Research On Fault Diagnosis And Fault Tolerant Control Of Three Level Inverter

Compared with the conventional two level inverters,three-level inverters have the advantages of lower voltage stress,better waveform spectrum,less voltage change rate and waveform distortion.Therefore,it is widely used in middle and high voltage,high-power electric power conversion and power transmission applications.However,there are many switching devices in three-level inverters,and the control of these switches becomes more complex,which lead to a significant increase in failure rate.Then the reliability of the circuit is greatly deteriorated.The short fault and open fault are the most common faults.In general,the fast fuse is added to the inverter circuit,when a device has a short fault,the short fault can be converted into open fault by fusing fuse.The open fault causes power devices to overflow,triggering problems such as torque reduction,heating and insulation damage.If the open fault is not dealt with in time,the entire circuit operates abnormally,causing serious accidents and huge losses.For some situations where the equipment can be fault-tolerant,the inverter system needs to be highly reliable.Therefore,the researches on fault diagnosis and fault tolerant control of three level inverters are of great significance to improve the reliability of the system.Firstly,in terms of Neutral-Point-Clamped(NPC)three-level inverter,this paper introduces a novel diagnosis strategy for open-switch faults.The switching states,load phase currents and bridge arm phase voltages are chosen as the fault characteristics.The change of the fault features is analyzed based on different current flow paths,and the basis of fault diagnosis is obtained.The feasibility of the method is verified by MATLAB simulation.Compared with other open circuit fault diagnosis methods,this method does not need complex algorithm,with smaller computation and faster diagnosis.Secondly,in order to improve the reliability of the inverter system,the fault tolerant topology and control strategy of the NPC inverter are further studied.The open circuit fault of inner switches occurs in the NPC three-level inverter,some switching states are lost,which leads to the failure to use redundant switching states for fault tolerant control.Therefore,a two-level leg is added to the NPC inverter.When the device open fault occurs,the fault phase is isolated by the bidirectional thyristor,and the load of this faulty phase is connected to the neutral point of this two-level leg,so that the NPC inverter can operate at full power after the failure.An improved Space Vector Pulse Width Modulation(SVPWM)strategy called "addition and subtraction substitution SVPWM" is proposed to effectively suppress fluctuation in capacitor neutral point voltages by readjusting the sequence and action time of voltage vectors.The fault-tolerant topology in this paper has the advantages of fewer switching devices and lower circuit costs.Experimental results show that the proposed fault-tolerant system can operate in balance of capacitor neutral point voltages at full output power and the reliability of the inverter is greatly enhanced.Lastly,a dSPACE real-time simulation system and main development board DS5202 ACMC were recommended particularly.Then,based on the system,a fault tolerant control strategy verification platform of NPC three-level inverter is built.The experimental resultsfurther illustrated the feasibility of the fault-tolerant control method based on the strategy called "addition and subtraction substitution SVPWM".