Transformer Differential Tripping on Energization with a Parallel Unit in Service
A 132/11.5 kV, 20/26 MVA ONAN/ONAF transformer T1 at a grid station was returned to service after tap-changer maintenance. Its 87T relay uses 500/1 CTs on the 132 kV side and 2000/1 CTs on the 11.5 kV side, a 30% slope, and second-harmonic blocking set at 15%. T2, an identical unit, was already carrying 12 MVA. On the first three energization attempts T1 tripped in 40–60 ms, one phase at a time, with no load connected and no Buchholz or pressure-relief operation. The disturbance recorder shows a large DC offset and a 2nd-harmonic ratio of 9–12% on the operating phase; the CT secondary leads are 180 m of 4 mm² cable. Insulation resistance, winding resistance, ratio and SFRA results are normal, and oil DGA shows no acetylene. The station has no spare CTs, and the next attempt must be made within the maintenance window.
1. What is the most probable cause of the trips, and what evidence in the record supports or contradicts it? Distinguish clearly between magnetizing inrush and an internal fault.
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Most likely magnetizing inrush, probably sympathetic inrush from T2, with CT saturation defeating the 15% second-harmonic block. Evidence: trips occur only at energization, in 40–60 ms, one phase at a time, with no load and no Buchholz/pressure operation. Insulation, winding resistance, ratio and SFRA are normal and there is no acetylene. Inrush has a large DC offset and even harmonics. The measured 2nd-harmonic ratio of 9–12% is below the 15% setting, so blocking should operate - but CT saturation distorts the waveform and reduces the measured 2nd-harmonic ratio, and 180 m of 4 mm² leads add burden. An internal fault would normally show sustained differential current, oil/pressure operation, gas, or changed SFRA/ratio; none are present. A turn-to-turn fault can be intermittent, so do not rely on one DFR record: compare all three phases, check DC decay and 5th harmonic, and repeat with T2 de-energized. If the trip persists with T2 out and no load, treat it as internal until disproved.
2. Propose a technically defensible setting and hardware change to allow energization without creating an unacceptable protection gap. State the trade-off and how you would prove it by test.
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Defensible package: (1) reduce CT burden - 180 m of 4 mm² is roughly 1.3 Ω loop; use 6 mm² or a lower-burden relay input, and verify the CT class is adequate for the DC offset (PX or 5P20 with connected burden below rating). (2) Raise second-harmonic blocking to 20% and enable cross-blocking; add a 100–150 ms definite-time delay active only during energization, not a permanent delay. (3) Prefer harmonic restraint with waveform/gap detection to simple blocking, because blocking can slow internal-fault clearance. Trade-off: a higher 2nd-harmonic threshold and delay increase security but may delay clearing a genuine fault. Keep the delay ≤150 ms and verify the differential element still trips within the transformer withstand. Prove by secondary injection: inject fundamental plus 10–20% second harmonic with DC decay, and by primary injection with the transformer disconnected. Then monitor the first live energization with DFR and a temporary sensitive earth-fault element.
3. T2 remains in service and the maintenance window is closing. Would you de-energize T2 for the next attempt, and what would that prove? What would you not conclude from a successful energization?
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Sympathetic inrush is plausible because T2 was already loaded. When T1 is energized, the voltage dip offsets T2 core flux and T2 draws a unidirectional current; this can appear in T1 differential if CTs saturate. Trips on one phase at a time and only with T2 in service support that mechanism. De-energizing T2 for the first T1 energization is a valid diagnostic: if T1 holds, the mechanism is confirmed. Do not treat permanent de-energization of T2 as the fix. If T2 must stay in service, apply the CT-burden and setting changes above. Also check whether T1 and T2 are connected through a closed bus-tie that allows circulating inrush; if so, energize T1 from the other source or open the tie during energization. A successful energization with T2 out does not prove the 87T settings are correct for all future energizations, nor does it prove the CTs will not saturate at a different system voltage or remnant flux.