P12x/EN T/Eb6 - Schneider - error

P12x/EN AP/Eb6Page 18/78Application GuideMiCOM P120/P121/P122/P1236. RESTRICTED EARTH FAULTMiCOM P120, P121, P122 and P123 provide Restricted Earth Fault protection. It should benoted that:The algorithms implemented in P120 and P121 for the first and second thresholds (I>, I0>and I>>, I0>>) are similar to the ones implemented in the P122 and P123 for the samethresholds. However the algorithm of the third threshold (I>>> and I0>>>) of P120 and P121is different to the one of P122 and P123.In fact, the algorithm of the third threshold of the P122 and P123 is based on a currentsample base in addition to the Discrete Fourier Transformation base. This implementationallows to trip faster on highly saturated current signals. The third threshold of P120 andP121- as it is for the first and second threshold of P120, P121, P122 and P123 products - isbased on the Fourier transformation;This explains the outstanding results obtained by the third threshold of P122 and P123compared to the other thresholds regarding the high impedance restricted earth faultapplication. So for:• P122 and P123: The user can use all the thresholds for REF application. Note that theresults of the third threshold will be greater due to the fact that a sample basealgorithm is used.• P120 and P121: The user can use all the threshold for REF application. The results ofthe third threshold will be similar to one of the first and second threshold (since all thethresholds are based on Discrete Fourier Transformation).NOTE:For P122 and P123, the maximum internal fault level for the thirdthreshold (for the 0.002 to 1In range) must not exceed 20In.6.1 IntroductionThe restricted earth fault relay is a high impedance differential scheme which balances zerosequence current flowing in the transformer neutral against zero sequence current flowing inthe transformer phase windings. Any unbalance for in-zone fault will result in an increasingvoltage on the CT secondary and thus will activate the REF protection.This scheme is very sensitive and can then protect against low levels of fault current inresistance grounded systems where the earthing impedance and the fault voltage limit thefault current.In addition, this scheme can be used in a solidly grounded system. It provides a moresensitive protection, even though the overall differential scheme provides a protection forfaults over most of the windings.The high impedance differential technique ensures that the impedance of the circuit is ofsufficiently high impedance such that the differential voltage that may occur under externalfault conditions is lower than the voltage required to drive setting current through the relay.This ensures stability against external fault conditions and then the relay will operate only forfaults occurring inside the protected zone.