Research On Stability Of Random Operating Condition Of Parallel Inverter Of Island Microgrid

In recent years,the microgrid as a carrier of energy innovation has developed rapidly,which has aroused great concern from scholars of various countries.This is because the microgrid is a clean and environmentally friendly,flexible and reliable distributed generation(DG).The island operation of the microgrid is equivalent to a self-sufficient system.Since there is no large power grid to support it,its ability to withstand disturbance is relatively weak,and it has great randomness.When the bus voltage is not ideal and other working conditions,the microgrid inverter that has lost the support of the large power grid must have a strong ability to maintain the stable operation of the island.In this paper,the islanded micro-grid parallel inverter is taken as the object of discussion to study the random operating conditions in the control operation.First,the three-phase voltage-type inverter is mathematically modeled,and the controllers of each layer are designed,including the power loop and the bottom voltage and current loop.Considering the output characteristics of the voltage source of the drooping inverter,the Thevenin equivalent output impedance model is constructed.The effect of different parameters on the output impedance is discussed in depth,which provides guidance for improving the inverter output impedance and controller parameters,so as to establish the theoretical basis for the inverter parallel research below.Secondly,an island microgrid impedance model with three drooping inverters is constructed,and the impact of the random operating conditions of line impedance inconsistency on the resonance mechanism of the source network is analyzed by impedance analysis.The PLECS simulation platform found that the line impedance inconsistency will produce high Frequency harmonics.A composite impedance control strategy combining internal and external virtual impedance is designed.The impedance analysis method is also used to analyze the effectiveness and stability.The simulation results show that high-frequency harmonics can be effectively suppressed,which verifies the correctness and superiority of the method.Thirdly,considering the effect of the local load switching on the island microgrid system,the power sharing conditions when the inverters are operated in parallel are discussed.When the traditional droop control strategy is used to randomly input local loads,the power is not very good The ground is equally divided and the dynamic response capability is poor,and the time to enter the steady state is longer.To solve this problem,a control scheme based on Cascaded Delayed Signal Cancellation(CDSC)and virtual impedance is designed,which can greatly improve the dynamic response capability and power sharing accuracy of the system.Finally,when the random switching of the load causes the power of the microgrid to be insufficient and the frequency exceeds the limit,a frequency modulation control strategy needs to be adopted.For the random operating condition with large frequency fluctuations,an improved scheme of frequency modulation based on model prediction is adopted,Realized frequency compensation,and verified through simulation that the strategy can not only improve the dynamic characteristics of frequency adjustment,but also reduce the sensitivity to system parameters,which can greatly enhance the island microgrid's random operating conditions under large frequency fluctuations.Stability and robustness.