Voltage unbalance in a three phase system is a condition when the magnitude of at least one phase voltage is not within the acceptable limits of nominal voltage.
There are several causes of voltage unbalance. 1) Variations in single phase loading cause the current in the three-phase conductors to be different producing different voltage drops causing the phase voltages to become unbalanced; 2) Blown fuses or overcurrent protective devices; 3) Unbalance single phase loadings; or 4) fault on one of the phases.
Three phase motors are most sensitive unbalance phase voltages. Unbalanced phase voltages applied to three-phase motors causes overheating in the rotor. The worst condition occurs when one phase is opened and the motor runs on single-phase power.
Generally the increase in the motor load current when subjected to unbalanced phase voltages is insufficient to trip current-responsive thermal or magnetic overload devices. The overheating caused by increase motor current in most cases lead to insulation breakdown of the motor.
Perfect unbalance can never be maintained as loads are continually changing causing phase-voltage unbalance continually. Proper design ensures that single phase loads are balanced into the three phases to ensure that load currents are balance. Normally, single loads are not connected to circuits supplying phase unbalance sensitive loads.
Another mitigation is to provide phase failure relays to detect voltages unbalances within the power system and trips the circuit protection device if the extent of unbalance voltage is beyond the preset limits.
1. IEEE Std 141-1993 - IEEE Recommended Practice for Electric Power Distribution for Industrial Plants
2. NEMA Standards Publication MG 1-1998 Motors and Generators