| Motor is an important field in the application of power electronics technology.In order to improve the control performance of motor and respond to the call of energy saving and emission reduction,the majority of motors adopt variable frequency speed control.In practice,the AC-DC-AC converter is the most widely used,therefore,the front-stage rectifier circuit is essential.However,the traditional rectifying circuit will cause the "pollution" of the power grid,and cannot realize energy feedback to the power grid,which is not in line with the development of the current era.The PWM rectifier can realize the unit power factor of the grid side,and can reverse the operation to feed back the electric energy,which can effectively control the "pollution" of power grid and reduce the input of compensation device.Therefore,the research of reversible PWM rectifier with high power factor has been paid more and more attention in recent years.In this paper,the topology of three-phase three-level NPC-I PWM rectifier is studied,and its working principle and dynamic commutation are analyzed.Then,the reversible operation conditions of three-phase three-level NPC-I type PWM rectifier are given,and the mathematical models and equivalent circuits in three different coordinate systems are established by introducing the switching function.In the d-q coordinate system,the double closed loop control strategy of PWM rectifier based on the current decoupled voltage vector oriented control is studied,in which the outer voltage loop is used to stabilize the DC bus voltage and the inner current loop is used to shave the sinusoidal current in phase with grid voltage on the grid side.The control model of PWM rectifier is obtained after reasonable simplification,and the design method of PI controller parameters in voltage and current loop is also presented.Compared with sinusoidal pulse width modulation(SPWM),Space Vector Pulse Width Modulation(SVPWM)has a 15%higher DC-side voltage utilization,good harmonic suppression,and is easily implemented by digital chips.In this paper,the pre-decomposing SVPWM modulation method is studied,and the design steps of the pre-decomposing SVPWM fast algorithm and the specific implementation process are given in detail.Based on three fixed reference vector,a seven-section time series table is designed,and the method of determining the operating time of the working vector is given.The digital implementation method of driving configuration is given based on DSP TMS320F28335 from TI corporation.Besides,the influence of small vector on neutral point potential is analyzed in this paper,and the mechanism of neutral point potential imbalance is revealed.Finally the control method of the neutral point potential balance based on the control factor is gotten.Aiming at the application of three-phase three-level NPC-I type PWM rectifier in motor driver load,a leakage current protection circuit is designed,and a fast leakage current detection and protection method is proposed,which can effectively improve the reliability of the system.This method consists of leakage current detection transformer,self-excited oscillator,second-order active low-pass filter circuit and differential detection and protection circuit.The leakage current signal is detected by the leakage current detection transformer,modulated by the high frequency carrier produced by the self-excited oscillator,then the modulated signal is sent to the active low-pass filter after filtering to obtain the low-frequency leakage current signal again,which is sent to AD sampling of TI’s DSP TMS320F28335 after the sample conditioning circuit,and the packet protection is implemented by the program algorithm according to the detected leakage current.At the same time,the output signal of the self-excited oscillator will also be sent to the differential detection and protection circuit.When the leakage current is large,the self-excited oscillator will output a higher frequency oscillation waveform,thus the differential detection and protection circuit acts to realize the fast protection function of leakage current.The main circuit parameters of three-phase three-level NPC-I type PWM rectifier are designed in this paper,and the corresponding parameter calculation methods are given,including parameters of input LCL filter,selection of power switching device,parameters of DC bus capacitance and design of auxiliary power transformer.Aiming at the common-mode leakage current problem existing in the non-isolated motor driver load,based on the traditional leakage current detection circuit the differential detection protection circuit is designed.At the end of this paper,the main circuit simulation model of three-phase three-level NPC-I type PWM rectifier is built by using the software PSIM,which verifies the correctness of the adopted circuit and control strategy.Based on the simulation verification,the power circuit and the digital control circuit experimental platform of the 16kW three-phase three-level NPC-I type reversible PWM rectifier is built,and the correctness of the theoretical analysis and design method is verified by the experiments. |
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