Asian international grid connection and potentiality of DC superconducting power transmission

1 Introduction

Many Asian countries have recently experienced natural and accidental disasters such as typhoons, floods,earthquakes, tsunami and nuclear plant breakdown.Disasters caused catastrophic damage to energy infrastructures.All these experiences made the residents in the regions keenly aware of the needs for energy infrastructure that is robust and resilient so that vital life-lines are secured in calamities. In the aftermath of the Great East Japan Earthquake of March 11 of 2011, the re-examination of infrastructure has led to a paradigm shift toward reforming the industrial organization on electricity with a full-fledged deregulation in Japan. Many new issues and challenges for redesigning infrastructure for energy are presented for the reform. In particular, with the start of cross-regional or international operation of power systems for effective use of sustainable energy, construction and expansion of interconnecting transmission lines have become an urgent issue. However, such renewable energy sources would affect the power grid through fluctuation of power output and the deterioration of power quality. Therefore, a new social infrastructure and novel energy management scheme to supply stable electric power would be required.

In this paper, proposals of the international electric power grid connection in Asia are discussed, and the features and issues of electric power grids to be solved are presented for realization of the international grid connection in Japan. As the essential technology for implementing the international grid connection, the present status of DC superconducting power transmission lines and power converters for long-distance power transmission lines is described. Finally, conditions and legal frameworks for the international grid connection are summarized.

2 International grid connections proposed in Asia

2.1 Proposals for Asian international grid connection

Renewable Energy Institute in Japan published “Asia International Grid Connection Study Group, Interim Report” in April 2017. The report clarified that an international power grid was economically reasonable in general and there is no particular technological difficulty in an international power grid compared with domestic grids[1]. Also, the report described that these aspects have been proved by the long history of the international power grid in Europe and moreover, interconnections are utilized not only in Europe but also in Southeast. From these outlooks,in light of global renewable energy expansion in the future,many international grid connections will be constructed across the world.

The report also summarized proposals and initiatives for international grid connections in Asia as follows. A study report on a Japan-Russia Power Bridge Project that would link a thermal power plant on Russian Far East Sakhalin Island to Niigata via Hokkaido, using submarine transmission lines was already released in the first half of the 2000s as shown in Fig. 1.

The feasibility study was conducted by Marubeni,Sumitomo Electric Industries, and Russia’s Unified Energy System [2]. This study report proposed the schedule to start 2 GW transmission in 2010 and 4GW transmission in 2012.This project has not been implemented as of 2017, however it is notable that schemes to export electricity from the Russian Far East to Japan have continuously been studied by multiple enterprises.

Fig. 1 Proposals for international grid connection in Asia

In the past, Jack Casazza, an internationally honored engineer, published a book in 2007 “FORGOTTEN ROOTS Electric Power, Profits, Democracy and a Profession”. This book is directly relevant to today’s national energy problems and explores the evolution and interrelation of the electric power industry, government policy, universities. It also discusses possible procedures for solving our problems in the future. He proposed the international grid connection between country/country and island/island in ASEAN nations for effective use of energy resources, reconciliation of neighboring countries through power transfer and revitalization of these regions [3] .

In 2002, the Korea Electro Technology Research Institute, KERI, and Energy Systems Institute, ESI, of the Russian Academy of Sciences proposed the Northeastern Asia Power System Integration Project [4]. In this joint scheme between the Korean and Russian research institutions, the plan to interconnect the Russian Far East and South Korea via North Korea has been considered.

Also, since 2009, the Hanns Seidel Foundation has proposed the GOBITECH Initiative to provide electricity to coastal areas in China, North Korea, and South Korea by utilizing solar PV in the desert areas in Mongolia and China.

2.2 Proposal of Asia super grid by Softbank Group

The triple-disaster of earthquake, tsunami and nuclear meltdown that hit the northeast of Japan in March 2011 was a wake-up call for many people. Having experienced the danger of nuclear power plants in Fukushima, Soft Bank Group felt the need to replace nuclear power with safer and cleaner renewable energy for a better future [5].To accelerate the deployment of clean, safe, and affordable renewable energy, Soft Bank Group founded the Renewable Energy Institute (REI), and in September 2011,the “Asia Super Grid (ASG)” was conceptualized. ASG goes beyond Japan and includes other Asia countries to further maximize the usage of renewable energy by taking advantage of diversity in loads and resources. The goal is to utilize renewable energy across Asia by connecting China,South Korea, Russia, and Japan via an international power grid using solar and wind power generated in Mongolia as the main power supply.

At the same time in 2011, the non-government council Japan Policy Council proposed “Asia Pacific Power Grid”as its first recommendation [6]. The Council specified that this scheme aims to overcome the issues of renewable energy as unstable power sources by interconnection and establish the Asia partnership in energy coordination.

Furthermore, since 2011, international power grid concept has been proposed mainly by power and transmission companies in Northeast Asia. Inspired in part by the Asia Super Grid vision mentioned above, major transmission companies in Asia started to formulate their own international power network scheme in Asia in conjunction with the vision [1]. The Korea Electric Power Corporation announced the Northeast Asia Super Grid plan in 2014 in cooperation with the Russian research institution Skoltech and other organizations. The company set out the concept of a Smart Energy Belt that connects Japan, China, South Korea, Russia, and Mongolia with a highly efficient electricity supply-demand system combining power storage technologies and smart grid [7].

2.3 Global Energy Interconnection proposed by SGCC

In September, 2015, Chinese President Xi Jinping proposed discussions on establishing a Global Energy Interconnection (GEI), to facilitate efforts to meet global power demand with clean and green alternatives at the UN Sustainable Development Summit and proposed to seize opportunities presented by the new round of change in energy mix and the revolution in energy technologies to develop global energy interconnection and achieve green and low-carbon development. This has been widely acknowledged and positively responded by the international community.

Under these circumstances, in 2015, the State Grid Corporation of China (SGCC), the world’s largest power transmission company, announced its Global Energy Interconnection (GEI) vision [8]. This is a vast vision calling for connecting the world through ultra-high-voltage transmission systems. Its proposed time schedule includes reaching a consensus by 2020, formation of grids on each continent by 2030, and completion of grids on a global scale by 2050. Also, the GEI vision stresses the goal of putting variable renewables such as solar and wind power to maximum use. In this respect, it shares a similar feature with “Asia Super Grid” introduced above. In addition, the target year of 2050 set for the realization of a global power grid is associated with the target of the Paris Agreement.

3 Features and issues of electric power grid in Japan

3.1 Responsible electric power supply by ten companies

In this section, some notable features of electric power grid and its interconnections in Japan are described. In 1950s, supply and demand for electricity had become very severe in Japan. A series of intense discussions were held on restructuring the electric utility industry as one of the measures for democratizing the economy. As a result, nine regional privately owned and managed General Electricity Companies, Hokkaido, Tohoku, Tokyo, Chubu, Hokuriku,Kansai, Chugoku, Shikoku and Kyushu Electric Power Companies were established in 1951 and assumed the responsibility of supplying electricity to each region as shown in Fig. 2 [8]. This fundamental structure remains to this day, and with the return of Okinawa to Japan in 1972, Okinawa Electric Power Co. joined as the tenth member.

They were also responsible for supplying electricity to consumers subject to retail liberalization, based on the provisions for last resort service, if they cannot conclude contracts with power producers and suppliers (PPSs).However, in April 2016, electric power companies are completely liberalized and all the customers became eligible (to freely select the suppliers of electricity) and Japanese electricity market introduced the complete competition mechanism.Before the liberalization of power market, electric power companies were basically obligated to operate independently by maintaining the supply and demand balance in their own areas, and then interconnection lines between areas were weekly coupled and a large amount of electricity could not be transferred through the interconnection lines. This insufficient transfer capacity problem of interconnecting lines between areas still exists in the present grid in Japan. Even if electricity is imported from foreign countries to Japan, it is difficult to transfer the imported electricity from the landed site to the center of consumption.

Fig. 2 Ten electric power companies as responsible suppliers of electricity

3.2 Control areas divided by two frequencies

The frequency of grid power differs between eastern and western interconnections in Japan, namely 50Hz and 60Hz respectively. This difference has a historical reason in that the Tokyo area adopted German-made generators at the beginning of the electricity business, while Osaka area adopted US-made GE generators. Therefore, as shown in Fig. 2, Frequency Converter Facilities (FCF)are necessary to transfer electricity between the eastern and western power grids. Three FCFs, namely Sakuma FCF and Higashi-Shimizu FCF in Shizuoka Pref. and Shin-Shinano FCF in Nagano Prefecture operate to convert the frequency.

The total transmission capacity of three FCFs is only 1,200MW and it is insufficient for this capacity to transfer a desirable level of electricity between two interconnection areas. The capacity of East-West Grid Connection is planned for expansion to 2,100MW in total by FY2020.This plan includes the increase in the capacity of Higashi-Shimizu FCF by up to 300MW in February 2013 by the Chubu Electric Power Company.

3.3 Necessity to improve the generation mix

The Agency for Natural Resources and Energy, METI,compiled a “Long-term Energy Supply and Demand Outlook (Energy Mix)” in July 2015 as the new energy policy toward FY2030 as shown in Fig. 3. According to the energy supply-demand forecast in FY2030, renewable energy power generation will expand to a range of about 22% to 24% of the total. Regarding renewable energies most suitable to the environment (photovoltaic, hydraulic,wind, biomass, and geothermal power generation), the government aims at the maximum introduction of each individual power source. Geothermal, hydraulic, and biomass energy sources in particular are expected to become alternatives to nuclear power, while wind and photovoltaic energy sources are expected to become alternatives to thermal energy [9].

In March 2011, units 1-4 of the Fukushima Daiichi nuclear plants were seriously damaged in a major accident caused by earthquake and tsunami and all Japan’s reactors were shut down. Hence the total capacity of nuclear power plants by 2719 MWe net was removed from the power system of Tokyo Electric Power Company. In 2014, units 5&6 joined them in being decommissioned. Since March 2016, Kansai Electric Power Company began loading the 157 fuel assemblies into the reactor’s core, after a court lifted an injunction that had kept both Takahama 3 and 4 off line, despite regulatory approval for their operation.Kansai Electric aims to resume generation at Takahama 4 firstly and at Takahama 3 in 2018. Of 42 operable reactors in Japan, only Sendai 1 and 2 and Ikata 3 are currently operating, although several are undergoing restart reviewsby the Nuclear Regulation Authority.

Fig. 3 Long-term energy supply and demand outlook (Energy mix) by METI, Japan

Fig. 4 Generation mix in 2020 (Estimate by OCCTO,2016, KWh-Base)

Presently, we are suffering from high price of electricity and low supply reliability. In generation mix, nuclear plants generate only one percent, and 87 percent of electricity is generated by fossil fuel as shown in Fig. 4. This is not acceptable for Japan from the viewpoints of energy security and stable electricity supply, therefore it is mandatory to increase sustainable energy and to decrease consumption of fossil fuel. The international grid connection will be one of countermeasures of these problems existing in Japan.

4 Present status of superconducting power transmission and its perspectives

4.1 Characteristics of superconducting power transmission

There are several projects of the experimental superconducting power transmission lines (SC-PTL) in the world, and their transmission power are 40 to 200 MW,and their lengths are several hundred meters to one kilometer. They cannot be installed into the sea now. The present main aim to develop SC-PTL is to install in the metropolitan area because the cross-section of SC-PTL is smaller than the copper and aluminum power cable and the civil engineering cost can be saved. In this meaning, we need more time to develop the SC-PTL for the power grid connection between China, Russia, Korea and Japan.

However, SC-PTL has high potential for a long transmission line because it is a low loss, small size and available to use in lower voltage. In the safety point of view, it is better to use the underground power cable but Ultra High Voltage DC power transmission (UHVDC)cannot be used because of the electric insulation problem of the power cable. Moreover, we can expect the cheaper instrumental cost and flexible operation of grid in future because of lower voltage and smaller size. These are fundamental reasons to develop SC-PTL.

The target parameters of SC-PTL are related with the heat leak of the cryogenic pipe and the cost of performance (COP) of refrigerator. They are 1 (W/m) and 0.1 respectively because the major loss of the long SCPTL depends on them. These requirements were fixed to compare the loss of copper and aluminum cables in 1990’s. The loss of SC-PTL almost does not depend on the magnitude of current. Even if the current is zero, the loss should be appeared because it should be kept in low temperature and the cooling system consumes electricity.It means that it is better to apply the SC-PTL when the operation ratio is high. Since the circulation circuit of the liquid nitrogen (LN2) usually needs two pipes, the heat leaks of two pipes should be added as the loss per unit length.

The efficiency of Carnot cycle is ～30% for the thermal cycle from 300 K to 70K, therefore the efficiency of the mechanical system is around 30%. However, it is not easy target values in the experiments, and the COP of refrigerator improved from 0.02 in 1990’s, and achieved to 0.08 in 2016, and it is almost saturated technically. Therefore,we need to realize lower heat leak of cryogenic pipe.

4.2 Results of Ishikari project

Fortunately, the heat leak of cryogenic pipe is improved in Ishikari project in Hokkaido, Japan. Two different designs for the cryogenic pipe are adopted [10]. The cryogenic pipe is composed of one outer pipe and two inner pipes, and one inner pipe is called cable pipe because the superconducting power cable is set inside.

The other inner pipe is called return pipe and it is used for LN2 circulation. One design of cryogenic pipe adopts to use the radiation shield, and it is connected to the return pipe thermally and is set to surround the cable pipe. This is effective to reduce the temperature rise the cable pipe even for a longer transmission. The experimental data of heat leaks are shown in Table 1 [11], and the LN2 is used as the cryogen. The temperature rise for the flow rate of 30 (L/min) and length of 1000m are also shown in the table.

The sum of heat leaks of the cable pipe and the return pipe for cryogenic pipe-2 is achieved to 0.885 (W/m), and it is lower than the targeted values of 1 (W/m) in 1990’s.Additionally, the temperature rise of cable pipe is 0.040 (K/km)for the flow rate of 30L/min. Therefore, we can extend the transmission line easily because the critical current of the cable at the end is not reduced so much.

Table 1 The heat leaks and its temperature rise of Ishikari project

Temperature rise [K/km] for 30L/min Cryogenic pipe-1 Cable pipe 0.818 0.928 Return pipe 0.462 0.525 Cryogenic pipe-2 Cable pipe 0.034 0.040 Return pipe 0.851 0.960 Pipe Heat leak[W/m]

4.3 Future plans from Ishikari project

Table 2 shows the calculated estimations of thermohydrodynamic parameters for 5 km and 10 km transmission lines based on the Ishikari data. These parameters are chosen to use the present instrumental devices. Therefore,the flow rate for 10km transmission line is 25 (L/min), and the output pressure from the cryogenic pump is lower than 0.2 MPa.

Table 2 Experimental data and estimated thermohydrodynamic parameters for 5km and 10km transmission lines

Ishikari data Length [m] 474 5,000 10,000 Flow rate [L/min] 30 20 25 ΔP [kPa]_cable 5.20 24.38 76.18 ΔP [kPa]_return 8.50 39.85 124.53 Cable ΔT [K] 0.019 0.301 0.481 Return ΔT [K] 0.46 7.20 11.52 Outlet temp [K] 70.47 77.50 82.00 Total pressure [kPa] 13.70 64.23 200.71 Heat leak [W/m]_cable 0.034 0.034 0.034 Heat leak [W/m]_return 0.851 0.851 0.851 Total heat leak [kW] 0.42 4.43 8.85 Radiation shielded cryogenic pipe

These parameters are the same as the present experimental values of the other projects for 0.25km to 1km transmission lines. Since the temperature rise of the cable pipe is lower than 0.5 K for 10 km, the critical current of the cable is not reduced so much, and we can use the rated current defined at temperature of 77 K. Depending on the heat leak analysis for cryogenic pipe, we believe that the heat leak can be reduced by half at least in the next project.This means that we can extend the transmission line from 20Km to 40km easily.

Since the temperature rise of the cable pipe is quite low,we can extend the transmission line longer. Fig. 5 shows one example, and it is 50km transmission line.

Fig. 5 Thermo-hydrodynamic estimation for 50 km SC-PTL

The refrigerator uses the cold from LNG to improve its COP because the temperature of LNG is ～120K, and LNG is a major energy source in the world now. Although huge energy is consumed to make LNG from the natural gas (NG, main component is methane), the cold of LNG is almost abandoned in the importing countries now. This is an unmeasurable major loss of energy and the CO2 production in the present time, and if we can use the cold from LNG, the total performance of LNG energy system including LNG production and consumption countries will be improved significantly. This kind of refrigerator is developed in Japan, and its COP exceed 1.0. Therefore,the loss of SC-PTL will be improved to 1/10. It means that the loss of SC-PTL will be only 1% of the copper cable system. The inlet temperature of cryogenic pipe and the terminal cryostat is 70K, and its flow rate is 40 (L/min)for cryogenic pipe, the temperature rise for 50km is only 1.5K, and the temperature of liquid nitrogen at the cryostat B is 71.5K. The heat leak to the return pipe is not small like the cable pipe, but the temperature would reach to 96K at 33km from terminal cryostat B and its pressure is around 0.6MPa. If some parts of LN2 are flushed into the air, the temperature of LN2 comes down to 77K because of the latent heat of LN2, and finally the flow rate of cryogenic pipe is ～32 (L/min). The temperature of LN2 at the terminal cryostat B is ～92K and the LN2 flows into the refrigerator to cool down again. We add some LN2 and send back to the inlet of cryogenic pipe. This system does not need the refrigerator within 50Km. Of course, we have several technical issues to solve for this system, but we almost reached to make the design of 50 km SC-PTL in cryogenic engineering.

4.4 Hybrid cryogenic pipe for electricity and LNG transports

Fig. 6 shows the other design option of the cryogenic pipe, called the hybrid cryogenic pipe [12]. The pipe can transport electricity and LNG.

Fig. 6 Hybrid cryogenic pipe to transport electricity by superconducting cable and LNG

Heat transport by radiation is proportional to the fourth power of absolute temperature, therefore if the cable pipe is surrounded by the low temperature wall like the radiation shield. The heat leak into the cable pipe is reduced significantly as mentioned in section 4.2. Therefore, we can save the power of refrigerator of LN2 to 90%. The temperature of LNG is ～120K, and it is too high to use SCPTL. But it is effective to reduce the heat leak to the cable pipe. Therefore, one inner pipe is connected to the radiation shield in Fig. 5, and LNG is transported by this pipe. The specific heat of LNG is almost double of LN2. Therefore,the temperature rise of LNG will be half of LN2’s, and we can expect that the distance of two terminals are extended to ～100km in Fig. 6.

Because of these considerations based on the experimental results in Ishikari, the structure of the radiation shield is effective, but this is not installed into an usual cryogenic pipes because they are composed of two corrugated pipes.

The Ishikari project got the data to be able to extend a longer cable system, but it is only 1 km system. Therefore,we should construct 10 km line as the next step because we have several technical subjects to solve. These subjects are related with the cost, too. For example, we should develop a submarine superconducting power cable. After the successful experiments of 10 km line, we will be able to connect to Beijing to Moscow, New Delhi, EU and the other Asian countries by repeating the unit 20km to 100km system. Moreover, the author expect that the heat leak of cryogenic pipe will be able to reduce by half at least.

5 Converter technology

5.1 Two types of DC power transmission technology

There are two technologies for DC power transmission：Voltage Sorce Converters and Current Source Converters.Fig. 7 shows the development of the rated voltage and rated power of the CSC type DC power transmission system.

5.2 Feasibility study of long distance direct current high temperature super conducting cable system

Fig. 7 Trends of HVDC voltage and capacity [15]

The design philosophy of the HVDC grid system and the HTS-HVDC is different. There is the reference Back to Back system using Thyristor as the switching device,and the rating of those DC main circuit is DC +/- 13kV /200MW [14].

The HTS transmission system with 20kV 50MW(2.5kA) is reported in Russia [15].

The assumption of this study is the rating of HTS cable has limitation of 100kV or 10kA. Table 3 shows the rating of Bi-Directional Thyristor.

Table 3 Example of Bi-Directional Thyristor. [16]500MW HTS-HVDC system is considered as below

Part number VRM (V) ITAVM (A) ITSM (kA) Package*(mm)5STB 24N2800 2800 3430 43 150/100 5STB 24Q2800 2800 2630 43 150/100 5STB 18N4200 4200 1920 32 150/100 5STB 17N5200 5200 1800 29 150/100 5STB 25U5200 5200 1980 42 172/100 5STB 13N6500 6500 1405 22 150/100 5STB 18U6500 6500 1580 29.7 172/110

Fig. 8 Configuration of HVDC system

The main circuit has Bipole configuration with DC main circuit rating of DC +/- 50 kV / 5 kA / 250 MW x 2 poles using 20 series x 2 Parallel 2.8 kV 2.6 kA Bi-Directional Thyristor.

This Bipole system has advantage that the transmission capacity can be increased according to the increase of the demand. The initial system is constructed as 250MW Assymetric Monopole, then it can be upgraded later as a Bipole system.

5.3 Historical trends of power semiconductor

Fig. 9 shows the historical trends of switching capacity of the Power Semiconductor [13]. Fig. 10 shows the outlook of the PressPak© IGBT unit.

Fig. 9 Trend of switching capacity

Fig. 10 Outlook of PressPak© IGBT unit

6 Conclusion

The population of Asian countries occupies about 2/3 of the world. Electricity generation in Japan, China,South Korea, and Russia accounts for 76 percent of total generation in Asia, and similarly, electricity consumption in these four countries attains to 77 percent of Asia’s total consumption. Japan, China, South Korea, and Russia have a vast majority of electricity generation and consumption of the most populated region in the world. If grids in the Northeast Asia can be joined together, there is a possibility of joining grids together worldwide to solve the global energy issues [5].

With the development of the international grid connection, electricity generated in other countries, such as China and Mongolia will be available in Japan. According to the studies conducted by Renewable Energy Institute(REI), the available electricity will be cleaner, stabler and less expensive but the obstacles to be solved are the regulatory issues on the import of electricity from foreign countries.

In the commencement of international grid connections with other countries, there have been many procedures where agreements were concluded under the leadership of national governments or government bodies.

In Europe, a common legal framework for international grid connections is formed by EU directives and regulations under the Treaties of the European Union. Therefore, there is not much need for agreements between member states on basic matters for installing international grid connections[1].

In other regions including Asia, there is no international legal framework, therefore bilateral agreements or multilateral agreements under the regional economic cooperation framework are to conclude to realize the international grid connections. The establishment of the common legal framework for international grid connections as in Europe will be a necessary requirement to promote the international grid connections in Asian countries

The vision of GEI is moving in the right direction, but some conditions are required for the international grid connection project to be successful. Renewable energy distributed among the participating countries must be generated from different sources to be advantageous to all cooperating and thus, ensure stable energy supply for all.Participating countries must be politically, economically,and socially stable, and they must have cordial relationships with each other.