POWER TRANSFORMER TEMPERATURE RISE AND THE THERMAL CAPABILITY BASIC AND TUTORIALS

Transformer KVA ratings have been alluded to on a number of occasions up to this point without explaining how the KVA rating is determined.

The KVA rating of a transformer is simply the steady-state KVA load applied to the output of the transformer at the voltage rating of the output winding that produces an average winding temperature rise (above the ambient temperature) equal to 65°C.

For older transformers, the rated average winding temperature rise was 55°C. Advances in insulating materials allowed a 10°C increase in average temperature.

The temperature rise of the winding is caused by all of the transformer losses that were previously discussed in this chapter. Therefore, the winding temperature is a function of load losses and no-load losses.


The thermal capability of a transformer is defined in a slightly different way from the rated KVA. Thermal capability is the KVA load applied to the output of a transformer that causes the hottest area in the windings, called the winding hot spot, to reach some limiting temperature.

The hot-spot temperature determines the rate of loss of life of the transformer as a whole, which is a cumulative effect. Therefore, the hot-spot temperature limit is usually based on a loss-of-life criterion.

POWER TRANSFORMER TEMPERATURE RISE TEST AT LOAD BEYOND NAMEPLATE RATING BASICS AND TUTORIALS

POWER TRANSFORMER TEMPERATURE RISE TEST AT LOAD BEYOND NAMEPLATE RATING BASIC INFORMATION

How To Conduct Temperature Rise Test For Power Transformer Beyond Nameplate Rating?

After completing the hot resistance tests data recorded during tests may be evaluated to determine preliminary exponents. The preliminary exponents may be used to evaluate whether an excessive top oil temperature or winding hottest spot temperature may occur during this test.

It is suggested that the winding hottest spot temperature be limited to 140 ˚C and top oil be limited to 110 ˚C, unless other values are agreed upon by the manufacturer and user. The top oil temperature and the measured rate of change of the oil level with temperature may be used to evaluate whether excessive oil levels may occur during this test.

If it becomes apparent that excessive values may be obtained, the load may be reduced from the 125% value, so the top oil temperature, winding hottest spot temperature, and oil level are limited to acceptable values.

After the evaluation of risk and the load beyond nameplate to be applied has been determined, proceed with the test as follows:

a) Short-circuit one or more windings, and circulate a constant current , at rated frequency, equal to

125% of rated current (1.25 x IR), plus additional current to produce losses equal to the rated no-load loss.

The current to be circulated may be determined using Equation (3). Continue applying this current until the top oil temperature does not vary by more than 2.5% or 1 ˚C, whichever is greater, in a time period of three consecutive hours.

b) Record all data listed in Clause 6 and Clause 7 after the top oil temperature rise has stabilized and

while is being applied:

c) Reduce the current to 125% of rated current ( ) and hold for a minimum time period of one hour. Calculate and record as measured current/ for later use in 9.8.5.

d) At the end of the one-hour period, while the current equal to 125% of rated ( ) is being applied, record all data.

e) Remove the load current, and measure a series of hot resistances of the windings at appropriate time intervals to determine the average winding temperatures using the cooling curve method in IEEE Std C57.12.90-1999. Only those windings found to be the hottest windings in item e) of 9.5 need be measured.