Chubu Electric utilizes pin insulators with a BIL of 65 kV for the fixed lead wires to the transformer. Moreover, the BIL for the primary bushing and for the transformer winding core are 60 kV and 90 kV respectively. Says Iizuka, “we differentiate the insulation level between the outside bushing and the internal transformer winding core and, in this way, we prevent breakdown on the inside of the transformer.” After protecting the main line with CLAHs and inducing surges to the transformers, the next countermeasure utilized by Chubu Electric was to protect the transformer. According to Iizuka, the rate of transformer breakages per year was more than 95 per cent of all lightning faults due to the power-follow current associated with the surge. This incidence was considered too high and in order to deal with this problem Chubu Electric engineers decided to protect pole-mounted transformers with special primary cutouts containing an integrated zinc oxide element.

Says Iizuka, “after installing this component, almost 100 per cent of the lightning-related damage suffered by pole-mounted transformers was eliminated.” However, he also points out that, based on reliability and economic considerations, installation of the zinc oxide cutout unit was restricted only to urban areas due to its higher cost compared to a normal primary cutout. Together, these urban areas represent some two-thirds of the total service area of Chubu Electric.

Kondoh points out that while such a primary cutout with built-in arrester function is also utilized by other Japanese utilities, the actual design may vary from one utility to the next. Similarly, the CLAH units employed throughout Japan vary in several ways, including the shape of the pin post insulator to which the current limiting element is attached. For example, one utility located in an area with heavy snowfall in winter requires an insulator with a more rounded shape than that employed by Chubu Electric.

Kondoh goes on to explain that variations in the design and the extent of application of CLAHs throughout Japan are due to different times of introduction as well as to different regional levels of isokeraunic activity. He also points out, “those utilities which have installed line arresters and which do not have particularly high lightning levels rely solely on the arresters to protect the line.” Monitoring and Prevention Programs in Place Apart from prevention, an important element in Chubu Electric’s program of maintaining high reliability on its distribution network is quick restoration of power in situations where the magnitude of a short circuit surge overwhelms the protective equipment in place. In this respect, the utility relies heavily on a network of over-current indicators installed on the main lines.
“Most accidents in our network” says Iizuka “are caused by short circuits, so we have to know what kind of current is flowing on the line.” He explains that these detectors are normally transparent under ordinary current but become red when a short circuit passes thereby helping maintenance personnel who patrol the lines to quickly determine where the short circuit has occurred.