Why is frequency of power system increase when demand is increase?

Question

Why is frequency of power system increase when demand is increase?

Answer 1

Your question, which should grammatically read,

"Why does the frequency of power systems increase when demand increases?"

is technically incorrect.

When demand increases, frequency DECREASES because the higher load slows the system turbines down. This, in turn, causes the load dispatch to open the speed/load control valves to correct the frequency deviation.

In normal conditions, frequency deviations are kept within +/- 0.1%.

On the other hand, when the system frequency increases, for whatever reason, demand increases because all motors connected to the grid will run  a little faster, thereby picking up more load.

.

Answer 2

When demand increases, Power Plants increase their output.

This does not change the power supply frequency of 50 or 60 cycles per second depending on the country.

Voltage and frequency remain constant.

UK is 220V at 50Hz/s and, US is 110V at 60Hz/s.