Because of the high concern for problems related to lightning, Chubu Electric has also set up a system for predicting the path of incoming thunder clouds. So far, a lightning alert system has been installed in all service offices covering a sample of about 300 representative distribution substations.

Basically, each instrument in the system measures sequence voltage (Vo) in the substation. Whenever a lightning stroke hits one of the main feeders, this voltage changes. The system detects this difference and automatically sends an alert to the service office. “This way,” says Iizuka, “we can know the direction of the incoming lightning and we are able to arrange to have a repair crew nearby in the event of a problem. Because Japanese customers require us to restore power interruptions as quickly as possible and usually in less than one hour” he continues, “it is necessary to have such a maintenance crew in place even before an accident has occurred.”

By all indications, these various countermeasures have proven very effective. In its 1982 fiscal year, the average time of power interruptions per customer due to lightning faults at Chubu Electric was about 50 minutes per year. As of fiscal 1998, this number had fallen to less than 5 minutes per year.

Looking back on their experience with the CLAH, Chubu engineers appear quite satisfied with the performance relative to their initial expectations. Says Kondoh, “our expectation of only some 10 accidents per year after application of these units proved correct. Moreover, after installation of the special primary cutout with integrated zinc oxide elements, failures of transformers due to lightning were reduced to virtually zero.”

Kondoh and Iizuka emphasize that the CLAH is not an arrester nor is it an arcing horn since, unlike a simple arcing horn, it can diminish the power-follow current and there is no interruption in service during its operation. Insofar as the gap type design of the unit, Kondoh sees a number of reasons why this is preferable to a gapless design. First of all, he notes that the gapped design allows the unit to be smaller since fewer zinc oxide blocks are needed. “Basically, the task of reducing the power-follow current is divided between the air gap and the zinc oxide” he says, “and we see this as an excellent design concept.”

Next, he finds that the gap design is the most practical way to add a CLAH to an existing solid core insulator in retrofit situations. Finally, since there is a gap, Kondoh notes that there is almost no deterioration of the zinc oxide since it is not continually energized. Indeed, Kondoh and Iizuka point out that in Japan utilities generally expect components such as the CLAH to last about 30 years.

Chubu Electric has taken down samples of CLAH units installed over the past 10 to 15 years and found some minor deterioration of the EPDM external housing with corresponding loss of hydrophobicity. However, Kondoh stresses that no difference in varistor voltage was detected. “We expected this type of performance when we purchased the units” he says. “Even though hydrophobicity has been lost, deterioration of the zinc oxide has not occurred.” Moreover, he states that he sees many opportunities for utilities worldwide which adopt insulated conductors to increase the reliability of their distribution networks by utilizing such devices as the CLAH.