| Microgrid clusters are an effective means of solving the problem of large-scale distributed renewable energy consumption.The emergence of microgrid clusters enables energy mutualisation between microgrids and improves the reliability of individual microgrid operations.When a microgrid cluster is connected to an unbalanced distribution network or its internal load is asymmetrical,it will cause a three-phase unbalance of the bus voltage at multiple levels within the microgrid cluster.Series and parallel compensation can effectively solve the multi-level bus voltage unbalance problem within the cluster,but the introduction of compensation will change the original impedance characteristics of the microgrid cluster,which will most likely lead to new resonance stability problems in the microgrid cluster.Therefore,the main research of this thesis is as follows:Firstly,this thesis introduces the characteristics of the microgrid cluster and the topology adopted in this study.A proportional-integral quasi-resonant control strategy is used to grid-connect the inverters under unbalanced external grid voltage conditions.The equivalent impedance model of the three-phase LCL-type grid-connected inverter under unbalanced grid conditions is established based on the harmonic linearisation method,and the equivalent sequence impedance model established is verified by simulation through the frequency sweep method.The output impedance characteristics of the three-phase LCL grid-connected inverter and its stability when connected to a weak grid are analysed.Secondly,the basic principles of commonly used unbalance compensation signal detection methods are introduced.On this basis,series and parallel compensation methods are investigated,and the effectiveness of series compensation for the compensation of three-phase unbalanced voltages and currents in the microgrid cluster under the condition of unbalanced external grid voltage and parallel compensation under the condition of asymmetric cluster-level loads is verified respectively.Further,the equivalent impedance models of power electronic equipment such as gridconnected inverters,series compensation and shunt compensation as well as basic power equipment such as lines,transformers and loads within the microgrid cluster are established,and the impedance characteristics of the equivalent models of power electronic equipment are analysed to lay the foundation for the subsequent resonance stability analysis of the microgrid cluster.Finally,a model of the cluster sequence impedance network of the microgrid in series compensation,parallel compensation and mixed series-parallel compensation states is developed.The resonance stability of the system under the three compensation states is investigated by using the modal analysis method with the nodal conductance matrix as the core.The effect of different parameters of the grid-connected inverters on the resonance stability of the system is also investigated in the series-parallel hybrid compensation state of the microgrid cluster.It is shown that the resonant mode of the microgrid cluster in the series-parallel hybrid compensation state combines the resonant mode of the system in the series-parallel compensation and parallel compensation states.The introduction of series compensation leads to new resonant frequencies in the microgrid cluster and has a greater impact on the resonance of low frequency harmonics in the system.Although no new resonant frequencies are added to the microgrid cluster after the introduction of parallel compensation,the parallel compensation impedance will change the resonant frequency distribution in the high frequency band of the system,which will still have a greater impact on the system harmonic resonance.When different parameters are used in the grid-connected inverters of the microgrid cluster,the resonant frequencies are also added to the high frequency band of the system,while no new resonant frequencies appear in the middle and low frequency bands. |
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