Mixing Delta-Wye and Delta-Delta Transformers in Large Charger Installations
note number - hce0001

The current drawn from the utility service by a Bitrode CDN or CRN charger (or any other power supply that uses semiconductor rectifiers to convert AC to DC) is not a pure sine wave; it consists of a series of pulses. Therefore, this current has a relatively high harmonic content, which produces a lower power factor than if the current was a pure sine wave.

To reduce the harmonic content of the current drawn from the utility service, large charger installations are sold with approximately half the chargers utilizing delta-delta transformers, while the rest of the chargers utilize delta-wye transformers.

The harmonic reduction results from the fact that the current drawn by the delta-wye transformers has a different waveshape from the current drawn by the delta-delta transformers. When both are operating from the utility service, the two currents combine to produce a total current waveshape that is closer to a pure sine wave than either current is individually. Therefore, harmonic content is reduced and power factor is improved.

Note that the above assumes that an approximately equal current is being drawn by each type of transformer. In large formation installations, this is a practical operating condition, particularly when the entire plant is operating near maximum capacity (where it is most important to maintain high power factor and low harmonic content).

Also note that a wye-delta/delta-delta combination would produce the same results.

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