Nuisance 87T Trips After a CT Replacement on a 5 MVA Transformer
A 5 MVA, 26.4/4.16 kV, Dyn1 transformer feeds a 4.16 kV switchgear lineup. The 87T relay uses 600/5 CTs on the 4.16 kV side and 200/5 CTs on the 26.4 kV side; the relay's ratio and vector-group compensation settings were verified as correct. During a recent protection upgrade the 26.4 kV CT was replaced with a 5P20, 15 VA unit and its leads were rerouted to a new junction box, producing a 300 m one-way run in 4 mm² copper. Relay burden is 0.05 Ω, and the CT's knee-point test data match its nameplate. Since the upgrade the transformer has tripped on all three energizations, each 2–3 cycles after the vacuum breaker closed. The latest record shows phase B differential current 5.8 pu against 6.2 pu restraint, second-harmonic content 9%, fifth harmonic 3%, and a visibly DC-offset high-side current. Winding resistance, insulation resistance and turns ratio are normal; last year's DGA was clean. The contractor has asked to raise the unrestrained pickup to 15 pu and lower second-harmonic blocking to 8%.
1. Using the data, decide whether the trips indicate an internal transformer fault or a CT performance problem. Support the conclusion with a burden calculation and an interpretation of the harmonic record.
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Conclusion: CT saturation, not an internal fault.
The arithmetic is decisive. A 5P20, 15 VA, 5 A CT has a rated burden of 15/5² = 0.6 Ω. The actual secondary loop is 600 m of 4 mm² copper (300 m one-way) plus the relay:
- R leads = 0.0175 × 600 / 4 = 2.63 Ω
- Total = 2.63 + 0.05 = 2.68 Ω, i.e. 4.5 times the rated burden.
The CT therefore reaches its accuracy limit at about 20 × (0.6/2.68) ≈ 4.5 × rated current, not 20 ×. Energization inrush with a DC offset readily exceeds that, so the high-side CT saturates and under-reproduces primary current - which the relay reads as differential current.
Saturation explains every feature of the record: a large spurious differential (5.8 pu against 6.2 pu restraint, so the slope-2 element operated, not the 10 pu unrestrained element), a degraded second-harmonic ratio of 9% instead of the 15–60% typical of genuine inrush (saturation clips the waveform), and the visible DC offset. Against an internal fault: normal winding resistance, insulation resistance and ratio, clean DGA, and a timing correlation with the CT replacement rather than with load. A real turn-to-turn fault would also tend to persist and grow across energizations, not repeat at the same instant.
2. Evaluate the contractor's proposal to set the unrestrained element to 15 pu and second-harmonic blocking to 8%. State what you would do instead.
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The proposal treats the symptom and weakens protection.
- 15 pu unrestrained pickup: this element exists to clear high-current internal faults fast, before the percentage slope can respond. A close-in 26.4 kV fault can produce 8–12 pu differential current; at 15 pu the relay may not trip instantaneously at all, and for a winding fault involving only part of the winding it may never operate. That is a direct reduction in protection for the fault most likely to destroy the transformer.
- 8% second-harmonic blocking: lowering the threshold does suppress the symptom (the relay becomes easier to block), but it does not cure the saturation, it leaves the differential measurement corrupted for every through-fault, and it increases the chance that a genuine internal fault accompanied by CT saturation is blocked. Harmonic blocking that is too permissive is a known failure mode.
Fix the measurement circuit instead: replace the 4 mm² leads with 10 mm² (R = 1.05 Ω, total ≈ 1.10 Ω ≈ 27.5 VA at 5 A) and specify a 5P20, 30 VA CT, or move to a 1 A secondary where the same 300 m run costs only about 1 VA of lead burden, or relocate the relay to shorten the run. Then verify by primary injection at the calculated maximum through-fault current with the real connected burden, and confirm polarity and ratio. Restore second-harmonic blocking to the manufacturer's recommended value (typically 15%) and keep the unrestrained element at its original setting.
3. List the checks and documentation you would require before the next energization, and state your action if the transformer trips again after the correction.
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Before re-energization:
- Prove no internal damage from the three trips - DGA (repeat after a few days of settling), winding resistance, insulation resistance and polarization index, and sweep frequency response against any baseline.
- Complete the CT circuit correction, then perform a primary injection test at the calculated through-fault level with the actual connected burden, recording composite error; verify polarity, ratio, and the relay's ratio/vector-group compensation.
- Re-verify the full 87T setting file: slope 1 and 2, breakpoint, unrestrained pickup, harmonic blocking, and any DC-offset/instantaneous elements.
- Stage the energization with the relay in service and a technician capturing the event record; if a temporary short-time delay on the unrestrained element is used to ride through inrush, document it as time-limited with an owner, an expiry date, and a return-to-normal procedure.
- Issue a signed commissioning record with test values, not just a pass/fail statement.
If it trips again after the correction: do not re-energize on the same settings. Treat the event as a possible internal fault until disproved - repeat DGA and electrical tests, review the new event record against the pre-trip baseline, and escalate to an internal inspection or a low-voltage excitation test. Repeated energization into an unresolved fault is how a repairable transformer becomes a scrapped one.