Research On Control Strategy For Virtual Synchronous Generator Under Non-ideal Condition

With the renewable power generators are connected to the grid through grid-connected inverters on a large scale,the inertia and the damping of the whole power system are relatively reduced.Thus,the voltage regulation and the frequcy regulation of the power grid are more difficult,bringing a series of new influences and challenges to the safe and stable operation of power system.The virtual synchronous generator is a voltage source type grid-connected power electronic device,which mimics synchronous generator and is expected to be a breakthrough to solve the stability problem of power system with high penetration.In this paper,the control strategy of the virtual synchronous generator is studied under the non-ideal power grid and the unbalanced load.First of all,this thesis focuses on the operation characteristics of the virtual synchronous generator under non-ideal grid condition.The grid-connected current formula of three-phase virtual synchronous generator is derived,on this basis,the impacts of the unbalanced and harmonic grid voltages to the grid-connected current are analyzed.A VSG balanced current control method that consists of improved algorithm of current loop reference and grid voltage feedforward control scheme is proposed.The improved algorithm of current loop reference based on the Double Second Order Generalized Integrator can suppress the negative influence of non-ideal grid voltage on the current loop.The grid voltage feedforward control scheme is adopted to suppress the harmonic and unbalanced components of grid-connected current affected by non-ideal grid.In addition,the structure and parameter of grid voltage feedforward control are analyzed and designed.The three-phase four-leg topology is often used to provide the zero-sequence current path for unbalanced loads when the virtual synchronous generator operates at standalone mode.An intuitional and clear conclusion about the unbalanced output voltages distortion mechanism under unbalanced loads is presented.Then,the working principle of three-phase four-leg virtual synchronous generator is analyzed,and it is concluded that three-phase four-leg VSG has the ability to supply unbalanced loads.In order to deal with the unbalanced loads more efficiently,an independent excitation control method is proposed.By controlling each phase voltage separately,the control between the three phases is completely decoupled.In this thesis,simulation model of the virtual synchronous generator is built in Matlab/Simulink.In addition,an experimental platform controlled by TMS320F28335 is established for experimental justification.The hardware and software is well designed based on the control requirements of the virtual synchronous generator.The simulation and experimental results are given to verify the effectiveness of theoretical analysis and control method presented above.