Modeling Analysis And Robust Frequency Control Of Virtual Synchronous Machine Based Microgrid

With the increased utilization of inverter-based distributed generation in microgrid,the overall inertia of microgrid decreases,which affects the stability of frequency and voltage of microgrid.Virtual synchronous machine(VSM)control enables the converter to simulate output characteristics of the synchronous machine and provide inertial active and reactive power response in case of abnormal frequency and voltage,so as to improve inertia and damping of microgrid.If the VSM control is applied to all inverter-based generation units and rectifier-based controllable loads in microgrid,i.e.,virtual synchronous microgrid(VSMG),the differences in stability and dynamic characteristics between VSMG and traditional droop microgrid(DMG),and potential defects in the VSMG need to be further analyzed.In addition,due to the coupling of frequency among microgrids,for the VSMG cluster(VSMGC)composed of multiple VSMGs,its secondary frequency control should be reconsidered among the cluster level to coordinate the frequency recovery.This dessertation summarizes the research status of VSM control and application,microgrid modeling analysis and reduction,and microgrid(cluster)secondary control at home and abroad.In view of existing deficiencies of VSM and its application in microgrid,the following research is carried out along the route of "VSM-VSMG-VSMGC".1)The VSM basic control strategy is introduced,and the low-voltage ride-through control as well as power quality composite control are proposed for VSM.The VSM low-voltage ridethrough control continuously provides inertial power support under symmetrical and asymmetric fault of power grid,and reduces transient impact on power grid after fault.The power quality composite control makes the VSM actively respond to frequency and voltage changes in the grid,and effectively compensate harmonic,reactive and unbalanced components in the load current,so as to improve power quality of the grid side current.2)The VSM control parameters tuning method considering its dynamic performance is proposed.By establishing the VSM power response dynamic model,the mechanism of VSM participating in power grid regulation is analyzed.The transfer function models of VSM participating in grid frequency and voltage regulations are deduced.Then,on the premise of ensuring the stability of active power loop,frequency loop and reactive power loop,the VSM control parameter tuning method considering dynamic performance is proposed,which makes the VSM inertial power response time coordinate with the response time of primary frequency regulation and area voltage control of power grid.3)The full-order nonlinear mathematical model and full-order small-signal model of VSMG are established.Participation factor analysis indicates that slow dynamics of the VSMG is mainly affected by state variables of synchronous machine and VSM power control.The small-signal stability and dynamic characteristics of both frequency-domain and time-domain are compared between VSMG and DMG.It is found that the VSMG has better robustness against load disturbance and parameters change,as well as better small-signal stability than DMG.Besides,the VSMG performs better dynamic and steady-state response under load power and grid frequency and voltage disturbances.4)The eqivalent synchronous generator(Eq SG)method is proposed for VSMG model order reduction.Based on the slow dynamics of VSMG,the Eq SG model structure is proposed.Based on the gray-box system identification technology,the time-domain dynamic response error between linear Eq SG model and VSMG full-order small-signal model is minimized by alternate iteration,such that realize the dynamic equivalence between Eq SG model(reduced model)and VSMG model.The comparison with three representative model reduction methods,i.e.,balanced transformation,singular perturbation,and poles clustering,shows that the Eq SG method performs better matching in the low-frequency band response and time-domain response with the VSMG model.Besides,the Eq SG model maintains equivalent physical meaning in the state variables,and can replace VSMG full-order mathematical model in the power system time-domain simulation under small disturbances scenarios.5)A robust secondary frequency control design method based on equivalent modeling is proposed for VSMGC.The Eq SG model is used to establish the VSMGC equivalent model.Modeling errors caused by equivalent modeling and model linearization are integrated into the VSMGC model as unstructured uncertainties.The distributed-centralized control structure is proposed for low-voltage microgrid cluster,and the structured μ-synthesis is used to design the secondary frequency control law,such that realize robustness in the case of predefined control structure and system model containing unstructured uncertainties.The event-triggered mechanism is introduced to reduce the communication bandwidth.By constructing Lyapunov function and solving linear matrix inequalities,the event-triggered parameters are obtained to ensure the robust stability.The proposed method is robust to load and renewable energy power disturbances,communication delay and packet loss,single communication link or controller failure while realizing coordinated frequency recovery of the VSMGC.