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Nevertheless, if agreed between the purchaser and the manufacturer, it is permissible to test 60 Hz transformers with a 50 Hz power supply and 50 Hz transformers with a 60 Hz power supply provided that the prescribed test current values, as required in 4.
This procedure requires that the voltage of the test supply is suitably adjusted with respect to the rated voltage of the transformer. If this is not the case, a single-phase supply, as described below, may be used. For delta-connected windings, the single-phase supply is provided between two corners of the delta and the voltage during the test has to be the same as the voltage between phases during a three-phase test.
For star-connected windings, the single- phase voltage is supplied between one line terminal and the other two line terminals connected together. Examples of two possible single-phase test arrangements simulating the three-phase test are given in figures 1 and 2.
NOTE 2 For star-connected windings with non-uniform insulation, it is necessary to check whether or not the insulation of the neutral is sufficient for single-phase testing. NOTE 3 If, for star-connected windings, the power supply is insufficient for the single-phase testing described above and the neutral is available, the manufacturer and the purchaser may agree upon the use of single-phase power supply between line terminal and the neutral, provided that neutral is capable of carrying the relevant current.
With this test arrangement, it might be convenient to mutually connect the corresponding terminals of the phases not submitted to test in order to better control their voltage, provided that this is feasible and circuitry correct. For categories I and II single-phase transformers, the number of tests shall be three. Unless otherwise specified, the three tests on a single-phase transformer with tappings are made in a different position of the tap-changer, i.
For categories I and II three-phase transformers, the total number of tests shall be nine, i. Unless otherwise specified, the nine tests on a three-phase transformer with tappings are made in different positions of the tap changer, i. For transformers of category III, an agreement between the manufacturer and the purchaser is always needed with regard to the number of tests and the position of the tap-changer. However, in order to simulate as closely as possible the effects of repetitive short-circuit events likely to occur in service, to allow a better monitoring of the behaviour of the unit under test and to permit a meaningful judgement in connection with possible variations of the measured short-circuit impedance, it is recommended that the number of tests is as follows: — for single-phase transformers: three; — for three-phase transformers: nine.
With regard to tap-changer position and test sequence, the same procedure as described for transformers of categories I and II is recommended. In general, such conditions are of a more complex nature in comparison with the three-phase short circuit which can be considered the reference case for two-winding transformers see 3.
Special testing circuits are often necessary in order to reproduce some of the fault events by means of tests. The choice of the test duties to be performed should be made, as a rule, on the basis of the analysis of the results of calculations of electrodynamic forces occurring in all possible fault cases.
The testing arrangements, the current values, the sequence and the number of tests are always subject to agreement between the manufacturer and the purchaser.
It is recommended that the tolerance on the agreed test current values and the duration of the tests are in line with those prescribed for two-winding transformers and that the test sequence is selected according to the expected increase of electrodynamic forces. These measurements and tests serve as references for the detection of faults. Furthermore, the outside of the transformer under test shall be observed visually and continuously video recorded.
NOTE 1 Additional means of detection may be used to obtain information and improve the evaluation of the test event, such as recording of the current between the tank insulated and earth, recordings of noise and vibrations, recordings of oil pressure variations occurring at different locations inside the tank during short-circuit current flow, etc.
NOTE 2 Random gas-and-oil-actuated relay tripping may occur during the tests due to vibration. This circumstance is not significant for the ability of the transformer to withstand short-circuit unless combustible gas is found in the relay.
NOTE 3 Temporary sparks over tank joints may occur at the energizing stage and internal sparking at the frame joints at the energizing and short-circuit stages. NOTE 2 Any differences between the results of measurements made before and after the test may be used as a criterion for determining possible defects. It is particularly important to observe, during successive tests, possible changes in the short-circuit reactance measured after each test, which may be progressive or tending to vanish.
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