| Under the carbon emission peak and carbon neutralization(bi-carbon)goal,the AC microgrid(hereafter referred to as microgrid)has become the focus of current research due to its advantages of flexible control and high penetration of new energy resources.A microgrid is generally connected to the main grid through a point of common coupling(PCC)and can operate in both grid-tied mode(GTM)and islanded mode(ILM).In GTM,the voltage and frequency of the AC bus are provided by the main grid,and the converters of the distributed resources act as grid-following converters and transmit appointed power.When the main grid is disconnected due to maintenance or failure,the microgrid enters the ILM.In this case,the voltage and frequency of the AC bus are provided by the energy storage converter(ESC).In the whole operation process,the microgrid needs to always meet the load’s demand for power quality.In addition,when multiple ESCs are operated in parallel,it is necessary to achieve the proper power distribution between ESCs according to their capacities.Since the load’s requirements on voltage and frequency in GTM can be satisfied by the main grid,the current researches focus on the control strategies in the mode switching process and ILM.Although many methods have been proposed to tackle the above issues,they are mainly based on the traditional control methods such as power droop control,which endow the inherent nonlinear characteristics and poor dynamic performance of the traditional control methods.Therefore,this paper focuses on the mode switching process and ILM,proposing novel control strategies for a single-ESC-operated scenario and a multi-ESC-operated scenario respectively.In the mode switching process,for a single-ESC-operated scenario,a seamless transfer control strategy based on power adaptive tracking characteristic is proposed,to achieve the smooth transfer between GTM and ILM.Firstly,by deducing the power balance equation of the microgrid,it is revealed that the cause of voltage and frequency offsets during the mode switching process is the mismatch of the ESC output power and the rated load power.Then,the power adaptive tracking loop is introduced into the controller,so that the output power of the ESC can adaptively track the rated load power when islanding occurs,so as to ensure the load voltage and frequency remain within the allowable range.In addition,a universal parameter design method is presented based on the stability limitations of the control system and the power quality requirements of AC loads.Finally,experimental results clearly validate the proposed control strategy and parameter design method.In the mode switching process,for a multi-ESC-operated scenario,a seamless transfer control strategy based on voltage-current(V-I)droop characteristic is proposed,to achieve the smooth transfer between GTM and ILM and realize the proper power sharing between ESCs.Firstly,the output power of ESC is analyzed both in GTM and ILM,and it is pointed out that the key factors affecting the power sharing performance are the combined resistance and the no-load output voltage phase of ESC.Then,the operating principles of V-I droop control both in GTM and ILM are presented straightforwardly with combined resistance.Besides,a communication-free synchronization algorithm enables both synchronization and pre-synchronization capabilities is elaborated in detail.The proposed synchronization strategy adaptively adjusts the phase steps of ESCs according to the phase differences between ESCs and the AC bus,to mitigate the phase differences of ESCs.Finally,the proposed strategy is validated experimentally by developing a 2-ESC-parallel-operated microgrid system.In ILM,for a single-ESC-operated scenario,a load current feedforward control method based on disturbance-observer(DOB)is proposed to improve the power quality.This method regards the load current as system disturbance,and feeds forward it to the output of the compensator in the voltage loop after it is measured by the disturbance observer,thereby the influence of the load current on the output voltage is suppressed.The fast-tracking of the load current is realized without adding additional current sensors,signal transmission or processing circuits,the dynamic performance of the system is improved and the steady-state error of output voltage is reduced.Based on the requirements of rapid response capability and stability of the control system,a parameter design method with universality and portability is given for the control system.The analysis shows that compared with the traditional dual loop control,the proposed DOB-based load current feedforward control method is less sensitive to parameter perturbation and the system is more robust.Simulations and experiments validate the feasibility and effectiveness of the proposed control strategy and the parameter design method.In ILM,for a multi-ESC-operated scenario,a DOB-based V-I droop control method is proposed to improve the power quality and realize the proper power sharing of ESCs,Initially,the principle of V-I droop control is presented straightforwardly in ILM,and it is revealed that the premise of accurate power sharing is that the combined resistances are inversely proportional to the ESC capacities,and the no-load output voltage phases of ESCs are synchronized.Then,a DOB-based V-I droop controller is proposed.On the one hand,the output impedance of ESC is eliminated by DOB-based current feedforward control.On the other hand,the combined resistance is properly designed by DOB-based virtual impedance control.Furthermore,a synchronization algorithm is proposed based on the real-time tracking of the AC bus voltage phase.This algorithm adaptively adjusts the phase step of each ESC according to the phase difference between ESC and the AC bus,so as to realize the synchronization of ESCs.A detailed parameter design method is implemented considering both system stability and power quality requirements.Finally,the proposed strategy is validated experimentally by building a 3-ESC-parallel-operated microgrid system. |
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