Short-circuit capacity at the point of common coupling?
The power grid, whether 50Hz or 60Hz, are not SPICE power sources; the grid cannot provide infinite current at zero impedance.

Loads with large changes in current ------ large industrial plants, or even small industrial plants ------ will cause "flicker" in the grid, and nearby users will need to tolerate that flicker. Using a UPS may be needed to sustain reliable computer operations.

Suppose your grid support 1 Megawatt power consumption, with 10% drop in line voltage. 1Megawatt at 100 volts requires 10,000amps. To produce only 10% drop, as load increased from 0 amps to 10,000 amps tells us the grid could produce 100,000 amps if short-circuited. The grid resistance (impedance) is V/I ==100volts/100,000amps == 0.001 ohm.

Any fluctuation in load causes a voltage change: deltaI * 0.001 ohm.

The wiring of the grid also is inductive, at 1microHenry/meter approximately, for a single wire in free space. Of course, the power company runs wires in duos or triplets or quadruplents, for several reasons. Such groupings reduces the inductance. Transformers also introduce inductance.

Thus the grid is a combination of resistance + inductance (+ capacitance).

When loads are switched, the fluctuation will be deltaI * Z (R + 2*pi*L).