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Protection Riddle No.76 - Neutral CT rating
Rating of the CT provided in the neutral of the transformer/generator for the earth fault protection is 1/5 of the rated current of the transformer/generator. How this 1/5 is arived? why?
Author : Deepa
Sat, August 14th, 2010 - 14:07
According to IEEE 242, when the generator neutral is grounded through an impedance limiting the maximum ground-fault current to a value less than the primary rating of the CTs, a differential ground fault scheme (Device 87G) should be considered. A differential ground-fault scheme, however, is capable of detecting faults to within 10% from the generator neutral. A neutral CT, separate from the ground overcurrent relay CT, may be necessary for maximum relay sensitivity. The ground differential CT should have a primary current rating of 10% to 50% of the neutral resistor-current rating.

The neutral-CT-turns ratio and tap setting of the overcurrent relay should be selected to provide an operating current of 5 to 10 times its pickup setting for a bolted line-to-ground fault at the generator line terminals. The pickup setting should be at least equal to and preferably greater than the pickup setting of downstream ground overcurrent devices for selectivity between those relays. The pickup setting should also be set above the anticipated level of any harmonic current flowing in the neutral during normal conditions. 
Author : Hamid - From: Iran- Firouzabad in Fars
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