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SummaryThree-phase unbalance is one of the most important power quality problems in the power system. When the system is working under unbalanced conditions, more losses and heating effects will be induced, and the system will become less stable, which is harmful to both utilities and customers. The most important thing for a seriously unbalanced system is to find out the main unbalanced source to take proper measures to mitigate the problem. In this paper, a method is proposed to indentify the main unbalanced source at the point of evaluation (POE) in the distribution power systems. Two extreme cases are analyzed in the paper by assuming that the unbalanced source to be totally from the POE upstream network or downstream network. Therefore, 2 corresponding analytical formulas are derived to reveal the relationship between the POE voltage unbalance factor (VUF) and the current unbalance factor (CUF). A method is then proposed to determine the contribution of the unbalanced source at the upstream and downstream networks based on the comparison between the derived VUFs and the actual VUF. Sensitivity analysis is implemented to show the robustness of the proposed method. The effectiveness and validity of the proposed method is illustrated by the IEEE 13-bus test system and IEEE 37-bus test system. It is shown that the estimation error of the proposed method is generally within 5%.

Three-phase unbalance is an important power quality issue that can cause many negative effects to the power system. Effective mitigation and management of voltage unbalance will benefit from the knowledge of how the unbalanced sources contribute to the voltage asymmetry at the point of evaluation (POE). In this paper, a method is proposed to assess the overall unbalance contribution of the multiple unbalanced sources at the point of common coupling (PCC). Firstly, the equivalent circuit for analysis is established and the unbalance contribution indices are proposed. Then a method is proposed to determine whether the dominant unbalanced polluter is at the upstream or downstream of POE. If the main unbalanced source is identified to be downstream of POE, a procedure is further proposed to determine the individual contribution of the multiple unbalanced sources. Moreover, for the multiple unbalanced source condition, the current flowing in each feeder is proposed to be used for the contribution estimation instead of the current actually emitted by the unbalanced source. Finally, a method is proposed to estimate the equivalent negative sequence impedance of the aggregate loads. Simulation and field analysis results validate the effectiveness and accuracy of the method.

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