Study On Optimal Control Of Neutral-point Voltage Balance Of Three-level Energy Storage Converter

With the decrease of fossil energy and the serious problem of environmental pollution,development and utilization of wind energy,solar energy and other new energy become the key of the global energy strategy.However,energy storage is a bottleneck restricting the development and widespread application of new energy,so the research of energy storage system is urgent.As the interface between the storage medium and power grid/load,the two-way energy conversion system(PCS)is the core of the entire energy storage system,which is essentially a power electronic converter for bidirectional power regulation.At present,there are two topologies of PCS,including high-voltage chain energy storage topology and low-voltage modular parallel topology.Low-voltage modular parallel topology becomes the main choice of large-capacity power storage because of its mature control and easy modular expansion.Currently,efficiency is the focus of attention for modular parallel PCS.Compared with the two-level topology,the three-level topology can reduce the loss and the size of the grid filter,improve the equivalent switching frequency,which is conducive to reducing the size and modular integration,so the three-level topology of the PCS will dominate the energy storage market.To this end,this paper focuses on solving the voltage balance problem of three-phase three-wire and three-phase four-wire of the three-level topology.Firstly,based on the two-level three-phase three-wire topology,the principle of SVPWM is introduced in detail,and the implementation method and steps of SVPWM are summarized.Then,the method is extended to the two-level three-phase four-wire capacitance midpoint topology,and the realization principle of three-dimensional SVPWM is analyzed,and the connection and difference of vector modulation between the three-phase three-wire and three-phase four-wire system are pointed out,laying the foundation for the following research on the vector modulation and voltage balance of the three-level topology.Secondly,in order to study the vector modulation and voltage balance of the three-level topology in three-phase three-wire system,starting from the recent three vector modulation,the algorithm of redundant small vector action time distribution based on nearest three vector VSM is introduced.It is pointed out that the algorithm has the shortcoming of introducing the low-frequency ripple voltage at the output side when the modulation ratio is high or the power factor is low.In order to suppress this phenomenon and further improve effect of neutral-point potential balance control on the DC side,the principle of virtual space vector algorithm is analyzed.It is pointed out that the application of the virtual space vector can eliminate the low frequency oscillation problem of the nearest three vector VSM,but at the expense of increasing the number of switching actions.Beside,the virtual space vector does not belong to the nearest three vector modulation,so it will result in the worse harmonic characteristics on the output side of the converter.In order to make full use of the advantages of the redundant small vector action time adjustment method and the virtual space vector method,a hybrid modulation strategy is proposed to improve the output harmonic characteristics of the converter while eliminating the low frequency oscillation of the midpoint potential and the validity of the hybrid modulation strategy is verified by simulation.Then,aiming at the research on the vector modulation and voltage balance of the three-level three-phase four-wire capacitive mid-point topology,the spatial vector is divided into seven sub-spaces based on the intrinsic relation and regularity of the SVPWM modulation of the two-level topology.The new coordinate origin is defined for each subspace,and the vector relocation is carried out so that the zero-axis component of the redundant small vector in each subspace is reversed in the new coordinate system,which is equivalent to positive and negative zeros in the vector modulation of the two-level topology,thus simplifying the implementation of the algorithm.In addition,in order to suppress the neutral-point potential offset,the concept of vector neutral-point potential control grade is proposed.According to the different working condition of the converter,the neutral-point potential control grade of the redundant vector is solved and the optimal vector selection is further obtained to achieve the greatest degree of suppression of the midpoint potential shift.Finally,the effectiveness of the neutral-point control strategy based on the three-dimensional vector optimization is verified by simulating the different working states of the converter.