A new graphic evaluator of the voltage unbalance in power networks

Voltage unbalance is a common phenomenon in AC power network. This disturbance can bring to the three-phase induction motors connected to the power supply system important drawbacks such as overheating, drop of efficiency and reduction in output torque. Therefore, operation of an induction motor with voltage unbalance above 5% is not recommended and therefore continuous monitoring of voltage unbalance o the supply system is need. In this paper a new method of evaluating the voltage unbalanc index in power distribution networks 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 estimate the severity of the voltage unbalance.