Research On Stability Analysis And Improvement Method Of Virtual Synchronous Generator Grid-Connected System

In recent years,as the growing requirement in lessening emission of carbon dioxide,large-scale renewable energy generation units are integrated into power grid.The modern power systems are gradually dominated by the synchronous generator（SG）and power electronics equipment.The power electronic converters,as the interface between renewable energy generation units and power grid,is playing a significant role in distributed generation grid-connected systems with penetration of distribute renewable energy.However,penetration of large-scale power electronics converters brings huge challenges for the stable operation of the system,as it is lack of the required inertia and damping as well as motion characteristics,which does not resemble as the conventional synchronous generators.Virtual synchronous generator（VSG）control,which emulates the external characteristics of SG and thereby providing the support of virtual inertia and virtual damping for the systems,has been one of the hot topics in the current research fields.The introduction of virtual inertia and virtual damping can strengthen stability of power systems and make transient response of the system much smoother.However,stability issues still exist caused by the dynamic couplings between grid-connected inverter and power grid as well as comparatively large resistance and inductance of the lines.The research findings of this paper is aided by the fundings of National key Research and Development Program“Key technology and applications in Medium and low voltage DC distribution systems”and“Key technology and equipments of DC distribution system with integration of distributed photovoltaic of multi-port”.Specifically,this paper investigates dynamic characteristics and stability of virtual synchronous generator grid-connected system from three perspectives,i.e.,power coupling characteristics and oscillation mechanism,stability analytical method of grid-connected system,and strengthening VSG control for suppressing oscillation,where VSG grid-connected system with multi-mode control architectures is recognized as the background and research object.Finally,the dynamic interactive influence between AC and DC subsysetms is studied for the system of bi-polar distributed photovoltaic grid-connected architecture.In addition,the collaborative power relationship between AC and DC subsystems are analyzed.Furthermore,the application of strengthening VSG control in hybrid AC-DC interconnection system is investigated.The major research contents and the innovative contributions of this paper include:（1）Aiming at power coupling characteristics and oscillation problem of VSG grid-connected system,the small signal model which considers the power coupling is built.In addition,the power coupling mechanism of grid-connected system is investigated,and the active-power magnitude-phase analytical method is put forward with taking reactive power coupling term into account.Additionally,the mechanism of oscillation instability of distribution network of large ratio of inductance divided by resistance with integration of single-stage AC distributed photovoltaic is analyzed from two perspectives,i.e.,damping effect and synchronizing effect.Aiming at negative damping effect brought by the reactive power coupling term,this paper put forwards reactive power feedforward decoupling control to implement thorough decoupling between active-and reactive-power,which mitigates negative damping oscillation instability issues caused by power coupling and strengthens stability and reliability of grid-connected system.Finally,simulations and experiments validates the accuracy of the oscillation mechanism analysis as well as the effectiveness of reactive power decoupling control strategy.（2）This paper puts forward a stability analytical method for the VSG grid-connected system based on the coupling feedback between voltage and frequency,and a frequency small signal model which considering loop coupling is established.In particular,the stability of the grid-connected system is studied by the analytical object which connects the frequency between the i^th and the（i+1）^th period,whether it forms negative feedback or positive feedback of the system can be judged as well.It offsets the shortages of the active power magnitude-phase analytical framework that do not takes the impact of virtual inertia in feedforward channel on system stability into account.To further strengthen stability of grid-connected system,this paper puts forwards virtual inductance control.It can implement power decoupling and provide enough damping support for the system to guarantee stable operation of the system.In the meantime,it lessens the synchronizing component.Besides,a calculation method of equivalence of inertia and damping is put forward to quantify inertia and damping of the system,which is used for the future optimization design of system parameters.The accuracy of proposed stability analytical method as well as virtual inductance control is verified by simulations and experiments results.（3）According to the above oscillation mechanism analysis as well as stability analytical method of VSG grid-connected system,this paper puts forward series active power magnitude compensation control strategy to reshape the VSG control loops.The proposed control aims to decline open-loop magnitude of the system and thus decline the cutoff frequency of the system.It can bring several advantages,e.g.,implementation of strengthening of system phase margin,lessening negative damping effect of the system,and avoiding risk of loss of synchronization brought by virtual inductance control.The equivalence of inertia and damping is calculated with the introduction of the series active power magnitude compensation control strategy,which aims to provide the guidance of control design of the grid-connected inverter.In addition,this paper combines the above three stability analytical methods（active power magnitude phase motion equation（MPME）,feedback analytical method based on the coupling between frequency and voltage,equivalence of inertia and damping）to clarify the mechanism of instability of the VSG grid-connected system.In addition,the applicability of strengthening VSG control strategy,which aims to mitigate oscillation of the system,is investigated.The proposed series active power magnitude compensation control strategy is inserted in the active power control loops of VSG,to effectively mitigate negative damping oscillation of the system and meanwhile provide inertial support for the system.The proposed control has several advantages,i.e.,convenient design,easy implementation,and strong robustness.Finally,the effectivenss of the proposed control is demonstrated by the experimental results.（4）Focusing on the stability issue of VSG grid-connected system with penetration of double-stage distributed photovoltaic units,this paper puts forward power coordination control of VSG grid-connected system based on DC capacitor inertial support characteristics.Specifically,the frequency deviation is passing through frequency regulator and then output extra DC voltage component.By means of dynamic interaction between DC voltage and AC frequency,the coordinated effect between AC-and DC-side is realized,which can provide supplementary inertial support for AC system.In addition,a proportional-integral frequency regulator is proposed.Supplementary inertia and damping support is realized from view of power coordination.This paper investigates the application of proposed strengthening VSG control in hybrid AC-DC interconnection system.Simulation results verifies effectiveness and robustness of the power coordinated control as well as the strengthening VSG control in the application of the hybrid AC-DC interconnection system.