Frequency variation in the power system exists due to the mismatch between the supply of power and demand for the power. Voltage variations exist in the power grid is due to the mismatch in the reactive power between demand (MVAR) and available.
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In spite of all these variations, there is a certain limit for the operation limits (variations allowed) for voltage and frequency parameters dictated by the Grid Code. Any variations in the parameters (voltage and frequency) below operating limits considered as power grid are unhealthy and restoration steps will be taken to make the power grid healthy.
In India according to Electricity Grid Code, the operation variation in the frequency and voltage allowed is given as below.
Frequency Operating variation limits
49Hz to 50.5 Hz
Voltage operating variation limit
| Voltage Rating | Maximum Voltage | Minimum Voltage |
| 400 kV | 420 kV | 360 kV |
| 220 kV | 245 kV | 200 kV |
| 132 kV | 145 kV | 120 kV |
