Unbalanced voltage compensation using grid-connected photovoltaic system

Appropriate power quality is indispensable for electrical distribution networks especially in microgrids including renewable energy sources. One of the critical issues affecting power quality is the unbalanced voltage. Voltage imbalance in microgrids spreads due to proximity of loads and power sources, causing instability and eventually interruption. In general, unbalanced voltage can be compensated by using reactive power compensators, e.g. series active power filter and dynamic voltage restorer. However, these equipment imposes additional costs, which is not economically viable. Recently, the surplus capacity of inverter-interfaced distributed generations is used to improve voltage quality. In this regard, the control methods based on proportional-resonance controllers in αβ frame or PI controllers in a dual-frame synchronous reference frame (DSRF) are the usual methods that are used. The problem of limiting the output current in αβ frame and the problem of oscillating components in DSRF are weakness of these methods. To resolve the mentioned drawbacks, in this paper a control method based on decoupled double synchronous reference frame (DDSRF) is proposed for unbalanced voltage compensation using a two-stage grid-connected photovoltaic system. The simulation results show that despite the dynamic changes and the alternating nature of the photovoltaic system in power generation, the proposed control system appropriately compensates the unbalanced voltage and controls DC link voltage and the photovoltaic output power. After compensation using the proposed control strategy, the voltage unbalance factor (VUF) and the total harmonic distortion (THD) at the PCC are under the permissible range accounting for two percent and five percent, respectively.