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Countermeasures for substation equipment against various special phenomena in Japan's longest (54 km) 66 kV AC cable transmission system

Kyushu Electric Power Co. has commenced construction of substations for the 54-km, 66 kV AC submarine cable transmission system, which will be the longest in Japan. The system is scheduled to go into operation in 2005. There are many problems attendant to the use of a long-distance cable, including voltage fluctuations, current-zero-miss phenomena accompanying reactor equipment, absorbing the discharge energy caused by large capacitance to ground, and resonance overvoltages. Japan's 66-154 kV systems are non-effectively grounded systems. An overvoltage of the sound phases due to a single-phase ground fault is liable to become particularly high in a cable system. Also, the current power factor when a ground fault occurs deteriorates, and detection of the directional ground relay becomes difficult. A study of various special phenomena was carried out with the aim of preventing overvoltage in the event of a single-phase ground fault, preventing resonance, and ensuring correct operation of the directional ground relay, by means of appropriate zero sequence compensation. Also, economical positive sequence compensation was adopted. The current-zero-miss was prevented by a modification in the zero sequence compensation grounding transformer. The surge arrester used had special specifications in order to cope with large discharge energy. The 220/66 kV-150 MVA main transformer was designed to have high impedance, in order to avoid resonance at the 5/sup th/ harmonic (300 Hz).