Research On The Dynamic Regulation Strategy Of Virtual Synchronous Generator Based Photovoltaic Power Plant

In recent years,the issue of environment and energy has drawn wide attention from all countries in the world.The replacement of energy-intensive thermal power units with renewable energy power generation has become the trend of the times.Photovoltaic(PV)power generation is an effective way to address environmental problems and accelerate energy transformation due to its advantages such as safety,reliability,widespread distribution,and high efficiency.However,with the gradual expansion of PV installed capacity,the impact of PV power plants on the power system is increasingly apparent.In order to improve the accommodation of PV and the stability of the power system,this dissertation studies the problems of frequency,voltage and transient stability caused by PV power plant,and proposes the corresponding control strategies for PV power plants participating in the regulation of power system.The main research contents include the following:Firstly,the virtual synchronous generaotor based PV plant is modeled.The mathematical models of the main equipments and control systems in PV plants are studied.Through the improvement of the control strategy of the energy storage system,the virtual synchronous operation of PV power plants is realized.The paramters of the equipments in the PV plant are selected based on the actual conditions of the project,and a detailed simulation model of the 5MW PV plant is built in Matlab/Simulink.Since the simulation speed of the detailed model is slow,the equivalent modeling method of PV power plant is proposed through the equivalent of inverter and DC capacitor.Without considering the electromagnetic transients of PV power plants,the equivalent model can greatly increase the efficiency of simulations while maintaining the output characteristics.Secondly,the frequency regulation strategy for virtual synchronous generaotor based PV plant is studied.The control strategies of the inverters of PV and energy storage systems are improved,and the frequency regulation characteristics of the improved active power generation units and their influencing factors are analyzed.According to the traditional process of frequency regulation of power systems and the frequency regulation characteristics of each active power generation units,the sequence of energy storage systems and PV units participating in/exiting frequency regulation is reasonably arranged.Then,a hierarchical frequency regulation strategy including inertial response,primary frequency regulation and emergency frequency control is proposed for the PV plant,so that the PV plant can exhibit a frequency response similar to that of a synchronous generator in the transient process.Thirdly,the voltage regulation strategy for virtual synchronous generaotor based PV plant is studied.The mechanism of the voltage fluctuation at the point of integration(POI)of PV plant is analyzed.The grid losses caused by the distribution of reactive power and the sensitivity of each reactive unit to the voltage of POI are studied.According to the reactive power control strategy based on the voltage of POI,the reference value of the reactive power of the PV plants is determined.In the case of ensuring the minimum grid loss and the highest voltage regulation sensitivity,the rational allocation of the reference value of reactive power among the reactive power generation units is achieved by the coorperation of the droop curves,and thus a coordinate control strategy for PV plants participating in voltage regulation is proposed.Finally,the active damping control strategy for virtual synchronous generator based PV plant is studied.The influence of the virtual synchronous generator based PV plants on the damping characteristics of the power system is analyzed.The limitation of this control strategy in improving the system damping ratio is pointed out by the results of eigenvalue analysis.Adopting the damping torque analysis,the theoretical conditions of the damping ratio of power system influenced by the virtual synchronous generator are obtained.On that basis,an additional damping control strategy which can actively suppresses low-frequency oscillation is proposed by referring to the structure of traditional power system stabilizer,and the method of parameter design is given by phase compensation method.All the theoretical analysis and the proposed control strategy in this dissertation are verified by simulation results.