Neutral point is not directly grounded system, single-phase fault grounding can continue to operate, but with a sound phase voltage rise, if the Lmin internal energy to remove the grounding fault, the voltage rise of the impact on the insulation is generally negligible.
However, according to the existing automatic device and operation level of our country, it is difficult to Lmin the feeder circuit with single-phase grounding fault.
The 6oko 35kv system in China is not directly grounded in neutral point. In the past, some engineering cables were selected according to the rated line voltage only, in practice some cable "relative" voltage is the insulation level of the rated phase voltage value, and the insulation breakdown accident caused by the source is repeated, which causes great loss.
The relative safety and reliability of the cable with high grade rated phase voltage is improved. In view of the design phase is difficult to anticipate the failure of the removal time, it is generally clear to the power supply system to enhance insulation, but to some industry systems using rated phase voltage after the operation of the practice is not a problem, can be allowed to discriminate.
As for the 133% phase voltage and 8h boundaries, with the issued SDJ26-89, Dl401-91, Sd289 88 standards, and the United States Astmde 470-81, IECl83 similar, but not the United States AEICCS5 82 conservative.
The generator circuit is important, the fault time is long, the cable length is limited, and the 173% phase voltage is suitable.
The maximum electric field strength in the cable insulating layer of HVDC transmission system depends not only on the external voltage, but also on the cable core load, and if the power transmission direction is changed in the operation, the internal electric field strength may increase with the polarity switching of the cable.
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