B4-106 APPLICATION OF HVDC FOR LARGE POWER SYSTEM INTERCONNECTIONS. (Germany)

Transcription

1 21, rue d'artois, F Paris 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 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, Erlangen, Germany. dusan.povh@ptd.siemens.de

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 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. GW Sources: IEEE T&D Committee Cigre WG B 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

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. Possibility to use larger and more economical Power Plants Reduction of the necessary Reserve Capacity in the Utilization of most favorable Energy Resources 1 2 a) AC Interconnection Flexibility of building new Power Plants at favorable Locations b) DC Interconnection Increase of Reliability by mutual Support in case of Emergency Reduction of Losses by an optimized Operation Reduction of Generation Costs by optimal Unit Commitment c) Hybrid AC/DC Interconnection Fig. 2: Advantages of Interconnections Fig. 3: Alternatives of 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 MAGHREB Fig. 4: European Power s (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

4 Interconnected s o Load Flow Problems (needs Management 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 between Power s Long Distance Transmission s o Voltage Stability o Reactive Power Problems o Steady-State Stability o Transient Stability o Subsynchronous Oscillations Fig. 5: Technical Problems in Large Power s Bottlenecks in the UCTE Source: UCTE - 5 / 2003 NTC Values for East- West 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 Adjustments (Frequency Controls, Generation Reserve) Need for close Co-ordination of joint Operation AC Interconnections normally weak at the beginning (additional Lines needed to avoid dynamic Problems) Bottlenecks in the because of uncontrolled Load Flow Spinning Reserve in the 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 kv) In liberalized Markets Disadvantages and higher Costs when transmitting Power through a Number of s and Involvement of a Number of Partners Stability Problems become more complex Phenomena spreading through the large and complex difficult to be managed Fig. 7: Problems of large synchronous Interconnections 4

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. a) Alternatives of HVDC interconnection a) Back-to-Back Solution b) 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 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 s can be determined exactly by the 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 s in Emergency Situations Fig. 9: Advantages of HVDC for 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 s 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

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. A) B) A) HVDC Transmission B) Long Distance AC Transmission C) AC Transmission through Interconnected Power s 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. HVAC cents/kwh Cents/kWh Loss costs Loss Costs AC AC 1,5 costs Investment Costs 1,5 1,0 1,0 AC HVDC AC 0,5 HVDC HVDC + ± kv kv 650 kv HVAC3 HVAC3 X 500 X kv 500 kv HVAC2 HVAC2 1x X X kv 735 kv Source: Siemens PTD SE NC 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

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 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 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

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. Share in installed wind energy of 12,223 MW Share in installed Wind Energy of 12,223 MW E. ON Netz: 48 % E. ON Netz: 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 Fig. 14: Integration of HVDC Transmission into a synchronous AC - 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 Nayong Anshun Power Anshun Guiyang Anshun Huishui Guizhou Power flow in one line Huishui -Hechi (MVA) HVDC GuiGuang a a Time (s) Dynamic Results b Hechi Guangdong Luoping TSQBASUO TSQDAW_27 TSQ-II Yantan Liuzhou Hezhou Guangzhou Beijiao Lubuge Yunnan Baise TSQ-I Guangxi Pingguo Laibin Wuzhou HVDC TSQ Nanning Yulin Zhaoqing Luodong Gaomin Fig. 15: Examples of Power Oscillations without a) and with b) Power Modulation of HVDC Control in the Chinese Grid [12] 8

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. AC double-link 1 A 1200 MW Ac double-link 2 B 200 GW Grid A 200 GW Grid B AC Link 1 DC Link 1 AC Link 2 AC Link 1 A DC-link MW AC double-link 2 B a) b) Only AC - instable after Fault Hybrid AC/DC - remains stable Fig. 16: Comparison of Stability for synchronous AC and synchronous Hybrid Interconnection a) Set-up of the s 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 s, [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. A B C D E F Large Large Hybrid Hybrid Interconnections with with HVDC HVDC and and FACTS: FACTS: HVDC - Long Distance DC Transmission DC Interconnection ( B2B) High Voltage AC Transmission / FACTS Fig. 17: Hybrid Interconnection for large Power s 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

10 Russian Power Grid 18 HVDC Interconnections Existing New Gezhouba-Shanghai TianGuang 3G-ECPG I GuiGuang 3G-Guangdong 3 x B2B 10 x HVDC Long Distance Transmission NWCPG CSPG NCPG Wangqu Plant Yangcheng Plant CCPG Three Gorges NECPG SPPG North Power Grid ECPG Center Power Grid Jinshajiang River a) Example China Tailand Power Grid SCPG Lanchangjiang River HPPG South Power Grid Source: SP China, ICPS - 09/2001 DEVELOPM ENT OF NATIONAL GRID URI WAGOORA DULHASTI PHASE - III (By 2012) KISHENPUR RAVI SATLUJ JULLANDHAR MOGA TEHRI CHICKEN NECK BALLABGARH A'PUR (DELHI RING) MEERUT BHUTAN HISSAR DIHANG TALA DAMWE RANGANADI LUCKNOW ARUN TEESTA BHIWADI M'BAD G'PUR KATHAL- NR BONGAIGAON GURI JAIPUR AGRA ALLAHABAD /UNNAO MARIANI PURNEA MISA SHIROHI MALANPUR VARANASI SILIGURI/BIRPARA SINGRAULI BADARPUR 500MW TIPAIMUKH B'SHARIF ZERDA /BARH KAHALGAON DEHGAM NAGDA BINA SATNA VINDHYA- CHAL NER KRISHNA N.K. LIMBDI GANDHAR/ SEONI SIPAT NAGAR AMRELI KAWAS ROURKELA BANGLA JETPUR CHEGAON KORBA VAPI DESH BHANDARA RAIPUR HIRMA PIPAVAV TARAPUR AKOLA BOISAR PADGHE AMRAVATI CHANDRAPUR TALCHER ER 1000MW JEYPORE WR DHABOL LONIKAND KOYNA KARAD RAMAGUNDAM LAKSHADWEEP KOLHAPUR PONDA KAIGA MANGALORE BANGALORE KOZHIKODE COCHIN 300 0M W NARENDRA SIRSI SR PUGALUR 2000MW HOSUR 2000MW CHITTOOR KRISHNAPATNAM SOUTH CHENNAI CUDDALORE VIJAYAWADA SINGARPET GAZUWAKA LEGEND 765 KV LINES 400 KV LINES HVDC B/B HVDC BIPOLE EXISTING/ X PLAN XI PLAN IX PLAN NICOBAR ANDAMAN AND KAYAMKULAM TRIVANDRUM KARAIKUDI KAYATHAR b) Example India Source: Power Grid Corporation of India Fig. 18: Hybrid Interconnections of large Power s in China a) and India b) Both technologies, HVAC and HVDC are used taking into account their technical and economical 10

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. 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 ] [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 s, [Cigré Symposium, Shanghai, China, April 2003] [4] H. Brumshagen, et al., Limits of Interconnection - Criteria for and Examples of Reasonable Extension, [Cigré Report , Paris, Session 1990] [5] H.-C. Müller, H.-J. Haubrich, J. Schwartz, Technical Limits of Interconnected s, [Cigré Report , 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, [Cigré Report , 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 and CIS & Baltic States, [Cigré Conference, Sept. 2003, St.-Petersburg, Russia] [9] M. Luther, U. Radtke, Betrieb und Planung von Netzen mit hoher Windenergieeinspeisung, [ETG Kongress, 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 , 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, [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 Interconnections, [Cigré Conference, Sept. 2003, St.-Petersburg, Russia] 11