<p style="text-align:justify"><span style="font-size:14px">When power demand is more than the generation and spinning reserve is not available, the frequency and voltage will drop to cope with the increased load demand. Reduction in frequency is harmful for generators as the steam will condense in last stages of LP turbine damaging them (in thermal power plant). Even if the frequency drops by very small percentage of normal frequency, the life expectancy of steam turbines is reduced. The good power quality demands that there should be no frequency variation, no voltage variation (or extremely little variation) and THD (Total Harmonic Distortion) not greater than 3 %. The numerical relay connected for protection of 11 kV feeder measures df/dt and dv/dt and trips the 11 kV line if df/dt value is more than certain pre-set value. Thus, tripping is established even before the frequency drops below the value that can damage the generator unit. This is known as automatic load-shedding.</span></p>