Research On The Modulation Strategy And Fault Tolerant Control Of Five-level Converter Cascaded By Dual Three-level Converters

Open end winding(OEW)load fed by dual converters is an effective topology used in medium-voltage high power and high reliability applications,owning to its simple topology,multi-level output characteristics and redundant space vectors.Output currents are distorted by the zero sequency current(ZSC)generated in the dual converters,when dual converters are fed by a single DC voltage source.Both high frequency pulse common mode voltage(CMV)and unbalanced neutral point voltage(NPV)affect the performances of the dual converters.Some detailed research mainly focusing on zero sequence voltage(ZSV)elimination,common mode voltage(CMV)suppression,active neutral point voltage control(ANPVC),phase leg fault tolerant control have been studied in this dissertation by taking the five-level converter cascaded by dual three-level neutral point clamped(3L-NPC)converters with a single DC voltage source as the research object.Firstly,by analyzing the multi-level output characteristics and establishing the model of the cascaded dual 3L-NPC converters with a single DC voltage source,the definitions and measuring circuits of the electrical variables in the multi-level converter cascaded by dual converters were given.The designed measuring circuits were used widely in the cascaded dual converters,furthermore,too many instruments were avoided.Double Fourier integral algorithm was employed to analyze the harmonics of output voltage affected by NPV.Output voltage,CMV and ZSV affected by the NPV was relevant to special modulation strategies.To simplify the modulation strategy used in the cascaded dual 3L-NPC converters and implement the modulation as two identical modules,the decoupling modulation strategy decoupled at the pulse generator of single three-level modulation algorithm was proposed.The conventional three-level modulation algorithm was extended directly to the cascaded dual 3L-NPC converters by the proposed decoupling method.The modulation strategy using 180° decoupling method decoupled at the pulse generator was proposed,which zero CMV was achieved.The output voltage,CMV were not affected by the proposed 180° decoupling method,the ZSV was maintained at zero in the average value during the switching period through adjusting the offset time in the three-level modulation algorithm.The modulation strategy using 120° decoupling method decoupled at the pulse generator was proposed,which ZSV was eliminated completely and CMV was maintained the same as the single three-level converter.The adjustment of NPV was accelerated by modifying neutral point current in single three-level modulation algorithm.The 120° decoupling method decoupled at the pulse generator was more appropriate to the cascaded dual 3L-NPC converters,which the maximum linear modulation index,DC voltage utilization,ZSV,CMV,ANPVC and universality were compared between 180° and 120° decoupling method.In order to suppress CMV of the cascaded dual 3L-NPC converters with 120°decoupling method,an improved virtual space vector pulse width modulation(IVSVPWM)was proposed,which the IVSVPWM was used in single three-level converter.ANPVC and CMV suppression were obtained simultaneously by adopting low CMV vectors and setting continuously adjustable variable coefficients in the proposed virtual vectors.Failure of CMV suppression caused by equal phase duty-ratio sequency optimization method was analyzed,and avoided by limiting the range of the variable coefficients in virtual vectors.Then,IVSVPWM was extended to the cascaded dual 3L-NPC converters with 120° decoupling method.ZSV was eliminated completely,ANPVC was obtained,and CMV was suppressed to 16.7% of the DC voltage source.A fault tolerant control method based on the ±120° decoupling method decoupled at the pulse generator was proposed,to solve the phase leg fault in the cascaded dual 3L-NPC converters.The fault tolerant topology was restructured by adding a few thyristors.Then,the fault phase legs were connected to the redundant phases.All of single phase leg,double phase legs,and most of three phase legs faults were solved by using the proposed fault tolerant control method.Meanwhile,the ANPVC was maintained.The maximum output voltage was the same as the one of normal status,and the output current was 57.7% of the one of normal status,when the proposed fault tolerant control method was implemented.Finally,experiments were carried out on the test bench of cascaded dual 3L-NPC converters to validate the proposed modulation strategies and fault control methods.There are 143 figures,22 tables and 167 references in this dissertation.