OTI & WTI What they measure
Dr K Rajamani
Co-Founder KNOPOWER
OTI (Oil Temperature Indicator) and WTI (Winding Temperature Indicator) are the two temperature parameters with which practicing engineers handling power transformers are most familiar. However, many engineers remain unaware of the subtle yet important differences between them.
Of the two, oil temperature is a directly measurable quantity, whereas winding temperature is a calculated value derived using a thermal model. Oil temperature is continuously measured throughout the transformer’s service life. In contrast, winding temperature is actually measured only once—during the temperature rise (heat run) test conducted at the manufacturer’s works.
During the heat run test, the average winding temperature is determined. From this, the average winding gradient is calculated. The hot-spot gradient (Hgr) is then obtained by multiplying the average winding gradient by a factor, typically around 1.3. In normal operation, the WTI indication is therefore a derived quantity based on measured top oil temperature, measured load current, and the hot-spot gradient obtained from the heat run test.
For older transformers, heat run test records are often unavailable. In such cases, only an estimated value of Hgr is entered into the device. As a conservative practice, a higher Hgr value (for example, 25 °C) is frequently assumed. Consequently, the use of an “advanced” digital RTCC does not necessarily guarantee an accurate WTI indication if the Hgr value programmed into the RTCC itself is significantly off the mark.
For this reason, the following approach is recommended for power transformer tripping logic to minimize nuisance tripping, particularly in utilities supplying power to urban load centers:
OTI: Stage 1 as alarm and Stage 2 as trip
WTI: Both Stage 1 and Stage 2 as alarms only, intended to alert the operator to reduce load
For the complete discussion on this intriguing topic, refer to the attached article.
Of the two, oil temperature is a directly measurable quantity, whereas winding temperature is a calculated value derived using a thermal model. Oil temperature is continuously measured throughout the transformer’s service life. In contrast, winding temperature is actually measured only once—during the temperature rise (heat run) test conducted at the manufacturer’s works.
During the heat run test, the average winding temperature is determined. From this, the average winding gradient is calculated. The hot-spot gradient (Hgr) is then obtained by multiplying the average winding gradient by a factor, typically around 1.3. In normal operation, the WTI indication is therefore a derived quantity based on measured top oil temperature, measured load current, and the hot-spot gradient obtained from the heat run test.
For older transformers, heat run test records are often unavailable. In such cases, only an estimated value of Hgr is entered into the device. As a conservative practice, a higher Hgr value (for example, 25 °C) is frequently assumed. Consequently, the use of an “advanced” digital RTCC does not necessarily guarantee an accurate WTI indication if the Hgr value programmed into the RTCC itself is significantly off the mark.
For this reason, the following approach is recommended for power transformer tripping logic to minimize nuisance tripping, particularly in utilities supplying power to urban load centers:
OTI: Stage 1 as alarm and Stage 2 as trip
WTI: Both Stage 1 and Stage 2 as alarms only, intended to alert the operator to reduce load
For the complete discussion on this intriguing topic, refer to the attached article.
PS: Direct measurement of winding temperature is possible by embedding fibre-optic sensors. However, their use is limited, largely confined to very high-capacity transformers. Hence, this aspect is only briefly mentioned in the article. click here to open file
Originally published Dec 29, 2025