Circulating Current – load switch open

Circulating Current load switch closed

I circulating adds to the load current in one transformer and subtracts from the load current in the other transformer. At rated load, the transformer with the higher secondary voltage will be overloaded!

Example 3.4 Two 100 kVA single-phase transformers A and B are to be operated in parallel. The respective no-load voltage ratios and respective impedances are obtained from the nameplates as –Transformer A 2300 – 460 V%R = 1.36%X = 3.50 –Transformer B 2300 – 450%R = 1.40%X = 3.32

Example 3.4 continued Determine –the circulating current in the paralleled secondaries –the circulating current as a percent of the rated current in transformer A –the percent difference in secondary voltage that caused the circulating current.

The rated low-side currents are

The equivalent resistance and reactance of each transformer referred to the low side

The impedance of the closed loop formed by the two secondaries The circulating current

The circulating current as a percent of the rated current in transformer A The percent difference in secondary voltage that caused the circulating current