Technologies Applied to Japan s HVDC Links. T. SAKAI, Y.MAKINO, H.KOSAKA Electric Power Development Co., Ltd. JAPAN.

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1 http : // B AORC Technical meeting 2014 Technologies Applied to Japan s HVDC Links T. SAKAI, Y.MAKINO, H.KOSAKA Electric Power Development Co., Ltd. JAPAN Takehisa_sakai@jpower.co.jp SUMMARY Many HVDC systems have been applied to various kinds of AC power systems worldwide since 1954 when the first HVDC system was put into commercial operation. Main purposes of HVDC systems can be roughly divided into two categories which are firstly long distance and bulk power transmission, and secondly interconnection between power networks. Former HVDC systems recently have aimed to increase DC system voltage such as ±800kV and much higher voltage and to enlarge capacity of 6400MW and much larger capacity per bi-polar configuration. On the other hand, latter HVDC systems have aimed to support AC system reliability with using flexible operation method. This paper mainly introduces the applied HVDC technologies of latter case to Japan s HVDC system which have been improved for AC system reliably operating performances. KEYWORDS HVDC control system, Simultaneous Bi-directional Power Flow Control (SBPFC), Stable operation, Bi-polar control, Flexible operation, AC power system quality, Metallic Return Circuit Protection, Ne 1. Introduction HVDC Systems have been applied worldwide for the cases which HVDC operational characteristics are superior to AC system s characteristics for the individual project. The main purposes of applying HVDC systems can be roughly divided into two categories which are firstly long distance and bulk power transmission including submarine systems, and secondly interconnecting between AC power systems. For the long distance and huge power transmission of HVDC system, required technologies are to apply higher reliable and efficient transmission system and for the submarine cable system, new type insulation cable has been developed and applied to achieve simpler maintainability and more operational capabilities. In case of interconnecting AC systems, various kinds of technologies are required in accordance with operational requirements and conditions of each project. This paper describes brief history of HVDC and introduces the new technologies applied to Japan s HVDC systems interconnecting AC power systems. Takehisa_sakai@jpower.co.jp

2 2. Transition of HVDC projects 2.1 Beginning Phase The first HVDC project in the world was Gotland HVDC Link which connected between main land of Sweden and Gotland located far from east coast of the peninsular. This project was put into commercial operation in 1954 and Mercury Arc Valves were used as the state of art technology at that time. The feature of the project was 98-kilomiter-long submarine cable with a voltage of 100kV and the capacity of 20MW. Projects following to the Gotland HVDC Link using Mercury Arc Valves technologies were Cross Channel HVDC Link between UK and France in 1961, New Zealand Inter Island HVDC Link in 1965, Sakuma BTB in 1965 Japan and Vancouver in Canada 1968 commissioned respectively. However, up to the HVDC project of Pacific Intertie commissioned in 1982 USA, Mercury Arc Valve Technologies were replaced with Thyristor Valve Technologies because Mercury Arc Valves had a fatal flaw as Consequential Arc Back so called CAB which once occurred HVDC Link should be tripped for clearing the fault. 2.2 Applying Thyristor Valve Technologies In the 1970 s high voltage and large current Thyristor devices were developed for applying converter system of HVDC installations. In accordance with recognizing satisfactory reliabilities and expectable operational performance of Thyristor Valves technologies, from later period of 1970 s to 1980 s as the primal peak period, a number of HVDC schemes using Thyristor Valve technologies were completed and put into commercial operation for the economical solution to transport bulk power between long distance. Typical HVDC schemes are to transport large power from the mine mouth thermal power plants to load centers in USA, and from remote large scale hydro power station to load centers in South America. 2.3 Bulk Power Transmission Beginning of 21 st century in China, huge capacity power stations such as Three Gorges Power Station and inland mine mouth coal fired power stations have been commissioned to meet the rapidly increasing demand. Since almost power plants are located in western area, and load centers such as Shanghai, Guangdong and Changzhou are located in west seacoast, transmission line lengths are very long as 1000km. As same as Chinese HVDC project circumstances, also the feature of Indian HVDC projects are similar nature conditions as large transportation capacity and long distance transmission line. Therefore, HVDC system ratings have been selected the voltage of 500kV and the current of up to 3000A for system ratings. 2.4 UHVDC Project By the year of 2010, the highest voltage of HVDC system was 600kV used for Itaipu HVDC Link having the capacity of 6300MW, two bi-polar with 800km long transmission line in Brazil. Enumerating the situations on power system planning in China and India to introduce UHVDC system, first geographical and power system conditions such as distribution and density of load centers and energy resources are typically different from other countries. That is, the required amount of capacity of demand and supplies are very large and the distance between load center and power supplies are very far. Considering these circumstances of project requirements, in order to achieve higher effective HVDC transmission system, DC voltage has been preferable to increase as far as possible within the restriction of relevant technologies available. Based on the result of technical and economic survey and preliminary study executed by CIGRE and other organization, rated voltage of 800kV was firstly selected to apply the coming UHVDC system. 1

3 And the first commissioned UHVDC project was Xiangjiaba Shanghai HVDC Link of which feature is rated voltage of ±800kV, capacity of 6400MW, and circuit configuration of one bi-polar having transmission length of 2000km. In the next stage of UHVDC system voltage, ±1100kV will be selected and already the project has been moved into action for implementation. 2.5 HVDC System for interconnection between power systems. In the planning of interconnection power systems, there are two options of HVDC system and HVAC system which are different essentially each other. Comparing to each other, HVDC interconnection is an asynchronous connection system through AC/DC converter system, whereas HVAC interconnection system is synchronous connection. Therefore, HVDC system can always interconnect between power systems very easily without adjustment of both frequencies and phase angle differences, and also can connect without increasing short circuit current. Other significantly important characteristic of HVDC interconnection is exchanging power flow controllability which can manage any arbitrary value within the rage up to thermal capability and power system acceptable conditions. For these reasons many HVDC links have been applied in the world to meet various purposes and requirements on AC system operation. 2.6 VSC HVDC System As mentioned above, HVDC system with Line Commutated Converters (LCC) was developed in 1950 s. In 1970 s, HVDC application evolved with the introduction of Thyristor Valves which were applied over the years with high reliability. In the mid 1990 s a new type of valves for the HVDC converters based on transistors (Insulated-gate Bipolar Transistors: IGBT) were developed. This type of converters is called as Voltage Sourced Converter, VSC. The first VSC HVDC system was applied at Gotland Link in Sweden, Usually, LCC can steadily operate within the condition having short circuit ratio more than 2.5times to HVDC transmission capacity. However, since VSC does not require such operational requirements and its superior functionalities such as AC side voltage controllability and less harmonic generation, over 10 projects have been put into commercial operation. Compared to LCC in flexible operational functionality, VSC may easily form multi terminal HVDC. Therefore recently HVDC grid using VSC has been actively studied for applying power system especially to apply to Europe. In accordance with developing control & protection system and DC high voltage facilities for VSC HVDC application, VSC HVDC will be more used in power system. 2.7 Transition of Japan s HVDC Links So far in Japan, four (4) back to back (BTB) HVDC link and two (2) point to point HVDC links were built and put into commercial operation. The first HVDC technology was used for interconnection between 50Hz power systems and 60Hz power system at Sakuma BTB HVDC link having 300MW. From the beginning of power system development in Japan, 50Hz power system in eastern area and 60Hz power system in western area have been independently expanded to meet power demand increasing in each area. However, in order to manage power system operation especially securing efficiently reserved power in each power system, Sakuma Frequency Converter Station, BTB HVDC link was completed and put into commercial operation in Since Sakuma BTB HVDC link was fifthly commissioned Figure1 Mercury Arc Valves in Valve Hall 2

4 in the world, it applied Mercury Arc Valve technology which was used common technology for this field as the art of the technology at that time. Figure 1 shows Mercury Arc Valves installed in Valve Hall at Sakuma BTB The second HVDC technology was also applied to Shin-Shinano Frequency Converter Station interconnecting between 50Hz and 60Hz power system commissioned in This HVDC project was fourthly put into commercial operation in the world in case of using Thyristor Valves technologies. Most advanced technology used in this project was domestically developed power electronics technology such as high voltage and large capacity oil immersed thyristor valves exclusively developed for this project, DC main circuit equipment and control & protection system. Two years later, in 1979 the point to point HVDC project, Hokkaido Honshu HVDC Link having 134km long overhead line and 43km long submarine cable was completed and commissioned. This project was the first entire HVDC project in Japan. The rated voltage was the highest level as 250kV at that time. Of special note on advanced technology was high voltage newly designed indirect light triggered thyristor vales system which had the seismic withstand capability. And high voltage large current submarine cable had been exclusively developed for this project. In addition, applied circuit configuration was bi-polar with metallic return system for avoiding any environmental impact due to earth return current. The cable landing work at that time is shown in Figure2. In the year of 2000, Kii Channel HVDC Link, the largest HVDC link in the world using submarine cable, was completed and commissioned. For this project, many kinds of DC main circuit facilities have been developed such as high voltage large capacity thyristor valve using 6 inches thyristor devices, 500kV DC Gas Insulated Switchgear, AC 500kV dead tank type AC Filters, 500kV submarine cable having the largest cross section of 3000 square mm and newly designed control & protection. Figure 3 shows the Network of Kii Channel and related AC system Figure 2 Cable Landing Work for Hokkaido-Honshu HVDC Link Figure 3 Kii Channel HVDC Link and related AC System 3. Main purposes and Roles of HVDC system From the view point of main purposes and roles of HVDC system, there are two kinds of HVDC system: one is power supply transmission line which mono-directly transmits generated power from the power plant to the power network of load center; and the other as interconnecting transmission line which bi-directionally exchanges power between AC power systems in accordance with both power systems benefits. Usually former HVDC link is firstly required to economically transport power as far as efficiently, therefore higher voltage is applied to ensure high efficiency transmission. For the later HVDC link are required various kind of requirements from each power system characteristics. Very important view point for planning interconnecting power system by HVDC link is that each AC connected power system conditions will drastically change with high possibility during HVDC link 3

5 life as 40 years. Therefore, especially HVDC links connecting power system are required to have very flexible system controllability which can accept variable requirements of power system operational requirements. A number of interconnecting HVDC links in the world perform on not only exchanging daily-ordered power but also contributing power system quality control during power flow control and emergency states so called as ancillary services. In addition to requirements mentioned for power system, operational circumstances have been significantly changed under the conditions of the deregulation on power system market. In the opened power market, HVDC operation modes including low power are strongly influenced by many power suppliers who own various kinds and with wide capacity range from very small to bulk power. Therefore, it is strongly expected that present and future HVDC links interconnecting AC power systems are greatly preferable to have flexible controllability. 4. Newly applied HVDC technologies to Japan s HVDC links In this section newly applied HVDC technologies to Japan s HVDC are introduced. Because all HVDC systems in Japan employ Line Commutated Converter (LCC), these new technologies have been developed for LCC HVDC system. 4.1 Flexible Power Flow Control by HVDC Link As generators having restriction on generating steady power, HVDC systems also have the adequate steady power range usually from 10% of Power Flow to Honshu Direction rated power to continuous rated power in Rated Power order to avoid DC current intermittence under the small current operation condition affected by AC system s voltage fluctuations in the steady state and control 0.1pu Power Setting to Honshu Direction error. In other words, HVDC link cannot interconnect power systems under the condition of below minimum steady operation current. Power Setting to Hokkaido Direction 0.1pu Especially in case of interconnecting very Rated Power Power Flow to Hokkaido weak power system, during power Figure 4 Available operation range of conventional HVDC reversal operation large frequency fluctuation occurs due to exchanging power flow variation as two times of minimum power. Available operation range of conventional HVDC system is shown in Figure 4. Although interconnecting by AC transmission line can connect without exchanging power, interconnecting by HVDC system minimum power flow is absolutely required. This operational condition is one of disadvantage of HVDC interconnection compering to AC interconnection. However, using HVDC link that consists of bi-polar configuration, there is practical solution which has been already employed for Hokkaido Honshu HVDC Link in Japan. Bi-polar system consists of two (2) poles of Pole#1 and Pole#2. In the combination of operation mode that Pole#1 exchanges minimum power from Hokkaido to Honshu as south direction and Pole#2 exchanges minimum power from Honshu to Hokkaido as north direction. This result shows that Pole#1 and Pole#2 operate with each independent power order and total power is completely compensated to 0MW. Figure 5 shows power flow controlled by Pole#1 and Pole#2 independently. This operation method is called Simultaneous Bi-directional Power Flow Control (SBPFC) shown in Figure 5. In this operation mode, when only power order of Pole#1 is increased with keeping Pole#2 power flow at minimum power order, exchanging power flow appears to Honshu direction. In the 4

6 similar way, exchange power flow to north direction is available with increasing power order of Pole#2. Figure 6 shows the combination operation of two poles with SBPFC. Power Flow to Honshu REC INV 0.1pu 1.0pu Pole#1 #P1 In case of Pole#1operation independently Rec Inv Power Flow to Hokkaido 0MW INV REC Pole#2 1.0pu 0.1pu In case of Pole#2 operation independently Figure 5 Independent Operation in Bi-polar System Inv N End Power Flow Power Flow Rec 0.9pu 0.9pu 0MW #P2 S End Figure 6 Combinational Operation of SBPFC Power system controller such as Automatic Frequency Control (AFC) can be only used during HVDC operating state. Usually, HVDC interconnection link only operates when the contracts are agreed between system operators on exchange power of which value is more than minimum operation power. However, after introducing SBPFC to HVDC system, even if power exchange contract is not agreed or agreed with 1MW exchange power, HVDC Link always can operate with AFC for improving power system quality as ancillary services. Figure 7 shows available operation range of conventional HVDC system with AFC, and Figure 8 shows available operation range of SBPFC with AFC. Power Setting to Hokkaido 1.0pu 0.9pu Power Flow to Honshu 0.2pu 0.1pu 0.1pu 0.2pu Power Flow to Hokkaido 0.9pu 1.0pu Output of AFC : +/- 0.1pu Power Setting to Honshu Figure 7 Available operation range of conventional HVDC system with AFC From the view point of efficient HVDC operation, SBPFC mode is suitably applied to lower power order range for reducing transmission loses as much as possible. In middle power order range mono polar operation mode can be used, and in higher power order range conventional bi-polar operation mode is used. Introducing SBPFC mode, it is possible to operate HVDC system with flexibility as AC interconnecting line. Figure 9 shows the typical operation record using optimal combination among SBPFC, mono-polar and bipolar operation mode. Power Setting to Hokkaido 1.0pu 0.9pu Bi-Polar Operation Power Flow to Honshu Power Flow to Hokkaido 1.0pu 0.9pu Output of AFC : +/- 0.1pu Power Setting to Honshu SBPFC Operation Bi-Polar Operation Figure 8 Available Operation range of improved SBPFC with AFC 5

7 This operation method is additionally installed in bi-polar control system which is the highest level in control hierarchy structure. Other purpose of SBPFC is to reduce the number of polarity reversal which may affect insulation stress of submarine cables used for HVDC circuit. In case of applying LCC to HVDC link, during power reversal operation in emergency state or reverse power order mode, Simultaneous Bi Directional Power Flow DC voltage is reversed. During this polarity reversal phenomenon, submarine cable is affected on insulation stress. To avoid this kind of stresses by the power reversal operation, usually the operation procedure is that first the HVDC link is blocked and after several hours for discharging, the HVDC link is restarted with reverse power flow direction. However, using SBPFC, polarity reversals can be avoided during power reversal operating. As the result of employing SBPFC, submarine cable life is expected to become longer without operational restrictions. 4.2 Technologies applied to Embedded HVDC Link An embedded HVDC system is a DC link of which ends are connected with at least a single synchronous AC network. This kind of HVDC link performs not only basic function of exchanging power but also some additional control function within the AC network such as power voltage control, system stability improvement and the mitigation of system cascading failure. Japan has two embedded HVDC links which are Kii Channel HVDC Link providing bulk power transmission and Minami Fukumitsu BTB HVDC Link providing strictly power flow control among three 500kV networks. Kii Channel HVDC Link connects Kansai Power Network and Shikoku Power Network and has many additional functions using various HVDC technologies such as Power Oscillation Damping Control, Automatic Frequency Control in case of Shikoku Power Network isolated and Islanding Transition Control in case of large scale coal fired power station isolated from AC network. In the actual AC network operation, a power oscillation sometimes occurs following disturbances in AC system. This means that the equivalent inertia of one part of the AC system is temporarily accelerated while the equivalent inertia of the other system temporarily decelerated and vice versa. Using superior controllability of HVDC technologies, the power oscillation can be damped within the duration of the initial state which has a possibility to amplify disturbance in whole AC network, if not controlled. Usually, the Power Oscillation Damping function becomes active automatically when the fluctuation value due to this phenomenon is beyond the preset value under emergency conditions or major disturbance. There were three optional control methods applicable to Kii Channel HVDC Link such as: Frequency difference between two converter stations; Generator rotor speed difference between two systems; Power fluctuation of AC interconnecting line. Through the technical evaluation using digital simulation on affection of three control methods, it was confirmed that all methods could improve AC system stability. Therefore the most practical method as frequency difference of two converter stations was selected since both frequencies should be send to each opposite converter station for other purposes control. Conceptual diagram of this damping control and related power networks are shown in Figure 10. Power system oscillation frequency of Shikoku Power Network is the range between 0.3 to 0.4Hz and Kansai Power Network associated with other 60Hz power is the rage of between 0.5 to 0.65Hz. The Mono Polar Operation Bi Polar Operation Figure 9 Typical Operation Record with Flexible HVDC Control 6

8 actual effect of damping control was determined at the event of large capacity power plant failure which is shown in Figure 11. The failure of this event was not so severe degree compared to the trunk line fault, therefore the output peak value required by the damping controller was up to 114MW. Figure 11 The Record of Oscillation Damping Control 4.3 Metallic Return Circuit Protection In order to avoid any environmental impact due to earth return current, Japan s HVDC systems have applied metallic return circuit even if the HVDC main circuit is bi-polar configuration. However, it is very difficult to detect return circuit fault because during bi-polar operation DC current does not flow due to balance of each other pole current. In this operational condition even if a fault occurs in the section of return circuit, DC current and DC voltage will not change from steady value as zero ampere and zero volte. For this phenomenon it is impossible to detect the fault state of return transmission line with ordinal relay system. As the countermeasure to solve those issues, Hokkaido Honshu HVDC Link has employed exclusive protection system with superimposing AC current which monitors whether return circuit is faulted state or not. The used frequency for this protection system has been carefully selected to avoid the influences from fundamental frequency used in power system. Used frequency for the protection system is 125Hz which is kept away from the fundamental frequency of 50Hz and the value of multiplies of 50Hz. Although superimposed frequency is optimized, the value of AC 125Hz current is not stable during HVDC system operation and the value is significantly changed by the operation modes such as mono-polar operation and bi-polar operation. For these severe circumstances of metallic return circuit protection, the adjustments of criteria values are very difficult to distinguish the fault state from sound conditions. Since the original protection system was installed in 1970 s, more than 30 years before, the types of protection relay and telecommunication technologies were very limited in their functions. Therefore, it is needed to develop new protection system. New algorism used for fault detecting and telecommunication system for real time data acquisition have been investigated, evaluated, optimized and newly installed. Figure 12 shows the new metallic return circuit protection circuit system including 125Hz monitoring current. The new detecting algorism was drastically changed from the method used in original protection 7

9 system which simply compares preset level to measured absolute value, to the new method which calculates each proportional value of each measured value at converter stations and cable landing points. The new protection system using above mentioned algorism can exactly detect fault section of overhead line section or submarine cable section. The new system was installed in Hokkaido Honshu HVDC Link in Hakodate C/S OH T/L Data Acquisition & Transfer Panel Furukawa Cable L/P Submarine Cable Sai Cable L/P 125Hz Monitoring Current OH T/L Kamikita C/S Return Circuit 125HzTr Fault Section Detection and Protection Mode Selection Panel Figure 12 Metallic Return Circuit Protection MRTB D Application of DC-XLPE Cable for Hokkaido Honshu HVDC Link Since 1954 when putting the first HVDC project into commercial operation, LCC HVDC systems using submarine cables have employed oil insulation cable such as Mass Impregnated paper insulated (MI) cable or Oil Filled (OF) cable in the world. MI cables have been used mainly in Europe and OF cables have been used mainly in Japan depending on manufacturing technologies, and project requirements and conditions. Generally, MI cable can be applied to longer distance without oil pressure equipment and it can be operated up to the maximum temperature of Degree Celsius resulting lower ampacity. Compared to MI cable, OF cable can be applied up to approximately 50km with employing oil pressure equipment, and it can be operated up to the temperature of Degree Celsius resulting higher ampacity. That is to say both type oil insulation cables have advantages and disadvantages. Instead of these oil insulated cables, recently XLPE cables having electrically higher performance are widely used for HVAC system in the world. However, so far XLPE could not be used for LCC HVDC system because it was believed that severe insulation stress appears during polarity reversal operation due to space charge accumulation. Electric Power Development Company (EPDC) and Japanese cable manufacturer have solved this longstanding problem with developing new insulation materials involving effective additive for DC-XLPE cables. Hokkaido Honshu HVDC Link firstly applied DC- XLPE cable for LCC HVDC system in the world. Prior to manufacturing DC-XLPE cable, fundamental electric characteristics of DC-XLPE and AC- XLPE cable were carefully investigated, evaluated and confirmed especially on space charge distribution. Since accumulated space charge leads to large enhancement in the electric field, conventional XLPE is significantly affected by polarity reversal on severe voltage stress. Newly developed DC-XLPE has the conductor size of 600 square mm for the submarine use and 900 square mm for land use. To avoid mechanical damage due to external force as far as possible, double steel-wire armour is applied. Figure 13 shows the structure of the submarine cable laid in the Hokkaido Honshu HVDC Link. A 14-mm-thick insulator is adopted. The cable incorporates optical fiber and is armored with double layers of steel wire. Its unit weight is approximately 48 kg/m in air and approximately 33 kg/m in seawater. Figure 13 Structure of the DC-XLPE Submarine Cable 8

10 Figure 14 shows also the configuration with three-dimensional cut model of DC-XLPE submarine cable used for this project. Before laying DC-XLPE cable, electric and mechanical capabilities were carefully confirmed with carrying out type tests and acceptance tests in the manufactures works. This cable was laid and put into commercial operation in December 2012 through the onsite tests including insulation withstand tests and HVDC system operation tests consisting of various operation modes including polarity reversal operation. And this cable has operated without any problems since commissioning. This kind of technologies will be used for worldwide LCC HVDC projects through the technical evaluation based on actual operational performance in Hokkaido Honshu HVDC Link. Figure 15 shows route location of DC-XLPE cable laid for Hokkaido Honshu HVDC Link. Figure 14Three-demensional cut model of DC XLPE Submarine Cable 5. Conclusion New HVDC technologies introduced in this paper are useful not only for Japanese power network but also for any power networks which have similar characteristic and operational requirements. Furthermore, upgrading and modifying these technologies, there are many possibilities that the technologies are applicable to number of existing and future HVDC projects. Figure 15 Route Location of Submarine cable for Hokkaido Honshu HVDC link References [1] T. Sakai, K. Takahashi New Operation and Control Scheme of HVDC Link under Power Market Environment CIGRE, 2010 Paris Session. [2] Joint Working Group C4/B4/C1.601, CIGRE Technical Broacher Influence of Embedded HVDC Transmission on System Security and AC Network Performance April 2013 [3] C. Watanabe, Y. Ito, H. Sasaki, Y. Murata, M. Suizu, M. Sakamaki, M. Watanabe, S. Katakai. IEEJ Transaction on Power and Energy, Vol.134 No.1 pp64-75, DOI: /ieejpes Practical Application of +/-250kV DC-XLPE Cable for Hokkaido Honshu HVDC Link 9