In case of 3 phase short circuit in a system the power fed into the system is

In case of 3 phase short circuit in a system, the power fed into the system is

Right Answer is:

Mostly Reactive


In case of 3 phase short circuit in a system, the power fed into the system is mostly Reactive.

Nature of power during Short circuit:

  • Under normal circumstances, the load current is mostly resistive with a high power factor.
  • Since the impedance of the reactor is mostly reactive with a very low power factor and the magnitude of current drawn by the load is within nominal values, the voltage drop of the reactor is very small.
  • In the event of a fault at the load, the fault current increases in magnitude to values several time nominal.
  • The fault current is mostly reactive since the fault bypasses the load ( the largest resistive part)
  • Now the entire transmission line is loaded with a high reactance because of the high frequency ( harmonics especially for unsymmetrical faults) and large transient current ( higher flux- larger the reactance) flowing.
  • The inductance is caused by supply line spacing and leakage inductance of the supply transformer. Sometimes the inductor is deliberately included to help in limiting the fault current.
  • So as a result, the fault current lags behind the supply voltage, at a lagging power factor. So, the power is partly active and partly reactive power flow.
  • Generally, the higher the fault capacity of a system, the more the reactive current is in proportion to active current so the lower the power factor of the fault current.
  • Additionally, fault currents are very inductive with a low power factor. Therefore, the reactor impedance presents a substantial portion of the overall system impedance and the value of the fault current is reduced.
  • Due to the voltage drop in the transmission line, the Capacitance effect decreases. So the fault current is the current drawn by the transmission line which makes this current mostly reactive.
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