| The Static Synchronous Compensator(STATCOM)is one of the indispensable devices in flexible AC transmission system(FACTs).Compared with traditional reactive power compensation devices,the STATCOM can continuously adjust the reactive power with faster adjustment speed,wider operating range and lower harmonic content of compensation current,which makes it more and more widely used.The 3-level neutral-point-clamped(3LNPC)converters has been widely applied in medium/high voltage and large-capacity STATCOM due to their advantages of high voltage resistance,low harmonic contents of output voltages and relatively simple structure.The power grid has extremely high requirements for safe and reliable operation.The failure of a STATCOM will lead to voltage flicker at point of common coupling(PCC)and power oscillation,which will bring serious negative impacts on the power grid.The 3LNPC converter has the highest failure rate in the STATCOM due to its long-term operation in a high voltage and high current environment.Therefore,it is an effective way to improve the reliability of STATCOM and ensure the stable operation of the power grid by adopting proper fault diagnosis and fault-tolerant control to ensure the continuous and stable operation of the system after a failure in the 3LNPC converter.This thesis will carry out theoretical and experimental research on the real-time simulation,fault diagnosis and fault-tolerant control of the 3LNPC STATCOM under an IGBT open-circuit(OC)fault,the research results have certain theoretical significance and practical reference values for improving the reliability and fault resilience capacities of 3LNPC STATCOM.A mathematical modeling method based on sub-model switching is proposed,and the digital real-time simulation model of a fault-tolerant 3LNPC STATCOM under an IGBT OC fault are established,which lays the foundation for the safe and efficient development of the control and protection programs of the system.Aiming at the problem that the mathematical model of the system under fault condition cannot be unified by one expression,after analyzing in detail the impact of the fault on the system operation,the system model is decomposed into three sub-models,and the switching conditions between the sub-models are derived,to achieve a unified system modeling under faulty condition.Based on the dSPACE platform,a real-time digital simulation model with rapid control prototype(RCP)and hardware-in-the-loop(HIL)is established,where the simulation step could be shortened to 1?s.Comparative experiments show that the similarity between the digital real-time simulation and the the actual system is more than 97%.For the IGBT OC fault diagnosis of 3LNPC STATCOM,an OC fault diagnosis method based on residual values and root-mean-square(RMS)values of grid-side currents is proposed,which has the advantages of high real-time performance,fast diagnosis rate and strong robustness.The method uses the normalized current residual value to detect the occurrence of the fault,and locates the faulty half bridge.Moreover,by partitioning the grid-side current,the faulty IGBT is locates based on the normalized RMS value of the fault phase current in the partitions.The experimental results demonstrate that the proposed fault diagnosis algorithm can locate the faulty IGBT rapidly and accurately within 10ms,and is robust to load variation.Aiming at the difficulty and narrow applicability range of the existing multi-level STATCOM fault diagnosis algorithm,an IGBT OC fault diagnosis method based on composite feature extraction and support vector machine(SVM)is proposed.The design process of the algorithm is standardized and generalized,and the diagnostic accuracy is high.This method combines wavelet packet energy spectrum and normalized average current to extract fault characteristics of grid-side currents in both frequency domain and time domain.A multi-class fault classifier under optimal parameters is established based on cross-validation and grid search method by using "one-to-one" multi-classification SVM.The results of the real-time simulations and the semi-physical simulations reveal that the diagnostic accuracy of the proposed fault diagnosis method is more than 96%.For the fault-tolerant control of the 3LNPC STATCOM after an IGBT OC fault,a topology reconstruction scheme of the three-phase four-leg structure is designed,which can perform topology reconstruction on IGBT open-circuit faults of single-phase and two-phase legs.Aiming at the complex controller design process caused by numerous topological types after topology reconstruction,,a controller design method based on dynamic weighting factor finite-control-set model predictive control(FSC-MPC)is proposed,which simplifies the fault-tolerant control algorithm,effectively balances the neutral-point potential of the DC-side of the system after fault-tolerance,and solves the unstable problem of system switching frequency caused by FSC-MPC,realizing the stable control of switching frequency.The experimental results illustrate that the proposed fault-tolerant control strategy can be accomplished without interruption after fault diagnosis,and the STATCOM can operate normally and stably after fault-tolerant control.In this thesis,the effectiveness of the real-time simulation model,the accuracy and real-time capability of the fault diagnosis algorithm,the feasibility and stability of fault-tolerant control strategy are key issues to improve the reliability of 3LNPC STATCOM.By utilizing the dSPACE real-time simulation system as the experimental platform and focusing on the real-time simulation,fault diagnosis and fault-tolerant control of the fault-tolerant 3LNPC STATCOM under an IGBT OC fault,the in-depth theoretical and experimental research has been carried out,and the reliable and stable operation of 3LNPC STATCOM has been achieved.The research results have guiding significance of theories and application values in engineering fields for improving the reliability of a class of 3LNPC grid-connected converters. |
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