Zero-sequence Circuting Current Suppression Research On The Parrel System Of The T Type Tri-level Inverter For Energy Storage

The rapid development of the smart grids and the distributed new energy makes the energy storage technologies become more prominent. As the core equipment for connecting the power grid and the energy storage devices, energy storage inverter could ensure the energy flow smoothly between the energy storage device and the power grid.Compared with the two-level energy storage inverter, the tri-level energy storage inverter has the advantages of a smaller output voltage harmonic, higher conversion efficiency and smaller EMI, which has become a hot research and application of the high-power energy storage inverters so far. But due to the limit of the current stress of the power switch tube, a single inverter power rating is difficult to meet the requirement in the energy storage system, paralleling more than one inverter has become the key technology to solve this problem. But this would cause zero-sequence circulating current between the parallel energy storage inverters, and the existence of the zero-sequence circulating current will reduce the efficiency of the system, seriously affecting the stable operation of the parallel system. This paper selects the T tri-level energy storage inverter as the specific research object and focus on the suppression of the zero-sequence circulating current problems in the parallel operation system.Firstly, it analyzes the topology structure, working principle and modulation algorithm of the T tri-level, and then makes a modeling analysis on T tri-level energy storage inverter, and analyzes the grid-connected control algorithm based on the model.In accordance with the parallel topology studied in the paper, it puts forward the main problems to be solved in this paper.Then, it analyzes and explains the zero-sequence circuit of two T tri-level inverters’ parallel system in detail and makes a modeling analysis on the zero-sequence circulating current of more T tri-level inverters’ parallel system, and then makes a modeling analysis of the zero-sequence circulating current for one energy storage inverter in the parallel system. Different zero-sequence circulating currents are defined and classified according to different exciting sources of the zero-sequence circulating current, then it correspondingly adopts the restraining strategy of the topology optimization and control on the zero-sequence circulating current with different components and has an analysis respectively, finally verifies the effectiveness of the strategy by simulation results.Finally, based on the zero-sequence circulating current model of the parallel system consist of two T tri-level energy storage inverters, it points out that the zero-sequence circulating current exciting source is zero-sequence voltage difference between the paralleled inverters. According to the conclusion and the characteristics of T tri-level,this paper puts forward a kind of modulation strategy(2MV1Z), which only adopts the zero vector and medium vector measures to keep the zero-sequence voltage of each T tri-level inverter with the same constant value, thereby eliminating zero-sequence voltage and zero-sequence circulating current, and proposes the zero-sequence circulating current reduction method that is combined with the neutral potential control to ensure the neutral potential balance and to prevent the possible offset zero-sequence circulating current. Finally, it has a simulation verification on the effectiveness of the algorithm.