A power transformer is the most expensive single item in a substation, and it can fail in more ways than almost any other apparatus: slowly from inside the oil, violently from a winding fault, suddenly from a tank fault, or gradually from overload and overvoltage. Each failure mode needs its own protection. This post groups transformer faults into four families and matches each one with the scheme that detects it.
Figure 6(b): Transformer faults and their protection schemes. One zone, four fault families — each gets its own protection.
The four fault families
| Fault family | What goes wrong | Protection scheme |
|---|---|---|
| 1. Incipient internal faults | Slow-developing trouble inside the oil: core bolt insulation failure, local hot spots, oil decomposition, low oil level. | Buchholz relay — gas float gives an alarm, and the oil-surge baffle trips the breaker; plus oil and winding temperature indicators. |
| 2. Winding phase & earth faults | Phase-to-earth, phase-to-phase and inter-turn short circuits in the HV or LV windings. | Percentage biased differential relay (87T) with harmonic restraint, plus Restricted Earth Fault (REF) protection. |
| 3. Tank & tap-changer faults | Violent internal arcing that builds pressure in the tank; a fault in the on-load tap-changer (OLTC) diverter switch. | Sudden Pressure Relief Valve (PRV), frame leakage (tank-earth) relay, and OLTC surge relay. |
| 4. External faults & overfluxing | Sustained external short circuits, overload, overvoltage and under-frequency, giving V/f > 1.1. | Backup IDMT overcurrent and earth fault relays (51 / 51N) and the overfluxing (V/f, device 24) relay. |
Why the Buchholz relay comes first
Most serious transformer failures begin as small, slow faults that no electrical relay can see: a hot spot decomposing the oil, a loose core bolt, a slowly falling oil level. The Buchholz relay sits in the pipe between the tank and the conservator and watches the oil itself. Gas released by a slow fault collects in the relay and drops a float, raising an alarm; a severe fault drives a surge of oil through the pipe and pushes a baffle that trips the breaker. It is the only protection that catches a fault while it is still incipient — before it becomes a winding short circuit.
Why the differential relay needs bias and restraint
The percentage biased differential relay (87T) compares the currents entering and leaving the transformer, exactly as the Merz-Price scheme does — but a transformer is not a plain wire. Its ratio, tap position and magnetising inrush all create a standing difference current even when healthy. The percentage bias raises the trip threshold in proportion to the through-current so that small mismatches are tolerated, and the harmonic restraint blocks the relay during magnetising inrush (which is rich in second harmonic) so that switching the transformer on does not cause a false trip. The Restricted Earth Fault (REF) relay covers the one thing the differential scheme sees poorly: an earth fault close to the winding neutral, where the fault current is small.
Key points to remember
- Four fault families: (1) incipient internal, (2) winding phase & earth, (3) tank & tap-changer, (4) external faults & overfluxing.
- Incipient faults → Buchholz relay (gas alarm + oil-surge trip) + temperature indicators.
- Winding faults → percentage biased differential 87T with harmonic restraint + REF.
- Tank & tap-changer faults → PRV, frame-leakage relay, OLTC surge relay.
- External faults & overfluxing → backup IDMT 51 / 51N and overfluxing relay (24, V/f); overfluxing when V/f > 1.1.
Based on Power System Protection & Switchgear, Paper II, Q6(b), Exam 11 June 2026.