Voltage unbalance is the difference in phase voltage among the three phases, compared to the average, as shown in the formula.
Large unbalance (over several percent) can cause excessive heating in motors, VFD controllers, and other 3 phase electronic power supplies.
The recommended limit from ANSI C84 is 3%.
Voltage unbalance may be local (limited to the secondary of a distribution transformer), mostly caused by unbalanced secondary loading, or present on the primary, caused by unbalanced feeder loading across all three phases...
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