Research On IGBT Open-circuit Fault Diagnosis And Fault-tolerant Control For Four-leg Active Power Filter

With the widespread use of various power electronic equipment,the power quality problems about harmonic current and three-phase unbalance in three-phase four wire distribution system have become more and more serious.Active power filter(APF)has become the important means of harmonic suppression in distribution system.The IGBT open-circuit fault in APF will affect the effect of harmonic suppression.When the fault is serious,it will bring the secondary harm to the APF and distribution system.In order to improve the reliability and safety of APF,This thesis introduce the harmonic current detection method and current tracking control strategy,focuses on the IGBT open-circuit fault diagnosis method and fault-tolerant control strategy of three-phase four-leg shunt APF.Firstly,the mathematical model of four-leg APF is established in the abc coordinate system and the ??? coordinate system.The current tracking strategy based on quasi-VPI controller is introduced,it provides the basis for the research of fault diagnosis method.Next,an improved qd harmonic current detective method based on sine amplitude integrator(SAI)is given.In this method,positive sequence component extraction link based on SAI and multi-channel decoupling structure are adopted,which improves the adaptability of the dq detection method to the distorted grid voltage.At the same time,the sliding window integral filter is introducted to remove the DC component of the grid voltage,and the effect of normalized frequency locked loop(FLL)eliminate the influence of frequency fluctuation.Simulation results show the effectiveness of the improved DQ detection method.Secondly,an IGBT open-circuit fault diagnosis method based on reference voltage is presented.The variation rule of reference voltage after IGBT open circuit at different positions is found out.The change rate of the DC component of reference voltage in each axis are used as the diagnostic basis,and the neutral compensation current is used as an auxiliary diagnostic basis for the N-phase bridge leg,so as to realize the open-circuit faults diagnosis of all IGBT without the external sensor.The simulation results of several typical faults show that the algorithm can be applied to the open-circuit faults diagnosis of single and multiple IGBT,and isn't affected by the load mutation.Thirdly,an open-circuit fault diagnosis method based on converter output voltage and reference voltage is given.This method diagnoses the IGBT open-circuit fault by the symbol relation between the converter output voltage and the reference voltage,and also uses the neutral line compensation current to assist the diagnosis of the N-phase bridge leg.This method uses the discrete mathematical model of APF to calculate the converter output voltage the according to the current sampling value,so it needn't the external sensor.This method also needn't set the diagnostic threshold,so it has higher reliability.The simulation results show that the method is effective for single and multiple IGBT open-circuit faults and can adapt to different operating conditions.Finally,the fault-tolerant circuit structure and its modulation and control method for two level four-leg APF are designed.The isolation and fault tolerance of single bridge leg are realized by passive isolation technology and switch redundant structure.The 3D-SVPWM modulation algorithm suitable for asymmetric fault-tolerant circuit structure is given.The method of DC-link voltage stability and balance control is also studied,and the controller parameters are designed.The simulation proved that APF used the asymmetric fault-tolerant circuit structure is still good effect of harmonic suppression.DC-link voltage controller can effectively maintain the stability of the DC-link voltage and the balance of the capacitor voltage.