Voltage stability assessment of photovoltaic energy systems with voltage control capabilities

This paper studies the impact of large-scale PV generation, up to 50% penetration level, on distribution system voltage regulation and voltage stability. The system voltage profiles are computed using power-flow calculations with load variation of a 24-hour time scale. The voltage stability is examined at different times of the day using a developed continuation power-flow method with demand as continuation parameter and up to the maximum loading conditions. The load-flow analysis implemented for both voltage regulation and voltage stability analysis is performed by using the forward/backward sweep method. The secant predictor technique is developed for predicting the node voltages which are then corrected using the load flow solver. Three models of the PV interface inverter are implemented in this study with full set of data representing environmental conditions. The voltage profiles are regulated using the PV interface inverters which support reactive power at unavailability of sun light. The available inverter capacity is utilized for regulating the system node voltages. The most possible scenarios of system voltage collapse are investigated at different times of the day. The developed methods and models are used to assess the performance of a 33-bus radial distribution feeder with high level of PV penetration. The results show that the PV interface inverters have to be designed to operate for reactive power support in order to improve voltage profile, secure power systems operation, and increase the lifetime of the online tap changing transformers.