FACT ON GROUND POTENTIAL RISE
What Is Ground Potential Rise?
When a ground fault occurs, the zero-sequence fault current returns to the power system ground sources through the earth and also through alternate paths such as neutral conductors, unfaulted phases, overhead ground wires, messengers, counterpoises, and metallic cable shields. The ground sources are the grounded wye-connected windings of power transformers, generator grounds, shunt capacitors, frequency changers, etc.
The GPR is equal to the product of the station ground grid impedance and that portion of the total fault current that ßows through it. Also, the GPR is equal to the product of the alternate path impedance and that portion of the conductively coupled fault current that ßows through it. The volt-time area of GPR to be determined is given in volt-seconds for the duration of the fault.
Ground grid impedance
Since the station ground grid impedance to remote earth is needed to calculate the GPR, the ground grid impedance shall be obtained either by the calculation or measurement methods described in 4.3.
Ground fault studies
A study should be made of various ground faults in order to determine the one that produces the highest GPR and volt-second area . The station ground grid impedance, as well as power overhead ground wire and telecommunication grounding networks, tend to limit the fault current and should be included in the calculations.
Power system generators
In the ground fault study, the power system generators are usually represented by their subtransient reactances. As the time progresses until fault clearing, their reactances increase to their transient values and possibly to their steady-state synchronous reactances. This change can be neglected in most cases and the initial subtransient reactance retained. All signiÞcant impedances should be included, such as for transmission lines.
The initial magnitude of the dc offset should be calculated as a function of the voltage magnitude at the time the fault is initiated. The highest dc offset occurs when the change in current, from just before fault initiation to just after fault initiation, is maximum.
Since the alternating current cannot change state instantaneously due to the inductance of the circuit, initially the dc offset counter balances the change in alternating current.
The dc offset then decreases to zero at a rate determined from the effective reactance-impedance ratio of the power circuit at the fault. With a highly inductive circuit, the maximum dc offset will occur when the fault is initiated close to a voltage zero crossing, a condition that is most unlikely for faults resulting from insulation breakdown.
Also, a highly inductive circuit will have a prolonged dc offset. For the application of a multiplication factor for dc offset.
Ground potentials differ from magnetically induced voltages. The transient dc component (dc offset) of the ground fault current produces a proportional but decaying ground potential. Equation (24a), used to determine the instantaneous current considering both the symmetrical and dc offset components, is included.
For induced voltages, the dc component is of minor importance since the induced voltage varies as di/ dt. In HV networks when the fault impedance is negligible, the time constant varies; but the rate of decay of the dc component is usually within 5Ð40 ms and is determined by the effective power system X/R ratio.