Trip the breaker that removes every source feeding the relay’s protected zone. Transformer differential, ground differential or REF, Buchholz, and pressure protection normally trip the HV breaker and intertrip the LV breaker; LV feeder-side overcurrent and earth-fault protection normally trip the LV breaker alone. HV overcurrent serves as coordinated backup, so it trips the HV breaker and commonly intertrips the LV breaker. Thermal protection needs an explicit design decision because some schemes unload the transformer through the LV breaker while others isolate both sides.
Operating-Element Interpretation
The number that matters is the measured quantity relative to its pickup or operating characteristic: current for overcurrent elements, differential current for zone protection, and temperature or pressure for transformer condition devices. A relay target identifies the element that operated, but breaker selection also depends on where its current transformers and sensing devices define the protected zone.
Start with the relay event record or mechanical target, then trace the element’s trip contact through any lockout or intertrip relay to the breaker trip coils. Compare the relay operate time, auxiliary-relay time, and breaker opening time. An “instantaneous” overcurrent element is not physically instantaneous: typical electromechanical and digital relay response can be in the 1–2 cycle range, and total clearing time also includes circuit-breaker mechanical operation.
Digital relays must sample the waveform, estimate magnitude, filter harmonics and DC offset, process the trip decision, and energize an output contact. Some can intentionally reproduce an older electromechanical delay to preserve an established coordination scheme.
Protection-Zone Mechanism
The transformer zone normally lies between the HV-side and LV-side current-transformer boundaries. A fault inside that zone must be disconnected from both systems because opening only the LV breaker can leave the transformer energized from the HV source. The normal internal-fault action is therefore an HV trip plus an LV intertrip.
Differential protection compares currents entering and leaving the zone after accounting for transformer ratio and connection effects. Ground differential or restricted earth-fault protection covers a defined ground-fault zone determined by phase and neutral current-transformer placement. REF in its usual form requires a usable grounded winding neutral; it is not applied to a delta winding in the same manner.
A fault beyond the LV breaker lies outside the transformer differential zone. LV overcurrent, instantaneous overcurrent, and earth-fault elements should clear that fault by opening the LV breaker first. HV overcurrent remains backup for an LV relay, LV breaker, trip circuit, or downstream-device failure.
Quantities, Limits, and Trip Destinations
| Protection element | Quantity or condition | Normal trip destination | Where to read the deciding value |
|---|---|---|---|
| Differential | Operate current exceeds the biased differential characteristic inside the CT-defined zone | HV breaker plus LV intertrip | Relay event record, differential characteristic, CT ratio and polarity data |
| Ground differential or REF | Residual or neutral differential quantity exceeds its setting within the ground-fault zone | HV breaker plus LV intertrip | Element target, neutral arrangement, phase and neutral CT drawings |
| LV overcurrent or instantaneous overcurrent | LV current exceeds pickup, with or without intentional delay | LV breaker alone for an external LV fault | LV relay pickup, time curve, event current, and breaker record |
| LV earth fault | LV residual or neutral current exceeds pickup | LV breaker alone | LV grounding diagram, earth-fault setting, and event record |
| HV overcurrent, instantaneous overcurrent, or earth fault | HV-side measured quantity exceeds its coordinated characteristic | HV breaker; commonly LV intertrip | HV relay settings, coordination study, and trip matrix |
| Buchholz or pressure device | Internal gas accumulation, oil movement, or pressure condition reaches the device operating point | HV breaker plus LV intertrip | Device indication, wiring diagram, lockout target, and breaker status |
| Oil or winding temperature | Thermal condition reaches the alarm or trip threshold | Scheme-dependent: LV unloading trip or complete isolation | Temperature-device setting, control narrative, and trip matrix |
Overload and overcurrent are different conditions. Overcurrent protection responds to measured current and its time characteristic; thermal devices respond to accumulated heating or a temperature proxy. This is heat, not logic: the selected breaker action must prevent continued damaging thermal input while matching the plant’s isolation philosophy.
Breaker Trip-Logic Procedure
- Draw the transformer, both breakers, all sources, winding grounding, current transformers, neutral current transformer, and transformer-mounted devices on one protection-zone diagram.
- Mark each relay element’s measurement boundary. Classify it as internal-zone protection, LV external-fault protection, HV backup protection, or transformer condition protection.
- Assign internal-zone elements to the HV trip path and the LV intertrip path. Include differential, applicable ground differential or REF, Buchholz, and pressure devices.
- Assign LV overcurrent, LV instantaneous overcurrent, and LV earth-fault elements to the LV breaker when they protect faults beyond that breaker.
- Assign HV overcurrent and earth-fault elements as backup trips to the HV breaker. Add the LV intertrip where the operating philosophy requires complete transformer isolation.
- Coordinate time-overcurrent elements so the closest downstream device clears first. Check the entire relay and breaker clearing sequence, not just the relay curve.
- Choose the thermal action explicitly. Use an LV trip when removing load is the intended response; use full isolation when the condition or equipment design calls for de-energization.
- Document every element, output contact, lockout function, intertrip path, breaker trip coil, and breaker auxiliary-status input in a cause-and-effect matrix.
Fault-Scenario Verification
Test the logic by scenario before energization or after any settings, wiring, relay, or breaker change. For an internal phase fault, confirm that differential operation commands the HV breaker and LV intertrip. For an internal ground fault within the applicable REF or ground-differential zone, confirm the same result.
For an LV bus or feeder fault beyond the transformer zone, verify that the LV protection operates first and that the HV backup delay preserves selectivity. Then simulate failure of the LV trip path and verify that the HV backup clears the source according to the approved coordination study.
Record relay pickup, relay operate time, output-contact operation, trip-coil energization, and breaker auxiliary-contact transition. The two breakers can receive commands together without opening at exactly the same instant because their auxiliary relays, trip coils, mechanisms, and interrupting times differ. Confirm the final state: both breakers open for an internal fault, but only the LV breaker open for a normally cleared downstream fault.
Recurring Coordination Pitfalls
Delta-star transformer connections change how ground faults appear across the transformer. A ground fault on an earthed-star LV winding can appear as overcurrent in two phases of a delta primary, so the HV earth-fault element does not necessarily see the same quantity as the LV earth-fault element. Review winding connections and sequence-current paths before treating the two elements as equivalent backup.
Another recurring error is treating instantaneous pickup as zero clearing time. Relay response may take 1–2 cycles, followed by output-relay and breaker operating time. Use recorded oscillography or event timing and the breaker’s documented operating data when checking total clearing time.
Relay replacement can also disturb coordination. A microprocessor relay may operate differently from an electromechanical unit unless its characteristic and intentional delay are configured to reproduce the original scheme. Compare pickup, curve shape, instantaneous behavior, filtering, output delay, and total clearing time rather than transferring only the headline settings.
Temperature-device action is not universal. Resolve conflicting expectations by tracing the actual wiring and approved cause-and-effect matrix: an LV-only trip unloads the transformer but leaves it energized, while an HV trip with LV intertrip isolates it.
Frequently Asked Questions
How do I decide whether a transformer relay trips the HV or LV breaker?
Locate the relay’s protected zone. Internal-zone protection trips the HV breaker and intertrips the LV breaker; protection for faults beyond the LV breaker normally trips the LV breaker alone.
How do I trip breakers from transformer differential protection?
Route the differential trip to the HV breaker and the LV intertrip path so neither side can continue feeding an internal transformer fault. Verify both breaker auxiliary contacts reach the open state during the scheme test.
How do I coordinate HV and LV overcurrent relays?
Set the LV device to clear downstream faults first and use the HV element as delayed backup. Compare relay operating curves plus output and breaker times across the applicable fault-current range.
How do I verify an instantaneous relay trip time?
Measure from fault-current application to relay output, then from output to breaker opening. A typical relay response can be 1–2 cycles, but the event record and breaker operating data decide the actual total clearing time.
When should I stop testing and contact official support?
Stop when the drawings, relay settings, winding connection, CT polarity, trip matrix, and observed breaker action disagree, or when a test would energize an unverified trip circuit. Keep the transformer out of service until the protection engineer resolves the discrepancy; escalate unresolved relay behavior through the manufacturer’s official technical-support channel with settings, event records, oscillography, wiring drawings, and test results.