GRID
The Tata Power Companies (TPC) and the Maharashtra State Electricity Board (MSEB) have their generating stations located in different parts of Maharashtra State and form an interconnected transmission system grid in the Mumbai-Pune region. Electric Power from this system is transmitted at 220/110kV through overhead conductors and underground cables amongst others to TPC’s sub-stations at Carnac, Parel, Dharavi,Backbay & Mahalaxmi. From here power is made available to BEST at 110/33/22kV.

The BEST Undertaking on behalf of BrihanMumbai Municipal Corporation (who are the licensees for the distribution of electric power within the City Limits of Mumbai) receives power in bulk from the TPCs. At 110 / 33 / 22kV, 3 Phase, 50Hertz.

BEST SYSTEM
110 / 33 / 22 kV Distribution System:

The 110 / 33 / 22 kV substations are completely indoor equipped with power transformers, switch-gears, control panels, reactors, capacitors, batteries and other auxiliary equipment.

Bulk power at 110 / 33 / 22 kV is transmitted from Tata’s five sub-stations through underground cables to BEST’s receiving sub-stations situated at different localities in Mumbai where the BEST Undertaking has installed 110 / 11 kV,  110 / 33 kV, 33 / 11-6.6 kV or 22 / 11-6.6 kV or 33/11 kV power transformers. The Power transformers have been provided with On load Tap Changing (OLTC) gear for regulating the voltage for giving stabilised supply to the consumers.

11/6.6 kV distribution system:

Each distribution substation comprises one or more incoming and outgoing feeders and one or more transformers of 1600/1000/630 kVA.  The outgoing feeders and primary of the transformers are controlled and protected by 11 KV oil circuit breakers in conjunction with their associated trip coils and current transformers. The distribution substations are normally equipped with 11 kV switch-gear, distribution transformers and Low Voltage Distribution boards or pillars. 56 kVAr capacitor banks are connected on the L. V. side of transformers for power factor improvement.

The distribution substations in the city are of the following three types :

Indoor Type – Where the switchgear and the transformers are located inside a building.

Outdoor Type – Where the switchgear and the transformers are located in open land and directly subjected to outside atmosphere.

Indoor/Outdoor Type – Where the switchgear is located inside a building and the transformers are located in an outdoor plot open to sky.

11 / 6.6 kV Cable Network: 
Underground 11kV cables radiate from the BEST receiving sub-stations to supply power to a large number of distribution substations and to certain consumer’s substations.  These feeders form a radial network under which each feeder supplies on an average  four to five substations in series.

Power at 6.6 kV or 11 kV is stepped down to 415 / 240 V at the distribution substations where the various sizes of 6.6kV / 415-240 V and/or 11-6.6 kV / 415-240 Volts  delta/star transformers of vector group 41DY11 are installed.  The star point of these transformer is solidly grounded and is also brought out to an insulated terminal for the 3 Phase, 4 wire distribution system.

415 / 240 Volts distribution system:

The 415 / 240 V secondary distribution system comprises of a vast network of underground four core cables, suitably sectionalised by means of distribution pillars. The  service cable lines are teed off, and taken into meter rooms, situated in the building premises, to supply power to medium and low voltage consumers.