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Perspectives of Power System Interconnections

D. Povh*, D. Retzmann

Siemens, Erlangen, Germany Introduction Development of electrical power supplies began more than one hundred years ago. At the beginning, there were only small DC networks within narrow local boundaries, which were able to cover the direct needs of industrial plants by means of hydro power. Surrounding residential areas and neighboring establishments that did not have their own power supplies were then increasingly also supplied via short lines. Longer DC transmission lengths have been achieved as early as 1882 (2 kV, 57 km from Miesbach to Munich, Germany), [2].

In 1891, Oskar von Miller achieved a breakthrough in AC transmission, with the first three-phase AC transmission system from the hydropower station in the German town of Lauffen, covering a distance of 175 km with 15 kV to Frankfurt am Main. At a frequency of 40 Hz, 210 kW was sent "on its way" and the system was a commercial success.

The growth and extension of AC systems and consequently the introduction of higher voltage levels have been driven by an extremely fast growth in power demand over the decades, and have been followed by the development of new technologies in the field of high voltages and innovations in design and manufacturing of equipment. Increasingly higher voltages were used for AC transmission, first in Europe at the 110 kV (1911) and 220 kV levels (1929), then in USA with 287 kV (1932), at 380 kV (1952) in Germany and finally with 735 kV in Canada (1965) and 1150 kV (1985) as a trial transmission project in the erstwhile Soviet Union.

In the meantime, DC transmission has also entered completely new dimensions. From the first local "mini networks", to some extent with batteries connected for storage, there are now ways of transmitting 3 to 4 GW over large distances with only one bipolar system: 1,000 to 2,000 km or even more are possible with overhead lines. Transmission levels of up to 600-800 MW over distances of just less than 300 km have already been attained with submarine cables, and cable transmission lengths of up to about 1,300 km are in the planning stage [9]. As a multiterminal system, High Voltage DC Transmission (HVDC) can also be connected at several points with the surrounding three-phase network [3].

Increased engineering knowledge and a greater understanding of the phenomena in the systems made it possible to create larger and larger systems, which operate more efficiently and more reliable [5, 6, 10]. The development of electric power systems and the steps in the technology are demonstrated in Fig. 1.

Fig. 2 shows the corresponding development of voltage levels over decades [2]. In the various countries the maximal voltage level became different, depending on the power to be to be transmitted and the transmission distance. A further parameter for selection of the voltage level is the maximum of

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power which is acceptable to be lost at line outages, for stability reasons of the grid. Therefore, it can be expected that worldwide the maximal voltage will remain at 800 kV [1].

Fig. 1: Power Consumption per Inhabitant

Fig. 2: Development of AC-Voltage Levels - Milestones Since the last two decades, this general development has been superposed by new global trends towards liberalization of the electric power markets. The aim of this development is to open up the

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6 1200 kV Ekibastuz – Kokchetav / USSR (1985)

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markets and to give consumers the opportunity to purchase energy at more favorable prices. Various countries feature different kinds of market liberalization models. In some countries the liberalization of the power market has already taken place; in others it will still take years before an acceptable solution is found. For all systems, however, the principles are similar: Generation is separated from transmission, to enable competition among the power plants in order to achieve the lowest price. The high voltage system becomes the network simply to “transport” the energy from the generating plant to the regional networks. The transmission system takes over the role of meeting contractual agreements among different parties for delivering power.

This changing environment decisively influences further development and optimization of transmission networks, since the load flows existing today can change considerably. The ancillary functions required for smooth operation of the networks, such as frequency control, load-flow control, reactive-power and voltage control, as well as the responsibility for system security, are in the hands of the system operator. To support the operation and to increase the reliability of heavily loaded networks FACTS (Flexible AC Transmission Systems, [4]) devices need to be installed. Higher investments into interconnections must be made to achieve cost benefits. For this task HVDC transmission will remain the favorable solution [3]. Power System Development The development of power systems is driven by the increasing demand on electric power. Global studies show that power consumption in the world follows closely the increase of population (Fig. 3). In next 20 years, power consumption in developing and in emerging countries is expected to increase for 220%, in developed countries, however, only for 37%. It means that fast development of power systems can be expected mainly in the areas of developing and emerging countries. This can be also seen from Fig. 4 where the worldwide installed generation capacity is shown by regions and by energy sources. The importance of gas as energy source will further increase, the relative importance of coal, oil and nuclear power will decrease. The regenerative energy sources without hydro are expected to increase fast, however, in total they will be still only in the range of about 4 %.

25,000 TWhpower consumption

1980

15%

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29%

2020

60%

40% Developing andnewly industrializedcountries

Industrialized countries(OECD), CIS,Eastern Europe

8 billionpeople

6.1 billionpeople

+1.2%/a

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Sources: IEA; UN; Siemens PG CS4 - 08/2002

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Sources: IEA; UN; Siemens PG CS4 - 08/2002

Fig. 3: Development of World Population and Power Consumption, 1980 to 2020 The development of power systems takes into account locations of expected load requirements on one hand and the suitable location of power stations on the other hand, to transport the energy from generation to consumers. However, on a long-term basis, it can be expected that the transmission systems will stagnate in their development, since an increasing part of power generation will be

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transferred into the distribution or low voltage networks in future.

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Source: Siemens PG CS4 - 08/2002

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MiscalleneousRegeneratives

Source: Siemens PG CS4 - 08/2002

Fig. 4: Development of installed Generation Capacity - Worldwide Based on different studies for power systems in different world regions, following general trends can be expected:

• Further extension of interconnected systems with the increase of power exchange between the systems to gain the advantages of interconnections

• Transmission of large power blocks over long distances, mainly from Hydro Stations and other renewable power sources via power transmission corridors or through the interconnected systems

• Increasing part of power generation will, however, be connected to the distribution systems.

Development of Distribution Systems Major changes can be expected in distribution networks where additional investments are also likely. Fig. 5: Perspectives of Distribution Network Development

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The task of distribution networks in the past was to receive power from the high voltage system and to distribute it to industrial customers and households. Due to pressure to reduce costs in the liberalized markets, more decentralized power generation, mainly from renewable sources if economically justified, will be fed directly into distribution and low voltage networks. Also political support and ecological advantages of renewable generation will contribute to increase the portion of generation on distribution levels. New developments such as fuel cells will be used. Power Electronics will increasingly be applied for connection of wind, solar and fuel-cell generation to the networks via medium or low voltage DC links, as well as to increase the level of power quality. Optimization of operation using decentralized generation, along with the ability to store energy and reduce the peak power taken from the interconnected system, will save costs and contribute ecological benefits through new technologies. The configuration of distribution systems will change as shown in Fig. 5.

Interconnected Power Systems 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 an increase in system reliability. A similar development is in progress also in the emerging countries of Asia and Latin America, where the demand for power is still growing fast, ref. to Fig. 3 and 4. The national power grids are in some large countries still at the formation stage, however also under the influence of upcoming changes in the market. The above discussed advantages of power system interconnections are listed in Fig. 6. They are generally valid and do not depend on the kind of the interconnection.

Fig. 6: Advantages of interconnected Power Systems Fig. 7: Alternatives of Power System Interconnections Fig. 7 shows schematically different alternatives of system interconnections. In the past, synchronous operation of power systems was the normal way to build an interconnected system (Fig. 7a). Best example for this way is the development of the UCTE system in Western Europe, which has been extended in steps to the to-day very complex configuration [10], with the expected further extension to Romania and Bulgaria. Fig. 8 shows the interconnected power system of West Europe with its stages of extensions to Central and Eastern Europe.

However, 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. 9. With the increased size of the interconnected system over thousands of kilometers most of the advantages offered by the interconnection reduce. To avoid these problems additional improvements of the system are needed and operation of joint systems becomes very complex and less reliable [6, 10]. The technical limitations of very large interconnected systems have also impact on the cost benefits of the interconnection. The reasons for these limitations are listed in Fig. 10.

The easiest way to interconnect large systems is to use HVDC (Fig.7b). This interconnection can be either long distance transmission or a back-to-back link. In such cases there are no special needs to coordinate the behavior of both systems and to provide strong interconnection to establish a dynamic

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Possibility to use larger and more economical Power Plants

Reduction of the necessary Reserve Capacity in the System

Utilization of most favorable Energy Resources

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Reduction of Losses by an optimized System Operation

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stable interconnection. The advantages of the interconnection can fully be utilized without to coordinate the operation of both systems. Fig.12 summarizes the advantages of HVDC as interconnection. Further, HVDC can be built in stages, following closely the demands of the interconnection, thus saving investment costs.

Fig. 8: Trans European interconnected System

Interconnected Systems

o Load Flow Control

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o Subsynchronous Oscillations Fig. 9: Technical Problems in AC systems Economical limitations of synchronous interconnections are summarized in Fig. 11, ref. to [1, 6, 10]. When medium sized power systems are interconnected, high economic advantages can be expected as the benefits are much larger compared to the measures to synchronize the networks. This could be the case also in the area of Central America. However, above a certain size of the interconnected AC systems, no further advantages can be expected when synchronization is extended to additional networks. In such cases the use of HVDC technology offers technical and economic advantages, as this way of transmission can be performed between systems without the need to adjust the operating conditions.

An AC interconnection asks for coordination of a number of system parameters. An important one is the strength of the interconnection. If few lines connect two strong systems, the interconnection becomes unstable. Only if additional lines are built, which increase the strength of the link, the interconnected system becomes stable. In large interconnected systems, power which should be transmitted over longer distances has to flow through the meshed systems producing additional losses and overloading possibly existing bottlenecks. Using the HVDC solution, it is, however, possible to install the link directly between the locations which require power exchange, thus bypassing the system.

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Fig. 13 shows both alternatives of long distance transmission, HV AC and HVDC. For HVDC, a +/- 500 kV bipolar line and for AC three 550 kV, alternatively two 765kV lines have been assumed. These transmission configurations are nearly equivalent, also with respect to the reliability. Costs in Euro-Cents/kWh, including cumulated loss costs have been evaluated for 2000 MW transmission power over a distance of 900 km. It can be seen that the DC alternative offers much lower costs. Results of other similar studies show that in general the HVDC interconnection is the cheapest solution at transmission distances over about 700 km at a rating of 1000 MW and above (break-even distance). This is also valid if the energy is transmitted through the AC interconnected system. It means, that the integration of an HVDC transmission into the interconnected system is in total more economical than the power transmission through the AC system [1].

Fig. 10: Problems of large synchronous System Interconnections

Fig. 11: Features and Limitations of Interconnected Operations

Fig. 12: Advantages of HVDC Interconnections

o Lower Costs (depending on specific conditions)

o No need for common Frequency Control

o Stable Operation also at small Power Rating of Interconnection

o Improvement of dynamic Conditions in the AC systems

o Transmission of scheduled Power independent of System Conditions in the AC Systems

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Reduction of economic Advantages because of large TransmissionDistances at relatively low Transmission Voltage (e.g. 400-500 kV)

In liberalized Markets Disadvantages and higher Costs when transmittingPower through a number of Systems and involvement of a number ofPartners

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Fig. 13: Costs of High Voltage Transmission The third possibility is a hybrid interconnection, consisting of an HVDC transmission and a parallel AC interconnection. The interconnection could start with HVDC and be later extended by additional AC links. Advantage of such a solution is that the HVDC can additionally support the operation of the AC interconnection at faults in the system and can control the load flow.

The hybrid solution could offer therefore the best possibilities for large power system interconnections. In case of continental, interconnected networks, consisting of a number of smaller systems, a configuration according to Fig. 14 would be technically and economically the best choice. An AC interconnection between the neighboring areas of the interconnected systems enables power exchange among these systems. Transmission of larger power blocks over long distances is, however realized by HVDC transmissions directly to the locations of power demand. The HVDC at the same time can strengthen the synchronous interconnections to avoid possible dynamic problems, which exist in such huge configurations.

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Fig. 14: Large Power System Interconnections Examples for HVDC and Hybrid Interconnections USA and Canada In North-America, large interconnected systems exist. In the USA and Canada there are a number of systems which are separated and which are operated asynchronously. Fig. 15 shows the situation in USA with 3 separated power systems. They are, however, interconnected by HVDC links to enable power exchange.

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Fig. 15: US Grid Areas and DC Energy Bridges Europe In Europe, the large UCTE interconnected system unifies most countries of the European Union and has been recently extended even to countries of Central Europe. Also here, numerous HVDC interconnections exist to the neighboring systems of Great Britain and Northern Europe as shown in Fig. 16.

Fig. 16: HVDC in Northern Europe However, further extensions of these interconnected systems or mergers between such systems by way of AC links can not be expected, with the exception of a few specific cases where political decisions and not economic reasons are prevailing to build national systems or where small additional networks can be integrated into the larger system. Reasons for this are shown in Fig. 11. High investments in new equipment and an adjustment of the organizational structure would be needed to enable synchronous operation of the interconnected systems under all operating conditions [1, 6, 9, 10]. Coordination of frequency controls, reactive power control adjustment, a common strategy for outages, comparable reliability and joint protection principles are prerequisite. As shown in the Fig. 11, the total benefits of an AC interconnection therefore diminish with the size of the network. The investments needed to enable synchronous operation soar, and the above-mentioned complex technical problems related to the operation of such systems have to be solved. China China is one of the countries with the fastest increase of power demand. The installed generation

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capacity in the last 10 years, from 1990 to 2000, grew from 135 GW to 319 GW; this is more than 230 %. Main energy resources in the country are hydro power in the Central and South areas and coal in the North [7]. In addition, some nuclear power stations are built close to the load centers. Because of large distances in the country, in the beginning of the system development, smaller isolated regional systems have been built up and developed to seven large independent regional systems, in addition to some small local systems in less populated areas of Tibet and Xinjiang (Fig. 17).

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TibetTibet

North eastNorth east

NorthNorthNorth w estNorth w est

SichuanSichuan CentralCentral

EastEast

SouthSouth

Power network coverage 96.4%Source: SP China, ICPS - 09/2001

Main AC transmission voltage levels in China are 330 kV and 500 kV. Large power blocks, produced by hydro power stations, e.g. at Three Gorges, have to be transmitted to the load centers over distances of 1000km and more. For this task mainly HVDC transmission is used. However, for the transmission inside of the regional systems also 500 kV AC is utilized.

Coal fired power stations can partly be built closer to the load; however they are more concentrated in the North of the country. Power Exchange among the regional systems is still relative low in relation to the installed capacity of the systems. Therefore, only few AC lines would be sufficient for such interconnections to cover needs for power exchange. However, because of dynamic problems and the needed additional costs for adjustment of the systems to enable synchronous operation, mainly HVDC transmission is used for the interconnections.

In future, the today existing seven independent regional systems will be merged to three large interconnected systems (North, Center and South Power Grid). The interconnection among these grids will be done mainly by HVDC as shown in Fig. 18, ref. to [7]. In addition to the existing five HVDCs, further 13 HVDC transmission links are planned. Only for some smaller and remote local networks, the connection to the larger regional grids should be realized by AC, and, due to the long distances, probably using 765 kV. Possible interconnections to neighboring countries should also be realized by HVDC.

India India is the second large country, a subcontinent with a population of over 1 billion people and in the last decades a very rapidly growing economy. In the 1960’s with, at that time, still low demands for energy, the Indian power industry consisted of individual isolated grids within each state, with local state power plants. In 1970’s some of these state grids were interconnected to form regional grids. With the increasing power demand, the step was envisaged in 1980's to build a national grid by the interconnection of the regional networks to gain the advantages of power exchange, sharing generation resources, to increase the reliability and to reduce the power outages in some areas. At present, the installed generation capacity is 110 GW, 71% produced in coal fired plants, 25% in hydro plants and 4% in nuclear plants and others, in which the wind generation is the most important one [8].

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

Gezhouba-ShanghaiTianGuang3G-ECPG IGuiGuang3G-GuangdongNWCPG-CCPG B2BNCPG-CCPG B2B

NWCPG-CSPGSPPG-ECPG B2B3G-ECPG IIXiaowan-GuangdongJinghong-ThailandNWCPG-NCPGHPPG-SCPG

and 4 more ...... until 2020

Source: SP China, ICPS - 09/2001Ex

istin

gN

ew

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

Gezhouba-ShanghaiTianGuang3G-ECPG IGuiGuang3G-GuangdongNWCPG-CCPG B2BNCPG-CCPG B2B

NWCPG-CSPGSPPG-ECPG B2B3G-ECPG IIXiaowan-GuangdongJinghong-ThailandNWCPG-NCPGHPPG-SCPG

and 4 more ...

Gezhouba-ShanghaiTianGuang3G-ECPG IGuiGuang3G-GuangdongNWCPG-CCPG B2BNCPG-CCPG B2B

NWCPG-CSPGSPPG-ECPG B2B3G-ECPG IIXiaowan-GuangdongJinghong-ThailandNWCPG-NCPGHPPG-SCPG

and 4 more ...... until 2020

Source: SP China, ICPS - 09/2001Ex

istin

gN

ew

Fig. 18: China goes hybrid: AC plus 18 HVDC Interconnections

Fig. 17: Regional Power Systems in China

11

Large hydro energy resources in India are available in the North and North-East of the country, near the Himalayan Mountains. 25 GW hydro potential is already used, 19 GW in implementation and further 66 GW still available. Huge coal reserves are available in the West and East areas of the country. Power produced in hydro stations and coal fired plants has to be transmitted to the loads over large distances of 1000 km and more. The interconnection of the regional grids started by the realization of HVDC back-to-back stations and the HVDC long distance transmission Rihand-Delhi. The status of interconnections in the year 2002 is shown in Fig. 19.

Fig. 19: India Power Grid 2002 Fig. 20: India Power Grid in the Future Main reason for the decision towards HVDC was that the costs to improve the regional systems to enable them for synchronous operation would be very high and would need long time to build up sufficient generation reserve. The second reason was that HVDC offers also technical advantages supporting the operation of the AC systems.

At present, the cumulative interconnection capacity is about 5 GW, about 6% of total peak power. Also in the next development step towards a national grid, the interconnections between the regional systems will be realized mainly by HVDC. In the phase II of building the national grid, however, also the AC interconnections at a voltage level of 400 kV will be extended and the higher voltage level 765 kV will be introduced to build high capacity “transmission highways”. The capacity of the interconnections in this phase (years 2006-07) should reach 23 GW.

The interconnection capacity will further be increased to 30 GW in the years 2011-12. In this phase, a strong AC interconnection will be established on 765 kV level, synchronizing Northern, Western and Eastern regions. However, also in this stage further HVDC links will strengthen the interconnections to the Southern region (Fig. 20). Conclusions Power systems develop on line with the increasing demand on energy. With time, large interconnected systems come into existence. Further development will be also influenced by the liberalization of power industry. Transmission systems have to be improved by new investments, including the use of HVDC, FACTS and other new technologies.

DEVELOPMENT OF NATIONAL GRID

NER

ER

NR

WR

SR

EXISTING

500MW

500 MW

MW1000

MW500

GAZUWAKA

CHANDRAPUR

VINDHYACHAL

SASARAM

UNDERCONST.

EXISTING

400 kV

220 kV

BONGAIGAONBIRPARA

MALDA

KORBA BUDHIPADAR

KOLHAPUR

BELGAUM

U.SILERU

BALIMELA

DEHRISAHUPURI

MALANPUR

AURAIYA

PHASE-I

AN

DA

MA

N &

NIC

OB

AR

LAK

SH

AD

WE

EP

(By 2002)

DEVELOPMENT OF NATIONAL GRID

NER

ER

NR

WR

SR

EXISTING

500MW

500 MW

MW1000

MW500

GAZUWAKA

CHANDRAPUR

VINDHYACHAL

SASARAM

UNDERCONST.

EXISTING

400 kV

220 kV

BONGAIGAONBIRPARA

MALDA

KORBA BUDHIPADAR

KOLHAPUR

BELGAUM

U.SILERU

BALIMELA

DEHRISAHUPURI

MALANPUR

AURAIYA

PHASE-I

AN

DA

MA

N &

NIC

OB

AR

LAK

SH

AD

WE

EP

(By 2002)

ROURKELA

RAIPUR HIRMA

TALCHER

JAIPUR

NER

ERWR

NR

SR

B'SHARIF

ALLAHABAD

SIPAT

GAZUWAKA

JEYPORECHANDRAPUR

SINGRAULI

VINDHYA-

2000M

W

2000MW

3000MW

1000MW

500MW

LUCKNOW

DIHANG

CHICKEN NECK

KRISHNA

TEESTA

TIPAIMUKHBADARPUR

MISA

DAMWE

KATHAL-GURI

LEGEND

765 KV LINES 400 KV LINES

HVDC B/B

HVDC BIPOLE

EXISTING/ X PLAN XI PLAN

ZERDA

HISSAR

BONGAIGAON

DEVELOPMENT OF NATIONAL GRID

KOLHAPUR

NARENDRA

KAIGA

MANGALORE

PONDA

IX PLAN

MARIANI

N.K.

KAHALGAON

RANGANADI

SEONI

CHEGAONBHANDARA

DEHGAM

KARAD

LONIKAND

VAPI

GANDHAR/

TALAARUN

BANGLA

BALLABGARH A'PUR(DELHI RING)

BANGALORE

KOZHIKODE

COCHIN

KAYAMKULAM

TRIVANDRUM

PUGALUR

KAYATHAR

KARAIKUDI

CUDDALORE

SOUTH CHENNAI

KRISHNAPATNAM

CHITTOOR

VIJAYAWADA

SINGARPET

PIPAVAV

LIMBDI

KISHENPUR

DULHASTIWAGOORA

MOGA

URI

BHUTAN

RAMAGUNDAM

SATLUJRAVI

JULLANDHAR

DESHNAGAR

VARANASI

/UNNAO

M'BAD

PURNEA

KORBA

NAGDA

SILIGURI/BIRPARA

PHASE - III

NIC

OB

AR

A

ND

AM

AN

AN

D

LAK

SH

ADW

EE

P

TEHRI

MEERUT

BHIWADI

BINA SATNA

MALANPURSHIROHI

KAWAS

AMRAVATI

AKOLA

(By 2012)

AGRA

SIRSI

CHAL

JETPURAMRELI

BOISARTARAPUR

PADGHE

DHABOL

KOYNA

/BARH

G'PUR

HOSUR

12

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 become lower with the increasing size of the systems to be interconnected and on the other hand, the costs to adjust systems for synchronous operation increase.

An HVDC interconnection, however, does not need system adjustments and it provides therefore a more economic solution. 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 interconnections.

There are no technical limits to build continental and even intercontinental interconnections by hybrid solutions. However, the exchange of electric power over distances of more than 3000 km seems not to be economically justified at the to-day cost relations.

The presented examples in USA, Europe, China and India show in a very clear way that HVDC or hybrid interconnections offers best possibilities to build a large interconnected system using a pragmatic solution, at the beginning the HVDC interconnections, and later, depending on the requirements, to apply also the AC interconnection. Acknowledgments The Authors would like to thank the State Power Company of China and the Power Grid Corporation of India for the permission to use the presented figures of their power system development. References

[1] Povh, D., Ehmcke, B.: Möglichkeiten und Grenzen beim Stromtransport. VDI Berichte Nr. 1129, 1994

[2] Kahnt, R.: Entwicklung der Hochspannungstechnik – 100 Jahre Drehstromübertragung. Elektrizitätswirtschaft 90 (1991), H. 11, S. 558-576

[3] Economic Assessment of HVDC Links. CIGRE Brochure Nr.186 (Final Report of WG 14-20)

[4] FACTS Overview. IEEE and Cigré, Catalog Nr. 95 TP 108

[5] Brumshagen, H. et al.: Limits of Interconnection - Criteria for and Examples of Reasonable Extension. CIGRE Report 37-203, Paris, Session 1990 [6] Müller, H.-C.; Haubrich, H.-J.; Schwartz, J.: Technical Limits of Interconnected Systems. CIGRE Report 37-301, Paris, Session 1992

[7] Lu Yanchang: China Power Industry Entering to the 21st Century. ICPS, 2001, Wuhan, China [8] Wheeler, J. D. et. al.: Building India's Grid: an Examination of the Infrastructure. Benefits of HVDC Transmission; CIGRE Report 14-114, Paris, Session 2002

[9] Povh, D.; Lei, X.; Retzmann, D.; Han, Y.; Wang, Zh.: Role of HVDC and FACTS in future Power Systems. Cigré Symposium, April 8-10, 2003, Shanghai, China [10] Breulmann, H.; Grebe, E.; Lösing, M.; Winter, W.; Witzmann, R.; Dupuis, P.; Houry, M. P.; Margotin, T.; Zerenyi, J.; Dudzik, J.; Machowski, J.; Martin, L.; Rodriguez, J. M.; Urretavizcaya, E.: Analysis and Damping of Inter-Area Oscillations in the UCTE/CENTREL Power System. CIGRE Report 38-113, Paris, Session 2000