In short. Transformer protection combines electrical elements (overcurrent, earth fault, differential for larger units) with device-level protections (gas, pressure, temperature). The scheme depends on size, importance and whether the transformer connects to a generator.
Protections by type
| Protection | Detects |
|---|---|
| HV fuse or overcurrent | Short circuits and overloads |
| Earth fault / REF | Earth faults in windings. See REF |
| Differential | Internal winding faults on larger units. See differential |
| Buchholz relay (oil-filled with conservator) | Gas from faults, oil surge |
| Pressure relief device | Internal pressure rise |
| Winding and oil temperature | Overload and cooling failure |
| Overexcitation (volts per hertz) | High voltage or low frequency |
Inrush
Energising a transformer draws a high current for a short time, many times full load, rich in second harmonic. Overcurrent and differential settings need to ride through. Harmonic restraint is common in differential relays. When a generator energises a transformer, the large inrush can also depress generator voltage. See transformer sizing.
Generator step-up and distribution
- Distribution transformers: HV fuses or breaker with overcurrent, plus temperature alarm and trip.
- Generator step-up: differential, REF, overexcitation, backup overcurrent and sudden-pressure protection.
Design steps
- Establish transformer size, vector group, impedance and earthing.
- Pick protections by size and criticality.
- Set the overcurrent above inrush and below damage curves.
- Configure differential with ratio and vector group compensation and inrush restraint.
- Wire non-electrical devices to alarm and trip, and test.
Illustrative example
A 2.5 MVA, 11/0.433 kV transformer is protected by an HV breaker with overcurrent and earth-fault relays, a Buchholz relay for oil faults and winding temperature trip. The overcurrent instantaneous element is set above inrush at a multiple of full-load current, and the earth-fault relay is graded with the LV main.
Protection scheme by transformer size
| Size or role | Typical protection |
|---|---|
| Small distribution transformer | HV fuse or breaker with overcurrent and earth fault; thermal alarm |
| Medium transformer | Overcurrent, earth fault, REF, Buchholz or pressure relief, winding temperature |
| Large or critical transformer | Differential, REF, Buchholz, overfluxing, backup overcurrent |
Non-electrical protections
- Buchholz relay: detects gas accumulation and oil surge in oil-filled units.
- Pressure relief device: vents sudden internal pressure.
- Winding and oil temperature: alarm and trip stages.
- Oil level: low-level alarm.
Inrush and energisation
Energising can draw inrush of several times rated current. Overcurrent settings must ride through it, while differential relays use harmonic restraint. Sequence energisation on a generator carefully, since inrush also dips generator voltage. See generator sizing.
Overfluxing
High voltage or low frequency raises flux in the core, causing heating. A volts per hertz function protects generator step-up transformers and others at risk.
Setting steps
- Obtain the transformer rating, impedance, vector group and tap range.
- Calculate full-load, inrush and through-fault currents.
- Set overcurrent above load and inrush and below damage curve.
- Coordinate with upstream and downstream devices.
- Commission with injection tests.
Maintenance
Test relays, oil, bushings and cooling. Thermography and oil analysis can reveal ageing before faults. Keep a record of trips and alarms.
Common mistakes
- Instantaneous set below inrush.
- Non-electrical devices not wired to trip.
- No overexcitation protection on generator step-up units.
What to send us
Send transformer nameplate and network data. Send your application details. See also coordination.
Questions engineers ask
What is a Buchholz relay?
A gas-and-oil flow relay on oil-filled transformers that detects internal faults.
When is differential required?
For larger or critical transformers, as set by standards and owner practice.
Why do transformers trip on energisation?
Inrush may exceed settings. Harmonic restraint and settings above inrush address it.
Do dry-type transformers need different protection?
They use temperature sensors in windings instead of oil devices.
This guide gives general principles. Scheme selection needs project data.