Fault Diagnosis And Tolerant Control Strategy In Shunt Active Power Filters

With the development of energy transformation strategy,more and more new energies are connected to the power grid through power electronic converters,which brings a lot of harmonics to the power grid.Harmonics can lead to abnormal heating of the circuit and resonance problems.Therefore,active power filter(APF)has been widely used due to its advantages of real-time dynamic harmonic compensation and fully controllable compared with the traditional passive filter.However,there are a large number of power switches in APF,which has poor shock resistance and is prone to failure.Once APF fails,not only can it not compensate the harmonic components in the power grid in real time,but it will become a harmonic source to produce unknown harmonic components,which will bring security risks.Therefore,the fault diagnosis and fault-tolerant control technology of power switch become one of the most important research contents to improve its reliability.It is of great significance to study how to detect the switch fault and carry out the corresponding fault-tolerant control strategy,so that APF can still be controlled to compensate the system in fault condition.Firstly,the working principles of APF and three-level converter are introduced,then working state and the mathematical model on the dq coordinate axis of the converter is analyzed.Based on the mathematical model,the fundamental frequency control technology based on dq decoupling algorithm and the harmonic control strategy based on predictive current algorithm are studied.At the same time,the working principle of single inductor voltage balancing circuit is studied to solve imbalance problem of capacitor voltage.Finally,in order to realize fault-tolerant control,three-level space vector pulse width modulation(SVPWM)technology with redundant vectors is studied.Taking a single bridge arm in the three-level neutral point clamped(NPC)converter as an example,the influence of open circuit fault on a single bridge arm is analyzed,and the expression of error current in non-zero current condition is derived.The mechanism of voltage distortion in bridge arm under zero current condition caused by fault is studied.On this foundation,the expression of error current under zero current condition is derived.According to the corresponding relationship between fault diagnosis variables and fault types under different reference voltage polarity,a fault diagnosis algorithm for shunt three-level APF is proposed.In order to improve the reliability of active power filter,the fault-tolerant control strategy is studied based on fast fault location.Firstly,the distribution of effective voltage vector in fault condition is analyzed.According to the severity of fault,the fault is classified into type I fault and type II fault.Then,the connection of clamping point is changed in case of fault by controlling the newly added switch to realize the initial topology fault tolerance.Then,the distribution of effective vectors after topology fault-tolerant is studied,and a modulation faulttolerant strategy is proposed to reduce the switching frequency and improve the performance of harmonic compensation.Finally,low-power experimental platform of shunt three-level APF is built,and the design and implementation of rectifier,APF and fault diagnosis program are completed based on FPGA.The experimental results show that APF can effectively compensate harmonics before fault,and the proposed fault diagnosis algorithm can quickly detect and locate faults after fault occurred.And there is no misjudgment in the process of load changing.The correctness of the fault diagnosis method is verified by the experiment.