Space phasor geometrical loci in polar coordinates as voltage unbalance monitoring tool

Voltage unbalance is a common phenomenon in AC power network. This disturbance can cause on the three-phase induction motors connected to the power supply system serious ill effects such as overheating, drop of efficiency and reduction in output torque. Therefore, operation of an induction motor for any length of time above 5% of voltage-unbalance condition is not recommended and on-line voltage unbalance monitoring is required from economical reasons. In this paper a new method of evaluating the degree of unbalanced voltage in power network is proposed. This new approach uses voltage space phasor geometrical locus in polar coordinates based on a modified “a-b-c” to “d-q” coordinate transformation which gives the possibility to localize the unbalanced phases and to appreciate the severity of the voltage unbalance.