Applications of HVDC for large power system interconnections

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B4-106
Session 2004
© CIGRÉ
APPLICATION OF HVDC FOR LARGE POWER SYSTEM INTERCONNECTIONS
W. BREUER, V. HARTMANN, D. POVH*, D. RETZMANN, E. TELTSCH
SIEMENS AG
(Germany)
0. ABSTRACT
AC power systems have been developed by applying interconnections, first to the neighboring systems
and then to larger complex systems covering parts of or even whole continents to gain technical and
economical advantages. With an increasing size of the systems, however, advantages of synchronous
AC interconnections diminish because of technical and co-ordination problems and because of high
costs for adjustments in the systems. The HVDC technology offers a number of technical and
economical advantages for interconnection. Adjustments in the AC systems are not needed. HVDC
provides therefore often an overall more economical solution. On long term, however, a hybrid
interconnection could be the favorable solution, starting with an HVDC interconnection utilizing its
advantages, and later be extended by an additional AC interconnection. In principle, a hybrid AC/DC
solution does not have any limitation for the size of the interconnected networks.
Keywords: Power System Interconnections - AC Solution - DC Solution - Hybrid Solution - Technical
Limitations - Economical Considerations.
1. INTRODUCTION
The development of electric power supply began more than one hundred years ago. Residential areas
and neighboring establishments were supplied first by DC via short lines. At the end of the 19th
century, however, AC transmission has been 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. It has been followed by the
development of new technologies in the field of high voltages, and by innovations in design and
manufacturing of the equipment [1]. Increasingly higher voltages have been used, first at 110 and 220
kV levels in Europe, then 287 kV in USA and 380 kV in Europe. Finally, the 735 kV level in Canada
and corresponding 765 kV level in other countries have been established. A transmission project with
1150 kV level has been built-up for testing purposes in the erstwhile Soviet Union.
Power systems have been extended by applying interconnections to the neighboring systems to
achieve technical and economical advantages. Regional systems have been built-up towards national
grids and later to interconnected systems with the neighboring countries. In industrialized countries
large systems came into existence, covering parts of or even whole continents. The liberalization in the
power industry additionally supports interconnections to enable exchange of power among the regions
* Siemens AG, Power Transmission and Distribution, High Voltage, P.O. Box 3220, 91050 Erlangen, Germany.
e-mail: dusan.povh@ptd.siemens.de
or countries and to transport cheaper and more ecologically suitable energy over long distances to the
load centers. Maximum reasonable distances to transmit power still economically are in the range of
up to 3000 km. In the future, the situation can, however, change if ecological and political terms or the
present cost conditions alternate.
In the second half of the past century, high power HVDC transmission technology has been
introduced, offering new dimensions for long distance transmission. This development started with the
transmission of power in an order of magnitude of a few hundred MW and was continuously increased
to transmission ratings up to 3 - 4 GW over long distances by just one bipolar line.
By these developments, HVDC became a mature and reliable technology. Almost 50 GW HVDC
transmission capacities have been installed worldwide up to now, ref. to Fig. 1 [3]. Transmission
distances over 1,000 to 2,000 km or even more are possible with overhead lines. Transmission power
of up to 600 - 800 MW over distances of about 300 km has already been realized using submarine
cable, and cable transmission lengths of up to about 1,300 km are in the planning stage.
60
GW
50
40
30
20
10
0
1970
1980
1990
2000
2010
Sources: IEEE T&D Committee 2000 - Cigre WG B4-04 2003
Fig. 1: Worldwide installed Capacity of HVDC Links
To interconnect systems operating with different frequencies, back-to-back (B2B) schemes have been
applied. As a multiterminal system, HVDC can also interconnect several locations in the surrounding
AC network [2].
2. LIMITS OF SYNCHRONOUS INTERCONNECTIONS
In industrialized countries extensive interconnected systems were built in the past to gain the well
known advantages, e.g. an ability to use larger and more economical power plants, reduction of
reserve capacity in the systems, utilization of the most efficient energy resources, and to achieve an
increase in system reliability. A similar development is in progress in emerging countries of Asia and
South America, where the demand for power is still growing fast.
As the regional networks have been built to supply energy from power stations relatively close to the
load centers, the voltage levels have been chosen according to these initial conditions. However, due to
the demand for interconnection to other systems and for exchange of power between them, these
conditions have been changed. Power has now to be transmitted over longer distances by insufficient
voltage levels and systems are in general not well developed at the system borders. This can produce
technical problems leading to bottlenecks when power has to be exchanged between the systems.
The benefits of power system interconnections are listed in Fig. 2. They are generally valid and do not
depend on the kind of the interconnection. However, some of these advantages diminish with an
increasing size of the systems to be interconnected.
The interconnection alternatives are schematically shown in Fig. 3 [3]. In principle, an interconnection
can be realized by establishing a synchronous link where such a solution is technically feasible and
economically justified. On the other hand, HVDC links offer often a technically better and even more
economical solution. A further solution is the hybrid interconnection, consisting of an AC connection,
2
supported by an additional HVDC link. In case that a synchronous interconnection is technically close
to its limits, the HVDC additionally can support the operation of the interconnected systems and
improves the transmission reliability.
System 1

Possibility to use larger and more economical Power Plants

Reduction of the necessary Reserve Capacity in the System

Utilization of most favorable Energy Resources

Flexibility of building new Power Plants at favorable Locations

Increase of Reliability by mutual Support in case of Emergency

Reduction of Losses by an optimized System Operation

Reduction of Generation Costs by optimal Unit Commitment
Fig. 2: Advantages of System Interconnections
System 2
a) AC Interconnection
b) DC Interconnection
c) Hybrid AC/DC Interconnection
Fig. 3: Alternatives of System Interconnections
An example for synchronous operation of very large power systems is the UCTE system in Western
Europe (Fig. 4), which has been extended step by step to the today very complex configuration, with
the extension to Romania and Bulgaria, and later reconnection of the Balkan countries [8]. Some of
the Maghreb countries in North Africa are already connected to the UCTE network and there are
further plans to interconnect Turkey through Balkan countries. Furthermore, discussion is in progress
on a possible interconnection to IPS/UPS system; however, it is open whether to apply an HVDC
interconnection which could be realized very fast to use its economic advantages or to wait for a
synchronous interconnection in the time horizon of 10 or more years.
NORDEL
IPS/UPS
UCTE - 1
UCTE - 2
Turkey
AL M AGHREB
Fig. 4: European Power Systems (2003) [8]
In large power systems technical problems occur resulting from meshed systems on one hand and
problems of long distance transmission on the other hand. They are summarized in Fig. 5 [4, 5, 6].
With an increasing size of the interconnected system over thousands of kilometers most of the
advantages offered by the interconnection will reduce. Large blackouts in America and Europe
confirm clearly, that the favorable close electrical coupling by AC might also include risk of
uncontrollable cascading effects in large and heavily loaded interconnected systems.
Results of UCTE studies on possible interconnections between the UCTE and IPS/UPS networks
show that additional power transfer through the existing system leads to bottlenecks or produces
insufficient n-1 conditions at different locations in the UCTE system [7, 8]. Fig. 6 shows that an
additional east-west energy transfer is limited (NTC values). To avoid congestion and problems,
additional investments and improvements for the system operations will be needed.
3
Interconnected S ystem s
o L o ad F low P rob lem s (n eed s
M anag em en t o f C o n g estion )
Long D istance Transm ission
S ystem s
o V o ltag e Stab ility
o F req u enc y C o n tro l
o V o ltag e S tab ility
o O scillatio n Stability
o In ter-A rea O scillatio n s
o B lackout R isk due to cascad ing
E ffects
o R eactive P ow er P ro blem s
o S tead y-S tate S tab ility
o T ran sien t S tability
o S u bs yn ch ronous O scillations
o P h ysical In teractio n s betw een
P ow er S ystem s
Fig. 5: Technical Problems in Large Power Systems
Bottlenecks
in the UCTE
System
Source: UCTE - 5 / 2003
NTC Values
for EastWest Power
Transfer
Fig. 6: Presently existing Bottlenecks (congestions) in the UCTE Grid with NTC (Net Transfer
Capacity) values for East-West Power Transfer [7]
The technical limitations of large interconnected systems have impact on the cost benefits of the
synchronous interconnection. These aspects are listed in Fig. 7. High costs are needed for system
adjustments and for co-ordination of joint system operation. If the AC interconnection is weak and
heavily loaded, stability problems will arise and the advantages of spinning reserve sharing diminish
as power has to be transmitted over long distances and can produce additional bottlenecks in the
system.
 High Costs for System Adjustments (Frequency Controls, Generation Reserve)
 Need for close Co-ordination of joint System Operation
 AC Interconnections normally weak at the beginning (additional Lines needed to
avoid dynamic Problems)
 Bottlenecks in the System because of uncontrolled Load Flow
 Spinning Reserve in the System to be transmitted over long Distances (additional
Loading and Cost Increase)
 Reduction of economic Advantages because of large Transmission Distances and
insufficient Voltage Levels (e.g. 400-500 kV)
 In liberalized Markets Disadvantages and higher Costs when transmitting Power
through a Number of Systems and Involvement of a Number of Partners
 Stability Problems become more complex
 Phenomena spreading through the large and complex System difficult to be
managed
Fig. 7: Problems of large synchronous System Interconnections
4
3. USE OF HVDC FOR INTERCONNECTION
The easiest way to interconnect large power systems is to use HVDC. Major benefit of an HVDC link
is its ability to control the power flow and its flexibility to adapt to different AC system characteristics
at both sides of the interconnection. The DC interconnection can be either long distance transmission
or a back-to-back link. Fig. 8 shows schematically both alternatives.
a)
Alternatives of HVDC interconnection
a)
b)
Back-to-Back Solution
Long Distance Solution
b)
Fig. 8: Alternatives of HVDC
Interconnection
The back-to-back solution is more suitable for exchange of moderate power, e.g. up to 1200 MW in
the areas close to the borders of both systems. If, however, a large amount of power should be
exchanged or transmitted over long distances, the HVDC point to point transmission offers more
advantages. Power can be brought directly to the spots in the systems where it is required without any
risk to overload the AC system in between. If the AC system voltage is relatively low for transmission
of large power over longer distance, e.g. 400 kV in the UCTE system, the HVDC transmission is a
more economical solution at distances of 700 km and above. A further advantage of such a solution is
the control performance of HVDC which can effectively support the AC system stability and damp
inter-area oscillations.
If an HVDC alternative is used, there are no special needs to co-ordinate the behavior of the AC
systems or to provide a strong AC link with several lines to establish a dynamically stable and reliable
interconnection. Hence, the advantages of interconnection can be fully utilized without additional
improvements of the AC systems. An additional advantage is that HVDC can be built in stages,
following closely the demands of the interconnection, thus saving investment costs. Fig. 9 summarizes
the advantages of HVDC used for interconnection.
 With DC Solution, Interconnection Rating is determined only by the real Demand
of Transmission Capacity
 With AC solution, for System Stability Reasons, AC Rating must be higher than
the real Demand on Power Exchange
 Increase of Power Transfer: With DC, staging is easily possible
 With DC, the Power Exchange between the two Systems can be determined
exactly by the System Operator
 DC features Voltage Control and Power Oscillation Damping
 DC is a Barrier against Stability Problems and Voltage Collapse
 DC is a Barrier against cascading Blackouts
 Predetermined mutual Support between the Systems in Emergency Situations
Fig. 9: Advantages of HVDC for System Interconnection
The economical benefits of an HVDC interconnection in liberalized markets can be demonstrated by
an example. An interconnection between two large power systems shall be done by means of a 600
MW back-to-back scheme. Hence, the AC Systems do not need any adjustments. The costs for a B2B
installation, including integration of the scheme into the system and financing are about 60 Mio EURO
[2]. In the liberalized market, these investments should be paid back fast, because long power delivery
contracts are difficult to be achieved. Hence, a return on investment within only 3 years shall be the
5
goal. Taking into account a power delivery at full rating of 7000 h per year (this corresponds to 80 %
utilization), the interconnection costs are only about 0.5 EURO-Cents/kWh on a 3 years contract basis,
including costs for operation, maintenance and losses. Nowadays, the cost-difference for electric
energy in different systems of the deregulated markets in many regions is typically 2 EUROCents/kWh, or even more, hence the HVDC interconnection would be economically extremely
effective. A back-to-back scheme can be built-up in a very short time, e.g. within 2 years. This is an
outstanding advantage of the DC-solution, as the time horizon to realize a synchronous
interconnection between very large systems is in the range of up to 10 years, or even more.
For long distance transmission, different alternatives are possible, which are depicted in Fig. 10.
A)
B)
A) HVDC Transmission
B) Long Distance AC Transmission
C) AC Transmission through Interconnected Power Systems
Subs.
Subs.
Subs.
C)
Fig. 10: Configurations for long Distance Transmission
In the case that power is transmitted through the subsystems (Fig. 10 c), existing networks are
additionally loaded, thus leading to unacceptable conditions in the system. Hence, additional
investments are needed to avoid bottlenecks. Losses in such a configuration are always higher
compared to transmission by a superposed AC link at a higher voltage level. It would therefore be
better to build a direct, superposed HVAC transmission between both power systems. However, the
HVDC transmission offers in such cases lower costs and additional technical advantages. Fig. 11
shows cost comparison for HVAC and HVDC transmission, transmitting 2000 MW over the distance
of 900 km using one 500 kV HVDC bipolar scheme and one single 765 kV AC transmission line.
Cents/kWh
€ cents/
kwh
HVAC
Loss costs
Loss Costs
AC
System
AC
System
costs
Investment
Costs
1,5
1,5
AC
System
AC
System
Source: Siemens PTD SE NC - 2003
HVAC2
735 kV
HVAC
1x
HVAC2
X 765
735XkV
kV
0,5
HVAC3
X 500XkV500 kV
HVAC3
HVDC
HVDC
+kV
650 kV
HVDC
500
kV
HVDC
+ ±500
1,0
1,0
Transmission Distance 900 km
Transmission Power 2000 MW
Fig. 11: Cost Comparison of HVDC and HVAC long Distance Transmission
4. BENEFITS OF HVDC INTEGRATION INTO A SYNCHRONOUS AC SYSTEM
The HVDC can be integrated into a synchronous AC network to reinforce the interconnection of
different parts of the system, when an increase of power exchange is requested without overloading
6
weak links or bottlenecks in the existing grid. Such a situation is expected in the German network,
when large amounts of renewable energy sources, e.g. wind parks, shall be connected to the northern
parts of the grid, ref. to [9]. At present, a total amount of about 12 GW wind power has already been
installed in Germany. A further increase of up to 45 GW wind power capacities can be expected in
next decades, from which about 50 % will be generated by off-shore wind parks in the north- and eastsea areas.
Fig. 12 shows a typical example of the present conditions. Plotted are wind power in-feed and the
regional network load during a week of maximum load in the E.ON control area. The relation between
consumption and supply in this control area is illustrated. In this part of the German power system, the
transmission capacity is already at its limits, especially during times with low load and high wind
power generation. Network overloading is identified, if the northern generation surplus should be
transmitted to the southern parts of the grid.
Source: E.ON - 2003
Fig. 12: Network Load and aggregated Wind Power Generation during a week of maximum Load in
the E.ON Grid
Power output of wind generation can vary fast in a wide range, depending on the weather conditions.
Hence, a sufficiently large amount of controlling power from the network is required to substitute the
positive or negative deviation of actual wind power infeed to the scheduled wind power amount. The
expected controlling power and the installed power are depicted in Fig. 13. It can be seen, that even at
present the need for controlling power is more than 2000 MW.
Fig. 13: Forecast of installed Wind Generation Capacity and required Controlling Power
Both tasks, to transmit surplus of power out of the northern wind generation area and to provide the
controlling power from the generation in central and southern grid parts, would additionally load the
existing network, thus leading to bottlenecks in the transmission system.
7
Upgrading of the high voltage transmission system will be essential. As an even more efficient
alternative, the integration of an HVDC long distance transmission link into the synchronous AC
system is schematically shown in Fig. 14.
Share in installed wind energy of 12,223 MW
Share in installed Wind Energy of 12,223 MW
E. ON Netz:
E. ON Netz:
48 %
Vattenfall
Europe
Transmission:
Vattenfall
Europe
Transmission:
37 %
RWE
Net:
14 %
RWE
Transportnetz Strom:
EnBW
Transportnetze:
1%
EnBW
Transportnetze:
48 %
37 %
14 %
1%
Source: E.ON - 2003
Fig. 14: Integration of HVDC Transmission into a
synchronous AC System - Example
5. SYNCHRONOUS HYBRID AC/DC INTERCONNECTION
The third possible interconnection alternative, as shown in Fig. 3, is the hybrid interconnection,
consisting of an HVDC link and an AC interconnection in parallel. Such system interconnection
should start with HVDC, B2B or Long Distance Transmission, to offer the above discussed technical
and economical advantages. Later, it could be extended to a synchronous interconnection by means of
additional AC links. An advantage of such a solution is fast realization of the starting phase for power
transfer. During further grid development, the AC systems could be improved and adjusted for the
later joint synchronous operation.
1500
Power flow in one line
Huishui -Hechi (MVA)
1200
Power System
900
600
300
b
b
0
aa
-300
-600
Nayong
Anshun
Guiyang
Guizhou
-900
0
5
10
15
20
Time (s)
HVDC GuiGuang
Anshun Huishui
Anshun
Hechi
Guangdong
Liuzhou
TSQBASUO
TSQDAW_27
Hezhou
Yantan
Luoping
Dynamic Results
Guangzhou
TSQ-II
Lubuge
Pingguo
Laibin
Beijiao
Wuzhou
Luodong
Baise
TSQ-I
Yunnan
Gaomin
Guangxi
HVDC TSQ
Nanning
Zhaoqing
Yulin
Fig. 15: Examples of Power Oscillations
without a) and with b) Power Modulation of
HVDC Control in the Chinese Grid [12]
8
The HVDC can additionally support the dynamic behavior of the AC interconnection during fault
contingencies and control load flow in the system. Fig. 15 shows an example of a large power system
in the Chinese grid [12], in which HVDC has been integrated. Because of long transmission lines, the
AC system experiences severe power oscillations after systems faults, close to the stability limits. In
the figure, oscillations are depicted, first for the case that HVDC is just transmitting power in constant
power mode (curve a). It can be seen, that strong power oscillations occur. If, however, damping
control of HVDC is activated (curve b), the oscillations are damped very effectively. Without HVDC,
e.g. with a fully synchronous interconnection, such a large power system would be unstable in case of
fault contingencies, thus leading to severe outages (Blackout). Such an example is shown in Fig. 16.
AC double-link 1
System A
1200 MW
System B
Ac double-link 2
200 GW Grid A
200 GW Grid B
AC Link 1
DC Link 1
AC Link 2
AC Link 1
DC-link 1
System A
1200 MW
System B
AC double-link 2
a)
b)
Only AC - System instable after Fault
Hybrid AC/DC - System remains stable
Fig. 16: Comparison of System Stability for synchronous AC and synchronous Hybrid Interconnection
a) Set-up of the Systems
b) Dynamic Results
The hybrid solution offers the best possibility for large power system interconnections. In case of
interconnected networks, consisting of a number of smaller systems, a configuration according to
Fig.17 is technically and economically the best solution. An AC or DC interconnection between the
neighboring areas of the different systems enables local power exchange among these regions. The
performance of the AC links can additionally be improved by means of FACTS (Flexible AC
Transmission Systems, [13]). Transmission of larger power blocks over long distances is, however
realized by HVDC long distance transmissions directly between the locations of power surplus and
power demand. The HVDC can at the same time strengthen synchronous interconnections to avoid
possible dynamic problems, which exist in such complex configurations. There are practically no
technical limitations for the size of the interconnected system when using such a configuration, as
shown in Fig. 17.
System
A
System
B
System
C
System
D
System
E
System
F
Large
LargeHybrid
HybridInterconnections
Interconnectionswith
withHVDC
HVDCand
andFACTS:
FACTS:
HVDC - Long Distance DC Transmission
DC Interconnection ( B2B)
High Voltage AC Transmission / FACTS
Fig. 17: Hybrid Interconnection for large Power Systems
The development of power systems in emerging countries like India and China is clearly focused
towards the hybrid interconnection, ref. to Fig. 18 a) and b).
9
Existing
Gezhouba-Shanghai
TianGuang
3G-ECPG I
GuiGuang
3G-Guangdong
NECPG
NCPG
Wangqu Plant
NWCPG
Yangcheng Plant
SPPG
North Power Grid
3 x B2B
New
18 HVDC Interconnections
Russian Power Grid
10 x HVDC
Long Distance
Transmission
CCPG
Three Gorges
CSPG
ECPG
Center Power Grid
Jinshajiang
River
SCPG
Lanchangjiang
River
HPPG
South Power Grid
a) Example China
Source: SP China, ICPS - 09/2001
Tailand Power Grid
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 AG OO R A
D U L H A S TI
R A V I S A TLU J
K IS H EN P U R
JU LL A N D H AR
T EH R I
M OGA
B A LLA B G A R H
( D EL H I R IN G )
C H IC K E N N E C K
A 'PU R
BHUTAN
MEERUT
H IS SA R
LU C K N O W
BH IW A D I
S HIR O H I
30
N A G D A B INA
S IL IG U R I/B IR P AR A
K O R BA
R A IP U R
TA R A PU R A K O L A
PA D G H E
C H A N D R A PU R
10 00 M W
MW
00
20
AM R AV A TI
WR
D HA B O L
LO N IK A N D
KOYNA
K AR A D
ER
T A LC H ER
JE Y PO R E
G AZ U W A K A
SR
X P LA N
X I P LA N
V IJA YA W A DA
00
765 K V L IN ES
20
S IR S I
L E G E ND
E X IST IN G /
IX P L A N
N A R EN D R A
K A IG A
H IR M A
NER
KR IS H N A
N AG A R
R O U R KE LA B A N G LA
DESH
R A M A G U N D AM
KO L H AP U R
PONDA
B A DA R P U R
T IP A IM UK H
N .K .
S E O N I S IP A T
G AN D H A R /
A M R E LI K A W A S
CHEGAON
V A PI
B H AN D A R A
P IP A V AV
B O IS A R
K A TH A LGURI
M AR IA N I
K A H A LG A O N
V IN D H YA CHAL
S A TN A
D IH AN G D A M W E
M IS A
50 0M W
B 'S H A R IF
/B A R H
DEHGAM
R A N G A N AD I
B O N G A IG A O N
V A RA N A S I
M A LA N P U R
S IN G R A U LI
MW
W
LIM BD I
J ET PU R
00
T AL A
PURNEA
M
ZE R D A
G 'P U R
M 'B A D
A G R A AL LAH A B AD
/U N N A O
N R J AIP U R
T EE S TA
AR U N
KR IS H N A PA TN A M
4 00 K V LIN E S
H V D C B/B
C H IT TO O R
H V D C B IP O L E
M A N G ALOR E
B A N G ALOR E
HOSUR
KS
HA
K A R AIK U D I
KA Y A M K UL A M
K A YA TH A R
R
EE
P
b) Example India
Source: Power Grid Corporation of India - 2002
AN AN D
DW
T R IV A N D R U M
N IC O B A
LA
P UG A L U R
AN D AM
S ING A R P E T
C U D D ALOR E
K O Z H IKO D E
C O C H IN
SO U T H C H EN N A I
Fig. 18: Hybrid Interconnections of large Power Systems in China a) and India b)
Both technologies, HVAC and HVDC are used taking into account their technical and economical
10
advantages [10, 11]. The hybrid solution seems also to be the best way for the realization of the future
interconnection between UCTE and IPS/UPS systems [14].
6. CONCLUSIONS
Power systems develop according to the increasing demand on power. Through this development,
large interconnected power systems come into existence. System interconnections offer technical and
economical advantages. These advantages are high when medium sized systems are interconnected.
However, when using synchronous AC interconnection, the advantages diminish with an increasing
size of the systems to be interconnected and on the other hand, the costs to adjust the AC systems for
synchronous operation increase. An HVDC interconnection does not need system adjustments and it
provides therefore an overall more economical solution. This technology enables power exchange in a
most flexible manner with high control performance, while the interconnected systems remain
physically independent. In addition, when power has to be transmitted through the system over longer
distances, the HVDC transmission is technically and economically the superior solution. On long term,
however, a hybrid solution, consisting of HVDC and AC links, is the most promising solution for
large national and continental interconnected power systems. There are no technical limits to build
continental and even intercontinental interconnections by hybrid solutions.
7. ACKNOWLEDGEMENTS
The Authors would like to thank the State Grid Corporation of China and the Power Grid Corporation
of India for the permission to use the presented figures of their power system development.
8. REFERENCES
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[Elektrizitätswirtschaft 90 (1991), H. 11, S. 558-576]
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[3] Han Yingduo, Wang Zhonghong, D. Povh, X. Lei, D. Retzmann “Role of HVDC and FACTS in
future Power Systems”, [Cigré Symposium, Shanghai, China, 8-10. April 2003]
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Report 37-301, Paris, Session 1992]
[6] H. Breulmann, E. Grebe, M. Lösing, W. Winter, R. Witzmann, P. Dupuis, P. Houry, T. Pargotin,
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Oscillations in the UCTE/CENTREL Power System”, [Cigré Report 38-113, Paris, Session 2000]
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