Research On Control Strategy Of Virtual Synchronous Generator Parallel Operation

With the increasing penetration of renewable energy in the grid,the difficulty of power system management and operation scheduling is increasing.Distributed generation is usually connected to the grid through the inverter.Although it has the characteristics of fast response,its large-scale access will have an impact on the stable operation of the entire power system because of its lack of inertia and damping characteristics.Virtual synchronous generator(VSG)control strategy can make the inverter simulate the external characteristics of the traditional synchronous generator by introducing the corresponding inertia and damping coefficient,so as to improve the stability of power system.In this paper,the power allocation problem of VSG controlled inverters in parallel operation is studied.Firstly,the VSG control strategy is adopted,and the specific implementation methods of active power-frequency control,reactive power-voltage control and voltage and current double closed-loop control are given.The small signal model of VSG is established,and the root locus of VSG system is drawn when the main control parameters are changed,and its influence on the stability of the system is analyzed.In order to solve the problem of large impact current when VSG switches from islanded mode to grid-connected mode,a pre-synchronization control strategy based on phase-locked loop is studied.Through frequency compensation and amplitude compensation,the stable operation of VSG is realized during the switching process of two modes.Secondly,The power transmission characteristics of VSG controlled inverters in parallel operation are analyzed,and a power distribution strategy based on proportional integrated(PI)regulator is studied to solve the problem that the VSG controlled inverters can not achieve accurate power distribution.The PI regulator is introduced into the reactive power-voltage control loop to realize the precise distribution of power among multiple inverters.However,if the line impedance changes,the parameters of the PI regulator need to be readjusted.In order to solve this problem,an improved virtual power control power allocation strategy which is not affected by the line impedance parameters is studied.The control strategy sets a unified rotation angle in the power conversion,so that the relationship between the virtual power and the actual power is independent of the line impedance angle.According to the Routh stability criterion,the optimal value of the rotation angle is derived.Meanwhile,a low-pass filter is introduced in the coupling path to further reduce the coupling between the output active power and the output reactive power of the inverter.The small signal model of the control strategy is established and its stability is analyzed.Finally,using Matlab/Simulink software,the VSG system simulation model is built,and the control strategies of multiple VSG parallel islanding operation and pre-synchronization are simulated and verified respectively.Using the semi-physical simulation experimental platform,the main circuit is running on the FPGA board.The above control strategy is implemented by the control board based on TMS320F28335,and the two control strategies are verified by the closed-loop system formed by the interface board.The simulation and experimental results verify the correctness and effectiveness of the proposed control strategy.