Research On Control Strategy Of The Wind-Solar-Storage-Inverter System Based On VSG

In recent years,with the increasing prominence of energy and environmental issues,the application of distributed generation（DG）based on renewable energy such as wind and solar power has gradually increased.However,the volatility and randomness of renewable energy sources pose significant challenges to the stability of the power grid system.To address this issue,this paper focuses on wind and solar DC microgrid systems,adds a hybrid energy storage system on the wind and solar side,and uses virtual synchronous generator（VSG）technology to control the microgrid inverter.On the one hand,this can mitigate wind and solar power fluctuations and enable wind and solar to operate at maximum power.On the other hand,it can give the grid-connected inverter of the wind and solar DC microgrid the damping and inertia characteristics similar to those of traditional synchronous generators（SG）,enabling it to participate in voltage and frequency regulation of the main grid and improve the grid-friendliness of the microgrid system.Firstly,this thesis derives the second-order mathematical model of SG.By analyzing its model,the mathematical model of a VSG is established,and the main design of the wind-solar-storage virtual synchronous generator system model is implemented,including the mechanical and electrical components of the VSG,as well as the active-frequency and reactive-voltage control parts.For the underlying control of the VSG inverter,the voltage and current double closed-loop are adopted.The voltage loop uses the PI controller to improve the voltage control accuracy,and the current loop uses the P controller to improve the current tracking speed.Furthermore,a wind-solar-storage virtual synchronous generator system is built.For the DC side of the less involved VSG inverter system,the respective mathematical models of photovoltaics,wind turbines,and hybrid energy storage and maximum power point tracking（MPPT）of the power generation module are studied and analyzed,introduced the wind-solar-storage VSG common DC bus topology and the effect of hybrid energy storage on wind-solar fluctuation power fluctuations under the VSG set power P_ref.To avoid the frequent over-limit phenomenon of the small-capacity supercapacitor in stabilizing the power fluctuations of the VSG DC side system and increase the stability of hybrid energy storage,a hybrid energy storage power allocation strategy with a second-order high-pass filter is proposed,In this case,the filter time constant varies in real-time with the supercapacitor′s charge state.In this strategy,two first-order filter allocation algorithms are connected in series to generate a second-order filter algorithm to restore the charge state.Meanwhile,the filter time constant is adaptively adjusted according to the charge state in the VSG DC side power fluctuation.Thus,when the supercapacitor’s charge value is in the buffer,the high-and low-frequency powers are allocated into each part of the hybrid energy storage for stabilization based on the variable time constant.while maintaining the stability of the VSG DC side system.MATLAB simulation results prove the effectiveness and correctness of the proposed strategy.Finally,since the VSG uses a medium-low voltage wind-solar DC micro-grid on the DC side,its output impedance is mainly resistive,which leads to active/reactive power coupling and affects the grid-connected control effect,and there will be a certain deviation in the phase of VSG and grid voltage when there is no pre-synchronization link.Aiming at the above problems,a VSG grid-connected control strategy with virtual impedance is proposed.Using the VSG mathematical model built above,a more accurate SOGI phase-locked loop is introduced and the virtual impedance is introduced into the VSG electrical part to improve the output characteristics of the grid-connected inverter.A secondary frequency modulation and voltage regulation strategy to improve VSG control is proposed to restore the frequency and voltage of VSG,and the SOGI phase-locked loop is used to collect the grid voltage phase to complete the grid-connected pre-synchronization design.MATLAB simulation results prove the effectiveness and correctness of the proposed strategy.