Dynamic Master-slave And Coordinated Fault-Ride Through Control Methods For Islanded Hybrid AC/DC Microgrids

Islanded hybrid AC/DC microgrids(HMGs)include both distributed AC and DC distributed renewable energy resources(DERs),which is an effective manner to independently supply power for remote areas,and to promote comprehensive reform of the energy industry.However,the islanded HMGs often have a small capacity,various types of DERs,loads,and operation modes.Therefore,inaccurate power-sharing,converter burnout due to overcurrent,large voltage deviations,and even system instability are prone to occur under loads/faults disturbances,which would seriously restrict the development of practical projects.This paper proposes dynamic master-slave control and coordinated fault-ride through(CFRT)control methods for HMGs.Three aspects of in-depth research including accurate power-sharing and voltage-frequency restorations under load disturbances,converter current-limiting protection,and system-level CFRT control under fault disturbances are carried out,relevant results are as follows.Focusing on the contradiction between the system power-sharing accuracy and the communication bandwidth under load disturbances,a robustnessenhanced dynamic master-slave control method is proposed for islanded HMGs.The concept of coordination factor(CF)is defined to overcome the difference between AC/DC DERs.A local differential time delay method(DDM)is presented to automatically select the DER with the largest CF as a unified leader within each microgrid,which greatly reduces the communication burden.After this,a dynamic master-slave architecture is constructed.In this way,the dynamic connections among all AC/DC DERs are established,which realizes the highprecise,strong-robust power-sharing between AC/DC DERs and system voltage and frequency restorations during load disturbances.Focusing on the overcurrent risk of key equipment during fault disturbances,the fault characteristics under different control strategies and bidirectional power flow are analyzed.Current limiting methods are also proposed.The equivalent network model and mathematical functions at the fault period are established to study the influencing factors and mechanism of fault characteristics.On this basis,a dual-coordinate proportional current-limiting method is designed for the bidirectional interlinking converter(BIC).Under the premise of equipment safety,the maximum power interaction on both sides and the maintenance of the DC bus voltage can be achieved.Also,it can be suitable for multiple types of control modes.Second,a quasi-PR mode-switch-based current limiting method is proposed for AC DERs to achieve a safe grid-connection operation.The safe operation of key equipment lay a solid foundation for system-level CFRT.Focusing on the difficulty of bidirectional AC/DC bus voltage support during fault disturbances,the coupling effects of converters’ FRT control on bus voltages are analyzed,and an efficient CFRT method is proposed for islanded HMGs.First,the fault interaction model of the AC/DC microgrids is established.It is found that the FRT control of AC DER will cause a large AC frequency deviation and the AC/DC bus over-voltage/low-voltage.In this regard,a CFRT method is proposed.The AC DERs are responsible to track the bus voltage through adaptive mode switching control,the DC DERs are responsible to reduce the DC bus voltage deviation through adaptive power balance control,and the BIC is responsible to provide the bidirectional bus voltage supports.The proposed method coordinates all the converters reasonably and orderly throughout the fault periods.Finally,the AC frequency support and the DC voltage maintenance under the premise of converter safety can be well achieved.Focusing on the instability issues of islanded HMGs with weak DC subsystems under fault disturbances,the fault coupling mechanism and FRT requirements under this condition are analyzed,and a dynamic reconfigurable CFRT control method is proposed.First,the multi-interactions model among AC DERs and AC/DC interaction model during fault periods are established.It is revealed that the line impedance and small capacity of the weak DC subsystem lead to the limited adjustable droop coefficient of DC DERs and increase the risk of system instability.In this regard,a dynamic reconfigurable CFRT control method for HMGs is proposed,which uses the local fault current differential delay method to autonomously identify the dynamically reconfigurable voltage reference unit under a random fault.All the AC DERs will be coordinated by the reference unit to realize the fault current limiting and AC voltage self-support.Then,the auxiliary decoupling control is presented for BIC to reduce the coupling effect of AC fault on the weak DC sub-system.Finally,high reliability,strong and stable power supply of HMGs with weak DC subsystems during fault disturbances can be achieved.This thesis studies the safe operation issues of islanded HMGs.Concerning the problems during load disturbances and fault disturbances,four research contents are carried out: the power-sharing and voltage/frequency restoration during load disturbance,the fault characteristics and current limiting of key equipment,the efficient CFRT control strategy,and reconfigurable voltage reference-based CFRT control method during fault disturbances.Relevant conclusions can provide technical ideas for the development of islanded HMGs engineering.