| As an important component of the renewable energy system,the energy storage system achieves energy transfer between the DC bus of the energy storage device and the public AC bus through inverters,thereby supplying energy to the power grid and load,ensuring the safety and reliability of the power system.As the core component of energy conversion,the inverter of energy storage system plays a decisive role in the safe and stable operation of energy storage system because it can operate in the multi-mode,such as single,parallel,grid-connection,islanding and grid-connected and off-grid.However,with the vigorous development of renewable energy and the expansion of the scale of energy storage system,the grid-connected current harmonic interference,multi-mode transition effects and parallel stability problems of energy storage system inverters are also aggravated.In view of the above problems,this paper aims at the multi-mode stable operation of the inverter in the three-phase energy storage system.Firstly,the applicable topology and grid-connected structure of the inverter in the energy storage system are introduced.The current harmonic suppression method of the grid-connected inverter is studied.Furthermore,a smooth off-grid method for the grid-connected master-inverter under unintentional islanding is proposed,and then the stability of the islanded parallel inverter system after off-grid is analyzed.Finally,according to the analysis conclusion,a multi-mode smooth operation strategy of parallel inverters with peer-to-peer architecture is proposed.The research work in this paper progresses layer by layer,providing important technical support for grid-connected harmonic suppression and multi-mode smooth and stable operation of energy storage system inverters.The main research contents are as follows:(1)The topology and principle of three-phase inverter suitable for energy storage system are introduced.The open-loop function of three-phase grid-connected inverter is derived.The control method of grid-connected inverter and the influence of neutral wire on harmonics are discussed.After analyzing the harmonic suppression ability of different controllers,repetitive control is selected as the basic method to suppress the harmonics of grid-connected current.(2)Aiming at the ability of repetitive control to suppress grid connected current harmonics of energy storage system inverter,an optimal design method of PI-repetitive controller is proposed,which effectively reduces the harmonic content of the gridconnected current compared with the traditional method.Furthermore,a variable factor passivity-based-repetitive control is proposed to significantly improve the harmonic suppression effect while ensuring the smooth start-up of the energy storage system.The experiments were conducted on two harmonic suppression methods to verify the effectiveness of the proposed methods.(3)Aiming at the unintentional off-grid problem of the master-inverter in the masterslave architecture energy storage system in the grid-connection,the reasons for the voltage impact and power mutation caused by the traditional unintentional islanding are analyzed.A nonlinear multimode controller is proposed for the smooth off-grid of the masterinverter in the energy storage system from the current-controlled grid-connected mode to the voltage-controlled islanded mode under the unintentional islanding.Based on this method,the stability of the main inverter during the off-grid is analyzed and verified by experiments.(4)Aiming at the islanding stability problem of parallel inverters with different control modes in the energy storage system after off-grid,the impedance models of voltage-controlled inverters and current-controlled inverters are established respectively.Based on the double closed-loop equivalent system,the impedance model of parallel inverters under islanding is derived.Using this model,the stability of the parallel system under different operating conditions is analyzed and verified,and the influence of inverters with different control modes on the stability of islanded parallel system is obtained.(5)Aiming at the problem of grid-connected and off-grid smooth operation of parallel inverters in the energy storage system with peer-to-peer architecture,an intelligent coordinated control scheme is proposed to realize the automatic adjustment of each inverter in grid-connected or islanded mode to grid-following control or grid-forming control.At the same time,the joint action of the power intelligent control loop and the coordinated control loop in the proposed scheme can realize the adaptive adjustment of the inverter control mode,so as to obtain smooth grid-connected and unintentional smooth off-grid.The experimental results show that the proposed scheme can achieve multi-mode smooth and stable operation of parallel inverters with peer-to-peer architecture. |
