Power Systems Lab · 18 modules · all computed in your browser
Inrush & 2nd-harmonic (87T)
Transformer energization flux and current, harmonic restraint, per-phase versus cross-blocking.
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Power Systems Lab · 18 modules · all computed in your browser
Transformer energization flux and current, harmonic restraint, per-phase versus cross-blocking.
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Energization
87T result
restrained / blocked
No phase satisfied operate AND not-blocked for a quarter cycle.
Inrush currents (pu of rated)
2nd-harmonic ratio of the differential currents (%)
Learn it
When a transformer is energized, the core flux has to start from its remanent value, but the applied voltage demands a flux that is offset by up to a full peak. The core is driven deep into saturation for part of every cycle and draws large, one-sided, peaky current pulses from one side only. To a differential relay that looks exactly like an internal fault. The pulses are rich in 2nd harmonic, so relays measure I2/I1 and restrain or block. Modern low-loss steels saturate more sharply and remanence can be high, so the 2nd harmonic can drop below 15 % in one phase. That is why cross-blocking, 4th/5th harmonic or waveshape methods exist.
Quiz · 0/3
1. Which closing angle gives the worst inrush with no remanence?
2. Why add cross-blocking?
3. What is the cost of cross-blocking?