External faults happen ten to hundred times more often than internal ones as far as the powertransformers are concerned. If a disturbance is detected and the internal/external faultdiscriminator characterizes this fault as an external fault, the conventional additional criteriaare posed on the differential algorithm before its trip is allowed. This assures high algorithmstability during external faults. However, at the same time the differential function is stillcapable of tripping quickly for evolving faults.The principle of the internal/external fault discriminator can be extended to autotransformersand transformers with three windings. If all three windings are connected to their respectivenetworks then three directional comparisons are made, but only two comparisons arenecessary in order to positively determine the position of the fault with respect to theprotected zone. The directional comparisons which are possible, are: W1 - (W2+W3) and W2 -(W1+W3). The rule applied by the internal/external fault discriminator in case of three-windingpower transformers is:• If any comparison indicates an internal fault, then it is an internal fault.• If any comparison indicates an external fault, then it is an external faultIf one of the windings is not connected, the algorithm automatically reduces to the two-winding version. Nevertheless, the whole power transformer is protected, including the non-connected winding.6.1.2.7 Differential current alarm M15266-337 v6Differential protection continuously monitors the level of the fundamental frequencydifferential currents and gives an alarm if the pre-set value is simultaneously exceeded in allthree phases. This feature can be used to monitor the integrity of on-load tap-changercompensation within the differential function. The threshold for the alarm pickup level isdefined by setting parameterIDiffAlarm. This threshold should be typically set in such way toobtain operation when on-load tap-changer measured value within differential function differsfor more than two steps from the actual on-load tap-changer position. To obtain suchoperation set parameterIDiffAlarm equal to two times the on-load tap-changer step size (Forexample, typical setting value is 5% to 10% of base current). Set the time delay defined byparametertAlarmDelay two times longer than the on-load tap-changer mechanical operatingtime (For example, typical setting value 10s).6.1.2.8 Open CT detection M15266-340 v5The Transformer differential function has a built-in, advanced open CT detection feature. Thisfeature can block the unexpected operation created by the Transformer differential function incase of open CT secondary circuit under normal load condition. An alarm signal can also beissued to station operational personnel to make remedy action once the open CT condition isdetected.The following setting parameters are related to this feature:• Setting parameterOpenCTEnable enables/disables this feature• Setting parametertOCTAlarmDelay defines the time delay after which the alarm signal willbe given• Setting parametertOCTReset defines the time delay after which the open CT conditionwill reset once the defective CT circuits have been rectified• Once the open CT condition has been detected, then all the differential protectionfunctions are blocked except the unrestraint (instantaneous) differential protectionThe outputs of open CT condition related parameters are listed below:1MRK 504 158-UEN A Section 6Differential protection69Application manual
