Research On Dual-output T-type Three-level Converter With Multiplexing Switches

In recent years,dual-output power conversion systems have been used in wind power generation,uninterruptible power supplies,and multiphase machine drives.Dual-output converter is the key,including the back-to-back converter（BTB）,the five-leg inverter（FLI）,and the nineswitch converter（NSC）.FLI and NSC reduce the cost and volume of the system by saving switches.And,studies show that they have higher efficiency than BTB,when operating in identified requirements.However,FLI and NSC are two-level converters,which restrict their applications in medium or high voltage and high power and strict power quality requirements.To solve this problem,based on the T-type three-level converter and the switch-multi-used method of nine-switch converter,a novel three-phase dual-output T-type three-level converter（DO-T-TLC）is proposed.The proposed DO-T-TLC consists of 18 switches（IGBT or MOSFET）and two capacitors,which has the dc-link and two sets of three-phase ac voltage output terminals.The working principle of the converter is studied,and the operating constraints caused by the multiplexing switches are found.The proposed topology and the circuit of the same type are compared in terms of the number of switches and the voltage stress of the switches.Through the basic laws of the circuit,the blocking voltage and instantaneous current of each switch are analyzed,which is helpful to guide the selection of the device.By transforming the operating constraints of the converter into modulation constraints,the sinusoidal PWM with dc offsets（SPWM-DCO）strategy is studied.In order to maximize the utilization of the dc-link voltage,the modulation index range of the SPWM-DCO strategy in three operating modes（in-phase common frequency,non-in-phase common frequency,and different frequency）are analyzed.A flow chart of the control scheme is developed to determine the feasible region of modulation parameters for different operating modes.The trigger circuit for generating switch PWM signals is designed by logical operation symbol.Based on the SPWM-DCO strategy and circuit structure,the root-mean-square current expressions of all switches are expressed to reflect the distribution of conduction losses in DO-T-TLC.Based on the simplified model of the dc-link,the capacitor voltage ripple is defined to quantify the unbalance of the capacitor voltage.Through the basic laws of the circuit and the characteristic equation of the capacitor,it is found that the neutral point current is the cause of voltage,then the problem of controlling the voltage is transformed into the problem of controlling the current.Four kinds of modulation index ranges are divided to solve the expression of the neutral point current,and a capacitor voltage balance method to control the average neutral point current is proposed.The effects and characteristics of the proposed control method are summarized and help guide the parameter selection of series capacitors.Finally,a hardware experiment platform is built.The FPGA is used to control the low-power prototype,and the results of output voltages,currents and capacitor voltages are analyzed.The experimental results show the feasibility of the circuit structure and the effectiveness of the modulation strategy to control the output voltages and capacitor voltages.