In the wake of the two mass breakdowns in the national electricity grid systems recently, the way Mumbai has been managing to ‘island’ itself in this regard has interested many, especially in other big cities.
This city has faced 37 major grid disturbances since 1981 and has survived successfully on 27 of these occasions, due to the islanding scheme. This was designed to intentionally isolate its network during widespread external grid disturbance, enabling supply to continue for essential category consumers, avoiding a tripping of thermal generators and quick restoration of the failed system.
Tata Power had designed the islanding scheme in 1981. It has been upgraded, with active participation of Reliance Infrastructure (R-Infra) and the BrihanMumbai Electric Supply & Transport (BEST), in line with the network development and generation additions, learning from each occurrence.
Thermal and hydro generating plants have been provided with islanding schemes to facilitate quick restoration in case of a blackout. A major disturbance is sensed by the frequency decay. At 48/47.9 Hz, under-frequency load shedding takes place by way of opening designated non-essential feeders, prior to islanding, to ensure load-generation balance and survival post islanding.
A Tata Power official told Business Standard: "The islanding scheme has been provided on all tie points to simultaneously operate and island the Mumbai system. After islanding, load generation balance is achieved and the Mumbai system continues to feed essential category consumers like the railways and others, depending on the available generation capacity within the Mumbai Island."
A Maharashtra State Electricity Distribution Company (MahaVitaran) official said Tata Power, a transmission system provider, has a standby agreement with the former, for which a reliability charge is paid by consumers. In the event of shortfall in the Mumbai system, this is made good by the state grid, so that the city does not face any outage.
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A Tata Power official said the Mumbai system comprised Tata Power and R-Infra generating units connected to the transmission network, connected in turn to the rest of the grid through 16 tie lines at four MahaVitaran receiving stations, at Trombay, Kalwa, Borivali and Boisar. Also, R-Infra’s Dahanu generating units (145 km away) and associated transmission network are connected to Tata’s network through two tie lines at the Borivali receiving Station. The MahaVitaran network is part of a bigger system consisting of the western, northern, eastern and north-eastern grids.
According to the official, a second stage islanding scheme is provided between Tata Power and R-Infra at Dahanu to insulate each of the systems against any further deterioration leading to a blackout in Mumbai.
In case of power flow from the Tata system to that of R-Infra’s, coupled with an under-frequency condition, the R-Infra system is isolated from Tata Power’s at Borivali to ensure continuity of supply in the Tata System. Mumbai’s power consumption is about 3,000-3,500 Mq. Tata Power’s capacity is 1,830 Mw and 500 Mw is from R-Infra.