Toward standardized recommendations for current unbalance limits in DER-rich unbalanced power systems: A comprehensive review

Article Layout • Proposes standardized current unbalance limits for DER-rich power systems. • Reveals that unbalance factors are not directly proportional under DER integration. • Reviews recent standards, grid codes, and research for assessing unbalance factors. • Recommends CUF < 5% and VUF < 2% as general guidance limits at DERs’ PCCs. • Suggests practical and policy-based mitigation strategies for reliable grids. Current unbalance (CU) can lead to numerous adverse impacts on power system components, reduce equipment lifespan, and weaken the overall reliability of the grid, particularly in distributed energy resources (DER)-rich unbalanced power systems. Although various existing standards primarily address voltage unbalance (VU) and define its acceptable thresholds, standardized limits for CU are still not properly defined. However, several standards provide case-specific recommendations under certain conditions, without having general standardized CU limits. This study emphasizes the critical role of CU as a key contributor to voltage deviations and other asymmetry metrics in active distribution networks. Therefore, this review article, drawing on mathematical analysis, previous research, and recent standards or grid codes, aims to establish standardized CU limits for various power system applications. The focus is placed on low- and medium-voltage distribution networks with high penetration of DERs and uneven load allocation, particularly in systems where DERs are connected through power electronic converters and interfaced with weak or stressed upstream grids. Based on the synthesis of the reviewed literature and analytical assessment, the study recommends (as guidance values) that both VU and CU should generally be maintained below 2% and 5%, respectively, under the considered operating conditions. Maintaining these limits can help mitigate adverse impacts on distribution system performance, particularly at the DER point of common coupling. Additionally, the study underscores the urgent need to develop comprehensive and multi-layer mitigation strategies to enhance system stability and ensure regulatory compliance. Furthermore, supporting the reliable operation of DER-rich power systems.