Fault Diagnosis And Fault Tolerant Control Of Cascaded H-bridge Photovoltaic Inverter

Cascaded H-bridge,as one of multilevel topology,has the characteristics of simple circuit and easy to expand.When it is used as photovoltaic inverter,the DC side of each H-bridge unit can be powered by an independent photovoltaic board.By adjusting the output voltage of each H-bridge unit,the output power of each unit can be changed,and then the DC side voltage of each H-bridge unit can be controlled to achieve the maximum of each photovoltaic board Power point voltage tracking,improve the power generation of the system.Therefore,cascaded H-bridge inverter is widely used in photovoltaic system.In the application of high voltage and high power,the increase of the number of Hbridge units will introduce a large number of power switch devices,which will increase the probability of switching failure of the inverter,and then affect the reliable operation of the inverter.In this paper,aiming at the common open circuit fault of the switch tube,the diagnosis methods based on software and hardware are studied successively.Among them,the diagnosis method based on software idea uses the original sensor of the system to collect the corresponding amount,and obtains the fault characteristic value through certain calculation,and then compares with the criterion value to obtain the diagnosis result.This method is convenient to realize and does not need to add additional sensors,and can complete the diagnosis in a working frequency cycle;the diagnosis method based on hardware idea needs to be AC transmission in each H-bridge unit The voltage sensor increases the complexity of the system hardware,but improves the diagnosis speed.The diagnosis can be completed in one carrier cycle,and the diagnosis of individual tube fault can be completed in half carrier cycle.Considering the importance of diagnosis speed to the reliable operation of the system,this paper studies fault-tolerant control based on the method of hardware.After the diagnosis,the corresponding fault-tolerant control scheme should be adopted to maintain the stable operation of the inverter.Fault tolerant control is also divided into software idea and hardware idea,in which the software idea uses the way of superimposing zero sequence components to improve the voltage utilization rate and realize power distribution,and the hardware idea usually uses the standby H-bridge unit to replace the fault unit to maintain the inverter operation.In this paper,a fault-tolerant control scheme is studied based on the idea of software.The power distribution is realized by superposing the zero sequence component of fundamental frequency in theoutput voltage of the three-phase inverter side,so that the normal H-bridge unit can still work at the maximum power point.By superposing the zero sequence component of triple frequency,the utilization ratio of DC side voltage is further improved,and the algebraic calculation method of zero sequence component is derived by using the idea of vector synthesis Without complicated angle calculation,it can save the operation time of the processor,and analyze how to improve the output power of the inverter and ensure the power stability of the network side in case of fault,and give the control scheme.In order to verify the effectiveness of the proposed open circuit fault diagnosis method and fault-tolerant control scheme,a 1.5KW three-phase 9-level cascaded Hbridge inverter prototype was built in the laboratory.The experimental results show that the scheme proposed in this paper is applicable to photovoltaic power generation system.