Why must power system frequency be maintained constant?

Power system frequency (50Hz or 60Hz) must be maintained within tight limits (typically plus/minus 0.5%) because: motors and clocks depend on accurate frequency, generators risk damage from off-frequency operation, and frequency deviation indicates generation-load imbalance. Frequency drops when load exceeds generation and rises when generation exceeds load. Automatic generation control adjusts output to maintain frequency, with primary response within seconds and secondary within minutes.