Virtual Synchronous Generator Technology And Its Application Under Unbalanced Power Grid

With the advancement of society,energy issues have become increasingly prominent,and the integration of new energy sources has become one of the research hotspots.However,with the increase of new energy penetration rate,due to its uncertainty and the low inertia of grid-connected inverters,it has a negative impact on the stability of the power grid.As a kind of inverter control strategy for simulating the rotor motion and external characteristics of synchronous generators,virtual synchronous generator control can improve the stability of grid connection of new energy.The research on virtual synchronous generator can be summarized as follows:(1)First,mathematical modeling of the synchronous generator.Simulate the external characteristics of the synchronous generator and the inertia and damping caused by the equation of motion of the rotor as the control core of the virtual synchronous generator.Virtual synchronous generator control is implemented according to the control loop of the power loop and the voltage and current double closed loop.By properly designing the virtual impedance,the line impedance ratio can be adjusted to make the active and reactive power better decoupling control.(2)The virtual synchronous generator draws on the traditional droop control small signal modeling method,and makes the variable parameter root trajectory through small signal modeling.The influence of the key parameters of the virtual synchronous generator on the stability of the system is analyzed.The response power command is established.The virtual synchronous generator is connected to the second-order small-signal model to study the dynamic response of different inertia and damping to the power command.(3)The analysis of the off-grid characteristics of the virtual synchronous generator includes one adjustment and two adjustments,one adjustment conforms to the droop characteristic,and the secondary adjustment can return to the rated value when the frequency or voltage amplitude deviates from the rated value.In the pre-synchronous mode,a self-synchronization scheme is designed.By using the characteristics of the virtual synchronous generator,the virtual impedance is set in the no-load state to generate a virtual current and the pre-synchronization is implemented by the power loop forming feedback to replace the phase-locked loop.In the grid-connected mode,experiments were carried out using a traditional three-phase inverter to analyze its power regulation characteristics.(4)Virtual synchronous generator application under unbalanced power grid.Firstly,the study of positive and negative sequence separation is carried out.The virtual synchronous generator modeling and improved control strategy for unbalanced grid conditions are established.Three control targets are obtained,and the negative sequence current command is inversely calculated by the control target.Simulation and experimental results verify the effectiveness of the proposed improved control strategy.