a transformer is energized with a more severe (minor faults cannot be discovered)internal fault (for example, forgotten grounding on transformer LV side for example,after a regular service). The feature is based on the waveform check. If a severeinternal fault exists, then, during energization the magnetic density in the iron corewill be low and high sinusoidal currents will flow from the very beginning. In this casethe waveform block algorithm removes all its three block signals in a very shortinterval of time. This quick reset of the waveblock criterion will temporarily disablethe second harmonic blocking feature of the differential protection function. Thisconsequently ensures fast operation of the transformer differential function for a switchonto a fault condition. It shall be noted that this feature is only active during initialpower transformer energizing, more exactly, under the first 50 ms. When the switchonto fault feature is disabled by the setting parameter SOTFMode, the waveblock andsecond harmonic blocking features work in parallel and are completely independentfrom each other.6.1.3.13 Logic diagramThe simplified internal logics, for transformer differential protection are shown in thefollowing figures.1MRK 502 043-UUS B Section 6Differential protection103Technical Manual
