because one or more of the fundamental frequency differential currents entered theoperate region on the operate - restrain characteristic. So, this protection is notindependent of the traditional restrained differential protection - it is activated afterthe first start signal has been placed.If the fault is positively recognized as internal, then the unrestrained negativesequence differential protection places its own trip request.If the bias current is higher than 110% of IBase of the power transformer winding W1,then any block signals by the harmonic and/or waveform blocking criteria areoverridden, and the differential protection operates quickly without any further delay.If the bias current is lower than 110% of IBase, the negative sequence differentialprotection is restrained by any harmonic block signal.This logic guarantees a fast disconnection of a faulty power transformer for any heavyfaults.If a fault is classified as external, the further analysis of the fault conditions is initiated.If all the instantaneous differential currents in phases where start signals have beenissued are free of harmonic pollution, then a (minor) internal fault, simultaneous witha predominant external fault can be suspected. If the differential current is above therestrain limit a trip will be issued.During external faults, major false differential currents can only exist when one ormore current transformers saturate. In this case, the false instantaneous differentialcurrents are polluted by higher harmonic components, the 2nd, the 5th and so on and thedifferential protection will block the trip operation based on the blocking criteria.Sensitive negative sequence based turn-to-turn fault protectionThe sensitive, negative sequence current based turn-to-turn fault protection detectsthe low level faults, which are not detected by the traditional differential protectionuntil they develop into more severe faults, including power transformer iron core. Thesensitive protection is independent from the traditional differential protection and is avery good complement to it. The essential part of this sensitive protection is theinternal/external fault discriminator. In order to be activated, the sensitive protectionrequires no start signal from the traditional power transformer biased differentialprotection. If magnitudes of HV and LV negative sequence current contributions areabove the set limit for IminNegSeq, then their relative positions are determined. If thedisturbance is characterized as an internal fault, then a separate trip request will beplaced. Any decision on the way to the final trip request must be confirmed severaltimes in succession in order to cope with eventual CT transients. This causes a shortadditional operating time delay due to this security count. For very low level turn-to-turn faults the overall response time of this protection is about 30ms. The sensitivenegative sequence differential protection is automatically deactivated if the biascurrent becomes higher than 150 % IBase. Further, this protection can always berestrained by any harmonic block signal. This because at rather low fault currents,which are to be detected by this protection, harmonic pollution is not likely.Section 6 1MRK 504 135-UEN ADifferential protection102Technical manual
