Control And Protection For PWM Rectifier Based On Silicon Carbide Power Device

The three-phase pulse width modulation(PWM)rectifiers has the advantages of bi-directional power flow,flexible and controllable power factor,sinusoidal current on the grid side,high-quality DC voltage,etc.This topology has been widely used for renewable energy power generation systems,high-performance electrical drives and other fields.For conventional single-vector model predictive direct power control(MPDPC)system of PWM rectifiers,the switching frequency is unfixed and the the total harmonic distortion(THD)of the current is high.In order to achieve better performance,a higher sampling frequency and a large computational burden is required.The third-generation wide-bandgap power device silicon carbide metal-oxide-semiconductor field-effect transistor(SiC MOSFET)can significantly improve the efficiency and power denisty of PWM rectifiers sharply.However,the characteristics of such power devices make the system vulnerable for over-current and short-circuit faults.In order to improve the performance and the reliability of PWM rectifiers,a control method based on model predictive control,the characteristics and protection of SiC MOSFETs are studied in this thesis.The main contribution and innovations of this thesis are given as follow.(1)An improved model predictive direct power control(MPDPC)for PWM rectifiers is proposed to solve the problem of high computational complexity and burden and large current harmonics in the conventional single-vector MPDPC.In the proposed method,one nonzero vector and a zero vector are applied in one control period,the duty cycles of nonzero vector are determined by an optimized calculation.Both the simulation and experimental results prove that,the proposed method has the advantages of lower computational burden and less dependence on rectifier parameters.Compared to the conventional MPDPC,the proposed control method can achieve a comparable dynamic response at lower sampling frequency.In addition,the proposed method can achieve better steady-state performance in terms of the reduced power ripples and the lower current THD.(2)The rated continuous drain current of SiC MOSFETs varies with the temperature.In the PWM rectifier based on SiC MOSFETs,the conventional constant threshold over-current protection cannot adjust the protection threshold according to the temperature,resulting in a reduced efficiency of power devices and the failure of protection.In this thesis,the relationships among the rated current,the maximum power loss,and the case temperature are fitted,and a novel case temperature-dependent over-current protection scheme is proposed for SiC MOSFETs.The protection circuit and control algorithm are designed.Experimental results prove that,the threshold of over-current protection can be adjusted with temperture in real time,which allows the SiC MOSFETs operating at higher current level.The scheme also improves the reliability of the PWM rectifiers.(3)In the PWM rectifier based on SiC MOSFETs,the shoot-through fault of the half bridge circuit may lead to the short-circuit current rising very fast and exceeding capability of the protection circuit.In order to improve the reliability of the PWM rectifier,a shoot-through protection method based on the gate-voltage detection of complementary SiC MOSFETs in a half bridge is proposed.In the propose protection method,the gate-source voltage of the SiC MOSFETs in a half bridge is detected to judge whether a shoot through occurs or not.The protection circuit's effectiveness is verified by the experimental results.Compared with conventional desaturation short-circuit circuits,the proposed method can take action and turn off the SiC MOSFETs in 0.2 ?s under the shoot-through condition,causing a smaller short-circuit current.The proposed method can provide effective protection for both planar-gate type and trench-gate type SiC MOSFETs.The proposed short-circuit protection method with the VDS(ON)detection protection method can be used with various short-circuit and over-current protection strategies for PWM rectifiers.