Three Phase Voltage Unbalance – Causes, Effects & Mitigation

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 conductos 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 unblanced phase voltages is insufficient to trip current-responsive thermal or magnetic overload devices. The overheating caused by increase motor current in most caseslead 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 ploads 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 unblance 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

About the Author

Ver Pangonilo
A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia and Professional Electrical Engineer (PEE) - Philippines with extensive experience in concept select, front-end engineering, HV & LV detail design, construction and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and waste water pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).
  • chris

    hallo sir,

    how do i get rid of voltage imbalance in a three phase system, mainly occuring when load lighting?

    • You need to investigate your lighting circuits to check which circuit is connected to which phase. Once you have all the data, balance the lighting circuit between your phases.

  • cesar

    Is the unbalance impedance is also considered as unbalance voltage, and how many percentage that my motor/equipment will handle the unbalance voltage.

    • Unbalance impedance is not synonymous to unbalance voltage but it may cause voltage imbalance depending on the magnitude of impedance imbalance. Unbalance impedance is the same as unbalance loads

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