with RTDSTM.
Fig. 15 shows the highlights of the HVDC Gui-Guang project, for which the POD scheme is
applied.
2004
View of the Thyristor-Module
Rating:
Voltage:
3000 MW
± 500 kV
Contract: Nov. 1, 2001
terminated 66 Months
Months
Project completed
ahead of Schedule by Sept. 2004
Fig. 15: Highlights of the GuiGuang HVDC Transmission
Project
Thyristor: 5" LTT with integrated
Overvoltage Protection
Similar studies for HVDC and FACTS in parallel operation have been carried out for a
number of large transmission projects worldwide.
5. UHV Solutions for Bulk Power Transmission
Bulk Power UHV AC and DC transmission schemes over distances of more than 2000 km are
currently under planning for connection of various large hydropower stations in China. Ultra
high DC voltage (up to 800 kV) and ultra high AC (1000 kV) are the preferred voltage levels
for these applications, to keep the transmission losses as low as possible.
In India, there are similar prospects for UHV DC as in China, due to the large extension of the
grid, ref. to Fig. 8. AC, they will, however, realize with EHV levels up to 800 kV.
An overview of existing UHV AC transmission schemes was presented in Fig. 2, section 1.
The figure shows, that – up today – all of these installations are now operating at a reduced
voltage of 500 kV. There are both technical and financial reasons, e.g. grid coupling
transformers using 500 kV are much cheaper. However, a lot of experience has been gained
and the engineers are ready for new challenges. China’s government undertakes strong
efforts, to overcome all outstanding issues – in close co-operation with international
manufacturers.
Specific issues for the necessary UHV technology developments are depicted in the
following, as seen from the Siemens perspective. It is obvious that the UHV insulation
requirements will lead to a huge increase of the mechanical dimensions of all equipment,
including PTs, CTs, breakers, disconnectors, busbars, transformers and reactive power
equipment. Some main equipment does not require detailed investigations since existing
technology basically enables to extrapolate from lower voltage applications. An example for
this type of equipment is the DC thyristor valve which is based on a modular design.
12/20
Additional thyristor levels to be connected in series are well feasible and do not require any
conceptual changes. However, for other equipment it has to be verified to which extent
existing technology and know-how are adequate for design and manufacturing process. This
includes the following equipment:
•
•
•
•
•
•
•
AC grid transformers and DC converter transformers including bushings
AC and DC wall bushings
DC smoothing reactors
AC reactive power equipment, including FACTS
AC breakers and disconnectors
DC bypass switches and DC disconnectors
AC and DC measurements
Regarding shunt-connected FACTS controllers, there are no specific additional efforts
necessary for the medium voltage equipment at the secondary side of the grid transformers.
For series connected FACTS, if applied, efforts will be needed for a robust construction of the
platforms matching the required seismic performance.
Converter transformers are one of the very important components for UHV DC application. It
is quite understood that the existing technology and know-how of converter transformers can
manage higher DC voltages. Yet, there are critical areas which need careful consideration and
further development in order to keep the electrical stresses at a safe level. Above all the
windings and the transformer internal part of bushings on the valve side of the converter
transformers with the barrier systems and cleats and leads require very careful attention.
In the following, design aspects for key UHV DC equipment are outlined. From Figs. 16-17 it
can be seen that for transformers the bushings will be a major issue with regard to mechanical
dimensions, including transportation to site.
¾ Existing Technology and KnowHow can well manage higher DC
Voltage Stresses
¾ Transformers for 800 kV HVDC
System are within existing
Manufacturing Capabilities
¾ Transportation Limits and Converter
Configuration will determine Type
and Size
¾ R&D in Progress in specific Fields
Works for 800 kV DC Transformer
Fig. 16: Transformer for UHV DC – In the State of Development
An example of the complete HVDC station layout is given in Fig. 18.
Main idea of this concept is to use two 12-pulse converters with 400 kV DC operating voltage
each and then to connect them in series in order to achieve the desired 800 kV arrangement.
13/20
Fig. 17: UHV DC Bushing at Test Lab TU Graz – Austria
Transformer Bushings
400 kV DC
800 kV DC
Each Pole can be operated with 400 kV DC
DC Neutral
800 kV-Valve Group
400 kV-Valve Group
DC Line
N-1 Criteria: Redundancy through Bypass-Breakers
Fig. 18: Fully redundant HVDC Scheme – with two 400 kV 12-Pulse Converters per Pole
14/20
A major benefit of this solution, as shown in Fig. 18, will be a smaller size of the converter
transformers, if transportation restrictions exist. Furthermore, it increases the redundancy of
the transmission: each of the 4 converters of plus and minus pole can be bypassed and the
assigned DC line will be operated at 400 kV reduced voltage level.
Due to this, the single line diagram of +/- 800kV UHV DC converter station will be mostly
the same as a +/- 500kV HVDC converter station. A configuration of two 12 pulse-groups per
pole has also a long term operation experience worldwide. It means there is no basic new
concept to be developed.
The arrangement of the valve-units in two 400 kV valve halls per pole is outlined in Fig. 19.
DC Neutral
400 kV DC
400 kV Valve Hall
400 kV DC
to 800 kV DC Line
“Ready for Transmission”
800 kV Valve Hall
Fig. 19: Valve Hall Configuration – for 800 kV HVDC
The 800 kV DC concept can be summarized as follows:
¾ UHV DC Valves using proven modular Design
based on existing Technology and Know-How
for DC Voltage 800 kV
¾ Valve Tower Configuration: Double or
Quadruple Valve
¾ Proven existing LTT Technology
Main benefit will be the use of proven modular technologies by just expanding them to the
new application.
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This is also valid for the AC and DC control and protection schemes. However, the
measurements will need to be adapted to the higher voltage level.
Based on the discussions and descriptions, following summary and conclusion can be made
for the design of UHV AC and DC bulk power transmission systems:
¾ From the main equipment point of view UHV DC systems of up to 800 kV and UHV
AC systems of up to 1000 kV are technically feasible
¾ In general, UHV equipment can be designed and manufactured on the basis of existing
technologies
¾ For most of the station equipment only some or even no R&D is anticipated
6. Prospects of the Brazilian Grid Development
In Brazil, there is a huge need for further system interconnections, both within the national
grid, and to the neighboring countries. Reasons for this are as follows: strong increase in
regenerative energy sources in Brazil, as well as creating new import and export capabilities
to the neighbors to meet the booming energy demand in the region.
Belo Monte
Madeira
from 2006 to 2015
Sources:
2006
2006
Fig. 20: Development of Hydro Generation in Brazil
Fig. 20 highlights the development of hydro sources in Brazil. Main increase in generation
capacity will be driven by two projects: first Rio Madeira and, at a later stage also by Belo
Monte. For these two projects different options for both AC and hybrid DC solutions are
under investigation.
16/20
Fig. 21 depicts an example of study alternatives for Belo Monte, and Figs. 22-23 represent
some of the alternatives for the Rio Madeira project, which are currently developed by EPE.
AC-Solution - 765 kV
DC-Solution +/- 600 kV
Fig. 21: AC-DC Study Alternatives for new Power Plant – Belo Monte
1,275 km
1,450 km
Generation
Capacity: 6.4 GW
Fig. 22: Initial Option of 750 kV AC Interconnection for Rio Madeira Project
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The hybrid solution, as shown in Fig. 23, is the most promising alternative for Rio Madeira,
by using the power oscillation damping features of the HVDC.
Benefits of Hybrid
Solution:
¾ Enhanced Stability
by POD with HVDC
¾ Reduction of Losses
DC Options:
ƒ 2 x 2.1 GW
ƒ 1 x 4.2 GW
+
AC Transmission:
ƒ 2 x 1.1 GW
Fig. 23: Hybrid AC-DC Options for Rio Madeira Project
In the on-going studies, it will be further investigated, whether a new 500 kV AC double line
or even the existing 230 kV transmission grid can be used. An additional B2B nearby the Rio
Madeira substation is also under consideration as a “flexible” interconnector to the
surrounding AC grid. Key-issue of the Rio Madeira project will be the use of a large number
of relatively small bulb generation units (each with ratings of 40 to 70 MW only) which have
a very low inertia time constant in the range of 1 to 2 s only.
In addition to this, if the interconnection uses the existing 230 kV AC system without
implementation of a new 500 kV or 750 kV AC “backbone”, the overall system stability will
be a crucial issue.
In the Cahora Bassa project (Fig. 24), similar stability issues were investigated, when a large
HVDC was operating in parallel with a very weak 330 kV AC system. The solution consisted
in the implementation of the GMPC (Grid Master Power Controller) which provided
coordinated control and damping facilities for the HVDC to stabilize the parallel AC system
as well as for remote system re-synchronization after AC line tripping. This was done by
means of GPS-satellite synchronized AC system phase angle measurements, as shown in the
figure.
The option for a use of thyristor switched braking resistors (TSBR) at Cahora Bassa hydro
power plant was also investigated during the stability studies. However, in the end, they were
not implemented in the project, because the HVDC Performance was good enough even
without TSBR, and the inertia time constant of the large hydro generators (484 MW each)
was not a crucial issue.
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Braking
Filters Resistors
Bus Split
AC
DC
Cahora Bassa
Mozambique
Loads
220 kV
Songo
GMPC:
GMPC:
1998
PAC
Grid
GridMaster
Master
Power
Power
Controller
Controller
using
using GPS
Technology
PAC
500 MW
f
533 kV DC
330 kV AC
P
GMPC
+
EC
Bindura
P
DC
PDC
Zimbabwe
1920 MW
PLC Signal
Transmission
GPS
Insukamini
1500 km
Interconnected
Grids
∆ϑ
400 kV AC
Signal Processing
for Control and
Protection
Apollo
Matimba
South Africa
Fig. 24: Upgrade of Cahora Bassa HVDC by means of GMPC for
Stability Enhancement of the Hybrid AC-DC Interconnection
7. Conclusions – With High Voltage Power Electronics towards a “Smart Grid”
Deregulation and privatization is posing new challenges on high voltage transmission
systems. System elements are going to be loaded up to their thermal limits, and wide-area
power trading with fast varying load patterns will contribute to an increasing congestion.
Environmental constraints will also play an important role. Additional problems are expected
when renewable energies, such as large wind farms, have to be integrated into the system,
especially when the connecting AC links are weak and when there is no sufficient reserve
capacity in the neighboring system available. In the future, an increasing part of the installed
capacity will, however, be connected to the distribution levels (dispersed generation), which
poses additional challenges on planning and safe operation of the systems, ref. to Fig. 25.
The loading of existing power systems will further increase, leading to bottlenecks and
reliability problems. As a consequence of “lessons learned” from the large Blackouts in 2003,
FACTS and HVDC will play an important role for the system developments, leading to
“Smart Grids” with better controllability of the power flows (Fig. 26).
FACTS and HVDC provide the necessary features to avoid technical problems in the power
systems, and they increase the transmission capacity and system stability very efficiently and
they assist in prevention of cascading disturbances.
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Bulk power DC transmission will be applied in emerging countries like Brazil, China and
India, to serve their booming energy demands efficiently.
Tomorrow:
Today:
G
G
G
G
G
G
G
G
G
G
G
Use of Dispersed Generation
G
Load Flow will be “fuzzy”
Fig. 25: Perspectives of Transmission and Distribution Network Developments
Power System Expansion …
… with Advanced Transmission Solutions
Fig. 26:
From Congestion, Bottlenecks and Blackout
towards a “Smart Gird”
20/20