Illustration: BRYAN CHRISTIE DESIGN

Mumbai’s peak load of about 2600 megawatts now substantially exceeds local generation capacity, which is only 2277 MW.

Five major grids—located in the north, east, northeast, west, and south—serve India. Except for the southern grid, all the others, totaling more than 100 000 MW in generation capacity, are on a synchronous tie, meaning that they are interconnected and share a common system frequency. Maharashtra state, of which Mumbai is the capital, is part of the western grid, which also includes the states of Gujarat, Madhya Pradesh, Chattisgarh, and Goa, an area of 11 million square kilometers with an installed capacity of around 47 000 MW.

Because Mumbai is connected to the western grid and receives some electricity to meet peak demand from the Maharashtra State Electricity Transmission Co., the city’s load directly affects conditions elsewhere in the state. Mumbai’s peak load of about 2600 MW now substantially exceeds local generation capacity, which is only 2277 MW. Tata provides 1777 MW to its direct customers, which include the commuter railway and heavy industry and whose peak load is about 500 MW. Tata also supplies the Brihanmumbai Electric Supply & Transport Undertaking (BEST), a public company that runs the city’s bus system and that distributes power to the southern portion of Mumbai. BEST’s customers’ peak load is about 800 MW. Reliance Energy, Mumbai’s other major utility, which supplies the northern half of the city, also buys electricity from Tata to supplement its own 500 MW of generation and meet its customers’ peak load of 1300 MW. To make up the shortfall, Tata often purchases power from the state of Maharashtra, for which the company pays an annual standby fee of $94 ­million on top of the actual cost of the electricity it imports.

Peak demand in Maharashtra, about 17 000 MW, exceeds generation capacity by about 5000 MW. That means planned load shedding goes on every day in parts of Maharashtra, where many rural areas have power for only 10 hours. Such privations are a fact of daily life that most Indians outside Mumbai have learned to live with and plan for, like the monsoons.

Increasing the availability, reliability, and quality of electricity is a long-term goal that will be met only with tens of thousands more megawatts of generation capacity. In the near term, administrators and engineers from both Mumbai power companies, Tata and Reliance, along with officials from the Maharashtra Electricity Regulatory Commission, are doing their best to stave off planned load shedding and avoid a catastrophic blackout that could cost the local economy billions of rupees.

Blackouts occur when load so far outstrips generation, either because of the loss of a generator or a major transmission line, that the frequency dips to 47.5 Hz. At that point, the generators automatically trip off and a blackout ensues. For example, on 25 February India’s western grid went down, and the citizens of Maharashtra were without power for several hours.

But as always, Mumbai was mostly spared. Both Tata and Reliance invoked “islanding” schemes, whereby they take the city off the national grid and supply customers with power from their own local plants until the larger network comes back up. First deployed by Tata in the late 1960s, the separation happens only when the integrity of the system cannot be maintained despite automatic emergency load shedding. The islanding does not require centralized computer control. Instead, reverse-power and underfrequency relays sense that the system frequency has dipped to 47.6 Hz and independently trip ­breakers on the six power lines that connect Tata and Reliance with the regional grid.

Most times the Tata and Reliance systems survive together as one island, but occasionally Reliance separates into an independent grid, as happened this past February. On that day, Tata generation was low due to the outage of a 500-MW coal-fired generator and a 150-MW gas turbine. According to K. Rajamani, chief consultant for Reliance, the utility had been importing electricity from Tata over tie lines with a capacity of 30 MW. That same day, a problem with a 400-kilovolt transmission line located near Mumbai destabilized the grid.

When system frequency dipped to 47.6 Hz, Tata and Reliance separated from the sinking western grid. Under normal conditions, power flows from Tata to Reliance over the interconnecting tie lines. But on 25 February, the problem with the western grid, coupled with generation deficiency in the Tata system, created a huge power swing that reversed the normal flow of electricity from Tata to Reliance.

The reverse-power condition fooled the load-shedding logic in Reliance’s Supervisory Control And Data Acquisition (SCADA) system, which is based on importing power from Tata. As a result, the SCADA system did not initiate automatic load shedding, and the Reliance generators fed Tata loads. This led to a steep decline in frequency, which tripped the generators at Reliance’s 500-MW plant at Dahanu and shut off power to about 1.7 million customers for 1 to 4 hours. Once Reliance’s system began to fail, Tata became its own island within an island. Despite its lower-than-normal generating capacity, Tata managed to keep the lights on in the southern part of the city served by its customer BEST, the 14th time it has islanded successfully since the last citywide blackout in 1997.

Mumbai’s dramatic increases in load over the past five years—averaging 5 percent annually—have strained the existing power system to the breaking point. Only a small part of that growth can be attributed to the influx of perhaps 1000 ­immigrants a day, most of whom are destined to sleep on the footpaths or seek shelter in the slums. Some of the recent demand comes from new air conditioners and washing machines, purchases enabled by a robust national economy, which expanded at a rate of 8.5 percent last year. And then there is the building boom. Much like Manhattan, Mumbai can only accommodate a swelling and increasingly affluent population by going vertical. Everywhere you look you see tower cranes helping buildings shoulder their way into the skyline. In addition, the mass conversion of 243 hectares of old textile mills in the heart of the city into commercial and residential space is creating glitzy, fully air-conditioned malls that rival the suburban temples of consumption found all over the United States. Add it up and it’s no wonder that the power companies, which have not built a new plant in a decade, are scrambling to keep the lights on.