Before putting any engineering component on the job, it has to go for several tests for its satisfactory performance. The transformers are in no way exempted. In fact a transformer after designing, manufacturing and finally assembling, has to go for the tests to assess its performance and predetermination of the losses and efficiency.
These are the following tests:
Transformer Megger test
(a) Megger is used to perform the insulation resistance test. The megger should be taken of suitable range. These tests are:
(i) Insulation resistance between the windings and body of the transformer. Connect ‘E’ terminal of the megger with the body and ‘L’ to the windings separately. The result, after rotating the handle up to the required speed, should be either infinity or at least one mega Ohm or as per specifications 0f the manufacturer.
(ii) Insulation resistance between the windings. Check the continuity of the windings. Short the terminals Of primary winding and secondary winding, separately. Connect ‘L’ terminal of megger to one winding and ‘E’ to another winding. On rotating the handle the megger should show infinity, or at least one mega ohm or as per the specifications of the manufacturer.
Transformer Core loss test
It is also known as the open circuit test. In this test the primary is connected to the rated voltage and rated frequency through ammeter and wattmeter as shown in Fig. 18.30. The secondary winding (generally h.t. is taken in this test) is left open. As the core losses are independent of the load, so the power shown by wattmeter with secondary as open circuited are the core losses. The copper losses being very less are neglected. In case of three phase transformer the core test is performed in the same manner with secondary opened. In this test i.e., the open circuit test the X . R I I I and cos can be determined.
Transformer Copper loss test
This test is also known as the short circuit test. It is used to find out the impedance, the copper losses, equivalent impedance and current transformation ratio also. In this test generally the low tension side is short circuited; with a thick conductor or a suitable ammeter. The reduced voltage is applied on the primary side to enable the full load or any desired load current in the transformer windings as shown in Fig. 18.33. In this test being low voltage the core losses are neglected and the wattmeter reading gives away the copper losses at that load. In case of three phase transformer the test is carried out and losses are measured by means of two wattmeters method.
Transformer Load test
To check the performance of the transformer on load, the load test is conducted. By this test, the efficiency, temperature conditions etc. are determined. In transformers of low capacity the arrangement of load is easy but for big transformer it is difficult and the power consumed is nearly a wastage.
Transformer Sumpner test
lt is also known as the Regeneration test or back to back test. In this test two similar transformers are taken. The primary windings are connected in parallel across the supply. The secondaries are connected in opposition to enable the transformer for parallel operation. In ideal conditions when the secondaries are connected there will not be any circulating cur-ent in secondary windings of the transformer. Now a voltage from outside is injected by means of an injecting transformer as shown and the current starts flowing in the transformer. The regulation is stopped as the required current is obtained. The wattmeter as connected in Fig. 18.34, WI shows the core losses and W2 gives away the copper losses of both the transformers. The transformer can be loaded for several hours without consuming much power only the losses are feed from the supply.
In this single test the core losses, copper losses and temperature rise for the performance and efficiency of the transformer are known. (D High voltage test. It is a test by direct application of the voltage from a suitable source or an induced voltage test in which the transformer is operated at a voltage and frequency sufficiently in excess of normal values.
Transformer Impulse test
This test is carried out in order to determine the ability of transformer to withstand the effects of high unidirectional voltage resembling the surge lightening.
Transformer Phasing out test
This test is conducted on a three phase transformer to identify primary and secondary windings belonging to the same phase. The primary and secondary windings of all the phases except the two under test are shorted as illustrated in Fig. 4.13. The arbitrarily chosen primary and secondary windings are assumed to belong to the same phase. A low voltage dc supply is given to the primary winding through a switch and a voltmeter is connected across the secondary. The momentary deflection on the voltmeter when the primary current is interrupted is carefully observed. The voltmeter is connected across the other secondaries in turn and the procedure is repeated. The winding across which maximum deflection of the voltmeter is observed is confirmed to be the desired secondary winding. The test is repeated for other primary windings to find the corresponding windings on the secondary sides.
Fig. 4.13 : Phasing out test
Transformer Polarity test
Normally terminals are distinguished by suffix numbers in such a way that the same sequence of number represents the same direction of induced e.m.f. in both primary and secondary windings. For a three phase transformer, this test is carried out for primary and secondary windings of each phase in turn in the same manner as for a single phase transformer. This test is essential to find the terminals having the same instantaneous polarity for each phase and is carried out as a routine test on all the transformers newly manufactured.
Transformer DC resistance test
Any suitable method for testing the d.c. resistance may be employed.
Transformer Voltage ratio test
It is directly found out from the readings of the voltmeters connected to the primary and secondary windings.