Impacts of Large-Scale Integration of Solar Photovoltaics and Load Characteristics on Power System Voltage Stability

In this thesis work, the voltage stability and post disturbance voltage recovery times are investigated for the Nordic 32 test system for different load combinations with and without large scale integration of solar PV power plants. In addition, the mitigation of the voltage stability and voltage recovery time phenomena through various Dynamic Var Compensators are investigated. The study is commenced with detailed analysis of the Nordic-32 test system that mimics the Swedish power grid through a Power System Simulation Software (PSS/E). The dynamic analysis was then done through PSS/E dynamic simulation capability to investigate the fault induced VRT (Voltage Recovery Time) through different load combinations in the system. It has been found that the voltage recovery time when induction motor loads are introduced beside complex (constant impedance and constant current) loads is significantly higher than that of the case when having complex loads only. Moreover, the system is more likely to have a voltage collapse with less number of cascaded disturbances in the case when having induction motors in the system. Also, if induction motor loads are dispersed throughout the weak buses in the system instead of being concentrated into a single bus, the voltage recovery time increases even more and the system becomes much more prone to a voltage collapse. The improvement of the voltage stability of the system has been done through the installation of Dynamic Var Compensators at the point of common coupling with the induction motor loads. The performance of various Dynamic Var Compensators such as Static Var Compensators (SVC) and Static Synchronous Compensators (STATCOM) has been investigated in the mitigation of the voltage recovery time with different load combinations and it has been found that the STATCOM was superior to the SVC in reducing the voltage recovery time and postponing the voltage collapse. The base case with the induction motor loads was able to withstand N-1 contingency. SVCs and STATCOMs are installed at the induction motor load buses with the same rating of 300 Mvar capacitive power for both devices such that the minimum voltage sag induced due to the fault doesn’t decrease below 0.8 p.u. It has been noticed that the system was able to withstand N-2 contingency for both compensators cases. For the case with SVC the voltage recovery time was reduced from 0.9 seconds in the base case to 0.665 seconds which corresponds to a reduction of 26% of voltage recovery time with a minimum voltage sag of 0.8 p.u. However, for the case with STATCOM, the voltage recovery time was reduced to 0.364 seconds that corresponds to a reduction of 60% of voltage recovery time with a minimum voltage sag of 0.87 p.u. The effects of large scale solar PV plants penetration has been investigated. It has been found that the maximum allowable solar PV penetration is 30% of the total generation for the base case due to the limits imposed by the Nordic Grid Code for frequency deviations due to the lack of inertia for PV plants. It has been also found that the voltage stability gets worse due to the increase in the voltage recovery time in the case of 30% PV penetration, this is because the time constant for the exciter models of the conventional generating units in PSS/E is less than that of the PV model. Adding to that, the significant reactive power losses in the transmission lines since the PV plants located far away v from the load center. The effect of irradiance changes with load profile changes throughout year has been investigated. It has been found that the system was more prone to a voltage instability if the share of the PV plants to the total generation of the system is significant.