residual fundamental current will however be significant. The inrush current of thetransformer in service before the parallel transformer energizing, will be a littledelayed compared to the first transformer. Therefore, we will have high 2nd harmoniccurrent initially. After a short period this current will however be small and the normal2nd harmonic blocking will reset.en05000136_ansi.vsdPower System51N51NANSI05000136 V1 EN-USFigure 68: Application for parallel transformer inrush current logicIf the BlkParTransf function is activated, the 2nd harmonic restrain signal will latch aslong as the residual current measured by the relay is larger than a selected step currentlevel. Assume that step 4 is chosen to be the most sensitive step of the four stepresidual overcurrent protection function EF4PTOC (51N_67N). The harmonic restrainblocking is enabled for this step. Also the same current setting as this step is chosen forthe blocking at parallel transformer energizing.The settings for the parallel transformer logic are described below.BlkParTransf: This is used to Enable blocking at energising of parallel transformers.Use_PUValue: Gives which current level should be used for the activation of theblocking signal. This is given as one of the settings of the steps: Step 1/2/3/4.Normally, the step having the lowest operation current level should be set.8.4.3.4 Switch onto fault logic M15282-106 v4In case of energizing a faulty object there is a risk of having a long fault clearancetime, if the fault current is too small to give fast operation of the protection. The switchon to fault function can be activated from auxiliary signals from the circuit breaker,either the close command or the open/close position (change of position).This logic can be used to issue a fast trip if one breaker pole does not close properly ata manual or automatic closing.SOTF and under time are similar functions to achieve fast clearance at asymmetricalclosing based on requirements from different utilities.Section 8 1MRK 511 401-UUS ACurrent protection174 Bay control REC670 2.2 ANSIApplication manual
