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The voltage dip incident on February 8 on Hong Kong Island was caused primarily by malfunctioning of an earth switch on a gas insulated switchgear (GIS) at the North Point Substation of the Hong Kong Electric Company (HEC), according to the findings of the company's report and investigation by the Electrical and Mechanical Services Department (EMSD).

The failure resulted in a short-circuit fault on one of the 132kV circuits supplying Morrision Hill Substation.

The built-in protection system had immediately tripped off the circuit and isolated the supply to the faulty GIS without interrupting power supply.

The incident caused a voltage dip lasting for less than 0.1 second, which was felt by consumers on Hong Kong Island. The faulty GIS was replaced and the circuit was put back into normal service on February 19.

An EMSD spokesman said today (Wednesday) that the malfunctioning of the earth switch was due to failure of its electromechanical locking device to keep the switch at the open position when the 132kV circuit was energised.

"The failure resulted in movement of the earth switch leading to the eventual short-circuit of the GIS," he explained.

The spokesman said to avoid a recurrence of similar incident, HEC had taken remedial actions to temporarily fasten all similar earth switches at the open position to prevent any abnormal movement of the switches at the North Point 132kV Substation.

The company will also install additional mechanical locking device at similar earth switches as a permanent measure to provide double protection against any abnormal movement when the circuits are energised.

"The modification work will take three months to complete," he added.

"We are satisfied that the remedial measures taken by HEC will further enhance the reliability of the GIS concerned to prevent recurrence of similar incident," the spokesman said.

He said EMSD would keep in close touch with the company over the implementation of the remedial measures and will monitor the progress of the modification work on the earth switches.

GIS is an advanced type of equipment widely used in local and overseas high voltage electricity supply systems with a life expectancy of some 30 years.

The spokesman noted that according to the manufacturer's record, the malfunctioning of the locking device on the earth switch had not been experienced before and the probability of its failure was extremely rare.

The GIS concerned was regularly inspected and maintained according to manufacturer's recommendation ever since its installation in 1986. The last inspection was carried out in November 1999.

Ends/Wednesday, March 8, 2000

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