of the two transformers will be in phase opposition. The summation of the two currents willthus give a small 2nd harmonic current. The residual fundamental current will however besignificant. The inrush current of the transformer in service before the parallel transformerenergizing, will be a little delayed compared to the first transformer. Therefore we will havehigh 2nd harmonic current initially. After a short period this current will however be small andthe normal 2nd harmonic blocking will reset.en05000136.vsdPower SystemIN>IN>IEC05000136 V1 EN-USFigure 49: Application for parallel transformer inrush current logicIf theBlkParTransf function is activated the 2nd harmonic restrain signal will latch as long asthe residual current measured by the relay is larger than a selected step current level. Assumethat step 4 is chosen to be the most sensitive step of the four step residual overcurrentprotection function EF4PTOC. The harmonic restrain blocking is enabled for this step. Also thesame current setting as this step is chosen for the blocking at parallel transformer energizing.Below the settings for the parallel transformer logic are described.UseStartValue: Gives which current level that should be used for activation of the blockingsignal. This is given as one of the settings of the steps: Step 1/2/3/4. Normally the step havingthe lowest operation current level should be set.BlkParTransf: This parameter can be setOff/On, the parallel transformer logic.7.4.3.5 Switch onto fault logic M15282-106 v3In case of energizing a faulty object there is a risk of having a long fault clearance time, if thefault current is too small to give fast operation of the protection. The switch on to faultfunction can be activated from auxiliary signals from the circuit breaker, either the closecommand or the open/close position (change of position).This logic can be used to issue fast trip if one breaker pole does not close properly at a manualor automatic closing.SOTF and Under Time are similar functions to achieve fast clearance at asymmetrical closingbased on requirements from different utilities.The function is divided into two parts. The SOTF function will give operation from step 2 or 3during a set time after change in the position of the circuit breaker. The SOTF function has aset time delay. The Under Time function, which has 2nd harmonic restrain blocking, will giveoperation from step 4. The 2nd harmonic restrain will prevent unwanted function in case oftransformer inrush current. The Under Time function has a set time delay.Below the settings for switch on to fault logics are described.SOTF operation mode: This parameter can be set:Off/SOTF/Under Time/SOTF+Under Time.1MRK 504 158-UEN A Section 7Current protection107Application manual
