IEC 60076-2:2011 pdf – Power transformers – Part 2: Temperature rise for liquid-immersed transformers.
• the top-liquid and average liquid temperature rises in a steady-state condition with dissipation of total losses;
• the average winding temperature rise at rated current for the average liquid temperature rise as determined above;
• the hot-spot winding temperature rise at rated current and for the top-liquid temperature rise as mentioned above.
This is achieved in two testing steps.
a) Total loss injection
The top-liquid and average liquid temperature rises are established when the transformer is subjected to a test current corresponding to the total losses of the transformer (see IEC 60076-1). The test current will be above rated current to the extent necessary for producing an additional amount of loss equal to the no-load loss at rated voltage, and the winding temperature rise will be correspondingly elevated
The top-liquid temperature and cooling medium temperature are monitored, and the test is continued until steady-state liquid temperature rises are established.
The first part of the test may be terminated when the rate of change of top-liquid temperature rise has fallen below 1 Klh and has remained there for a period 01 3 h. If discrete readings have been taken at regular Intervals, the mean value of the readings during the last hour Is taken as the result of the test. If continuous automatic recording is applied, the average value during the last hour Is taken.
b) Rated current injection
After the top-liquid temperature rise has been established, the test shall be continued without a break with the test current reduced to rated current for the winding combination connected. This condition is maintained for 1 h, during which time continuous temperature records of top-liquid, windin9 hot-spot (if measured) and external cooling medium should be taken at least every 5 mm.
At the end of the hour. the resistances of the windings are measured, either after a rapid disconnection of the supply arid short circuits (see 7.8 and Annex C) or, without switching off the supply, by means of the superposition method which consists of Injecting Into the windings and measuring direct current of low value superimposed on the load current.
The values of average temperature of the two windings are determined from the resistance variations, and in addition by taking into account the liquid temperature decrease when the current is reduced to the rated value, as well as the variation of the external cooling medium temperature.
If the direct measurement of the hot-spot winding temperature Is provided, the corresponding temperature rise Is obtained by taking the highest reading before disconnection and applying the correction indicated in 7.10.3 because the liquid temperature decreases when the current is reduced to the rated value.
By agreement, the two steps of the test may be combined In one single application of power at a level between load loss and total loss. For liquid immersed transformers, the temperature rise values for top-liquid, average liquid and for the windings shall then be determined using the correction rules given in 7.13. The power injected during the test shall however be at least 80 % of the total loss value.
7.3.3 Test modification for particular transformers
Two-winding transformer with a tapping range larger than ± 5 %, or having a rated power exceeding 2 500 kVA.
Unless otherwise specified, the temperature rise test is conducted with the transformer connected on the maximum current tapping (see IEC 60076-1) and the tapping current for that tapping is used during the later part of the test (see 7.3.2 b)).
The total losses to be injected during the first part of the test (see 7.3.2 a)), shall be equal tothe highest value of total loss appearing at any tapping (corresponding to its tappingquantities). This tapping is also often, but not always,the maximum current tapping.This partof the test determines the maximum top-liquid temperature rise. For the determination ofwinding temperature rise at the maximum current tapping, the value of liquid temperature riseto be used in the evaluation shall correspond to the total losses of that tapping. The valuefrom the first part of the test will be recalculated if obtained with other data.