The maximum temperature rise, order to determine. it is necessary to conduct a full-load test on a transformer. The small transformers full-load test is conveniently possible, but for large transformers full-load test is very very difficult. A suitable load to the absorb full-load power for large transformer may not be easily available. It will also be very expensive as a large amount of energy will be wasted in the load during the test. Large transformers can be tested for determining the maximum temperature rise by the back-to-back test. This test is also called the Regenerative test or Sumpner’s test.
The Back-to-back test on single-phase transformers requires for two identical transformers. Figure below, shows the circuit diagram for the back-to-back test on two identical single-phase transformers T1 and T2. The primary winding of the two transformers are connected in parallel and supplied at rated voltage and rated frequency. A voltmeter, an ammeter and a watt meter are connected to the input side as shown in figure below.
The secondaries are connected in series with their polarities in phase opposition, which can be checked by the voltmeter V2. The range of this voltmeter should be double the rated voltage of either transformer secondary. In order to check that the secondaries are connected in series opposition, any two terminals (say B and C) are joined together and the voltage is measured between the remaining terminals A and D. If the voltmeter V2, reads zero, the two secondaries are in series opposition and terminals A and D are used for test. If the voltmeter reads a value approximately equal to twice the rated secondary voltage of either transformer, then the secondaries are acting in the same direction. Then terminals A and C are joined and the terminals B and D used for the test.
The circuit is now closed for primary , the total voltage across the two secondaries in series be zero. There will be no current in the secondary winding. The transformers will behave as if their secondary winding are open circuited. Hence, the reading of watt meter W, gives the iron losses of both the transformers.
A small voltage is injected in the secondary circuit by a regulating transformer T excited by the main supply. The magnitude of the injected voltage is adjusted till the ammeter A, reads full-load secondary current. The secondary current produces full-load current to flow through the primary windings. This current will follow a circulatory path through the main bus-bars as shown dotted in figure. The reading of wattmeter W2 will not be affected by this current. Thus, watt meter W2, gives the full-load copper losses of the two transformers.
The ammeter A, gives total no-load current of the two transformers. Thus, in this method we have loaded the two transformers to full load but the power taken from the supply is that necessary to supply the losses of both transformers.
The temperature rise of the transformers can be determined by operating these transformers back-to-back for a long time, say 48 hours, and measuring the temperature of the oil at periodic intervals of time, say every one hour.
Also Read- Characteristics of Potential Transformer