An Experimental Research On Hierarchical Cooperative Control Strategy Of Ac Microgrid Clusters

With the constant development of power electronics technology,the demand for distributed generation(DG)is increasing in many fields,and the requirements for DGs capacity,reliability and stability are also increasing.Therefore,microgrid(MG)systems including distributed energy sources such as wind energy and renewable energy have been widely recognized.With the depletion of non-renewable energy sources and the increasing demand for electricity consumption,the demand for distributed generation(DG)is increasing in many fields.Microgrid(MG)systems including wind power,water conservancy,and renewable energy have been widely recognized by researchers because of diversified energy supply and low environmental pollution.However,the disadvantages of MG systems are caused by poor anti-interference ability and slow transient response,and the system stability cannot be guaranteed.The microgrid cluster(microgrid cluster,MGC)system is composed of MG systems and distributed power sources,and the reliability of power supply and distribution is greatly improved.In this paper,the hierarchical cooperative control strategy for MGC systems have been investigated,which can be summarized as follows:(1)The parameter design and control parameter optimization of single-phase inverter are established.In order to improve the control accuracy and stability,an improved second-order active low-pass filter(ISOALPF)circuit is proposed,which is used in the closed-loop feedback loop of single-phase inverter.Then,the cut-off frequency and quality factor of the ISOALPF are analyzed and the transfer function model of single-phase inverter with ISOALPF is studied,which is carried out by the s-domain modeling method.Furthermore,a control parameter optimization method is proposed,and then the relationship between amplitude margin,phase margin,amplitude crossing frequency,phase crossing frequency and control parameters are established.The feasible range of crossing frequency including control parameters is determined and the selectable range of control parameters is obtained to achieve control parameters optimization.In addition,the accuracy of control parameters optimization is verified by analyzing the closed-loop zero-pole diagram.Finally,a simulation model and an experimental platform are built to verify the theoretical analysis of the control parameter optimization.(2)The control strategy and stability analysis of the MG system are conducted.By analyzing the Thevenin equivalent circuit of the single-phase inverter,the equivalent impedance model of MG system is proposed,and the stability judgment standards of impedance criteria ?,? and ? are obtained.Moreover,the impedance criteria ? is utilized to analyze the stability of the small-scale MG system consisting of two inverters,and then the range of line impedance and control parameters are ensured.Additionally,the droop control and secondary control in the hierarchical control strategy are explained and an equivalent droop control method without communication delay is used to improve the dynamic response capability of the system.Finally,the accuracy of the stability analysis is validated by simulation and experimental results.(3)Research on the structural framework and hierarchical coordinated control strategy of the MGC system is carried out.For a MGC system,the hierarchical coordinated control strategy is adopted,and the primary control,secondary control and tertiary control are introduced,respectively.Moreover,the interaction between different control strageties is analyzed and the coupling between MGs is explained.The small-signal model of MG under the hierarchical coordinated control strategy is established.By analyzing the optimal scheduling problem of the MGC system,the economic benefits of MGC system are described.(4)Based on the mathematical modeling and stability analysis of the MG system,an AC MGC hardware experiment platform with three sub-MGs and six single-phase inverters is built,and then the hardware circuit structure and working principle are explained.Besides,the program design of the hierarchical coordinated control strategy is completed.Experiments under voltage reference disturbance condition,linear load disturance condition are carried out on the hardware platform.Finally,the accuracy and effectiveness of the control strategy and theoretical analysis of the MGC system are verified by experimental results in this paper.