Site:           Tokyo Electric Power Company (TEPCO)
                Power Engineering R&D Center
                Insulation and Power Apparatus Department
                4-1 Egasaki-cho Turumi-ku
                Yokohama 230, Japan
                http://www.rd.tepco.co.jp/
Date Visited:   4 June 1996
WTEC Attendees: R. Schwall (report author), 
                R.D. Blaugher, 
                J. Daley, 
                G. Gamota, 
                H. Morishita, 
                R. Sokolowski
Hosts:          Dr. Hara Tsukushi, Group Manager, 
                   Insulation and Power Apparatus Department
                Hideo Ishii, Senior Engineer, 
                   Insulation and Power Apparatus Department

BACKGROUND

TEPCO is the largest utility in Japan, providing roughly one-third of the country's power. Its service area is centered around Tokyo and covers roughly 10% of the area of Japan. Peak demand is 69 GW, and generation is from a mixture of hydroelectric, fossil fuel, and nuclear plants.

Tables TEPCO.1 and TEPCO.2 and Fig. TEPCO.1 show TEPCO's Power Engineering R&D Center staffing, budget, and general history; areas of activity; and organization. Staff effort in superconductivity (SC) is approximately 10 people: 5 in the lab and 5 outside. Yearly R&D expenditures are about $1 million on power lines, $1 million on fault-current limiters, and $0.5 million on Super-GM support.

Table TEPCO.1
Power Engineering R&D Center Statistics

Fig. TEPCO.1. Organization of TEPCO R&D Center.

Table TEPCO.2
History of TEPCO Power Engineering R&D Center

SUPERCONDUCTIVITY PROGRAM

TEPCO is active in three SC areas: power transmission lines, fault-current limiters, and the Super-GM generator.

Superconducting Power Transmission Lines (SPTL)

Requirements

A much larger fraction of TEPCO's transmission and distribution (T&D) lines are underground than for a comparable U.S. utility. This is due to the fact that transmission from hydroelectric plants must be accomplished via tunnels through mountainous terrain, as well as to the need to bury lines in TEPCO's largely urban service area. Lines up to 275 kV are oil-filled, and those at higher voltages (up to 500 kV) are dry (cross-linked polyurethane, XLPE). Capacity is from 250 to 1,000 MVA/circuit using cable ducts up to 150 mm ID. The objective of the superconducting cable program is to provide additional capacity in the existing ducts, thus avoiding the cost and disruption of excavating to install additional ductwork. Figure TEPCO.2 provides a size comparison of HTS, LTS, and conventional cables for 1,000 MVA transmission. TEPCO analysis indicates that the design providing 1,000 MVA in a 130 mm dia. duct will require a current density in the superconductor of 10⁵ A/cm².

Fig. TEPCO.2. Comparison of relative sizes of 1,000 MVA cables using various technologies.

Project Overview and Funding

TEPCO has a cooperative SPTL program with Sumitomo Electric and Furukawa, which began with Furukawa in 1987. Funding is $2 million/year and is flat; $1 million of that is provided by TEPCO and $500 thousand each by Sumitomo and Furukawa. This level of funding appears to support the fabrication and test of SPTL prototypes and not the underlying work to improve the performance of the superconductor.

Demonstration Cables

Tables TEPCO.3 and TEPCO.4 give specifications for 4 demonstration cables, designated as A, B, C, and D.

Table TEPCO.3
Specification and Testing of the High Tc Superconducting Cable Prototypes

Table TEPCO.4
Specification of 50-Meter-Long Conductors

Technical Challenges

The primary technical challenges are seen as critical current density and ac loss. TEPCO researchers have doubts about the ability of BSCCO to provide the required 10⁵ A/cm² current density. Their feeling is that acceptable ac loss can be achieved if the cable strands are transposed to force current sharing.

Fault-Current Limiters (FCL)

System Requirements and Current Program

TEPCO researchers see a need primarily for transmission-level fault-current limiters because they continue to add generating capacity to the TEPCO grid, thus increasing the available fault current at existing breakers. At the time of the WTEC team's visit, the maximum system voltage was 500 kV, and the maximum circuit breaker interrupt capacity has recently increased from 50 kA to 63 kA. Development of a 80 kA breaker was underway, but TEPCO management wanted to limit fault current to < 63 kA.

In about 1989, TEPCO started a program with Toshiba Electric to develop an LTS parallel inductor limiter. The primary application is transmission at 500 kV, but the 1996 prototype was rated at 2 kA_rms and 6.6 kV. The choice of a parallel inductor design was driven by the desire for a limiter that does not require a trigger signal. In addition, a large premium is placed on compactness, given the limited space in Japanese substations. Work to date has concentrated on reducing the refrigeration requirement for the FCL, and this is now at 3.4 W/phase, which is described as "almost acceptable." The limiter goes normal in about 150 ms, and the voltage transient associated with the quench is such that metal oxide varistors (MOVs) will be used to suppress it in the high voltage design. The TEPCO system employs transmission breakers that open in 100 ms and reclose after one second. Distribution breakers open in 100 ms and reclose after one minute. A distribution limiter must, however, survive 6 to 10 reclosures. Recovery time of the 1,000 A 6.6 kV prototype coil was two seconds.

Future Plans

The TEPCO strategy for controlling fault currents is to install solid state breakers initially, then migrate to FCL as the technology matures. The acceptable cost for an FCL in the intertie position is the cost of ten breakers, and that in the feeder position is "much lower." A very limited number of applications is seen for distribution-level current limiters. The transmission-level FCLs are seen as HTS because they can be conduction-cooled in a helium-free system using an 8 W refrigerator that was developed for the Maglev program. The voltage breakdown of helium in a transmission system is seen as a serious problem. The TEPCO program plan calls for development of a 3-phase distribution-level limiter in three to four years and a field test in this century. The transmission level limiter is predicted to be available in 2010.

Super-GM

TEPCO has assigned three persons to Super-GM. TEPCO management anticipates that the generator will be the first power application of superconductivity to be tested, but the FCL will be the first to be installed in the grid. TEPCO interest in superconducting generators is likely to be confined to units of 1,000 MW and higher.

SMES and Flywheels

TEPCO management sees SMES of interest only if it is applied to diurnal load leveling at power levels of several GW. They do not see an application for smaller SMES, and the WTEC team's hosts commented that power system stabilization controls are less costly than micro-SMES and work well. Even at 10% of its current cost, they would regard micro-SMES as expensive. They are actively following the Nippon Steel/ISTEC flywheel project but have no internal effort.