Oil and winding temperature indicators (OTI/WTI) — thermal monitoring of an oil-filled power transformer

The guide on the Buchholz relay covers the mechanical gas protection of an oil-filled power transformer. This article covers a complementary, also non-electrical, form of monitoring on the same transformer: thermal monitoring via an oil temperature indicator (OTI) and a winding temperature indicator (WTI).

Why thermal monitoring is needed alongside the Buchholz relay

A power transformer generates heat in service through copper losses (in the windings) and iron losses (in the core). This heat must be removed via the oil and the cooling system (for example ONAN or ONAF cooling). A gradual, general overload or deteriorated cooling performance (for example due to fouled radiators or a failed cooling fan) does not cause the sudden gas formation or oil surge that the Buchholz relay reacts to, but it does cause a slowly rising operating temperature — a creeping process that accelerates degradation of the winding insulation's service life without the Buchholz relay signalling it.

OTI: direct measurement of the top oil temperature

The oil temperature indicator (OTI) measures the temperature of the oil in the upper part of the main tank — where the oil is hottest, because warm oil rises — via a temperature sensor (typically a capillary thermometer or a resistance sensor) inserted into a sealed pocket in the tank wall, so the sensor can be replaced without opening the tank. Because the OTI measures the bulk oil temperature and that oil has a large thermal mass, the OTI responds relatively slowly to load changes: a time constant on the order of a few hours is typical.

WTI: the "thermal image" principle for the winding hot spot

A temperature sensor cannot be placed directly inside the winding itself — the winding is at high voltage and a built-in sensor would weaken the insulation. Instead, the winding temperature indicator (WTI) uses the "thermal image" principle: the WTI combines the OTI's oil-temperature measurement with an additional heating element fed by a current proportional to the transformer's load current (via its own current transformer). This heating element simulates the extra temperature difference (the "gradient") between the top oil and the hottest point in the winding (the hot spot) that corresponds to that load. As a result, the WTI reading follows a load change faster than the OTI — a time constant on the order of a few minutes is typical — and approximates the actual winding hot-spot temperature better than the oil temperature alone would.

Alarm and trip levels

Both indicators are typically fitted with adjustable electrical contacts for an alarm level and a higher trip level (and often also an intermediate level that forces the cooling fans or pumps on). The exact setpoint values are not fixed in a single universal standard, but follow from the specified temperature-rise limits of the transformer in question (as specified per IEC 60076-2, the standard for temperature rise of oil-immersed transformers) and the nameplate/manufacturer's specification; they therefore differ per manufacturer and per transformer design.

Note: the temperature-rise limits of IEC 60076-2 are rise values relative to ambient temperature, not absolute alarm or trip temperatures; the actual absolute alarm and trip setpoints of the OTI/WTI on a specific transformer follow from the manufacturer's specification for that particular design and the prevailing ambient conditions, not from a single fixed table that applies equally to every transformer.

Practical relevance

During periodic maintenance of an oil-filled power transformer, it is worth checking whether the OTI and WTI readings are plausible relative to the actual load and ambient temperature, whether the alarm contacts are actually connected to the monitoring or SCADA installation, and whether a previous alarm was investigated rather than simply reset. A persistently high WTI reading at a normal load can indicate deteriorated cooling performance (fouled radiators, a failed fan or pump) that, if not addressed, accelerates the degradation of the insulation's service life.

Common mistakes

Interpreting the WTI reading as a direct measurement of the winding temperature — it is a simulated approximation via the thermal-image principle, not a direct measurement inside the winding itself.
Resetting a persistent temperature alarm without investigating the cause (for example fouled cooling or structural overload) — as with the Buchholz relay, an alarm is a signal requiring investigation, not a nuisance that can simply be dismissed.
Thinking that thermal monitoring (OTI/WTI) makes the Buchholz relay redundant, or vice versa — the two signal different failure mechanisms: gradual thermal overload versus gas-producing internal faults.
Confusing the slow response time of the OTI with that of the WTI — during a rapid load change, the WTI gives a far more current indication of the winding hot spot than the OTI, which can lag by hours.