Fault Localization Strategy Of VSC-HVDC Converters Based On Isolation Forest

Voltage-source-converter based high-voltage direct-current（VSC-HVDC）transmission has become the most effective technology for realizing large-scale renewable energy power generation and transmission due to its large transmission capacity,long transmission distance and low operation loss.The modular multilevel converter（MMC）has become the main VSC topology in VSC-HVDC due to its modularity,high efficiency and scalability.Since the MMC adopts a cascaded structure of submodules（SM）and consists of a large number of power switches,its reliable operation is crucial.The reliability research of the MMC has become a research hotspot in both academia and industry,and the detection and localization of SM open-circuit fault has become an important topic in MMC reliability research because of its inconspicuous fault characteristics and difficulty in being detected in time.The paper takes the three-phase six-arm MMC with half-bridge submodules（HB-SM）as the research object,and proposes an Isolation Forest（IF）based SM open-circuit fault diagnosis and localization strategy for the MMC.The proposed strategy utilizes the statistical characteristics of data for fault localization and provides new ideas for the MMC fault localization research.The research contents and primary contributions are listed as follows:（1）The paper analyzes the working principle of the MMC and establishes the detailed mathematical model of the MMC.Then,the control system of the MMC is analyzed including capacitor voltage balance control,circulating current suppression control and overall control.The SM open-circuit fault characteristics is analyzed in detail,both T₁ open-circuit fault and T₂open-circuit fault are analyzed,which reveales the characteristics that SM open-circuit fault will affect its capacitor voltage and make it deviate from normal SMs.（2）The paper proposes an IF based SM open-circuit fault diagnosis and localization strategy for the MMC,which can localize the SM open-circuit fault based on the IF constructed by capacitor voltages.First,Isolation Trees（IT）are constructed based on SM capacitor voltages to distribute SMs,and the SM depth is used to describe the relative positions of all SMs in the IT.Second,IF is constructed based on ITs and SM average depth is used to describe all SMs in the IF.Then,the output of the IF is defined as the serial number of the SM corresponding to the minimum SM average depth,and this SM is considered to be suspicious.The outputs of several continuous IFs are stored in the output buffer,and the fault flag is generated based on the output buffer,accordingly the faulty SM is localized.Finally,the multiple SM open-circuit faults can be localized through the cyclic execution of the fault localization strategy.（3）The proposed IF based SM open-circuit fault diagnosis and localization strategy is analyzed comprehensively in the paper.The selection of several key parameters in the proposed fault localization strategy is discussed,and the corresponding parameter selection methods are proposed.The performance of the proposed fault localization strategy is evaluated from the following four aspects:discussion of fault localization time,analysis of the implementation time of the proposed strategy,comparison of existing fault localization strategies and evaluation of the classification effect based on indexes in the field of artificial intelligence and statistical.The proposed IF based SM open-circuit fault localization strategy constructs concise low-data-volume tree models only depend on SM capacitor voltages,and uses sparsity and difference properties of outlier data to localize faulty SM.The proposed strategy has linear time complexity,simple calculation process with low calculation complexity.The proposed strategy does not require detailed mathematical models of the MMC and manual setting of empirical thresholds.In addition,the proposed strategy does not require any modification to the hardware circuit.（4）The proposed IF based SM open-circuit fault diagnosis and localization strtegy is verified by both simulations and experiments.The simulation is based on the professional simulation tool PSCAD/EMTDC,and the experiment is based on the small-sized prototype built in the laboratory.Through simulations and experiments,the effectiveness of the proposed IF based SM open-circuit fault diagnosis and localization strtegy is verified in several different cases.