| As the link between the Distributed Generation(DG)and the AC bus or the grid,the control strategy of microgrid inverter is crucial to the stability of the microgrid system.Traditional microgrid inverter control strategies such as constant power control,constant voltage and constant frequency control,and traditional droop control cannot provide the system with "inertia" and "damping" characteristics.The Virtual Synchronous Generator(VSG)control strategy can not only realize the frequency and voltage regulation of the microgrid,but also provide the required inertia and damping for the system.It is an ideal microgrid inverter control strategy.However,When the load suddenly changes,the output frequency stability of the inverter is poor,which seriously affects the power quality.In addition,parallel inverters are the main way to realize power expansion,while inverters based on traditional virtual impedance control cannot achieve power sharing when the line impedances mismatch.How to realize the parallel power sharing of microgrid inverters based on VSG control and ensure the excellent frequency performance of the microgrid system has become a research focus of domestic and foreign scholars in recent years.Therefore,this paper focuses on the research of the frequency stability improvement of the microgrid inverter and the accurate power distribution method of the parallel system when the line impedance is mismatched.First of all,this paper adopts the Neutral Point Clamped(NPC)three-level inverter as the topology of the microgrid inverter,introduces the working principle of the NPC three-level inverter in detail,and deduces the static coordinate system and rotation mathematical model in the coordinate system.The control strategy of NPC three-level inverter based on VSG is analyzed,mainly including power outer loop,voltage and current inner loop control strategy and three high-efficiency modulation methods of zero-sequence injection,which lays the foundation for the theoretical analysis in subsequent chapters.Secondly,in view of the problem of poor frequency stability of the microgrid inverter using traditional fixed-parameter VSG control during islanding operation,the influence of inertia coefficient(J)and damping coefficient(D)changes on the system frequency is firstly analyzed,and then the rate of change of frequency and frequency offset information are introduced into the values of inertia coefficient and damping coefficient at the same time,and the threshold value of frequency information is set simultaneously,so that the segmented J parameter and the exponential D parameter can be changed in real time according to the power disturbance,thereby enhancing the anti-disturbance capability of the island microgrid system,improving the frequency stability of the system.Finally,the effectiveness of the proposed control strategy is verified by simulation results.Thirdly,a multi-parameter cooperative adaptive control strategy is proposed to solve the problem of uneven power distribution and circulating current caused by line impedance mismatch of parallel VSG systems.The paper firstly analyzes the matching method of droop coefficient,inertia damping coefficient and line impedance parameters in the parallel VSG system,establishes the small signal model of the parallel system,and provides a theoretical basis for the selection of key parameters.Secondly,using the voltage and output power information at the beginning and end of the line to accurately calculate the line impedance,effectively avoid errors caused by external factors such as line measurement.Under the premise of ensuring that the equivalent output impedance of the inverter is configured in inverse proportion to the capacity,the line impedance of the parallel VSG inverter is corrected in real time by using the adaptive virtual impedance to achieve accurate power distribution.In addition,in order to avoid the power oscillation of the parallel system,improve Frequency stability,a microgrid inverter control strategy based on J,D and virtual impedance three parameters synergistic adaptation is proposed,which effectively improves the parallel power sharing of microgrid inverters and the system frequency stability.Finally,the simulation is used to further verify the feasibility of this strategy.Finally,based on the DSP controller TMS320F28335 of TI and the real-time simulator HIL of Yuankuan Energy Co.,Ltd.,a Hardware-In-the-Loop experimental platform is built to verify the theoretical analysis in this paper.The experimental results verify the correctness and effectiveness of the proposed control strategies. |
