Research On Fault-Diagnosis And Fault-Tolerant Control Strategies For T-type Three-level Converters

In today's world,energy crisis and environmental pollution have seriously restricted the development of social economy.Development of renewable energy is a key way to solve this crisis.As the core equipment of power conversion,power converter plays a key role in the renewable energy generation system.The T-type three-level converter has been widely used for its high efficiency and low harmonic.However,its working condition is quite complex and harsh,and power converter failure occurs frequently.The latest data shows that the switching device is one of the main causes of power converter failure.Through on-line real-time diagnosis of faulty switching device and adopting appropriate fault-tolerant control strategy,the renewable energy generation system can effectively guarantee low cost,high performance and stable operation.The faulty condition can be divided into short circuit fault and open circuit fault.In order to realize the fault-tolerant operation when short-circuit fault occurs,fault isolation technology is often used to convert it into open circuit fault condition.Therefore,researchers have done a lot of research on open circuit fault diagnosis and tolerant control for T-type three-level converter.However,with the large-scale integration of renewable energy into the power grid,the fault-tolerant control of T-type three-level converter faces severe challenges in the aspects of scene adaptability,neutral point voltage balance and efficiency optimization.? The traditional fault diagnosis method of T-type three-level rectifier is only suitable for unit power factor.However,in order to achieve maximum power tracking in wind power generation system,the rectifier mostly works under non unit power factor,so the existing method is no longer applicable;? The traditional fault-tolerant control strategy will further aggravate the neutral point voltage imbalance,resulting in a series of problems,such as increased switching device stress,reduced capacitor lifetime and increased current harmonic distortion.? The existing fault-tolerant control strategy is inefficient,which is a bottleneck problem restricting the effective improvement of energy utilization Therefore,this paper focusing on the open circuit fault diagnosis and fault-tolerant control strategy of T-type three-level converter to improve its reliability and performance.The main research work and innovation of this paper are as follows:In order to solve the problem that the traditional open-circuit fault diagnosis method of T-type three-level rectifier is difficult to adapt to multiple scenarios,a new fault diagnosis method based on instantaneous voltage error is proposed.The new method uses the error between the measured voltage and the estimated voltage to realize fault diagnosis.Firstly,according to the absolute value of line voltage error,combined with hysteresis comparison and look-up table,the fault diagnosis is realized quickly.In order to avoid the adverse effect of high frequency component of line voltage on fault diagnosis,the measured value and estimated value are processed by low-pass filtering respectively,which enhances the reliability of diagnosis Secondly,combined with the amplitude of DC bus voltage fluctuation,the accurate location of fault devices is further realized.This method does not depend on the fault characteristics under specific conditions,and thus,it can realize fault diagnosis even when rectifier operated under non-unit power factor.The fault diagnosis method of T-type three-level rectifier is improved,which lays a solid foundation for the application of fault-tolerant control strategyNeutral-point voltage imbalance includes DC bias and oscillations.As an inherent problem of three-level power converter,it is easy to lead to a series of problems such as the stress increase of switching devices and high total harmonic distortion.The traditional fault tolerant control strategy has a large neutral point voltage oscillation.To solve it,this paper propose two fault-tolerant control methods are proposed,which are improved single-phase two-level switching tolerant control(I2LS-TC)method and hybrid switching tolerant control(HS-TC)method.Firstly,in order to realize the fault-tolerant operation of the power converter,O state in I2LS-TC does not participate in the fault phase voltage synthesis in the whole cycle,while in HS-TC,in order to keep the advantage of lower current total harmonic distortion of three-level power converter,O state only does not participate in fault phase voltage synthesis in the affected half cycle.Secondly,by skillfully selecting the three-phase conduction time,the two methods can suppress the neutral point voltage oscillation effectively.Simulation and experimental results show that the performance of the two methods is improved by more than 100%compared with the traditional fault-tolerant control algorithm.In addition,compared with the two fault-tolerant control strategies,HS-TC has a better harmonic performance,and is more suitable for occasions with higher power quality requirements.The I2LS-TC has a poor harmonic performance,but the calculation is simple,and it is more suitable for the application scenarios with low harmonic requirements and strict cost requirements.In order to eliminate the DC bias and oscillation of neutral point voltage simultaneously,the carrier based outer switch tolerant control(CB-OSTC)and carrier based outer switch tolerant control(CB-ISTC)are proposed respectively.In CB-OSTC,the single carrier instead of double carrier is firstly used in the faulty phase to avoid the use of the faulty switching state.Secondly,the DC bias compensation value and AC offset compensation value are introduced to effectively suppress DC bias and oscillation of neutral point voltage based on the zero-sequence voltage injection.Compared with the traditional fault-tolerant control strategy,the neutral point voltage balancing adjustment time is greatly reduced and the oscillation amplitude is reduced to less than 50%of the original.Compared with I2LS-TC and HS-TC,the CB-OSTC has the advantages of simple calculation and lower system hardware cost.In CB-ISTC,the neutral point voltage can be adjusted quickly and accurately by selecting the zero-sequence component and the appropriate neutral point voltage offset.Compared with the traditional fault-tolerant control strategy,the speed of neutral point voltage regulation is increased by an order of magnitude,and the oscillation amplitude is reduced to half of the original.In a word,the new fault-tolerant control strategy can effectively adjust the neutral point voltage,reduce the switch device stress,prolong the converter life and reduce the current total harmonic distortion.It has important academic and engineering value for the realization of high-performance fault-tolerant operation of power converters.The efficiency of power converter is one of the key factors.The traditional fault tolerant control for T-type three-level converter has the disadvantage of low efficiency,large size and high cost.To solve it,this paper proposes a new fault-tolerant control method which take the switching loss and neutral point voltage balance into consideration simultaneously.This new method considers the internal relationship between carrier type,carrier direction,zero sequence voltage component and output voltage waveform comprehensively.And then the high-quality and efficient operation of the power converter is realized by taking the neutral point voltage regulation and switching loss reduction as the control objectives.In this method,the fault-tolerant operation of power converter is realized by using single carrier firstly.Then,the zero-sequence voltage injection method is used to select different clamping states to realize the neutral point voltage regulation and the switching events reduction in the switching period.Finally,the switching events between the adjacent switching period can be effectively reduced by selecting the carrier direction reasonably.Simulation and experimental results show that compared with the traditional fault-tolerant control strategy,the proposed method can significantly improve the operation efficiency of the power converter,which is of great significance to maximize the power converter capacity and improve the energy efficiency.To sum up,this paper takes the T-type three-level power converter which is widely used in new energy power generation system as the research object,and makes a series of innovative research achievements in fault diagnosis and fault-tolerant control strategy when open circuit fault occurs at switching devices:? A new fault diagnosis method suitable for wide power factor range is proposed,and the existing fault diagnosis methods are improved and developed;?According to the operation mechanism of neutral point voltage imbalance,two kinds of fault-tolerant control strategies are proposed,which effectively suppress the neutral point voltage imbalance and improve the power quality for both AC and DC sides;? A new fault-tolerant control method to reduce switching loss is proposed,which improves the efficiency of power converter effectively.It provides an important technical support for the safe,reliable,high-quality and efficient operation of T-type three-level power converter.