Dynamic symmetrical components analysis and compensation for unbalanced distribution systems
The short circuit fault and the unbalanced three-phase loads are the common reasons causing unbalanced voltage distortion (UVD) in the distribution system. In the past, the decline in electric motor performance and life cycle was a well-known fact caused by UVD. However, the UVD deserves more attention in recent years due to the rapid growth of the wind power penetration in the distribution system. The wind power system operated under the UVD suffers not only the pulsation torque but also penetrates unbalanced current and consequently deteriorates the UVD of the distribution system. This paper presents an approach to dynamically resolve the symmetrical components from the three-phase instantaneous voltages in order to clarify the degree of the UVD for compensation. The vector representation for the symmetrical components involves the information of angular frequency into the steady-state phasor, which can push much of the complexity down on the controller design for compensating the UVD. A dynamic voltage restorer (DVR) is employed as a voltage compensator in the series path of the feeder for refining the percent voltage unbalance of the UVD. The impact of the UVD reduction on a grid-tied 3-kW squirrel cage induction generator (SCIG) shows the effectiveness of the proposed approach.