Resonance Suppression Strategy Of LLCCL-type Energy Storage System Grid-connceted Converter

With the increasing problems of energy security,ecological environment and climate change,accelerating the development of new renewable energy sources has become a consensus around the world.The energy storage system can effectively compensate for intermittent,volatility and other shortcomings of the output,improve the controllability and stability of the new energy generation system,and have wide application prospect in power system.As the interface between the energy storage system and the grid,the reliability and stability of the grid-connected converter will directly affect the normal operation of the energy storage system.Therefore,the grid-connected converter is of great importance to the energy storage system.This paper studies the single-machine and multi-machine resonance mechanism of LLCCL type energy storage system grid-connected converter.Furthermore,the resonance suppression strategies of LLCCL-ESSGCC are proposed.In this paper,the problem of grid-connected current resonance in single machine operation is analyzed.Firstly,the mathematical model of LLCCL-ESSGCC is established.Then the resonance suppression strategy of high-frequency harmonic attenuation branch current feedback LLCCL-ESSGCC is analyzed.Furthermore,it is concluded that the capacitor current feedback strategy does not affect the notch effect on the switching harmonic while suppressing the LLCCL-ESSGCC resonance.Finally,the simulation shows that the LLCCL-ESSGCC with this feedback method has the strongest harmonic suppression ability of the grid current.For the resonance of multiple LLCCL-ESSGCCs parallel operation,this paper establishes the model for the multiple parallel operation of LLCCL-ESSGCCs.The relationship between the grid-connected current and the output voltage of the gridconnected converter in LLCCL-ESSGCC is deduced.It is concluded that the equivalent impedance of the power grid will increase with the increasing of the number of gridconnected converter,and thus the resonant frequency of LLCCL-ESSGCC will move to the low frequency band.For the problem of grid impedance detection in engineering,the grid impedance measurement method using two harmonic injection is analyzed in this paper.When multiple LLCCL-ESSGCCs are operated in parallel,the method of injecting harmonics with single converter will have detection error due to the influence of the circular current between converters.Therefore,this paper proposes a method to eliminate the circular current between converters and improve the accuracy of impedance measurement that all the LLCCL-ESSGCCs inject the same harmonics into the grid.For the problem of resonance suppression for parallel operation of multiple LLCCLESSGCCs,the converter output impedance reshaping method using parallel virtual impedance is analyzed in this paper.Although this method can restrain the system resonance frequency from moving to the low frequency band,it will weaken the damping effect of the resonance peak of LLCCL-ESSGCC.Furthermore,the method of connecting a series virtual inductance between the inductance of the grid side of the LLCCLESSGCC and the equivalent impedance of the grid is proposed in this paper.This method can restrain the system resonance frequency from moving to the low frequency band while retaining the damping effect of the grid equivalent resistance to the resonance of LLCCL-ESSGCC.