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Protection Riddle No.112 - Knee point voltage for motor differential protection
how to determine the knee point voltage of ct  for motor differential protection protection?

i require formula for Vk & explanation.

please do the needful.
Author : Aditya
Mon, April 7th, 2014 - 17:43
Generally you must refer to relay manufacturer recommendation. The " art & sience of protective relaying" reference book has recommended some instruction as below:
High-speed percentage-differential relays having variable ratio-or percent-slope characteristics are preferred. At low values of through current, the slope is about 5%, increasing to well over 50% at the high values of through current existing during external faults. This characteristic permits the application of sensitive high-speed relaying equipment using conventional current transformers, with no danger of undesired tripping because of transient inaccuracies in the CTs. To a certain extent, poorer CTs may be used-or higher burdens may be applied-than with fixed-percent-slope relays.
Two different operating principles are employed to obtain the variable characteristic. In both, saturation of the operating element is responsible for a certain amount of increase in the percent slope. In one equipment, saturation alone causes the slope to increase to about 20%; further increase is caused by the effect on the relay response of angular differences between the operating and restraining currents that occur owing to CT errors at high values of external short-circuit current. The net effect of both saturation and phase angle is to increase the slope to more than 50%.
The other equipmentl4 obtains a slope greater than 50% for large values of through current entirely by saturation of the operating element. A principle called "product restraint" is used to assure operation for internal short circuits. Product restraint provides restraint sufficient to overcome the effect of any CT errors for external short circuits; for internal short circuits when the system supplies very large currents to a fault, there is no restraint.
Author : Hamid - From: Iran
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