Neutral earthing of three-phase systems, with particular reference to large power stations

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The paper reviews the various methods of earthing the neutrals of power systems and gives recommendations illustrating the latter by an example.Methods of analyses are considered, and the simplicity obtained by using symmetrical-component circuits is shown; attention is confined mainly to the zero-sequence circuit which can be varied to suit the earthing condition under consideration. The same circuits can be used also for the determination of restriking voltages.With an insulated neutral the effect of cyclic restriking in building up large neutral displacement voltages is shown.Reactance earthing is considered from a general aspect, particular attention being given to the effects of an unbalanced system and mutual reactance when using arc-suppression coils. The cause of neutral inversion when earthing through 3-phase potential transformers is dealt with.Earthing, both resistance and direct, is considered, special attention being paid to multiple earthing. The necessity of limiting the earthfault current, and methods of reducing the circulation of third-harmonic currents are indicated.Consideration is also given to the secondary effects of the method of earthing the neutral, inter alia, on the interference with communication circuits and insulation stresses.Suggestions are made regarding the ratings of neutral resistors, reactors and circuit breakers.