Modelling, Control and Stability Analysis of Photovoltaic Systems in Power System Dynamic Studies

This thesis investigates the impact of: i) the low voltage ride-through and dynamic voltage support capability; ii) the active current recovery rate; iii) the local voltage control; and iv) the plant-level voltage control of large-scale photovoltaic systems on short-term voltage stability and fault-induced delayed voltage recovery as well as transient and frequency stability. The power system dynamic performance is analysed using state-of-the-art methods, such as phasor mode time-domain simulations and the calculation of the critical clearing time that determines the stability margin. Moreover, the recently developed Kullback-Leibler divergence measure is applied to assess the quality of the voltage recovery. Drawbacks of this metric are outlined and a novel metric, the so-called voltage recovery index, is defined that quantifies the phenomenon of fault-induced delayed voltage recovery more systematically. The studies are performed with a generic photovoltaic system model, which was developed by the Western Electricity Coordinating Council. Typical model parameters are used that were determined in collaboration with a manufacturer. The implemented model is successfully validated against the Renewable Energy Model Validation tool that was developed by the Electric Power Research Institute. Moreover, the implemented photovoltaic model is open-source software and can be used by academia and industry. The stability analysis is performed in DIgSILENT PowerFactory using: i) a one-load infinite-bus system in order to show the impact of the photovoltaic system control modes on the fundamental concepts and principles of short-term voltage stability; and ii) an IEEE multi-machine voltage stability test system, namely the Nordic test system, that additionally illustrates interactions with other power system components, such as synchronous generators. The results show that without the low voltage ride-through capability, the multi-machine system is short-term voltage and transient unstable. Only the low voltage ride-through and dynamic voltage support capability help to avoid instability. The fastest active current recovery rate achieves the best voltage recovery. However, this recovery rate also leads to overfrequencies in the system that could cause disconnection of generation. Therefore, the rate should be tuned considering both, voltage and frequency dynamics. In case of local constant voltage control, photovoltaic systems try to restore their pre-fault voltage by increasing their reactive and reducing their active currents. However, due to the rather stiff grid behaviour, restoring grid voltage is impossible and the system collapses owing to the lack of active power produced by the photovoltaic systems. To overcome this problem, adequate reactive power limitation is required or the current limit logic needs to be changed. If plant control is used, plant-level constant voltage and local coordinated reactive power/voltage control should be applied. Finally, the results show that the voltage recovery index provides useful information about the delayed voltage recovery and helps to compare different short-term voltage controls of photovoltaic systems.