| Energy storage system can improve the stability and reliability of power grid.Thus,it is widely used in the power system.Energy storage converter connects battery to grid,so its control strategy directly affects the stability and reliability of energy storage system.In this thesis,the resonant suppression strategy of LCL-type energy storage converter,the multi-converter resonance suppression strategy,and virtual synchronous generator control strategy are studied.Firstly,passive damping and active damping are studied for the resonant problem of LCL type energy storage converter.The mathematical model of LCL energy storage converter is established,and the characteristics and loss of passive damping method are analyzed.In order to solve the problem of passive damping loss,capacitive-current-feedback active damping method is studied,and the control parameters are designed from the viewpoint of system stability.It is pointed out that the active damping method have strong suppression ability and good resonance suppression effect.Secondly,aiming at the resonance problem under the condition of multiple running LCL type energy storage converter,the parallel resonance generation mechanism and resonance suppression strategy are studied.The multi-converter parallel network is simplified by the Thevenin theorem and the superposition theorem.The transfer mechanism of the output admittance and control system of each converter is deduced.According to the actual grid impedance value and the multi-converter parallel network structure,the control parameters of capacitivecurrent-feedback active damping method are redesigned,which suppresses the multi-converter parallel resonance.Aiming at the problem of detecting the impedance value of the actual grid in the design process of active damping parameters,an improved grid impedance detection method is proposed.The traditional method of detecting grid impedance by injecting harmonic current through the single converter in the multi-converter network can generate circulating current and thus cause error of grid impedance detection.Therefore,an improved method is proposed that multiple converters inject non-characteristic harmonic current at the same time to suppress the multiconverter circulating current,and the accuracy of the grid impedance detection is improved.Finally,in terms of inaccurate current command for virtual synchronous generator control caused by the coupling problem of LCL type energy storage converter in the dq coordinate system,an improved virtual synchronous generator control current command calculation method is proposed.In this method,the dq axis independent output impedance model is obtained by eliminating the coupling amount,and the decoupling of the dq axis command current is realized to improve the control precision.Based on the improved virtual synchronous generator control,the energy storage system has output inertia,and can provide support for the grid frequency. |
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