Research On Power Coupling Characteristics And Decoupling Control Of Virtual Synchronous Generator

Due to the depletion of fossil energy and the accompanying ecological problems,distributed generation has developed rapidly in recent years.As the inertia of grid connected converter（GCC）is zero,with the continuous increase of new energy penetration,the inertia of power grid is gradually reduced.In order to ensure the stable operation of power system,the power electronic based distributed power generation equipment is expected to be involved in the inertial response.As one of the main methods for providing grid support,grid-connected inverter controlled by virtual synchronous generator has attracted much attention because it can simulate the inertia and damping characteristics of traditional synchronous machines.However,there exists the interaction between the active power loop and the reactive power loop.During normal operation,the power coupling issue limits the active power transmission capacity of grid connected inverter and aggravates the reactive power shortage of power grid.When the shortcircuit fault occurs,the power coupling degrades the transient stability of the system.Thus,this paper focuses on the power coupling of VSG,and the main research contents are shown as follows:1)In order to analyze the power coupling of VSG,a small signal model is proposed,which takes into account the equivalent grid impedance,phase difference and the reactive power loop.With the proposed model,the influence of virtual inductor on VSG power coupling is analyzed in detail.As a result of the control structure of VSG,there exists the inherent power coupling issue.Traditionally,when the line is pure inductive and the phase difference is very small,it is considered that the variations of active and reactive power are only related to the phase difference and voltage disturbance,respectively,i.e.,P-f,Q-V control mechanism.However,it is difficult to achieve the aforesaid two hypotheses at the same time.Hence,the traditional virtual inductor based decoupling method needs to be further studied.The proposed model indicates that the virtual inductor should not be considered as the real inductor in terms of decoupling.The decoupling ability of virtual inductor is limited,and an excessively large virtual inductance is not conducive to decoupling.In addition,when the grid impedance reaches a certain critical condition,it is not suitable to use virtual inductor for power decoupling.The research in this part lays a theoretical foundation for the further improvement of decoupling strategy.2)The q axis voltage drop of virtual inductor based power decoupling control（QVPDC）is proposed to eliminate the power coupling of VSG.The traditional virtual inductor based decoupling method only focuses on the effect of impedance angle.The reason that the virtual inductor can reduce power coupling is because proper virtual inductor can provide certain voltage compensation.Additionally,the limited decoupling capability of virtual inductor results from the limited voltage compensation.Firstly,the feed forward from active power output to voltage reference is introduced to further verify the influence of voltage compensation on power decoupling performance.Small signal analysis shows that enough voltage compensation can eliminate power coupling.Then,phasor diagram based qualitative analysis and semi quantitative calculation are given to study the influence of virtual inductor on the voltage compensation capability.The results indicate that the voltage output capability of VSG is limited by the d axis voltage drop of virtual inductor.Finally,to overcome the negative effect of d axis voltage drop,only the q-axis voltage drop is considered when using the virtual inductor based decoupling method,i.e.,the proposed QVPDC method.3)For the case of voltage sag,the transient stability characteristics of VSG are analyzed.And,based on the direct power control（DPC）,DPVSG control strategy is proposed.The above-mentioned QVPDC method can only improve the active power transmission capacity of VSG slightly and improve the power angle instability caused by the frequency integration of VSG active power loop.However,the active power transmission capacity is restricted by the port voltage when the short-circuit fault occurs.Unless the fault is cleared on time,the transient instability cannot be avoided.The traditional solution is to switch to the vector current control when the voltage fault occurs.Nevertheless,the vector current control based grid connected system cannot work in the island mode,due to the current source characteristic.In order to maintain the grid support capability and island operation capability in case of large disturbance,this paper analyzes the power control characteristics of VSG.It indicates that the Q_vsg-v_cd-i_od-P_vsg characteristics of VSG power loops will affect the active power output and intensify the interaction between power loops.Then,on the basis of DPC,the power loops of VSG are changed to the P_vsg-v_cd,Q_vsg-v（cq） control strategy,i.e.,DPVSG control.As a result,the grid connected converter will have good stability and power grid support capacity in case of short circuit fault.