"Except for a few islands and some small isolated systems, the entire electric grid in North America is really one big circuit. The humble wall outlet is actually a gateway to one of the largest and most complex objects ever built. The grid encompasses billions of individual components, tens of millions of miles of wire and thousands of individual generators " - Thomas Overbye, Reengineering the Electric Grid, American Scientist, 2000, Vol. 88, Iss. 3.

The Indian Power Grid is not quite as big as North American power systems, but still has a large generating capacity of about 100 GW (100,000 MW). It is actually made up of three synchronised grids:

Update: A very recent development has been the synchronisation of the WR-ER-NER grid with the NR grid (August 2006)

A synchronised grid implies that the generators in that grid are connected to one another by at least one AC transmission path. This also means that the synchronous generators in that grid are all operating at the same electrical frequency (what happens if they do not ?).

For a well designed and operated system, inter-connected synchronous generators are seen to "stick together" in synchronism (by virtue of the physical equations which govern their motion). However they may lose synchronism if subjected to large disturbances. A generator when connected to a power system has to be connected by a special procedure known as 'synchronisation'. You will learn some of these things in forthcoming lectures.

A load can be connected in a synchronous grid by simply connecting it in shunt (after suitable voltage transformation using transformation and ensuring that the system is capable of servicing that extra load).

Did you know? In the WR-ER-NER grid, a synchronous machine in Panandhro Thermal Power Station (Kutch, Gujarat) operates "in synchronism" with Ranganadi, a hydro-generator in Arunachal Pradesh, a distance of more than 2000 km!

There may be a very small number of induction generators in a synchronous grid. Induction generators do not run at synchronous speed and are sometimes used in conjunction with wind turbines. However, since induction generators always absorb reactive power, they are limited to small applications wherein reactive power is supplied externally by capacitors and synchronous generators.

The three synchronous grids in India have the same nominal frequency (50Hz) but may not operate exactly at the same frequency. They are operated independently. However, the synchronous grids are connected by DC links which allow for a limited and controlled power exchange. It is planned to make a national grid consisting of large capacity links between all grids in the future ( why is there such a high degree of interconnection?).