Voltage Imbalance :
The imbalance is expressed as a percentage of the amplitude of the positive sequence component of the amplitude of the negative sequence component in three-phase systems.

This definition applies to both voltage and current. Usually in three-phase systems the voltage unbalance is below 3%. The current imbalance is higher than the voltage, especially in the case of single-phase loads. Voltage unbalance may occur in three-phase systems due to the failure of one of the capacitor groups.