Research On Transient Characteristic Optimization Strategy Of Virtual Synchronous Generator Controlled Microgrid Inverter

Distributed power sources are connected to the AC microgrid through a power electronic converter,and the inverter is the key device for realizing power conversion.Equivalent voltage source inverters in microgrids,especially the virtual synchronous generators(VSG),show obvious advantages in many aspects,such as the grid voltage support capability,independent load capacity,fault current limiting capability,multi-units parallel flexibility,grid-tied/islanding switching flexibility,etc.The interface scheme of the distributed grid-connected power sources not only needs to meet the requirements of microgrid stability,but also needs to focus on the power characteristics of the distributed power sources.This paper mainly studies the VSG technology applied to photovoltaic(PV)and energy storage(ES)in AC microgrid,and its transient characteristic optimization strategy.The main function of the PV inverter is to transfer the PV power to the microgrid,and minimize the impact of the power change on the system performance and stability.At the same time,it should be well adapted to the microgrid environment and avoid the instability of the grid-connected PV system caused by microgrid disturbance.The PV VSG technology solves the problem of PV sources smooth access to the microgrid.However,the PV VSG has limited support ability for the microgrid frequency,and the microgrid frequency disturbance has a great influence on the power response characteristics of the PV VSG which results in the poor microgrid frequency adaptability of the PV VSG.Based on the functional positioning of PV sources in AC microgrid discussed earlier,this paper proposes a control strategy for the dispatching PV VSG(DPVVSG).Through the frequency feedforward technique,the frequency of the point of common coupling(PCC)is introduced into the power closed-loop control of the DPVVSG,which eliminates the dominant conjugate poles causing the power oscillation,reduces the sensitivity of the VSG output power responding to the microgrid frequency disturbance,and improves the PV VSG adaptability of microgrid frequency disturbances.The proposed frequency feedforward strategy does not change the power response characteristics of the power reference change,thus preserving the advantages of PV sources smooth access to the microgrid.In this paper,the small-signal model of the proposed strategy is established,and the influence of the equivalent gain and time constant of the frequency-locked loop on the system performance is analyzed.Finally,an experimental platform was built to verify the performance advantages of the proposed DPVVSG strategy.Unlike the dispatching PV VSG,the non-dispatching PV VSG(nDPVVSG)considers the control of the inverter DC bus voltage.Based on the functional positioning of PV sources in AC microgrid,this paper proposes an nDPVVSG strategy.The strategy makes full use of the buffer energy of the DC bus capacitor to solve the problem of PV sources smooth access to the microgrid.A frequency feedforward strategy is proposed to introduce the PCC frequency into the closed loop of the inverter DC bus voltage,which improves the damping characteristics of the frequency disturbance,and reduces the influence of the microgrid frequency disturbance on the DC bus capacitor voltage.In this paper,a phase-locked loop parameter configuration method is proposed to realize the decoupling control of the response characteristics of the power generation disturbance and the microgrid frequency disturbance.Based on the requirements of the power response peak time and the damping ratio of the frequency disturbance,a parameter design method for the nDPVVSG strategy is proposed.Finally,the response characteristics are evaluated to verify the performance advantages of the proposed nDPVVSG strategy under strong microgrid step disturbance,weak microgrid step disturbance and weak microgrid random disturbance.The conventional ES VSG strategy can effectively improve the suppression ability of the microgrid frequency disturbance,but the output power oscillation phenomenon of the VSG is also significant when the power reference changes.Aiming at the problem of poor power response performance,this paper proposes a VSG real power optimization(VSG-RPO)strategy based on the virtual inertia power control.This strategy introduces a low-pass filter in the virtual inertia power control loop.By adjusting the time constant of the low-pass filter,the damping characteristic of the power response is improved,thereby solving the problem of poor power response performance.In this paper,the small-signal model of the proposed VSG-RPO strategy is established.Based on the system phase margin and frequency response peak value,a simple parameter design method is proposed.In addition,this paper proposes two transient response performance evaluation indicators,and compares the response characteristics of the proposed VSG-RPO strategy and the other two optimization strategies in terms of power reference changes and microgrid frequency disturbances.The experimental results show that the proposed VSG-RPO strategy can improve the power regulation performance of the ES VSG,and the proposed indicators can comprehensively evaluate the transient oscillation phenomenon of the power response and the frequency response.The VSG-RPO strategy is proposed for improving the power regulation performance of the ES VSG,but the microgrid frequency support capability of the VSG-RPO strategy is reduced compared with conventional VSG strategy.To solve this problem,based on the VSG-RPO strategy,this paper proposes an enhanced VSG(eVSG)strategy with enhanced microgrid frequency support capability.The eVSG strategy introduces two PCC frequency information in the virtual inertia power control loop: the estimated PCC frequency and the measured PCC frequency.On the one hand,the eVSG strategy improves the power regulation performance of the ES VSG;on the other hand,the eVSG strategy provides additional power support for the microgrid frequency disturbance.In this paper,the small-signal model of the proposed eVSG strategy is established and the power regulation performance and microgrid frequency support capability are evaluated.Besides,the effect of the eVSG equivalent output impedance the and the microgrid equivalent line impedance on the virtual inertia control is analyzed.Finally,the experimental results verify the performance advantages of the proposed eVSG strategy in terms of power regulation capability and microgrid frequency support capability.