Research On Control Strategy Of Interlinking Converter In Islanded Hybrid AC/DC Microgrid

In microgrid,hybrid AC/DC microgrids have advantages of both DC microgrid and AC microgrid,which can enhance the flexibility and reliability of distributed generation absorption,effectively reduce power electronic power conversion links and improve the efficiency of renewable energy generation.The hybrid AC/DC microgrids is the mainstream direction of micro-grid technology development in the current and future decades.As the core device connecting AC and DC buses,the interlinking converter(ILC)plays an important role in coordinating the bidirectional power flow between the dual subgrids.The ILC's control strategy is very important for the stable operation of the system.The hybrid microgrid belongs to weak grid.Especially in islanded mode,AC bus loses its grid support,and ILCs need to take into account both AC and DC power balance and voltage stability,so the control strategy is more complex.Therefore,this thesis mainly studies the islanding operation control strategy of ILCs.The specific work is as follows:Firstly,the structure and operation principle of hybrid microgrid are described.And the outputted characteristics of AC and DC subgrids are analyzed.The application scenarios of ILCs in this research are defined.Then,the mathematical models of three-phase two-level ILC in three-phase stationary coordinate system,two-phase stationary coordinate system and synchronous rotating coordinate system are derived,and its power control strategy is deeply explored.That provides research basis for the optimal design of islanding operation control strategy of ELCs.Secondly,aiming at the dynamic frequency stability of hybrid microgrid in islanded mode,the causes of AC frequency oscillation in the transient response during large load switching disturbance on the AC side are analyzed.Referring to the operation mode of traditional power system,the mathematical model of synchronous generator is embedded into the control system of ILCs.An improved control strategy of ILCs with synchronous generator characteristics is proposed.The DC subgrid and ILCs are integrated into a virtual synchronous generator,which provides inertia frequency support for the AC bus,damps active power oscillation and improves the dynamic stability of the AC frequency.The effectiveness of the proposed strategy is verified by comparing simulation and experiment with traditional methodsThirdly,aiming at the complex condition of AC/DC hybrid microgrid accessing harmonic load and unbalanced load simultaneously in island mode,a unified control strategy of ILCs with power quality control function is proposed.The proposed strategy can be divided into two parts:voltage feedback virtual APF control and DC APF control without harmonic detection.The AC virtual APF can extract the harmonic and negative sequence current components by detecting the local AC-side voltage,compensate the AC voltage distortion and imbalance without affecting the basic functions of the power sharing.The proposed DC APF control strategy does not need a special harmonic extraction model.It can quickly compensate the DC side multi-frequency harmonic current,suppress DC bus voltage ripple,and realize decoupling control for power quality control of ILCs.The effectiveness of the modified strategy is proved by simulation and experiment.Finally,an experimental prototype of high efficiency ILC is developed by using wide bandgap semiconductor power device SiC Mosfet,and the hardware and software of the prototype are analyzed in detail.On this basis,a low-voltage hybrid microgrid experimenal platform is designed.Through nomolized control technology,ILCs are controlled to coordinate proportional power sharing between AC and DC subgrids,and autonomous operation of hybrid microgrid in islanded mode is realized.The experimental results prove the efficiency and feasibility of the prototype.