Research On The Optimized Modulation And Control Strategies Of Three-Level Converters In The Photovoltaic-Storage-Charging System

As bus voltage in photovoltaic-storage-charging system rises higher gradually,three-level converter is widely used.The performance of converter is greatly affected by passive components,such as filter inductors bus capacitors and flying capacitors in the cascading struct.Hence,the characteristic of current ripple is analyzed and a modified modulation is proposed in three-level ac converter for the optimization of filter inductor and bus capacitor.Moreover,the mechanism of flying capacitor is analyzed and a control strategy of flying capacitor is proposed in three-level dc converter for the optimization of filter inductor and flying capacitor.The main research contents of this paper are as follows:The unified analysis method of current ripple for three-level ac converter and the precise design process are proposed.The limitation of current ripple is one of the basic principles.Since current ripple of the ac converter is related to many factors such as modulation ratio,phase,and modulation method,it is difficult to obtain a reasonable value of inductor.Hence,the Generalized Space Vector Modulation(G-SVM)are defined in this paper at first.The commonly used modulation strategies are all unified and included in the G-SVM.Then the characteristics of current ripple with G-SVM are analyzed,the theoretical minimum value of current ripple is derived.On the basis,the unified expression of current ripple is obtained with several commonly used modulations.This method can be used to guide the design of inductor and the theoretical basis for the optimization of current ripple.An optimized discontinuous pulse width modulation(DPWM)for current ripple and neutral-point fluctuation reduction of parrallel three-phase three-level converter.Traditional DPWM strategy results in large inductor current ripple and neutral-point voltage fluctuation.In a single converter,it is difficult to optimize these two metrics at the same time.In addition,for Vienna rectifiers,the input inductor current will be distorted near the zero-crossing point.Hence,the characteristics of current ripple and neutral-point fluctuation with traditional DPWM are clarified at first.Then,the quantitative analyses on current ripple and neutral-point fluctuation with traditional DPWM,classical space vector modulation and optimized DPWM are presented in detail.To solve the issue of current distortion,hybrid DPWM is proposed based on optimized DPWM.An analysis method for the balancing mechanism of the flying capacitor clamped dc converter and the design principles of parameters considering self-balancing ability are proposed.Active control methods are usually applied for flying capacitor.In fact,the voltage of flying capacitor is able to achieve the balance naturally.However,this ability changes with converter parameters and working conditions,and it is difficult to evaluate the self-balancing capability.Hence,the harmonic model of the converter is established at first.Then,the self-balancing mechanism of flying capacitors is analyzed quantitatively.On the basis of this model,the effects of driving signal error,loop impedance,bus capacitor,load impedance and flying capacitor on the balance ability of flying capacitor are also quantitatively analyzed.A flying capacitor control strategy for hybrid clamped Boost converter is proposed.Since the flying capacitor self-balancing is affected by many factors,the flying capacitor voltage still needs to be actively controlled.However,the traditional control strategy requires the sampling of flying capacitor and independent drive signals,which greatly increases the demand for software and hardware resources especially in high voltage photovoltaic system with multiple dc inputs.Therefore,compared with the traditional control method,the proposed method does not need the sampling of the flying capacitor voltage and the switches can share the drive signals.Hence,the software and hardware resources can all be saved significantly.In this paper,the balancing mechanism and control parameter design of the proposed method are presented in detail.Then the state space average model and small signal model are established based on the detailed analysis of modes for hybrid clamped three-level Boost converter.With the proposed strategy,the flying capacitor voltage can be controlled around half of the bus voltage with error less than 2%.