RESISTANCE OF WINDING DURING PARALLEL OPERATION OF TRANSFORMERS

The debatable article expresses disagreement with existing opinion that compensating current (also known as balancing, cross, equalizing, phasing, restoring, circulating current – ed.note) which flows through primary and secondary winding of three-phase step-down parallel operated transformers of groups 12 and 11 (Y/Y-12, Y/D-11), is value-restricted by the sum of short-circuit resistances. In particular the inertia of customary approach of assessment doesn’t allow taking into account the well-known factor of coexistence within the same three-phase circuit of star (Y, can also be called T – ed.note) and triangle-shaped (Δ or D – ed.note) connections. Taking into consideration exactly this factor the author substantiates that the formulae, suggested by him to replace previously used equations, might reduce inaccuracy of computation during engineering of new transformers.

Following the computation of parameters of secondary D-shaped winding of three-phase step-down transformer, the author, in order to realize computations for Y-connections, has elaborated formulae of determination of balancing current for blank cycle of two parallel operated three-phase step-down transformers with diagrams and groups of windings Y/Y-12 и Y/D-11. Their difference from known formulae consists in the fact that resistance or relative value (in %) of a short-circuit current are reduced by a third. Denominator of formulae, used to determine balancing current, in view of assumed conditions, includes substitutional value of the sum of resistance or of relative values (in %) of short-circuit current of three-phase step-down transformer with Y/Y-12 winding connections or diagrams and 2/3 of the value of resistance or relative value (in %) of short-circuit current of three-phase step-down transformer with Y/D-11 winding connections or diagrams.

The reliability of suggested formulae is confirmed by measurements of balancing current under blank cycle mode of two parallel operated three-phase step-down transformers TS-2,5/0,5 with Y/Y-12 и Y/D-11 winding diagrams and connections. Difference between calculated value and measured value of current was of 1,93%.

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