Research On Weak Grid Modeling And Photovoltaic Penetration Level Improvement Strategy

With the high speed development of social economy and technology innovation,more and more people pay more attention on renewable energy. In China, PV power station have already played an important role in power grid construction. Things are especially true in the northern and western part where population density are exiguity, distance between neighbor stations are relatively long and the transmission radius are large which contain multi-level voltage transformation. These factors may results in the target grid have a weak connection with external grid, which are called ‘Weak Grid’. In weak grid the poor voltage stability cannot guarantee a reliable power networks. Voltage stability and power quality are worsen which hinder the next step of PV development. Focusing on the above issue, this thesis carries on research on weak grid characteristics, and optimize the PV weak grid in three point of view: additional electricity appliance, PV controlling scheme and weak grid power system optimization. Therefore, the penetration level of PV can be enhanced.Firstly, by clarifying the definition of PV weak grid, the system parameters are identified and weak grid model is built. Then the transmission line type, length and power factor of PV are investigated to built a complete weak grid model and by power flow analysis, PV grid connected point voltage stability is investigated, and penetration level of PV are observed.Secondly, focusing on the weak grid capacitive rise effect, a transmission line compensation model is built. The compensation factor are calculated and the problem are solved by Thyristor Controlled Reactor on the basis of weak grid. The steady state simulation results are to be compared to investigate the penetration level variation.Thirdly, by clarifying the principle of voltage rise caused by reverse power flow, a real power constraints and reactive power voltage regulation scheme is proposed which guarantee a steady state when special occasion occurs.By transient analysis, we can get the performance of inverter under the control scheme.Lastly, the PV allocation and sizing optimization is investigated by the means of Differential Evolution Multi-objective Optimization, where an IEEE 33 nodes model is utilized and modified into weak grid. Therefore, we can get the optimized PV allocation and capacity, and the PV penetration level can be maximized.