FACT ON GROUND POTENTIAL RISE
What Is Ground Potential Rise?
GPR
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.
DC offset
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.
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