Control Strategy Of Microgrid Inverter And Stability Analysis Based On Virtual Synchronous Generator

Due to the intermittence and randomness of distributed generation and the existence of a large number of power electronic devices in the process of its access to the microgrid,the microgrid is easy to generate power oscillations.At the same time,the moment of inertia and damping of the system are reduced,so that the strength of the microgrid system become weakened.However,the traditional control strategy is difficult to meet the autonomous operation requirements of islanded microgrid,and the possibility of instability is greatly increased.In order to meet the requirement of frequency and voltage support in islanded microgrid,the control strategy,virtual synchronous generator(VSG)is adopted for the inverter of microgrid in this paper.And then the impedance model of VSG inverter is established to be a basis to study and analyze the stability of islanded microgrid.First of all,according to the characteristics of photovoltaic cells and static energy storage cells,the two are combined to establish an islanded microgrid power system in this paper.The output of photovoltaic arrays is affected by the light intensity and the temperature,but it could be controled by the maximum power point tracking(MPPT)technology,so that the utilization of solar energy could be improved.The DC side voltage could be stabilized by the energy storage battery through the Bidirectional DCDC converter.It is conducive to improve the reliability of photovoltaic power.Secondly,based on the inherent characteristics of synchronous generator(SG),the VSG control principle is described well in this paper,and virtual inertia,virtual governor and virtual excitation controller are designed.Besides,imitating the SG stator equation,virtual impedance is designed to solve the problem of power distribution imbalance in parallel system caused by the mismatch of tie line impedance.The simulation results are shown that VSG control could support and stabilize the frequency of islanded system more effectively than Droop contro.And the accuracy of reactive power distribution and system stability of parallel system could be improved by the introduction of virtual impedance.Thirdly,the equivalent mean model and small signal model of the inverter for the isolated microgrid are derived by using the switching mean method.This method is easy to solve the circuit state,and the output impedance model of the VSG inverter is established on this basis.And then,the influence of the parameters of the current loop,voltage loop and power loop on the output impedance is analyzed by the Bode diagram of the output impedance.It is proved that the VSG control could significantly affect the low-frequency characteristics of the inverter output impedance.Finally,the impedance analysis model and stability criterion of single VSG inverter system and multiple VSG inverter systems are derived.Different from the complex model solving algorithm and criterion of the traditional eigenvalue method,the impedance analysis method combineing with the Nyquist criterion is adopted in this paper.Because it is easy to model and analyze.It is proved that VSG islanded system could maintain voltage stability with different load type.And the of microgrid stability could be increased by parallel connection of multiple VSG inverters;without adding low-pass filter and contact line inductance,islanded microgrid will loss of stability.The voltage and frequency fluctuate violently,so the power cannot be transmitted effectively.