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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