SINGAPORE - A faulty voltage transformer at a substation was the likely cause of a 1½-hour power outage in January that affected about 27,000 residents.

The transformer had started a fire in a substation in Bright Hill, and preliminary investigations by the Energy Market Authority suggest this led to the Jan 26 outage, said Senior Minister of State for Trade and Industry Koh Poh Koon.

Once the fire was detected, protective devices were automatically activated to isolate the fault, said Dr Koh.

This resulted in the power cut, he told Workers' Party Non-Constituency MP Leon Perera, who had asked about the extent and cause of the incident.

The power failure hit consumers in Ang Mo Kio, Bishan, Sin Ming, Toa Payoh and Thomson, where residents reported a cut in their home electricity supply. Lifts stopped as well, and traffic lights turned dark.

This power disruption came after another major outage in September 2018, which affected more than 146,000 residents in multiple parts of Singapore, ranging from Boon Lay to Aljunied.

On whether there is a rising trend of such power failures, Dr Koh told Mr Perera this was the first power failure in five years involving a faulty voltage transformer.

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"Other incidents may have happened in the recent past, but this does not necessarily indicate a systemic issue because they pertain to either different parts of the system, or could have been just due to human error," said Dr Koh.

He added: "We would be worried about a trend if the same component, or the same system within the electrical grid across different substations, failed repeatedly.

"While there may be a couple of incidents that happened in the recent past, they affect different parts, components, for which the (manufacturers) are still looking at... Until investigations are complete, I would not draw any premature conclusion."

On the Jan 26 incident, Dr Koh said national grid operator SP Power Grid restored the power supply to 80 per cent of affected consumers within 30 minutes.

The remaining consumers had their electricity supply restored in 90 minutes.

Stressing that Singapore's power system remains among the most reliable in the world, he noted as well that over the past six years, Singapore's average annual disruption per consumer have ranged from 12 seconds to 4.2 minutes.

This includes the most recent incident.

In comparison, major cities such as Osaka, New York, Hong Kong and London experienced an average disruption per consumer of between five minutes and 21 minutes in the financial year of 2017, he noted.

"EMA will thoroughly investigate each incident and take appropriate regulatory action if necessary," said Dr Koh. "SP Power Grid has also set up an internal investigation committee to investigate the incident."

It has engaged the original equipment manufacturer's experts to review and advise if the failure was due to the design, manufacturing or material defects, he noted.

An independent expert, Chubu Electric from Japan, has been asked to review the manufacturer's findings and recommendations.

SP Power Grid has since adopted a higher frequency of monitoring for the specific equipment that failed, and will prioritise replacements of that particular component in future rounds of substation maintenance, said Dr Koh.