Short circuit means that the power supply is short-circuited directly without passing through the load. But the winding of the transformer for 50HZ power frequency AC, the impedance of the winding is very large, a transformer as a power supply, another transformer as a load, as a transformer of the power supply through a large impedance of another variable winding form a loop, this is called short circuit?
Large and small ground current systems refer to the large or small current flowing through the ground point when single phase grounding occurs. If the neutral point of the transformer is directly grounded, when the occurrence of single-phase grounding, will constitute a loop, at the ground point through a large short-circuit current, it is called the high current grounding system. If the neutral point of the transformer is not grounded, when the single-phase grounding occurs, it does not constitute a fault loop. Only the capacitor current flowing through the system to the ground at the ground point is small, so it is called the small current grounding system.
Large current grounding system and small current grounding system have their own advantages and disadvantages, it can not be said that large current grounding system will lead to more accidents.
Large current grounding system for neutral point grounding directly, so than the small current grounding system has advantages in terms of overvoltage, equipment type selection by phase voltage, suitable for high voltage grade if take the small current grounding system will greatly increase the cost of insulation, 110 kv voltage class above is commonly so large current grounding system. The disadvantage is that when the system has a high fault probability of single-phase grounding, the protection will remove the fault, so the power supply reliability is reduced.
Small current grounding system is on the contrary, the disadvantage is that there may be a higher level of overvoltage in operation, equipment selection should be considered according to the line voltage, so it can only be used for low voltage level even if the insulation is considered according to the line voltage cost will not increase a lot, 66KV and below are generally used in small current grounding mode. The advantage is that when single-phase grounding occurs, the fault point only flows through a small system capacitive current, which does not damage the symmetry of the line voltage, and can continue to run (generally, the grounding point should not run for more than 2 hours), and the power supply reliability is high.
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