Research On Strategy Of Zero-Sequence Circulating Current Suppression And Power Coordinated Control In AC/DC Hybrid Microgrid

With the vigorous development of power electronics technology,AC/DC hybrid microgrids are widely used in the field of distributed generation due to their flexible operating modes and effective reduction of frequent load energy conversion,and will become the mainstream structure of microgrids in the future.In order to increase system capacity and conversion efficiency,paralleled three-phase pulse width modulation(PWM)converter technology is introduced into the microgrid.However,large differences in line parameters of paralleled PWM converters are liable to cause excessive zero-sequence circulating current(ZSCC),which not only raises the system losses,but also greatly increases the probability of converter failure shutdown.In addition,for the large scale of hybrid microgrid and complex control process,it is of great significance and value to design the reasonable power coordinated control strategy to maintain the power balance of the system.This thesis takes the hybrid microgrid based on paralleled interlinking converters(ILC)as the research object,and studies the underlying converter control strategy,the ZSCC suppression strategy and the overall power coordinated control strategy to achieve the system stability under different operating conditions.The main contents are as follows:First of all,this thesis introduces the hybrid microgrid topology with photovoltaic as the main generation and paralleled ILCs,and constructs micro-source model for system requirements.Based on the above topology and model,this thesis designs various control modes of underlying converter in different application scenarios,and verifies its correctness through simulation.Secondly,to solve the problem that ZSCC easily generates in two paralleled PWM converters under the condition of different line parameters,with the basis method of adjusting the zero-vector action time by traditional PI control,this thesis proposes a ZSCC suppression strategy based on proportional-integral quasi-resonant(PIQR)control and feedforward compensation control.To simultaneously reduce the amplitude of ZSCC and suppress the main low-frequency harmonics,this thesis proposes the method of combining PI controller,quasi resonant controller and feedforward compensation to generate the zero-vector duty ratio adjusting factor,and builds paralleled PWM converter simulation model.The simulation shows that compared with the traditional PI control method,the proposed ZSCC suppression strategy can more effectively suppress the amplitude of ZSCC,reduce low-frequency harmonics and improve the power quality.Lastly,by the hybrid microgrid model test,it can greatly reduce the ZSCC in paralleled ILCs,and maintain the DC bus voltage stability.Finally,based on the hierarchical control idea of hybrid microgrid and considering the system working mode,this thesis designs the power coordinated control strategy under the mode of grid-connected and off-grid.In grid-connected operation,based on the DC bus constant voltage control of the ILC,this thesis refers to the DC storage state of charge,the DC sub-grid power balance state and the maximum transmission power of the ILC,and stabilizes the DC bus voltage by adjusting the operating mode of the DC storage and the secondary load on-off.In islanding operation,this thesis takes DC sub-grid as main microgrid,DC storage as main power regulation equipment.In accordance with the principle of DC storage "late discharging and early charging",it refers to the DC storage state of charge,the overall power balance state of the system and the operation mode of each micro-source,and maintains the balance of overall power by regulating the control strategy of the micro-source,the reference power and the secondary load on-off,etc.Lastly,this thesis builds simulation model of hybrid microgrid and designs multiple operating conditions to verify the effectiveness and stability of the proposed power coordinated control strategy.