Study On Grid Connected Control Strategy Of Virtual Synchronous Generator Under Temporary Working Conditions

Due to its renewability and cleanliness,the proportion of distributed new energy power generation in the power generation side of the power system is increasing year by year,which will not only make the power system suffer from load disturbances,but also the generation will be affected by factors like fluctuations in new energy power generation output.The large-scale access to distributed power will reduce the proportion of synchronous generators,further weakening the inertia and damping in the system and weak load disturbances may lead to system instability.To solve this kind of problem,this paper studies the virtual synchronous generator control strategy of the grid-connected inverter,analyzes the mathematical model of virtual synchronous generator,and makes a collaborative adaptive improvement of its virtual control parameters,and makes it maintain stable operation when grid side voltage drop fault through the switching of the current command,so that the virtual synchronous generator can cope with more complex working conditions.First,this paper establishes the mathematical model of the grid-connected inverter,obtains the mathematical model of virtual synchronous generator by simulating the electromagnetic and mechanical characteristics of synchronous generator,and refers to the active power-frequency and reactive power-voltage droop characteristics of the synchronous generator for frequency modulation and voltage regulation control,and virtual synchronous generator control strategy is used to control the grid-connected inverter.The simulation results show the correctness of the virtual synchronous motor control strategy.Then,the paper analyzes the influence of frequency output characteristics and the virtual synchronous generator active power caused by virtual inertia and damping parameters by the small-signal model of virtual synchronous generator.Combined with the angular frequency oscillation characteristics of synchronous generator,the virtual inertia and damping parameters are improved cooperatively and adaptively,and set threshold for the adaptive control,so that the adaptive control of virtual synchronous generator can improve its output response by being flexibly adjusted according to the load fluctuation.The simulation results show the optimized virtual parameter cooperative adaptive control strategy of virtual synchronous generator can optimize inverter output active power overshoot and limit frequency fluctuation of the virtual synchronous generator.Finally,given that the traditional virtual synchronous generator has yet to have the low voltage ride-through capability,the low voltage ride-through control of virtual synchronous generator is improved by current command switching method.By analyzing the power angle characteristics and short-circuit current characteristics of the virtual synchronous generator,when a voltage drop fault occurs on the distribution network side,the synchronous operation of virtual synchronous generator can be maintained and the fault inrush current can be limited by reducing its active power command and adjusting its output voltage of the machine terminal.The virtual synchronous generator current command is switched to the command under a failure to limit the fault inrush current,and the self-synchronization virtual synchronous generator could provide the phase angle for coordinate transformation of low voltage ride-through control.The simulation results prove that the control strategy can limit the fault current and offer power support for the system when the fault occurs,and maintain the output current balance when the voltage is unbalanced.