Calculation of earth potential rise in the underground power distribution cable network

Earth potential rise (EPR) calculation

The author has observed young graduate engineers using available computer programs to model substation earth mats and calculate earth potential rise (EPR) without understanding the importance of the parameters involved.

In one instance, an engineer was calculating EPR for a 33/11kV zone substation earth mat. The calculated earth mat resistance was 3 Ohms, the source substation had an 11 Ohms neutral earthing resister and the length of the 33kV line to the zone substation was approximately 10km.

Obviously, the EPR of a 3 Ohm earth mat with 1732 A fault current is only 5196 Volts compared to 30kV calculated by the earth mat analysis software.

The author, in early days of exposure to such calculations, also made similar mistakes. His mistakes were associated with calculation of EPR in an 11kV underground cable network. The author had fault analysis software (PSSU) which had the facility to calculate high impedance phase to earth fault currents. He used this facility and calculated earth fault currents and EPR in a cable network and it was a mistake.

The fault current calculated by PSSU was equal to the current in the faulty phase conductor and there was no way of finding out the proportions that returned to the source via the cable sheath and the general mass of earth. It was during these calculations the author realised that special considerations are required to calculate EPR in a cable network.

The fault current analysis of a 3-phase 3-wire overhead line distribution network is relatively simple compared to that of an underground cable network, or a combination of overhead and underground networks.

In this paper, a method of calculating fault current split (between cable sheath and the general mass of earth), and the EPR and the transferred earth potential rise (TEPR) for the phase to earth faults in the overhead and the underground three core cable power distribution network will be presented.