New East-West HVDC Commissioned, Interconnecting Nagano and Gifu Prefectures

Kawasaki, Japan—Toshiba Energy Systems & Solutions Corporation (hereafter “Toshiba ESS”) today announced that the TEPCO Power Grid (hereafter “TEPCO PG”) Shin-Shinano frequency converter expansion project, which was designed, procured, and installed by Toshiba ESS (Nagano Prefecture, rated capacity 450 MW ×2, DC ±200 kV, DC 2,250A), has commenced commercial operation as of yesterday. This high-voltage direct current (HVDC) ^*1 system is the first HVDC system in Japan that interconnects the east-west grids operating in 50Hz and 60Hz, ^*2 respectively, via a DC transmission line. With delivery of the main equipment having started in 2017, commissioning tests and trial operations have been conducted by connecting to the east and west AC grids before commencement of commercial operation.

The transmission grid in Japan is operated in different frequencies (50 Hz in East Japan and 60 Hz in West Japan) where a large power interconnection between the grids had not been possible before the first frequency converter (FC) was constructed. The total interconnection capacity between Tokyo (50 Hz) and Chubu (60 Hz) regions was 1,200 MW, with FC systems installed in three substations, including the existing Shin-Shinano substation, before this project started operation. Due to the electricity security crisis following the East Japan Earthquake in 2011, it became more vital than ever to have a more interconnecting link between the Tokyo-Chubu grids, and it was decided that TEPCO PG would expand the Shin-Shinano FC in the east while Chubu Electric Power Grid would build the Hida FC (Gifu Prefecture) station in the west. With the commencement yesterday of commercial operation of the Shin-Shinano frequency converter expansion project, the total interconnection capacity has been increased by 900 MW (from 1,200 MW to 2,100 MW), contributing to the interconnectivity between different regions in Japan.

The HVDC converter stations at both ends are connected to 500-kV, high-voltage systems of 50-Hz and 60-Hz networks in order to have stable DC power transmission. Furthermore, Toshiba ESS is in charge of all AC/DC equipment at the Shin-Shinano substation and has provided all of the equipment, including AC/DC harmonic filters, air insulated switchgears, and control and monitoring systems in an optimal design.

In addition, according to the goal to install all the equipment effectively in a limited area in the existing substation while keeping costs reasonably low, the number of series connected thyristor elements for the thyristor valve, ^*3 which is the core of the converter system, is reduced by about 10% by reviewing the specifications of the components. The numbers of layers of the thyristor valves are also reduced and the valve structure is simplified, and finally, the valve hole, where the thyristor valve is installed, is smaller than in the conventional design.

Since 600 MW of the FC system, which is the sum of the first and second FCs, had already been in operation at the Shin-Shinano substation, adding another 900 MW of HVDC system to the same substation could affect many aspects of the power transmission system. Therefore, TEPCO PG and Toshiba ESS jointly conducted system studies, such as stability analyses, transient response considering the existing conversion equipment using up-to-date analysis methods, ^*4 and minimized the impact to the existing system by designing an optimal system.

Hideki Saito, Vice President of the Grid Aggregation Systems Division at Toshiba ESS, said that “due to frequent large-scale disasters in recent years and the expansion of renewable energy, the need for a power exchange between different regions is growing, and Toshiba ESS will continue to provide high quality products and services cultivated over years of expertise in developing and manufacturing large scale transmission systems.”

*1 The power on the sending side is converted from alternating current to direct current before being transmitted, and the system on the receiving side will return to alternating current to use the power.
*2 As of April 1, 2021; investigated by TEPCO PG and Toshiba ESS.
*3 A semiconductor device that can be used for creating alternating current with various frequencies from direct current.
*4 Response of current and voltage to change, such as the voltage in the circuit of a system which goes through an equilibrium state to a new steady state.

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