harmonic component. This component can be used to create a restrain signal toprevent this unwanted function.At current transformer saturation a false residual current can be measured by theprotection. Here the 2nd harmonic restrain can prevent unwanted operation as well.2ndHarmStab: The rate of 2nd harmonic current content for activation of the 2ndharmonic restrain signal. The setting is given in % of the fundamental frequencyresidual current.8.4.3.3 Parallel transformer inrush current logic M15282-97 v6In case of parallel transformers there is a risk of sympathetic inrush current. If oneof the transformers is in operation, and the parallel transformer is switched in, theasymmetric inrush current of the switched-in transformer will cause partialsaturation of the transformer already in service. This is called transferredsaturation. The 2nd harmonic of the inrush currents of the two transformers will bein phase opposition. The summation of the two currents will thus give a small 2ndharmonic current. The residual fundamental current will however be significant.The inrush current of the transformer in service before the parallel transformerenergizing, will be a little delayed compared to the first transformer. Therefore, wewill have high 2nd harmonic current initially. After a short period this current willhowever be small and the normal 2nd harmonic blocking will reset.en05000136.vsdPower SystemIN>IN>IEC05000136 V1 EN-USFigure 82: Application for parallel transformer inrush current logicIf the BlkParTransf function is activated, the 2nd harmonic restrain signal will latchas long as the residual current measured by the relay is larger than a selected stepcurrent level. Assume that step 4 is chosen to be the most sensitive step of the fourstep residual overcurrent protection function EF4PTOC. The harmonic restrainblocking is enabled for this step. Also the same current setting as this step is chosenfor the blocking at parallel transformer energizing.The settings for the parallel transformer logic are described below.BlkParTransf: This is used to On blocking at energising of parallel transformers.1MRK 505 393-UEN B Section 8Current protectionLine differential protection RED650 2.2 IEC 171Application manual
