Improving of STS algorithm to detecting voltage unbalance in low voltage distribution networks

Thyristor-based static transfer switches (STS´s) are feeding sensitive loads with two independent sources by monitoring voltage quality. STS is used in distribution networks to provide connection to alternate sources of ac power for critical loads when the main source fails. In distribution system sensitive loads are endangered by either voltage sag or voltage unbalance. Simulation results show that, the STS-2 of the IEEE benchmark in voltage sag and transient conditions operates properly; but do not detect voltage un-balance. In this paper, operating of the STS-2 under unbalance distribution network - as a scenario to improve the power quality for a three-phase critical load in industrial areas - is discussed. This paper presents an improved algorithm to detecting voltage unbalance with the STS-2 based on the ratio of the negative and the positive sequences. Simulation is presented in the well known software ATP/EMTP. Also load and network parameters are based on the industrial network of the Naghadeh - Iran.