Superfast fault foxes officials
SUVRO ROY/Telegraph
Calcutta, April 20: A fault that fattened itself within a fraction of a second and leapfrogged at lightning speed is being blamed for one of the worst blackouts in Calcutta.

A CESC panel of seven officials — eight if a representative of a state-run agency joins them — has been asked to investigate Sunday’s power outage and suggest remedial measures by May 6. The committee will be led by the CESC executive director (programme implementation), Dipesh Guha.

CESC officials described the probable cause as “a freak incident” that has not occurred in 20 years since an isolation system was put in place to protect the rest of the supply network. (See chart below)

The panel will have to crack two key mysteries. First, why did the fault affect three phases instead of one?

“We have had single-phase faults from current transformer bursts earlier. But yesterday, the single-phase fault rapidly developed into a three-phase fault. I have not witnessed anything like it during my tenure with the CESC during the past 19 years,” said Sumantra Banerjee, CESC managing director.

Second, what can be done to prevent a recurrence? This is a tricky question because preliminary investigations suggest that a state-run agency’s isolation system kicked in 0.17 second — as prescribed — after the fault struck.

But before CESC’s back-up circuit-breaker could get itself activated at the appointed time — 0.08 second later than the state-run system — the fault had developed into a three-phase menace and raced past the safety net. This led to the shutdown of all four CESC generation plants.

The seemingly obvious solution — reduce the activation time further to beat the super-fast fault — is fraught with pitfalls. “If we set the timer lower, say at zero, a slight fluctuation caused by the brushing of a tree branch against a wire or a sari blown over can activate the circuit-breakers and lead to frequent shutdowns,” said a CESC official.

Officials said the time fixed for the two isolation devices — set a little distance apart — was in line with established practices in the country.

Central Electricity Authority (CEA) officials in Delhi said a 0.17-second activation time for the protection systems appeared adequate. The protection systems need to be tested periodically, but the frequency of testing would depend on the criticality of the areas, said Pankaj Batra, director in the grid management division at the CEA.

The CESC panel will try for a solution to outpace future faults without activating shutdowns at the drop of a sari.

A separate investigation will be carried out into the trigger of the outage — the explosion of a current transformer under the West Bengal State Electricity Transmission Company Ltd (WBSETCL). The wild fluctuation in voltage was caused by this explosion which occurred outside the CESC purview.

A power department official said such explosions were not abnormal. “The cooling oil could have dried up, and the transformer burst because of heat or it could have been an internal fault.”

Power secretary Sunil Mitra has asked CESC to include an expert from the state agency. “It was purely a technical fault but we want to prevent such disasters,” Mitra said.