The Stability Control Of Hybrid AC-DC Microgrids System Considering Fault Types

The AC-DC hybrid microgrids combine the advantages of AC-DC microgrids and have been widely studied and gradually applied.The AC-DC hybrid microgrids system in island mode has limited capacity and lacks the support of the public grid,its antidisturbance ability is poor.Complex topologies and frequent energy exchanges can easily cause problems such as heating of power electronics in the system,and the probability of failure of AC-DC hybrid microgrids will also increase.This lead to huge changes in the topology of the microgrid system,voltage and current are more prone to disturbance,and the negative incremental impedance characteristics of constant power load will amplify the disturbance of the system,making the stability problem of the AC-DC hybrid microgrid system in island mode more prominent.In this paper,a stability control method is proposed for the AC-DC hybrid microgrids system after three-phase short circuit and single-phase ground fault,and the hybrid potential function derivation theorem is proposed.After the three-phase short circuit of the system,the protection device will cut off the fault,resulting in the topology change,and the proposed stability control method shows the influence of constant power load power,energy storage unit power,DC bus voltage,AC side filter inductance and resistance,DC side capacitance and DC-AC interconnection converter current inner loop proportional control parameters on the stable operation of the system.Aiming at the problem of unbalanced operation caused by single-phase ground fault,the symmetric component method is applied to convert the asymmetric system into a symmetrical sequential network,and based on the instantaneous power calculation formula,the symmetrical sequential network power is converted into three-phase power,so as to obtain that the stability of the AC-DC hybrid microgrids system with singlephase ground fault is related to the line equivalent resistance,the current inner loop proportional control parameters of the DC-AC interconnection converter,the filter inductance,filter capacitor,fault point voltage and injected power.Firstly,according to the working characteristics of distributed power supply and the control strategy of the converter,the Park transform is used to obtain the nonlinear equivalent model of the AC-DC hybrid microgrids system under the dq coordinate system.Then,considering the action of the protection device in the system,the internal faults of the microgrid are divided into two categories: changed system topology and unchanged system topology.Taking the three-phase short-circuit fault as an example,when the fault of changing the system topology occurs,the mixed potential function theory is used to establish the large signal model of the microgrid system,and the system stability control method is derived on the basis of considering the control parameters and system parameters of the DC-AC interconnection converter.When a single-phase ground fault occurs,the instantaneous power calculation formula is applied to convert the sequential power in the symmetric network into the phase power,and it is derived that the phase power is only related to the positive sequence and zero sequence power,and the symmetric component method is used to convert the asymmetric system into a symmetrically operated positive sequence and zero sequence network,and the stability control method of the microgrid system is derived respectively.Finally,the simulation model of AC-DC hybrid microgrids system is established using MATLAB,and an experimental platform for AC-DC hybrid microgrids system is built,and the stability control methods after three-phase short circuit fault and singlephase ground fault of the obtained AC-DC hybrid microgrids are verified.Simulation and experimental results show the correctness of the proposed stability control method.It can be seen that reducing the current inner loop control parameters,filter inductance value,and increasing the voltage regulator capacitance value of the DC-AC converter can help improve the stability of the AC-DC hybrid microgrids system after the fault.