It is imperative that an electrical transformer's temperature is monitored and maintained within design limits. Excessive operational temperatures are proven to lead to a shorter useful working life for the transformer and should be avoided.
Dry and Liquid Immersed Transformer Temperature Monitoring
Hot Spot - a hot spot temperature represents the highest temperature within the transformer windings and is measured directly, or indirectly. The hot spot area is estimated, but it is always located within the windings closest to the core.
Top Oil - the top oil temperature represents the highest temperature of the insulating liquid and is measured directly. The hottest liquid area is directly above the transformer windings and core. Note that the name 'Top Oil' refers to oil as the insulating medium, which is slightly misleading as about 5% of all liquid insulated transformers use liquids other than oil (silicone, synthetic esters, natural esters etc.).
Dry type transformers only require temperature measurement of the windings (hot spot). Liquid immersed transformers require winding and top oil temperature measurement. Dry type transformers measure the winding hot spot temperature directly.
Liquid immersed transformers measure the top oil temperature directly and then adjust this value to estimate the winding hot spot temperature (thus hot spot temperature is measured indirectly). The temperature off-set value between the top oil and hot spot is decided by the original equipment manufacturer (OEM).
How does transformer temperature monitoring work?
The below video is an extract from our Introduction to Electrical Transformers Online Video Course.
Dry Transformer Temperature Measurement
Temperature sensors for dry type transformers can be installed above the unit, or, installed within one, or, several of the low voltage windings; the low voltage windings are closer to the transformer core and thus represent the hottest area (hot spot). Modern dry type transformers are often fitted with fibre-optic cables which yield a much more reliable reading than traditional electronic sensors which are prone to inaccuracies due to the large magnetic fluxes created by the transformer.
Liquid Immersed Transformer Temperature Measurement
Local top oil temperature monitoring is usually achieved using a bellow and capillary arrangement whilst remote monitoring can be achieved using a standard Pt 100 sensor.
Modern transformers use fibre-optic cables for winding hot spot temperature monitoring, but this is a relatively new trend. Standard practice has been to install the top oil temperature sensor well within a coiled resistor. The resistor/heater coil is connected to the transformer LV or HV windings via a current transformer and this causes the resistor temperature to increase or decrease depending upon the transformer load. This setup is known as a thermal image device.
Note that it is the author's experience that traditional winding temperature sensors are difficult to calibrate in the field and are often found to be inaccurate.