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

21, rue d'Artois, F-75008 Parishttp://www.cigre.org © CIGRÉ

Session 2004B4-106

2

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.

1970 1980 1990 2000 2010

60

50

40

30

20

10

0

GW

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,

3

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.

Fig. 2: Advantages of System Interconnections 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.

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.

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

System 1 System 2

a) AC Interconnection

b) DC Interconnection

c) Hybrid AC/DC Interconnection

System 1 System 2

a) AC Interconnectiona) AC Interconnection

b) DC Interconnection

c) Hybrid AC/DC Interconnectionc) Hybrid AC/DC Interconnection

NORDEL

IPS/UPS

UCTE - 1

AL MAGHREB

UCTE - 2Turkey

NORDEL

IPS/UPS

UCTE - 1

AL MAGHREB

UCTE - 2Turkey

4

Source: UCTE - 5 / 2003

NTC Values for East-West Power Transfer

Bottlenecks in the UCTE System

NTC Values for East-West Power Transfer

Bottlenecks in the UCTE System

Fig. 5: Technical Problems in Large Power Systems

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.

Fig. 7: Problems of large synchronous System Interconnections

Interconnected System s

o Load Flow Problems (needs M anagem ent of Congestion)

o Frequency Control

o Voltage Stability

o Oscillation Stability

o Inter-Area Oscillations

o Blackout Risk due to cascading Effects

o Physical Interactions betw eenPower System s

Long Distance Transm ission System s

o Voltage Stability

o Reactive Pow er Problems

o Steady-State Stability

o Transient Stability

o Subsynchronous Oscillations

Interconnected System s

o Load Flow Problems (needs M anagem ent of Congestion)

o Frequency Control

o Voltage Stability

o Oscillation Stability

o Inter-Area Oscillations

o Blackout Risk due to cascading Effects

o Physical Interactions betw eenPower System s

Long Distance Transm ission System s

o Voltage Stability

o Reactive Pow er Problems

o Steady-State Stability

o Transient Stability

o Subsynchronous Oscillations

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 toavoid 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

5

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.

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.

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

b)

a)

b)

a)

Alternatives of HVDC interconnectiona) Back-to-Back Solutionb) Long Distance Solution

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

b)

a)

b)

a)

Alternatives of HVDC interconnectiona) Back-to-Back Solutionb) Long Distance Solution

6

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 EURO-Cents/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.

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.

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

A)A)

B)B)

C)

Subs. Subs.Subs.

C)

Subs.Subs. Subs.Subs.Subs.Subs.

A) HVDC TransmissionB) Long Distance AC TransmissionC) AC Transmission through Interconnected Power Systems

Source: Siemens PTD SE NC - 2003

HVDC

ACSystem

ACSystem

HVAC

ACSystem

ACSystem

HVA

C3

X 50

0 kV

HVA

C2

X 73

5 kV

HVD

C +

650

kV

HVA

C3

X 50

0 kV

HVA

C2

X 73

5 kV

HVD

C +

500

kV

Loss costs

costs

0,5

1,0

1,5

€ cents/ kwh

Loss Costs

Investment Costs

0,5

1,0

1,5

€ Cents/kWh

Transmission Distance 900 kmTransmission Power 2000 MW

HVD

C ±

500

kV

HVA

C 1

x 76

5 kV

Source: Siemens PTD SE NC - 2003

HVDC

ACSystem

ACSystem

HVDC

ACSystem

ACSystem

HVAC

ACSystem

ACSystem

HVAC

ACSystem

ACSystem

HVA

C3

X 50

0 kV

HVA

C2

X 73

5 kV

HVD

C +

650

kV

HVA

C3

X 50

0 kV

HVA

C2

X 73

5 kV

HVD

C +

500

kV

Loss costs

costs

0,5

1,0

1,5

€ cents/ kwh

Loss Costs

Investment Costs

0,5

1,0

1,5

€ Cents/kWh

Transmission Distance 900 kmTransmission Power 2000 MW

HVD

C ±

500

kV

HVA

C 1

x 76

5 kV

HVA

C3

X 50

0 kV

HVA

C2

X 73

5 kV

HVD

C +

650

kV

HVA

C3

X 50

0 kV

HVA

C2

X 73

5 kV

HVD

C +

500

kV

Loss costs

costs

0,5

1,0

1,5

€ cents/ kwh

Loss Costs

Investment Costs

0,5

1,0

1,5

€ Cents/kWh

Transmission Distance 900 kmTransmission Power 2000 MW

HVD

C ±

500

kV

HVA

C 1

x 76

5 kV

7

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 east-sea 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.

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.

Source: E.ON - 2003

8

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. 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.

Fig. 15: Examples of Power Oscillations without a) and with b) Power Modulation of HVDC Control in the Chinese Grid [12]

Power System

Dynamic Results

5 10 15 200

0

600

900

1200

1500

-600

300

-900

-300

Time (s)

Pow

er f

low

in o

ne li

neH

uish

ui-H

echi

(MV

A)

a

b

a

b

Dynamic Results

5 10 15 200

0

600

900

1200

1500

-600

300

-900

-300

Time (s)

Pow

er f

low

in o

ne li

neH

uish

ui-H

echi

(MV

A)

a

b

a

b

5 10 15 200

0

600

900

1200

1500

-600

300

-900

-300

Time (s)

Pow

er f

low

in o

ne li

neH

uish

ui-H

echi

(MV

A)

a

b

a

b

GuiyangAnshun

Nayong

Anshun

TSQBASUO

Anshun Huishui

Hechi

Lubuge

TSQ-I

Luoping

HVDC TSQ

Liuzhou

Yantan

Pingguo

Baise

TSQ-II

Nanning

Yulin

Laibin

Hezhou

Gaomin

Luodong

Zhaoqing

BeijiaoGuangzhou

Wuzhou

TSQDAW_27

YunnanGuangxi

Guizhou

Guangdong

HVDC GuiGuang

GuiyangAnshun

Nayong

Anshun

TSQBASUO

Anshun Huishui

Hechi

Lubuge

TSQ-I

Luoping

HVDC TSQ

Liuzhou

Yantan

Pingguo

Baise

TSQ-II

Nanning

Yulin

Laibin

Hezhou

Gaomin

Luodong

Zhaoqing

BeijiaoGuangzhou

Wuzhou

TSQDAW_27

YunnanGuangxi

Guizhou

Guangdong

HVDC GuiGuang

Power System

Dynamic Results

5 10 15 200

0

600

900

1200

1500

-600

300

-900

-300

Time (s)

Pow

er f

low

in o

ne li

neH

uish

ui-H

echi

(MV

A)

a

b

a

b

Dynamic Results

5 10 15 200

0

600

900

1200

1500

-600

300

-900

-300

Time (s)

Pow

er f

low

in o

ne li

neH

uish

ui-H

echi

(MV

A)

a

b

a

b

5 10 15 200

0

600

900

1200

1500

-600

300

-900

-300

Time (s)

Pow

er f

low

in o

ne li

neH

uish

ui-H

echi

(MV

A)

a

b

a

b

GuiyangAnshun

Nayong

Anshun

TSQBASUO

Anshun Huishui

Hechi

Lubuge

TSQ-I

Luoping

HVDC TSQ

Liuzhou

Yantan

Pingguo

Baise

TSQ-II

Nanning

Yulin

Laibin

Hezhou

Gaomin

Luodong

Zhaoqing

BeijiaoGuangzhou

Wuzhou

TSQDAW_27

YunnanGuangxi

Guizhou

Guangdong

HVDC GuiGuang

GuiyangAnshun

Nayong

Anshun

TSQBASUO

Anshun Huishui

Hechi

Lubuge

TSQ-I

Luoping

HVDC TSQ

Liuzhou

Yantan

Pingguo

Baise

TSQ-II

Nanning

Yulin

Laibin

Hezhou

Gaomin

Luodong

Zhaoqing

BeijiaoGuangzhou

Wuzhou

TSQDAW_27

YunnanGuangxi

Guizhou

Guangdong

HVDC GuiGuang

Share in installed wind energy of 12,223 MW

E. ON Netz: 48 %Vattenfall Europe Transmission: 37 %RWE Net: 14 %EnBW Transportnetze: 1 %

Source: E.ON - 2003

E. ON Netz: 48 %

Share in installed Wind Energy of 12,223 MW

Vattenfall Europe Transmission: 37 %

RWE Transportnetz Strom: 14 %EnBW Transportnetze: 1 %

Share in installed wind energy of 12,223 MW

E. ON Netz: 48 %Vattenfall Europe Transmission: 37 %RWE Net: 14 %EnBW Transportnetze: 1 %

Share in installed wind energy of 12,223 MW

E. ON Netz: 48 %Vattenfall Europe Transmission: 37 %RWE Net: 14 %EnBW Transportnetze: 1 %

Source: E.ON - 2003

E. ON Netz: 48 %

Share in installed Wind Energy of 12,223 MW

Vattenfall Europe Transmission: 37 %

RWE Transportnetz Strom: 14 %EnBW Transportnetze: 1 %

E. ON Netz: 48 %E. ON Netz: 48 %

Share in installed Wind Energy of 12,223 MW

Vattenfall Europe Transmission: 37 %Vattenfall Europe Transmission: 37 %

RWE Transportnetz Strom: 14 %RWE Transportnetz Strom: 14 %EnBW Transportnetze: 1 %EnBW Transportnetze: 1 %

9

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.

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.

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).

SystemA

SystemB

SystemC

SystemD

SystemE

SystemF

Large Hybrid Interconnections with HVDC and FACTS:Large Hybrid Interconnections with HVDC and FACTS:

HVDC - Long Distance DC Transmission

High Voltage AC Transmission / FACTSDC Interconnection (B2B)

SystemA

SystemB

SystemC

SystemD

SystemE

SystemF

Large Hybrid Interconnections with HVDC and FACTS:Large Hybrid Interconnections with HVDC and FACTS:

HVDC - Long Distance DC Transmission

High Voltage AC Transmission / FACTSDC Interconnection (B2B)

System A

AC double-link 1

1200 MW

Ac double-link 2

System B

200 GW Grid A 200 GW Grid B

System A

AC double-link 1

1200 MW

Ac double-link 2

System BSystem A

AC double-link 1

1200 MW1200 MW

Ac double-link 2

System B

200 GW Grid A 200 GW Grid B200 GW Grid A 200 GW Grid B

System A System B

AC double-link 2

DC-link 1

1200 MWSystem A System B

AC double-link 2

DC-link 1

1200 MW1200 MW

Only AC - System instable after Fault Hybrid AC/DC - System remains stable

AC Link 1

AC Link 2 AC Link 1

DC Link 1

Only AC - System instable after Fault Hybrid AC/DC - System remains stable

AC Link 1

AC Link 2 AC Link 1

DC Link 1

a) b) b)

10

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

Russian Power Grid

North Power Grid

Center Power Grid

LanchangjiangRiver

JinshajiangRiver

NWCPG

NCPGWangqu Plant

Yangcheng Plant

NECPG

SPPG

CSPG Three GorgesECPG

CCPG

Tailand Power Grid

SCPG

South Power GridHPPG

a) Example China Source: SP China, ICPS - 09/2001

b) Example India

R O U R KE LA

R A IPU R H IR MA

T A LC H ER

J AIP U R

N ER

ERW R

N R

SR

B 'SH AR IF

AL LAH A B AD

SIP A T

G AZ U WAK A

JE Y POR EC H AN D R A PU R

S IN G R A U LI

V IN D H YA -

2000M

W

2000MW

3000MW

10 00MW

500M W

LU C K N O W

D IH AN G

C H IC KEN N ECK

KR IS H N A

T EE STA

T IPAIM UK HB ADA R P U R

M ISA

D AMW E

K ATH A L-GU R I

LEG E ND

765 K V L IN ES 400 KV LIN E S

H V D C B/B

H VD C B IP OLE

E XIST IN G / X P LA N X I P LA N

ZE R D A

H IS SA R

B O N GA IGA O N

D E V E LO PM E N T O F N AT IO N A L G R ID

KO L H AP U R

N A R EN D R A

K A IG A

MA NG ALOR E

PO N D A

IX PLA N

M AR IA N I

N .K.

K A H A LG AO N

R AN GA N AD I

SEO N I

C H E G AO NB H AN D A R A

D E H GA M

K AR AD

LO N IK AN D

V A PI

G AN D H A R /

T ALAAR U N

BA N G LA

B A LLA B GAR H A'PU R(D EL H I R IN G )

B A N G ALOR E

KO Z H IKO D E

C O C H IN

KA YA MK ULAM

T R IVAN D R U M

PUG ALU R

K A YA TH AR

K AR AIKU D I

C U D D ALOR E

SO U T H C H EN N AI

KR IS H N A PA TN AM

C H IT TOO R

VIJA YA W A DA

S ING AR P E T

P IPAV AV

LIM BD I

K IS H EN PU R

D U L H ASTIW AG OO R A

M OG A

U R I

BH U TAN

RA MA G UNDAM

SA TLU JR AV I

JU LLAN D H AR

D ESHN AG AR

VA RA N ASI

/U N N A O

M'B A D

P U R N E A

KO R BA

N A GD A

S ILIG U R I/B IR P AR A

P H A S E - III

NIC

OB

AR

A

ND

AM

AN

AN

D

LAK

SH

AD

WE

EP

T EH R I

ME ER UT

BH IW AD I

B INA S ATN A

MA LA NP URS HIR O H I

K AWA S

AM R AV A TI

A K OLA

(B y 2012)

AG R A

S IR SI

C H A L

J ET PU RA MR E LI

BO ISA RTA R A PU R

PAD GH E

D HA BO L

KO Y N A

/B A R H

G'P U R

H O SU R

Source: Power Grid Corporation of India - 2002

Gezhouba-ShanghaiTianGuang3G-ECPG IGuiGuang3G-Guangdong

3 x B2B10 x HVDC Long Distance Transmission

Exis

ting

New

18 H

VDC

Inte

rcon

nect

ions Gezhouba-Shanghai

TianGuang3G-ECPG IGuiGuang3G-Guangdong

3 x B2B10 x HVDC Long Distance Transmission

Exis

ting

New

18 H

VDC

Inte

rcon

nect

ions

11

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

[1] R. Kahnt, “Entwicklung der Hochspannungstechnik – 100 Jahre Drehstromübertragung”, [Elektrizitätswirtschaft 90 (1991), H. 11, S. 558-576] [2] “Economic Assessment of HVDC Links”, [CIGRE Brochure Nr.186 (Final Report of WG 14-20)] [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] [4] H. Brumshagen, et al., “Limits of Interconnection - Criteria for and Examples of Reasonable Extension”, [Cigré Report 37-203, Paris, Session 1990] [5] H.-C. Müller, H.-J. Haubrich, J. Schwartz, “Technical Limits of Interconnected Systems”, [Cigré Report 37-301, Paris, Session 1992] [6] H. Breulmann, E. Grebe, M. Lösing, W. Winter, R. Witzmann, P. Dupuis, P. Houry, T. Pargotin, J. Zerenyi, J. Dudzik, L. Martin, J. M. Rodriguez, “Analysis and Damping of Inter-Area Oscillations in the UCTE/CENTREL Power System”, [Cigré Report 38-113, Paris, Session 2000] [7] “Load-Flow Anylysis with Respect to a possible synchronous Interconnection of Networks of UCTE and IPS/UPS”, [UCTE Study Report, Brussels, 8. May 2003] [8] F. Vandenberghe, “State of UCTE Studies on the Interconnection between the UCTE System and CIS & Baltic States”, [Cigré Conference, 17-19. Sept. 2003, St.-Petersburg, Russia] [9] M. Luther, U. Radtke, “Betrieb und Planung von Netzen mit hoher Windenergieeinspeisung”, [ETG Kongress, 23-24. Okt. 2001, Nürnberg, Germany] [10] Li Wengi, “Power Grid development and HVDC Transmission in China”, [ICPS, 3-5. Sept. 2001, Wuhan, China] [11] J. D. Wheeler, et al., “Building India's Grid: an Examination of the Infrastructure. Benefits of HVDC Transmission”, [CIGRE Report 14-114, Session 2002] [12] Jing Yong, Wu Xiaochen, Du Zhongming, Jin Xiaoming, Wang Yuhong, D. H. Zhang, J. Rittiger, “Digital Simulation of AC/DC Hybrid Transmission System”, [PowerCon, Oct. 2002, Kunming, China] [13] “FACTS Overview”, [IEEE and Cigré, Catalog Nr. 95 TP 108] [14] V. Sitnikov, D. Povh, D. Retzmann, E. Teltsch, “Solutions for large Power System Interconnections”, [Cigré Conference, 17-19. Sept. 2003, St.-Petersburg, Russia]