Coordinated Control Technology Of AC/DC Hybrid Microgrid Interlinking Converter

With the rapid development of new energy power generation and power electronic technology and the direct current of new power supply equipment,the research of AC and DC hybrid microgrid has attracted much attention.It combines the advantages of AC microgrid and DC microgrid and can flexibly access AC and DC power supply and load.In addition,it can effectively reduce the intermediate conversion link and the system cost,so the power flow and control mode more flexible,which is one of the important networking modes in the future.As the link between the AC subnet and the DC subnet,the interconnection converter is essential to realize the two-way flow and mutual support of power between the AC subnet and the DC subnet.Therefore,the coordinated control strategy of AC/DC hybrid microgrid interlinking converter is proposeded,three aspects of parallel circulating current suppression,power coordinated control strategy under different operation modes and stability analysis is analyzed and discussed,and through simulation and experiment is verified in this paper.Firstly,take the three-level TNPC interlinking converter as the research object,briefly analyze its topology and working principle,and establish a mathematical model to provide a theoretical basis for the subsequent chapters.Aiming at the problem of zero-sequence circulating current in the parallel connection of interlinking converters,a parallel circulating current model is established,its generation mechanism is analyzed,and the source and influencing factors of circulating current are given.Aiming at the high-frequency circulating current,taking a switching cycle as an example,the peak change rule of the high-frequency circulating current when the carrier angle difference is 180~o is analyzed,and the maximum stress caused by the high-frequency circulating current is given to guide the hardware filter design.Aiming at the low-frequency circulation,the average equivalent model is established.Through the Fourier decomposition of the zero-sequence excitation voltage source,the characteristics and composition of the low-frequency circulation components are deduced and analyzed.Based on the analysis results,a control strategy using PIR controller to suppress low-frequency circulation is proposed.Finally,the theoretical analysis and the proposed control strategy are verified by simulation and experiment.Secondly,research on the power coordinated control technology of interconnected converters is carried out.First,the operation mode of AC/DC hybrid microgrid is analyzed,and the control requirements of interlinking converters under different operation modes are summarized.Subsequently,the focus is on the research of power coordinated control strategies in the two operating modes of interconnection and off-grid.Aiming at the interconnected and grid-connected operation mode,a method for controlling AC power-DC bus voltage droop of interlinking converters is proposed,and the principle and strategy of power distribution are introduced.The proposed method can realize interlinking converters to adjust according to the preset curve.DC voltage,tracking dispatch command and power sharing among interlinking converters;for interconnected off-grid operation mode,a droop control based on P-ω,Q-u plus virtual impedance is given to achieve coordinated control of interlinking converter power,and the virtual The reason and influence of impedance introduction are given,and the method of selecting virtual impedance is given.Finally,research is carried out on the stability analysis of interlinking converters.The focus is on the stability of the DC bus when the interconnected converter adopts AC power-DC voltage droop control.The small-signal equivalent modeling of the DC/AC interconnected converter on the DC bus side is established,and the equivalent model is verified by simulation.Based on the impedance model,the analysis gives the change rule of the equivalent output impedance of the interlinking converter when the control parameters change.The impedance ratio criterion and the Nyquist curve are used to reveal the influence of the interlinking converter control parameters on the stability of the cascade system when the interlinking converter carries a constant power load.When the proportional parameter of the voltage loop of the interlinking converter increases,the cascaded system will tend to be unstable,which is verified by simulation,which provides guidance for the design and engineering debugging of the cascaded system control parameters.