Large Interconnected Systems for Economical and Secure Power Supply

Transcription

1 1 Large Interconnected s for Economical and Secure Power Supply Dusan Povh* and Dietmar Retzmann Siemens, Germany Abstract-- The demand for electrical energy in industrialized countries keeps on growing, whereas in emerging countries it increases very fast. National and regional power systems are gradually being interconnected to very large and complex systems, covering whole continents. Deregulation and privatization as well as ecological constraints became a new challenge for electric power supply. The goal is to provide sustainable delivery of the most economical and also environmentally-compatible energy. The interconnection should further improve the security of power supply, which became an important factor in the economy. Extension of large power systems can lead to congestion in the interconnections and operational problems which could diminish the security of power supply. Large investments into the power sector are required to achieve sustainable and secure power supply, which, along with the improvement of costeffectiveness, can be done by means of introduction of new transmission technologies. The paper provides a general overview of the developments in the power sector in Europe and discusses development and further extension of UCTE system. It discusses new technologies as well as new developments in system control and protection which can help to meet future demands. Smart Grids can help tackle these tasks. Index Terms-- Development of UCTE system, future trends, technical problems, solutions, AC and DC interconnections, limits of interconnected systems I. INTRODUCTION interconnections have a successful, over one century long history. They started with the implementation of the high voltage AC power transmission. The idea was to bring the energy to the cities from the remote hydro power generation. Starting with this development, power systems have evolved first to regional, then to national interconnected networks at high voltage levels. When establishing power supply interconnections to the neighboring partners, power industry intended to achieve the extension of power exchange. The operating voltage of these interconnected systems depends mainly on geographic conditions and average power transmission distance. The next step in the interconnections was then the integration of national and regional interconnected systems to very large networks which could cover whole continents. The final possible step in this development is probably a global network, which could be gradually built to expand over the continents. The driving factor for such networks could be the utilization of huge potential of renewable power resources worldwide, driven by the ecological requirements. Long-term developments in power industry depend on expectations for future political, financial and technical conditions. In the last decades, however, the developments were strongly driven by the globalization, leading to deregulation and liberalization. The world markets opened gradually, in different countries with different speed. This transition of economies brought many advantages, but also disadvantages in some fields. At the same time, social and environmental aspects became increasingly more important, even if they are, in some way, in contradiction with the globalization of the economy [1]. In Europe, 400 kv which has been introduced in the national systems remained the highest operating voltage, in the Far East countries mostly 550 kv, and in North America 550 kv and 765 kv are used. The level over 1000 kv is under development and should be implemented in years to come. HVDC (High Voltage Direct Current) projects as an alternative and a complement to AC systems reached the voltages of +/-600 kv and the levels of transmitted power of 3000 MW per bipole. However, new projects with +/-800 kv are on the way. The large interconnected networks in Europe (UCTE), the USA, China and India became huge and extremely complex systems, which leads to problems in terms of technology, security and organization. The question is where technical and economical limits of the interconnected systems are, and how the problems can be tackled. * Dusan.povh@t-online.de

2 2 II. THE UCTE SYSTEM A. Historical development and trends The UCTE (at the beginning termed the UCPTE) was founded in 1951 in Western Europe with the task to utilize in the best way the available generation. First, it consisted of the countries of central Europe. In 1987, the system was extended to Spain, Portugal, Greece and former Yugoslavia. This south-east Area of Europe was later temporary disconnected from the UCTE due to the war and for political reasons; however, since 1994 it is again the part of the system. From 1995 on, the Central system, which consists of the power systems of the countries of Eastern Europe, has been synchronized with the UCTE [2]. The UCTE is further interconnected with other large neighboring systems in Europe by HVDC and with the power systems in Northern Africa by a weak AC cable interconnection. Fig. 1 shows countries operating in the UCTE and the system areas responsible for the corresponding part of the primary reserve power. Coordination South Coordination North This sums up to 3,000 MW * Source: UCTE-GDR - Final Report 3 - Primary Reserve - Version 1 Fig. 1: The UCTE (situation in 2008) and values of primary reserve power for each part of the system The integration of electric power systems into large interconnected systems is based on benefits regarding energy costs, supply reliability, provision of reserve generation etc. Further important reason for the interconnection is the political wish to be integrated into a community. This is well understandable as new countries which are interconnected to UCTE get important advantages. The main advantages of interconnected power systems are listed below [3]: Possibility to use larger and more economical Power Plants Reduction in the necessary Reserve Capacity of the Utilization of most favorable Energy Resources Flexibility of building new Power Plants at favorable Locations Increase in Reliability of the s Reduction in Losses by an optimized Operation In recent decades the development of more economical power plants using larger thermal units, big nuclear power plants and large water power generation plants have been built. The systems interconnection enabled the use of these large units without the risk for operation. In case of outages, the surrounding systems can deliver the lacking power until the own reserve generation is activated. Another important advantage of the interconnection is the use of the most favourable energy resources, based on the contractual agreement between the partners in the interconnected system. The flexibility of constructing power plants in the favourable locations and the increase in total reliability of the interconnected system are further advantages. The general future trends in the power system development can therefore be summarized as follows [4, 5]: Further extension of interconnected systems by AC or HVDC interconnections Increased power exchange among the interconnected systems due to liberalization of the energy market Transmission of large power blocks over long distances (Hydro, Wind, and Solar Energy) Renewable Energy Resources at favorable locations, however, partly installed in distribution networks The trends mentioned above also mean that the interconnected systems gradually take over the task of power transmission through the network over long distances. This is particularly the case in the liberalized environment, where the customer can buy the energy from the cheapest seller which could be far away, and the power system operator has to provide technically reliable transmission facilities to meet the contracts. B. Problems in large interconnected systems and their solutions Main problems at the extension of the UCTE system are to enable the system configuration for the increased power exchange and for the transmission of larger amount of energy through the system: The regional and national systems have been designed to generate power as close as possible to the load centers. It means that the economic operating voltage of only 400 kv, as is the case in Europe, was sufficient. This voltage is, however, not suitable for power transmission over longer

3 3 distances. It is therefore not the optimum voltage for new transmission conditions any longer. As the regional and national systems were built in form of isolated networks, the line interconnections to other neighboring systems are weak. This leads to bottlenecks if power is to be transmitted through the systems. Fig. 2 shows an example of bottlenecks which may occur if larger power blocks are to be transmitted between or through the national power systems from the east to the west. These bottlenecks make it difficult to realize the idea of open market. Large power systems with weak interconnections tend to oscillations which could endanger the system operation. An example is the interarea oscillation experienced in the UCTE system. Fig. 3 shows these oscillations which reached the value of more than 1000 MW. However, such oscillations can, in most cases, be effectively damped by appropriate settings of the PCC on determined generators. A further solution to avoid these oscillations is FACTS (Flexible AC Transmission s). However, for effective damping the proper location of FACTS equipment has to be found. Larger systems are also more sensitive to local outages which can develop to large blackouts, as experienced in the past. Fig. 4 shows the comparison between the calculated and experienced power outages in the American systems [6]. The surprising result is that the experienced outages are much higher than calculations. Reasons for the differences are listed in the figure. Bottlenecks in the system resulting from insufficient investment are, however, the most important reason that faults cascade to large outages. Measures to prevent large outages and blackout are: Sophisticated Protection Schemes (SPS) Implementation of sophisticated controlled links to disconnect subsystems in case of danger to the whole system Use of FACTS equipment to control load flow Use of hybrid AC/HVDC interconnections Separation of subsystems by HVDC New ideas for detection of risky situations are available which enable operators to adjust the system to avoid cascading. One of these new solutions is simultaneous monitoring of rotor angles in the system and, after calculating the possible risk of outages, giving warning to the operator. Fig. 5 shows results of such calculations for the UCTE system. Red area means that this part of the system is endangered to collapse. Most effective solution to secure the system is, however, additional investment. Operation of the system became more difficult with the increasing number of participating countries and the liberalization of the power market. The organizations inside of individual countries, ownership of the networks, and the responsibility of Independent Transmission Operators (TSO) vary greatly. For practical reasons, the total system consists of two coordination areas (North and South) and a number of control areas as shown in Fig. 1, which are responsible for load flow management and minute reserves inside of these areas. Organizational aspects are also essential to the operation of the UCTE system. The main elements of the requirements are data exchange between control centres, coordination of outages for maintenance, incident analysis, access to and allocation of interconnection capacities. Further, the legally binding contracts for share of responsibilities between parties and relevant market players, with respect to technical and operational requirements are crucial to the membership in the UCTE. The idea of liberalization is to ensure free trading of electrical energy through Europe as well as with the neighboring systems. Due to extremely different starting conditions in these terms, the liberalization in different countries can be done only in steps and with different speed. Fig. 6 depicts plans for this development forecast by the UCTE organization. Bottlenecks in the UCTE Source: UCTE 5 / 2003 NTC Values for East- West Power Transfer NTC = Net Transfer Capacity Fig. 2: Bottlenecks in the UCTE system during power transmission through the system

4 4 An Example: Trip of 900 MW Power Station in Spain Δ f [mhz] Frequency Frequency & Power Spain Poland Active Power France-Germany Germany (border to France) (border to France) P [MW] (one 400-kV system) t [s] Signals: simulated & measured by WAMS Mode 1: f = 0.21 Hz, Damping ζ = -3.7 % Source: CIGRE Report , Paris Session 2000 Fig. 3: Interarea oscillations in UCTE The Grids are close to their Limits Source: UCTE Interim Report s too complex to be tested properly (Protection, Controls) Insufficient Investments into the (heavily loaded Network Elements) Lack of Maintenance Insufficient Training Human Errors Expected Actual Source: Fig. 4: Calculated and experienced outages in the American s

5 5 Fig. 6: Liberalization of the UCTE Market Fig. 5: Identification of rotor angle in the system (red stands for risk area) Out of Operation Dürnrohr 550 MW Wien SO 550 MW Etzenricht Existing Interconnections * Cross Channel 2000 MW Skagerrak 940 MW Baltic Cable Kontek Gotland 260 MW Fenno-Skan 500 MW (+800 MW) Konti-Skan 550 MW Volgograd-Donbass 750 MW Vyborg 1200 MW (B2B) SwePol Moyle 500 MW Estlink 350 MW NorNed 700 MW BridNet 1000 MW Storebælt Planned Interconnections Options Euro Link Viking Cable Norway-UK Iceland-UK (B2B) Ireland Wales 400 MW * or under Construction 4000 MW (TEN Studies) 1200 MW 1100 MW Fig. 7: HVDC interconnections with Scandinavia and the UK

6 6 C. Trend in further development of interconnected systems Taking into account the above features of the UCTE which are typical of other large power systems as well, the influence of environmental requirements and the liberalization of economy, the future trend in the development can be summarized to the following items: Distribution network will take over some of the characteristics of today s transmission systems with large amount of local generation. New technologies, particularly the HVDC, FACTS, GIL (Gas Insulated Lines) and energy storage devices will boost power transfer in the system, overcome bottlenecks, reduce losses and total transmission cost and further increase the security of power supply. Wide development of communication will bring about sophisticated system automation, adaptive system protection, and on-line services. III. EXTENSION OF THE UCTE INTERCONNECTION A. Existing Interconnections with neighboring systems In the UCTE, strong HVDC interconnections with the neighbors already exist, particularly with the NORDEL and with the power system in the UK. Fig. 7 depicts these interconnections which have in total over 6000 MW transmission capacity. The HVDC provides the interconnection with the same advantages as an AC interconnection with the difference that the systems can remain in the asynchronous operation. The decision which type of the interconnection is used depends on technical features and financial calculations only. Should longer submarine cables be required (Interconnections with Scandinavia and the UK), the HVDC is the only possible solution. Most of them are connected to Scandinavia. The only AC interconnection with another system is the cable connection between Spain and Africa; however, it is meant for relatively low power level. Furthermore, a small isolated area of the Ukrainian system is also connected to the UCTE system by AC. Middle East and Turkey. The idea of this connection is primarily of political interest and less of the economic one. The ring can probably be implemented only by means of the HVDC link inserted into it to get a stable operation also in case of outages. Interconnection with Turkey and further on with the countries of the Caucasus and Iran. The idea is to provide power transmission from this area, which is rich in natural resources, to Europe. Since decades the interconnection with the Eastern Interconnected system, consisting of Russian network and those of other countries of Eastern Europe attached to it, is under consideration. In terms of size, the system is comparable with the UCTE system; however, geographically it encompasses over thousands of kilometres to Siberia. However, even if this interconnection is technical possible, the reliability of the operation is doubtful. A faster and technically more suitable solution would be to interconnect both systems by back-to-back and long distance HVDC transmissions. IV. POWER TRANSMISSION THROUGH THE INTERCONNECTED SYSTEM A. Transmission alternatives The new task of the interconnected systems will be to transmit large power blocks over longer distances, for the liberalized trading enables purchasing the most economical power even at distant locations. Possibilities of power transmission are shown in Fig. 8. A) B) Subs. Subs. Subs. C) B. Planned interconnections There are, however, plans and ideas for further extension of the UCTE system: The interconnection with Northern Africa should be extended to the Mediterranean Ring which could be closed through Egypt, countries of the Fig. 8: Alternatives of power transmission over long distances Normally, in the interconnected systems power has to be transmitted through the subsystems (Fig. 8c). However, this solution has disadvantages. The

7 7 operating voltage of 400 kv in the UCTE system is too low to transmit large power blocks in an economical way. Furthermore, the transmitted power is superimposed to the load flow inside of the subsystem which increases losses and results in bottlenecks depending on the operational conditions (Fig. 2). An alternative would be to build a new, higher voltage transmission to carry power directly from power generation to the load centres (Fig. 8b). However, the AC transmission with a higher voltage level (e.g. 800 kv) could not be implemented in the UCTE system due to the rightof-way problems. Moreover, problems could occur through parallel operation of two voltage levels leading to load flow problems and additional bottlenecks. These difficulties could be, however, overcome by FACTS devices. From the economical point of view, HVDC transmission is less expensive compared with an AC alternative. The HVDC alternative (Fig. 8a) offers the best solution both technically and economically [7]. At transmission distances over 800 km, the HVDC is cheaper; it is easier to build DC overhead lines due to the narrow right-of-way. In the densely populated areas, DC cables could help avoid environmental problems. An important technical advantage is that the HVDC transmits power between the converter stations without having any influence on the load flow inside the AC system. It means that HVDC can avoid bottlenecks in the system. An example of this solution is the intention to interconnect wind generation in the north and the strong system in the south of Germany. Wind energy could be transmitted to the south and at times of low wind availability, power could be transmitted to the north. Vattenfall Europe Transmission Long-term: GW 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 % 48 % Vattenfall Vattenfall Europe Europe Transmission: Transmission: 37 % 37 % RWE RWE Net: Transportnetz Strom: 14 % 14 % EnBW EnBW Transportnetze: Transportnetze: 1 % 1 % Source: E.ON Benefits of this Solution: o Load Sharing o Generation Reserve Sharing Installed Generation Capacity: 120 GW (2006) very fast 2006: 20 GW! Furthermore, the system in the south could support the northern system with the required regulation energy for wind generation. Fig. 9 shows the possibility of this kind of transmission. B. Limits of interconnected systems When observing the sustainable growing of interconnected systems and new tasks in the liberalized environment, the question can be asked whether there are any limits in the geographical size of these systems. It is the fact that the technical and economical advantages of the entire system diminish on one hand with the size and the number of national systems, and the required additional investments for system enlargements and removal of bottlenecks increase on the other hand [8]. However, each new interconnected system gains important advantages. To avoid technical problems and to use the present system configuration without high additional costs for system improvements, the future strategies for the development of large power systems should go clearly in the direction of hybrid interconnections, consisting of the HVDC and HVAC which interconnect regional sub-systems, depending on the specific situation. Such interconnected systems, shown schematically in Fig. 10, have significant advantages in terms of technology and reliability. The idea of a hybrid interconnection is to exchange power between neighboring systems via an AC interconnection and to use the HVDC to transmit large power blocks over large distances. In addition to this, the HVDC has an advantage of supporting the reliable operation of the total system by its fast control. In some large systems, e.g. in China and India the development goes clearly in this direction. The costs of power transmission in a system increase with the transmission distance. It can be considered that the current transmission costs are approx. 1-2 Cents per kwh and 1000 km. This is valid also for the transmission through an existing system. This is economically valid independent from the regulation on the charges for power transfer through the system The advantages of the energy transmitted through the interconnected systems over very long distances are therefore less economical if transmission costs are taken into account. The reasonable distance for economical power transmission is in the range of up to 3000 km, at the to-days cost for power generation. These conditions, however, could possibly change to higher distances if energy cost will increase and strong political efforts will support the use of renewable energy in remote areas on large scale and cost to save CO 2 production. Fig. 9: Embedding of HVDC into AC power system

8 8 G A B C D E F Large Large Interconnections, using using HVDC HVDC and FACTS HVDC - Long Distance DC Transmission HVDC B2B -via AC Lines High Voltage AC Transmission & FACTS DC thestability Booster and Firewall against Blackout Fig. 10: Hybrid interconnected system Countermeasures against large Blackouts V. CONCLUSIONS VI. REFERENCES interconnections have a successful, over one century long history. The first step towards the interconnections was the integration to national and regional interconnected systems. Then, the systems developed to large and complex ones interconnecting continents. In the last decades the development was strongly driven by the globalization, leading to deregulation and liberalization. The world markets opened gradually, in different countries with different speed. This transition of economies brought many advantages, but also disadvantages in some fields. At the same time, social and environmental aspects became increasingly more important, even if they are, in some way, in contradiction with the globalization of the economy. The huge size of power systems also leads to some technical problems which can, however, be solved. One of the new tasks of the interconnected systems became the transport of large amounts of energy over long distances through the system. However, the system configuration has not been designed to tackle this task. Therefore, new solution is required. The HVDC in combination with an AC interconnection offers a future proved solution. With this solution, there is virtually no geographical limit to the extension of the interconnected system. The only reasonable size limit to this kind of interconnection is the costefficiency of power transmission over very long distances. [1] World Energy Investment Outlook-2003 Insight, IEA Publication, Paris 2003 [2] Vision and Strategy for Europe s Electricity Networks of the Future, Doc. EUR 22040, Luxemburg 2006 [3] Povh, D.; Retzmann, D.; Teltsch, E.; Kerin, U.; Mihalic, R.: Advantages of Large AC/DC Interconnections, CIGRE Report B4-304, Paris 2006 [4] Povh, D., Pyc, I., Retzmann, D. ; Weinhold, M. G. : Future Developments in Power Industry, The IERE Central and Eastern Europe Forum, October 17-21, 2004, Krakow, Poland [5] Povh, D.; Retzmann, D.: Perspectives of Power Interconnections, IERE Conference, San Hose, Costa Rica, November 2003 [6] Fairley, P.: Advanced Mathematical Modeling suggests that big Blackouts are inevitable, IEEE Spectrum, August 2004 [7] Economic Assessment of HVDC Links. CIGRE Brochure Nr.186, June 2001 [8] Müller, H.-C.; Haubrich, H.-J.; Schwartz, J.: Technical Limits of Interconnected s, Report , Cigre Meeting, Paris (1992).

9 9 VII. BIOGRAPHIES Prof. Dr. Dusan Povh was awarded the degree of Electrical Engineering by the University of Ljubljana, Slovenia in 1959 and his doctor degree from Technical University Darmstadt, Germany, in Since 1985 he is professor at the University of Ljubljana. In 1995 he has been appointed Guest-Professor at Tsinghua University, Beijing, PR China. He worked for Siemens for many years in different leading positions and is now an independent consulting. His main fields of interest are power systems and power electronics. He published in his carrier more than 200 international papers. He is Fellow of IEEE and Honorary Member of CIGRE. He served for 6 years as the Chairman of CIGRE Study Committee on HVDC and FACTS. He received from IEEE two prestigious awards: Uno Lamm Award for achievements in field of HVDC in 2001, and FACTS Award for his contributions in this technology in Prof. Dr. Dietmar Retzmann has been at Siemens since He received Dr.-Ing. degree in 1983 at the University of Erlangen- Nuremberg. His area of expertise covers project development; simulation and testing of HVDC, FACTS, Protection and Custom Power; system studies; innovations and R&D activities. Dr. Retzmann is active in Cigré, IEEE, ZVEI and VDE. He is author and co-author of over 170 technical publications in international journals and at conferences. In 1998, he was nominated Guest-Professor at Tsinghua University, Beijing, and in 2002 at Zhejiang University, Hangzhou, China. Since 2004, he is lecturer on Power Electronics at the University of Karlsruhe, Germany. In 2006, he was nominated Siemens TOP Innovator.