WORST FAULT TYPE BASIC INFORMATION (GROUND POTENTIAL RISE CONSIDERATION

Basically, three types of faults should be investigated:

a) Line-to-ground faults (L-G). These are predominant in terms of frequency of occurrence.  Zero sequence and positive-sequence currents will be required.

In practice, GPR is a function of zero sequence currents only, but positive-sequence currents are required to determine magnitudes of the individual zero-sequence currents ßowing in each phase of the faulted circuit.

b) Double line-to-ground faults (2L-G). These are statistically less frequent than L-G faults but could produce zero-sequence currents far exceeding those caused by L-G faults.

Theoretically, this is because of different connections of sequence networks during these faults. For an L-G fault, the positive-sequence, negative-sequence, and zero-sequence networks are connected in series and driven by the prefault voltage source; whereas, for a 2L-G fault, positive-sequence impedance is connected in series with the parallel combination of zero-sequence and negative-sequence impedances, with less overall impedance in the path of the fault current.

[For instance, many high MVA autotransformers may be added to power stations. These could have their primary-to-secondary, or primary-to-tertiary, zero-sequence reactance ratios so high that their primary current is small compared with the tertiary (ground) current.

In addition to this, if more such transformers are added to the station, the resulting tertiary currents will be very large due to further paralleling of reactances.]

c) Three-phase faults. These are statistically less frequent than L-G and 2L-G faults. Three-phase faults produce positive sequence currents, and detailed calculations are required to determine magnitudes of the individual zero-sequence currents ßowing in each phase of the faulted circuit.

If X1 and X0 are positive-sequence and zero-sequence reactances, respectively, of the system impedance at the point of fault and X1 is less than X0, the 2L-G fault will result in higher zero sequence fault currents, often twice as high as the L-G fault currents calculated at the same fault location.

The GPR produced by 2LG faults is not normally considered, due to its low probability. For an overview of the frequency of occurrence of different types of faults as a function of voltage levels on which they occur and other parameters.