| 3.2 |
System Protection Relays |
| 3.2.1 |
Underfrequency Relay and Rate of Change of Frequency Relay |
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In case of a grid failure (fig. 3.3), captive generators tend to supply power to other consumers connected to the substation. The load-generation imbalance leads to fall in frequency. The underfrequency relay R detects this drop and isolates local generation from the grid by tripping breaker at the point of common coupling. After disconnection from the grid, it has to be ascertained that there is load-generation balance in the islanded system. Because of the inertia of the machines, frequency drops gradually. To speed up the islanding decision, rate of change of frequency relays are used. |
| 3.2.2 |
Undervoltage Relay |
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Whenever there is an uncleared fault on the grid close to the plant, the plant generators tend to feed the fault, and the voltages at the supply point drops. This can be used as a signal for isolating from the grid. |
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