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Complications of Magnetizing Inrush |
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We now plan to show that even percentage differential protection scheme will misclassify the inrush current as fault current. This is because during inrush, secondary current is negligible (zero if secondary is open circuited), while primary current can be as high as 10 - 20 times the full load current. Thus some kind of restraint function is required to inhibit the pick up on inrush. Traditionally, this restraint is based upon second harmonic content in primary which discriminates a fault from the inrush condition. |
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Alternatives in numerical relaying also include voltage restraint used in integrated substation protection scheme and flux restraint scheme. Before, discussing these schemes further, we will review the origin of magnetizing inrush phenomenon. |
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Consider the circuit in fig 39.5. The switch is closed at t = 0. By Faraday's law, we have |
, with
and
