Tetsutaro Nakano

Results of Japan's First In-Grid Operation of 200-MVA Superconducting Cable System

A high-temperature superconducting (HTS) power cable demonstration project was started in 2007 to evaluate the cables performance, stability, and reliability. This project aims to operate a 66-kV 200-MVA HTS cable system in a real power grid of the Tokyo Electric Power Company. A 240-m-long HTS cable was successfully installed, and other system components, such as a cable-to-cable joint, terminations, and a cooling system, were also constructed at the Asahi Substation in Yokohama. After several completion and performance tests on the system, the HTS cable was connected to a real grid from October 29, 2012 to December 25, 2013. The in-grid operation had continued for more than one year without any accidental interruption of the operation or troubles of this system. The temperatures and pressures of liquid nitrogen flowing in the HTS cable were controlled to within the target values. After the in-grid operation, the critical current of the HTS cable was measured, and it was confirmed that there was no degradation compared with the initial results. In addition, no partial discharge was observed in periodical measurements. It is concluded that the HTS cable system has good performance and stability for long-term in-grid operation.

New HTS Cable Project in Japan: Basic Study on Ground Fault Characteristics of 66-kV Class Cables

In July 2014, a new high-temperature superconducting (HTS) cable project supported by the New Energy and Industrial Technology Development Organization began in Japan. The aim of this project is to verify and improve the safety and reliability of HTS cable systems. The main verification targets are system safety in the event of the following accidents: (1) ground fault; (2) short-circuit current; (3) cryostat failure; (4) a low heat loss cryostat; and (5) a high efficiency cooling system. If a ground fault occurs, it is a matter of concern that the pressure in the cryostat increases due to the arc energy. It is an additional concern if the arc penetrates the cryostat so that the liquid nitrogen leaks out of the cable. It is important to know the amount of energy in the arc in order to numerically predict the outcomes of a ground fault. We performed basic ground fault tests using sheet samples immersed in liquid nitrogen while measuring the arc energy and also examining the structure of a protection layer that can prevent arc penetration to the outside of the cable core.

Basic Study on Ground Fault Characteristics of 275-kV HTS Cable

The ground fault experiment for the 275-kV high-temperature superconductor cable was conducted in the project “Demonstration studies of the stability and reliability of next-generation transmission systems,” sponsored by NEDO. A short piece of the cable, which had an artificial ground fault point with an iron pin, was stood and poured with liquid nitrogen, and then conducted alternating current for three cycles, 60 ms. As a result, the current flowed in the ground fault path and caused an arc discharge. When the effective current was less than 5 kA, the arc lasted only half a cycle to invert the polarity. When the current was over 5 kA, the samples were damaged severely. The cryostat pipes were vented completely, and the cable cores were cratered deeply.

Shield Current of 3-in-One High-Temperature Superconducting Cables

This paper discusses the shield current of 3-in-One high-temperature superconducting (HTS) cables. The cable core consists of the HTS conductor and the HTS shield, which is coaxially wound around the former. By short-circuiting the three-phase shields at both ends, electromagnetic induction makes it possible to pass a current through the shield that is nearly identical to the conductor current and opposite in phase. In the case of 3-in-One cables, in which the three cores are tightly wound, the inductivity of the shield current is heavily influenced by the winding direction and the spiral pitch of the shield. Reversing the winding direction of the multi-layer shield or designing the shield layer with long spiral pitch are both effective ways to keep inductivity high. The shield current of a 240 meter cable was measured, and the measured inductivity was comparable to the numerically simulated value.

Fundamental Study of Ground Fault Accident in HTS Cable

High-temperature superconducting (HTS) power cable has significant merits of compactness and large power transmission capacity. Although the stability of the HTS cable system was verified by in-grid operation, the verification of its safety and reliability against various accidents is required for practical use of this system. A ground fault accident is one of the typical accidents of a conventional cable. If this fault occurs by breakdown of the dielectric layer, the generated arc energy is dissipated into the environment in various forms. As a result, the safety of the public may be jeopardized. Arc energy relates to arc voltage, which is dependent on the inherent physical properties of the faulted equipment. The use of coolant and the structure of the HTS cable differentiate it from a conventional cable, so it should be verified how these differences influence the arc voltage. Accordingly, ground fault tests using the HTS cable were conducted and the arc voltages were compared to that of a conventional cable. The results obtained proved that the arc voltage on the HTS cable is similar to that of a conventional cable, in spite of the differences concerning the use of coolant and the cable structure.

NEDO “High-Tc Superconducting Cable Demonstration Project”

Synopsis: The high-temperature superconducting (HTS) cable demonstration project is now underway at Asahi substation in Yokohama city. This project aims to operate a 66 kV, 200 MVA HTS cable system in a real power network of the Tokyo Electric Power Company in order to verify its reliability and stable operation. A 240-meter-long HTS cable was successfully installed and other system components – such as a cable-to-cable joint, terminations and a cooling system – were also constructed at the substation. After several completion tests and performance tests of the system, the cable was connected to a real grid, and the ingrid demonstration began on October 29, 2012. So far, the cable system has been operating quite well despite the fluctuations of the transport current or the climate change.