Transformer,three phase:Parallel Operation of 3-phase Transformers

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Parallel Operation of 3-phase Transformers

All the conditions which apply to the parallel operation of single-phase transformers also apply to the parallel running of 3-phase transformers but with the following additions :

1. The voltage ratio must refer to the terminal voltage of primary and secondary. It is obvious that this ratio may not be equal to the ratio of the number of turns per phase. For example, if V1, V2 are the primary and secondary terminal voltages, then for Y/D connection, the turn ratio is V2/ (V1 / 3) = 3V2 / V1.

2. The phase displacement between primary and secondary voltages must be the same for all transformers which are to be connected for parallel operation.

3. The phase sequence must be the same.

4. All three transformers in the 3-phase transformer bank will be of the same construction either core or shell.

Note. (i) In dealing with 3-phase transformers, calculations are made for one phase only. The value of equivalent impedance used is the equivalent impedance per phase referred to secondary.

(i) In case the impedances of primary and secondary windings are given separately, then primary impedance must be referred to secondary by multiplying it with (transformation ratio)2.

(ii) For Y/D or D/Y transformers, it should be remembered that the voltage ratios as given in the questions, refer to terminal voltages and are quite different from turn ratio.

Example 33.26. A load of 500 kVA at 0.8 power factor lagging is to be shared by two three- phase transformers A and B of equal ratings. If the equivalent delta impedances as referred to secondary are (2 + j6) W for A and (2 + j5) W for B, calculate the load supplied by each transformer.

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Example 33.28. A load of 1,400 kVA at 0.866 p.f. lagging is supplied by two 3-phase transformers of 1,000 kVA and 500 kVA capacity operating in parallel. The ratio of transformation is the same in both : 6,600/400 delta-star. If the equivalent secondary impedances are (0.001 + j 0.003) ohm and (0.0028 + j0.005) ohm per phase respectively, calculate the load and power factor of each transformer. (Elect. Engg-I, Nagpur Univ. 1993)

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Example 33.29. Two 3-phase transformers A and B having the same no-load line voltage ratio 3,300/400-V supply a load of 750 kVA at 0.707 lagging when operating in parallel. The rating of A is 500 kVA, its resistance is 2% and reactance 3%. The corresponding values for B are 250 kVA; 1.5% and 4% respectively. Assuming that both transformers have star-connected secondary wind- ings, calculate

(a) the load supplied by each transformer,

(b) the power factor at which each transformer is working,

(c) the secondary line voltage of the parallel circuit.

Solution. On the basis of 500 kVA,

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(c) Since voltage drop of each transformer is the same, its value in the case of transformer A would only be calculated. Now, for transformer A, kW = 400 for the active component of the current and kVAR = 351 for the reactive component.

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