Dynamic voltage stability of unbalanced DNs with high penetration of roof‐top PV units

Abstract

Dynamic voltage instability (DVI) issue is one of the primary concerns in distribution networks (DNs) due to the growing integration of low-inertia compressor motor loads. Moreover, the inherent unbalanced nature of DNs as a result of unbalanced load could make the DVI issues more severe. Furthermore, with the way rooftop photo-voltaic (PV) units are integrated, often in the form of single-phase units, DVI issues might be a threat to secure operation of DNs (both low and medium voltage) in the future. Though dynamic voltage stability (DVS) has been researched well in DNs, how imbalance system would influence the phenomena yet to be fully understood, especially with growing number of PV units. Although the roof-top PV units are connected at low-voltage DNs, their impacts on the DVS can propagate to the immediate medium-voltage DNs. Hence, this paper systematically examines the impacts of imbalance on the DVS of medium-voltage DNs with high PV penetrations first. The degree of unbalance is defined, and then DVS is thoroughly investigated considering various unbalance scenarios. Two indices, namely Voltage Nadir (VN) and Rate of Change of Voltage Recovery (ROCOVR) are proposed for clear understanding of the issue. It is observed that the increment of system imbalance can significantly influence the DVS in a deprived way. Finally, this paper has discussed and compared mitigation strategies with respect to cost and DVS improvement. Case studies are conducted on two IEEE benchmark test systems, namely, IEEE 4 bus and IEEE 15 bus systems. It is revealed that reactive power injection by rooftop PV inverters would be the most cost-effective solution to avoid a potential instability problem while mitigating the impacts of imbalance.