Theoretically, LV faults can be handled in
precisely the same manner as the HV faults. All differences between HV and LV
side construction (transformer connections, line conductors, length, pole
footing impedance, etc.) will reflect in the calculation of zero- and
positive-sequence LV L-G and balanced fault currents.

As opposed to HV systems, which usually
carry overhead ground wires, some LV lines, delta or wyeconnected, carry no
neutrals. When neutrals are present on LV systems, LV bus fault calculations
follow the same method, and LV line faults will be the same as HV line fault
calculations.

When neutrals are not present on LV system,
both LV bus and line-fault GPR can be calculated using the simplified method.
For a LV bus fault, ZL will consist of the parallel combination of
impedances-to-remote earth of all HV overhead ground wire-tower ladder networks
only.

The rest of the method still applies.

For LV line faults, assuming a radial LV
line, a single fault infeed can be assumed if no generation exists on the load
side of the line. This assumption is correct for most cases, but it should be
pointed out that, for instance, a large induction motor can become a
zero-sequence current generator at the instant of the fault, due to the inertia
of the rotor and the mechanical load.

If this can be neglected, the worst fault
then occurs outside and near the station. The HV bus fault is modified by
inserting, in series with Zs, the self-impedance of the faulted phase
conductor, and inserting, in series with Za, a faulted pole footing impedance.

The methods described in this subclause
should be used for hand calculation and estimation purposes only. For a more
complicated network, that is, the network with a high number of ROWs, circuits,
transformers, ground sources, and short lines such as could be found between
generating and switching stations, hand calculations cannot be used for either
exact or approximate solutions; a computer program shall be used.

In such a program, the theoretical approach
should include the effect of other forms of grounding, such as rails, pipes,
etc.; the effect of the length of lines; the effect of positive-sequence
current phase shift in certain transformer windings; etc.

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