7.1.3.9 Open CT detectionThe Generator differential function has a built-in, advanced open CT detectionfeature. This feature can block the unexpected operation created by the Generatordifferential function in case of open CT secondary circuit under normal loadcondition. An alarm signal can also be issued to station operational personnel to makeremedy action once the open CT condition is detected.The following setting parameters are related to this feature:• Setting parameter OpenCTEnable enables/disables this feature• Setting parameter tOCTAlarmDelay defines the time delay after which the alarmsignal will be given• Setting parameter tOCTReset defines the time delay after which the open CTcondition will 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) differentialprotectionThe outputs of open CT condition related parameters are listed below:• OpenCT: Open CT detected• OpenCTAlarm: Alarm issued after the setting delay• OpenCTIN: Open CT in CT group inputs (1 for input 1 and 2 for input 2)• OpenCTPH: Open CT with phase information (1 for phase L1, 2 for phase L2, 3for phase L3)7.1.3.10 Switch onto fault featureThe Transformer differential function in the IED has a built-in, advanced switch ontofault feature. This feature can be enabled or disabled by the setting parameterSOTFMode. When SOTFMode = On this feature is enabled. It shall be noted that whenthis feature is enabled it is not possible to test the 2nd harmonic blocking feature bysimply injecting one current with superimposed second harmonic. In that case theswitch on to fault feature will operate and the differential protection will trip. Howeverfor a real inrush case the differential protection function will properly restrain fromoperation.For more information about the operating principles of the switch onto fault featureplease read the Technical Manual.7.1.4 Setting example7.1.4.1 IntroductionDifferential protection for power transformers has been used for decades. In order tocorrectly apply transformer differential protection proper compensation is needed for:Section 7 1MRK 502 071-UEN -Differential protection136 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062Application manual
