The line impedance that we assume to be small resistor of 1/PSSC or perhaps 1/Zs depending where we think currents are going is only really correctly a low resistance at DC and up to 50Hz.
A better model for faster waveforms is to consider the mains as the same low resistance but with an inductor in series . this inductance is partly the substation transformer windings and partly from the un-cancelled magnetic fields around the cables to it.
The practical effect is that more impedance is presented to higher frequency events (and can be a lot higher - at 300Hz (so a half-sine pulse would be 1.5milliseconds) the reactive impedance of a transformer winding might be 6 times higher than it is at 50Hz) .
The upshot is that the voltage drop associated with fat rising currents or high frequency harmonics is disproportionately high compared to measurements at 50Hz - or if you are fast enough you can load the mains and see the voltage dip far more than you expect, and then recover to the steady state value.
At higher frequencies still (hundreds of KHz) the self capacitance of the lines conspires with the inductance to give you a more or less constant line impedance that is several tens to a few hundreds of ohms depending on the cable construction.
This is the ratio of current to voltage for fast events that are short compared to the propagation delay in the cable ( cable length and speed of light needed here, but 300m per microsecond is a good start), and in the short time that the source end not yet realised there has been a change at the load end.
In short voltage drops you would have measured at 50Hz are greatly exceeded during the first few microseconds of a load step.
The line impedance that we assume to be small resistor of 1/PSSC or perhaps 1/Zs depending where we think currents are going is only really correctly a low resistance at DC and up to 50Hz.
A better model for faster waveforms is to consider the mains as the same low resistance but with an inductor in series . this inductance is partly the substation transformer windings and partly from the un-cancelled magnetic fields around the cables to it.
The practical effect is that more impedance is presented to higher frequency events (and can be a lot higher - at 300Hz (so a half-sine pulse would be 1.5milliseconds) the reactive impedance of a transformer winding might be 6 times higher than it is at 50Hz) .
The upshot is that the voltage drop associated with fat rising currents or high frequency harmonics is disproportionately high compared to measurements at 50Hz - or if you are fast enough you can load the mains and see the voltage dip far more than you expect, and then recover to the steady state value.
At higher frequencies still (hundreds of KHz) the self capacitance of the lines conspires with the inductance to give you a more or less constant line impedance that is several tens to a few hundreds of ohms depending on the cable construction.
This is the ratio of current to voltage for fast events that are short compared to the propagation delay in the cable ( cable length and speed of light needed here, but 300m per microsecond is a good start), and in the short time that the source end not yet realised there has been a change at the load end.
In short voltage drops you would have measured at 50Hz are greatly exceeded during the first few microseconds of a load step.
