Voltage restrained Over Current Protection : Normal IDMT Over current is not work – when a over current fault occurs, due to higher current levels, there would be a drop in terminal voltage. For the same fault impedance, the fault current will reduce (with respect to terminal voltage) to a level below the pickup setting. Consequently normal IDMT may not pick up. It is necessary to have a relay shoes pick up setting will automatically reduce in proportion to terminal voltage. Hence the over current protection must be voltage restrained. Two levels of Over current protection is required-low set and high set (for short circuit protection). Thermal Overload: This protection is a must – it monitors the thermal status of machine for currents between 105% to the low set 0/C level (normally 150%). Current Unbalance : Generators are expected to feed unbalanced loads – whose level has to be monitored. If the unbalance load exceeds 20%, it may cause over heating of t...
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