Power outages of the kind that paralysed Mumbai on Monday are rare. The western power grid that serves these areas, plus “islanding” systems, are designed to supply power 24×7 to India’s financial nerve centre. The last big outage was in June 2018, but it was not as widespread as the one on Monday.
A statement from the Chief Minister’s Office (CMO) pieced together the factors that led to the outage. Four 400 KV power lines supply electricity to Mumbai. These four lines come together at the 400 KV Kalwa substation in Thane and from there power is distributed to the entire Mumbai Metropolitan Region (MMR).
Of the four lines, the 400 KV Kalwa-Talegaon line shut down on Saturday at 1:47 pm due to a broken conductor. The breakdown took place in a remote area at the top of the Sahyadri range, and was being repaired.
On Monday, a second line, Padgha circuit-1, was shut down due to an overload at 4:33 am. “Still the power supply was smooth through the two other power lines and power generation in Mumbai,” according to the statement.
But at 10:01 am, the 400 KV Padgha-Kalwa circuit-2 tripped, and led to the tripping of the 400 KV Pune-Kharghar line. Mumbai’s power supply went into “islanding” mode, a system by which power supply continues uninterrupted in spite of a grid shutdown, through a distributed generator. Tata Power and Adani have islanding systems to protect Mumbai’s power supply from grid troubles.
The statement from the CMO said power supply to the Island city was disrupted due to the shutdown of Tata Power’s 500 MW and Adani’s 250 MW generators at Dahanu.
Tata Power, in a statement, said a preliminary assessment had revealed that the power disruption began with the forced shutdown of MSETCL’s 400 KV Pune (Talegaon)-Kalwa line for repair work. MSETCL then did an emergency shutdown of the 400 KV Kalwa-Padghe line-1 at 6:54 am on Monday to attend to the fault. It was expected to be revived by 9:30 am but that did not happen.
At 9:58 am, the 400 KV Kalwa-Padghe-2 carrying 633 MW tripped. The flow on the Pune-Kharghar line rose up to 900 MW tripping it at 10 am, resulting in load drop in Mumbai system, the Tata Power statement explained. “Mumbai’s islanding system… could not hold, as an additional 900 MW load dropped at 10:05 hrs,” the company said.
After restoration work began to bring back supply from the three hydro units and Trombay gas and coal units as soon as the MSETCL transmission lines were connected, Tata Power started restoring supply to its consumers progressively from 12 noon onwards, the company said.
This “unforeseen event” due to the transmission system failure led to the cascading impact to all downstream suppliers of electricity including Tata Power, BEST and its consumers, Tata Power said.
Adani Electricity Mumbai (AEML) also put out a statement. “Due to transmission network failure outside of our system at around 10.05 this morning, power grid saw outages across utilities in Mumbai,” the company said.
It further said the distribution system operated the Islanding facility successfully and continued power supply to vital installations with the help of Dahanu Thermal Power Station (DTPS).