Modulation And Control Strategy Of Three Level Converter

With environmental pollution,energy shortage is increasingly serious,energy-saving carbon reduction has gradually become a human consensus.New energy power generation has been greatly developed in recent years for reducing carbon emission of thermal power generation.The high efficiency and excellent harmonic performance of the three-level converter make it widely used in active filtering and new energy grid-connected scenarios.However,the increase of the power level brings a higher requirement for the efficiency and current harmonics.This article focuses on the modulation strategy and control method of three-level converter,the main work is as follows:Proposed a new DPWM mode to solve the two problems existing in the DPWM application of the mid-point voltage imbalance,light load current harmonics.Firstly,a hybrid DPWM strategy based on modulation wave is proposed,which is denoted as MHDPWM.The modulation method varies with the modulation Index between the discontinuous and continuous PWM.This modulation strategy is compared with traditional DPWM in reducing switching losses and current harmonics.Then analyze the conflict with the midpoint voltage balance of conventional DPWM,DPWM failure problems.A midpoint voltage balancing strategy suitable for MHDPWM is proposed.Simulation results show that the strategy can achieve midpoint voltage balance and DPWM without conflict.Aiming at the problem of large harmonic current of DPWM at light load,this paper proposes a current reference hybrid DPWM strategy.Through theoretical analysis and simulation,this modulation strategy can reduce the switching loss under heavy load,sacrifice less switching losses at light load,improve the current quality,and have the advantages of no conflict between MHDPWM and mid-point balance.Adopting the compound control strategy of repetitive control and PI control,it can enhance the harmonic suppression ability of the traditional PI controller and effectively reduce the low harmonics.First,the mathematical model of NPC is established,and the dq decoupling PI controller parameters of the system are designed according to the model.Then introduced the internal model principle,and deduced according to the internal model principle that the repetitive controller has better harmonic suppression than the PI controller.Then,the parameters of repetitive controller are designed.Combined the dynamic response speed of the PI controller and the harmonic suppression capability of the repetitive controller,the PI controller and the repetitive controller are combined in series and parallel.The feasibility and harmonic suppression performance of the two compound control methods are analyzed and verified by simulation.Finally,according to the system index and topology,hardware parameters and software architecture of the converter are designed,and build a 3.6kW NPC three-level converter experimental platform.Based on the experimental platform,the feasibility of the control and modulation strategy proposed in this paper is verified.