Self regulating effect of loads for frequency control

Question

How does power absorption from loads depend on frequency? I know that during a frequency deviation in power grid, there are primary reserves which control the frequency, but I know there is also the contribution of loads (for example if frequency increases, power absorbed by loads increase, if frequency decreases, power absorbed by loads decreases). Another thing, is this true for all the loads? If not, which are these ones?

Answer 1

Synchronous machinery, such as large synchronous motors, synchronous condensers, etc are a common example of such a load. The motor's mechanical rotation is exactly synchronized to the grid frequency -- if the grid frequency begins increasing, then the power drawn by the motor increases as it accelerates its driveshaft and load (which have substantial mechanical inertia).

If the grid frequency starts to decrease, then the power drawn by the motor decreases (and the motor may perhaps begin acting like a generator in the sense of regenerative braking) as the kinetic energy of the mechanical loads decreases in sync with the decreasing frequency.

Answer 2

Current lags voltage in inductive machines and visa versa with capacitor loads but peaks in current with diode rectifiers from grid to PSU caps unless active PFC is used.

Since frequency is the derivative of phase over time, the frequency increase can be viewed as a voltage phase leading ramp that also leads inductive machines to increase current from their rotational back EMF steady state. This is multiplied by the number of machines in use.

We also know inductive impedance is ZL(f)= 2pi* f * L but work is modelled by a shunt R. If work increases with f and RPM then R reduces at steadystate.

So there are two effects. The rate of change (inertia) and the net load change steady state.

Answer 3

Besides synchronous machinery, other nonlinear loads also respond to frequency changes.

Consumer electronics and IT/telecom systems commonly use switching power supplies with input rectifiers that rectify AC mains voltage.

As the mains frequency increases, the rectification switching losses increase slightly. As the frequency goes sufficiently down, the rectifier forward losses grow due to higher average current. So, most switching power supplies have a “happy spot” in terms of mains frequency, and their input rectifier losses grow on either side of this frequency. When you got billions of such supplies plugged into the grid, the effects become measurable.