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^th harmonic (300 Hz).