[san]

We have a 11kV/400V 4MVA trafo with 97kA short ckt rating. Can u suggest for the neutral grounding methodology keeping high shrt ckt rating of the system

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[Hamid]

Usage of Solidly Grounding in low voltage systems is normal practice. But in high level short circuit current case (e.g. 97 KA which mentioned); some consideration shall be applied in grounding method and other equipment selection. Close to the grounding points, the ground-fault currents are high, in some cases exceeding the three-phase short circuit currents. In a few instances higher interrupting capacity breakers may be required over that necessary for three-phase short-circuit interruption. The higher currents also produce more conductors burning. The greater currents result in lower positive-sequence voltages with a tendency toward a lower stability limit for line-to-ground faults. The higher earth currents may in some cases interfere with communication circuits. Most unfavorable influences from the above high-current phenomena have largely been removed, so far as system extensions are concerned, by the availability of the newer high-speed relays and circuit breakers. These comments apply particularly to such items as stability, conductor burning and communication circuit influence. The interrupting requirements of circuit breakers can be brought to equality with that for the three-phase fault condition by the addition of a moderate-sized grounding reactor where necessary. When the reduction in current is no more than this, the system will still retain the classification of

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