Event

Voltage unbalance is defined as the largest difference between the average RMS voltage and the RMS value of any single voltage phase divided by the average RMS voltage, usually expressed as a percentage. (Example)

Cause

Unbalanced loads or single-phase loads that are not evenly distributed between the phases of a three-phase system will cause voltage unbalance. This loading can be inside of a facility or outside on the utility. Usually the unbalance is found within the facility. Other causes can be due to transformer impedance not matched on banked transformers or possibly a power factor correction bank with a blown fuse or bad capacitor.

Any voltage unbalance may contribute to premature equipment aging. Unbalance will cause power supply ripple, severe insulation degradation due to heat generation, and decrease mean time between failure (MTBF) on all affected equipment. Poly-phase induction motors should not be operated with a voltage unbalance greater than 5%. For a voltage unbalance between 1% and 5% the max load of the machine should be derated to reduce the possibility of damage.

Rule of thumb: If the voltage unbalance is greater than 2% it should be addressed. Also note that a three phase non-linear load will have a very large current unbalance with only a small voltage unbalance.

Voltage unbalance may be calculated by using a multimeter to record phase-to-phase voltages, and then expressing the maximum deviation from the average divided by the average voltage as a percent.