On October 12, 2020, Mumbai suffered a massive power outage that disrupted electricity supply in many parts of the city and its suburbs. The outage was a rare occurrence in the country’s financial capital, which has an islanding system in place to separate its electricity supply from the rest of the state in case of a grid failure. However, owing to a series of cascading failures in the state grid, the islanding system tripped as well, leaving the city’s 18.4 million residents in the dark for 2 to 15 hours. Mumbai’s suburban rail network was also affected, although the stock exchanges and Covid hospitals kept running without any untoward incidents. Both the central and state governments have ordered enquiries into the outage. A team comprising officials from the Central Electricity Authority has been formed to investigate the causes of the state grid failure and find possible solutions to prevent such an occurrence in the future. The Maharashtra Electricity Regulatory Commission (MERC) has also begun suo moto hearings in the matter, and has appointed a three-member committee to undertake a root-cause analysis and submit a report to the commission in three months.
Details of the outage
While the exact sequence of events leading up to the outage is still being investigated, preliminary reports suggest that it was triggered by the tripping of Maharashtra State Electricity Transmission Company Limited’s (MSETCL) 400 kV transmission system at Kalwa, which supplies power to Mumbai and adjoining areas, and was exacerbated by the failure of the city’s islanding system, which was set up in 1981 by Tata Power. There are four incoming circuits/lines connected to MSETCL’s Kalwa receiving station for further transmission of power to Mumbai. Of these four lines, the 400kV Pune-Kalwa line had been under forced shutdown since October 10, 2020. Two other lines tripped on October 12, 2020, leading to overloading of the fourth circuit, a sudden voltage drop and the subsequent outage.
As per the daily grid events report of the Western Region Load Desptach Centre, MSETCL’s 400 kV Kalwa-Padghe Line 2 tripped due to an R-ph fault at 09:58 hours on October 12, 2020. At 10:05 hours, the 400 kV Pune-Khargar line emergency had tripped due to heavy sparking on the isolator, while the 400 kV Kalwa-Kharghar line also hand tripped due to a CT jumper hotspot, leading to the Mumbai system blackout. Tata Power’s Trombay Unit 5 (500 MW) and Trombay Unit 7(A) and 7(B), along with Mahagenco’s Uran 5 (108 MW), Uran 6 (108 MW) and Uran (A0), also tripped at 10:05 hours.
Adani Electricity Mumbai Limited’s system got islanded and survived, with a total load of 400 MW (fed through the Dahanu power plant’s Unit 1 and Unit 2). Loads at the Bhiwandi area feeding from Padghe also reportedly tripped. The total load loss was observed to be around 2,600 MW, of which 2,200 MW was in Mumbai and 400 MW in Khargar, Navi Mumbai, Bhiwandi and Thane. The total generation loss was around 840 MW at Trombay and 220 MW at Uran.
According to a statement by Tata Power, one of the city’s distribution licensees with integrated generation assets, Mumbai’s islanding system was successfully separated at 09:58 hours; however, it could not hold the sudden 1,000 MW load drop at 10:05 hours. Tata Power rapidly initiated restoration by bringing in power from its three hydro units and activating its Trombay gas and coal units as soon as the MSETCL transmission lines were connected. The company started restoring supply to its consumers progressively from noon onwards and through the course of the day.
What next?
During MERC’s hearings on the matter, it was suggested that Mumbai’s islanding system should be upgraded. The system has been in place for the past 25 years. When islanding took place for the first time in October 1997, the city’s electricity demand was 1,415 MW while supply stood at 1,490 MW. Over the years, Mumbai’s demand has increased and its embedded generation has fallen short of supply, leading to overdependence on interchange power from outside the state transmission system. This time, when islanding kicked in, the generation capacity was reportedly 1,394 MW, while the demand touched over 2,800 MW. The MERC has taken note of the preliminary recordings of various stakeholders, but is yet to pass directives in the matter. The commission, however, highlighted that distress load shedding could have been carried out and that grid stability is the collective responsibility of all stakeholders. Following the incident, the state government has also asked Mahagenco to add up to 2,000 MW of gas-based capacity at the Uran thermal power plant, which can start generating power quickly and contribute to effective islanding.
Such incidents highlight that it is imperative for grid constituents to take measures to strengthen and stabilise the grid, especially as more and more infirm renewables are integrated and the load profile of consumers evolves with electric mobility and decentralised generation.
Neha Bhatnagar