Frequency all over a synchronous Power Grid is the same in steady state. Mind the word Synchronous Power Grid, as there may be two different grids operating at slight different frequencies. It is very important to maintaining a constant frequency or to frequency to vary over a very narrow band in a power system operation.
Frequency in a power system is intimately related to the electrical speed of synchronous generators. As we know that the acceleration of a Generator is solely governed by the difference between mechanical and electrical torques, therefore to maintain a constant speed, mechanical input and electrical output power need to be continually matched. Electrical load can vary randomly, but fortunately the total load versus time roughly follows a trend. Frequency of Grid is depends upon many factor like load variation, prime move control of Generator etc.
Frequency needs to be maintained near 50 Hz (For India) for the following reasons:
1) Steam turbine blades are designed to operate in a narrow band of frequencies. Deviation of frequency beyond this band may cause gradual or immediate turbine damage. Consequently, protective and control equipment take corrective action in case of under/over frequency. A 50 Hz steam turbine may not be able to withstand frequency deviation of +2 Hz to -2.5 Hz for more than an hour in its entire life.
2) Loads and other electrical equipment are usually designed to operate at a particular frequency. Off-nominal frequency operation causes electrical loads to deviate from the desired output. The output of power plant auxiliaries like pumps or fans may reduce, causing reduction in power plant output.
At the present, Grid frequency deviation in Indian Grids is generally in between 50.5 to 49.0 Hz under normal operating conditions. The figure below shows the typical frequency variation on 1st December 2016 in the Northern Grid of India (taken from the Northern Regional Load Dispatch Center website – www.nrldc.in).