Graduate Thesis Or Dissertation
 

Determination of the maximum loading of parallel connected three-phase transformers with unequal impedance

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  • The comparative values of impedance and resistance to reactance ratios are important parameters in determining the load capacity of an interconnection of power transformer, called a bank. The full design capabilities of parallel or three-phase connected transformers cannot be utilized unless the following constraints are applied. 1. All transformer voltage ratios must be identical. 2. All transformer percent impedances must be equal. 3. The resistance to reactance ratios of all transformers must be equal. Departure from these conditions involves either an uneconomical division of current, or a circulating current, both of which will lower the efficiency and decrease the load that the bank can carry without overheating. A transformer bank whose individual transformers do not have equal percent impedances will have a load distribution that is unbalanced. The transformer with the lowest percent impedance will supply its full-rated MVA capacity while the other transformers are underloaded. This condition represents a loss in the capacity of the transformer bank. Transformers having widely different impedance values can be made to divide their load in proportion to their rating by placing the proper impedance in series with those transformers that have low impedances. The replacement of a damaged transformer in a bank will produce the best transformer load distribution if the following constraints are applied: 1. The transformer should have impedance and voltage ratio equal to those of the existing transformers in the bank. 2. The voltage ratios and the percent complex impedances should be equal to those of the existing transformers even though the KVA ratings are not equal. 3. When the resistance and/or the reactance values are different, they should be larger than those values of the existing transformers. A difference in the voltage ratios of paralleled or banked transformers will produce a much greater reduction in load capacity than mismatched impedances. This condition should be avoided if at all possible.
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