3.18 Over frequency protection 81O
3.18.1 81O Overview
The feeder relay provides a two-stage over frequency protection with independent definite time
delay characteristics, and each stage has the same protection logics.
An increase in system frequency occurs, e.g. when large blocks of load (island network) are
removed from the system, or again when a malfunction occurs with a generator governor. This
entails risk of self-excitation for generators feeding long lines under no-load conditions.
The calculation of the frequency is based on the voltage sampled values.
3.18.1.1 Function Block
The function block of the protection is as below.