| How to guarantee and enhance the correct action rate of transformer protection in a variety of complex operating conditions and fault scenarios remains to be one of the few problems in need of an ideal solution within the field of main equipment protection. Compared to conventional transformers, converter transformers used for HVDC and UHVDC systems are located at the center of the AC/DC intersection and operate in unique modes, the protection of which, therefore, faces even more complex problems. With the development and application of UHVDC transmission technology, UHV converter stations present differentiated connection modes in AC/DC fields, and its electromagnetic environment becomes more complex, the impact of fault and its recovery process on the converter transformer protection will become even more severe. However, existing researches on the abnormal operation behavior of the converter transformer differential protection are quite rare, with only accident reports and superficial analysis. The mechanism of impact of disturbance under a variety of special operation conditions on the performance of converter transformer protection is unclear. Therefore, it is particularly important to thoroughly study the scenarios and mechanism of the abnormal actions of converter transformer protection under various complex transient disturbances, to seek out the pattern and characteristics of fault and disturbance, and accordingly put forward solutions to improve the correct action rate of the converter transformer protection, which would further enhance the overall correct action rate of transformer protection.Accordingly, this thesis focuses on the study of the mechanism of abnormal operation of converter transformer differential protection caused by a variety of faults and disturbances. Furthermore, and the corresponding protective strategies are surveyed as well.In respect of the impact of complex inrush on transformer differential protection exacerbated in deeply AC/DC coupling scenarios, this thesis established comprehensive simulation models of practical EHV AC and UHV DC system for in-depth analysis of the impact of AC/DC connection topology and control logic of convert stations on the inrush evolution and transfer process, based on the actual parameters of the Tian-Guang HVDC project and the Yun-Guang HVDC project and the actual control strategy and logic of each AC/DC control units, which may serve as foundations of the follow-up studies on the various inrush features and fault characteristics. In order to reveal the mal-operation and fail-to-trip mechanisms of the longitudinal differential protection of conventional converter transformer under certain energization conditions and fault conditions, the thesis simulated and analyzed the transient features of the various special inrushes and fault currents of converter transformer based on the above simulation model, and analyzed the limitations of the second harmonic blocking criterion in the scenarios above, which may provide references to studies on the mechanism analysis of unexplained abnormal operation behaviors of converter transformer longitudinal differential protection. In order to study the impact of the recovery inrush caused by fault removal on the conventional zero-sequence differential protection, this thesis quantitatively explained the cause and effect of the recovery inrush after the fault occurrence and subsequent fault removal of one of the converter transformers connected to the same bus from perspectives of both analytical analysis and time-domain simulation, based on the mathematical equations and the electromagnetic transient model. Furthermore, a relationship between the TA magnetic bias cumulative effect caused by inrush and the mal-operation of zero-sequence differential protection was established, which may serve as reference in studying the potential mal-operation risk of zero-sequence differential protection caused by TA magnetic bias cumulative effect. In order to reveal the mal-operation mechanism of the differential protection for UHV dual-converter-transformers due to sympathetic inrush, this thesis clarified the this type of mal-operation behavior actually results from second harmonic restraint criterion losing blocking capability, based on the analysis of the occurrence process and deviation characteristics of the symmetrical inrush during UHV converter transformer energization using the simulation model proposed above. This provided a means for enhancing the operation reliability of the differential protection for dual-converter-transformer of existing and future UHV converter stations.In respect of studying the crucial techniques of improving the operational performance of differential-type protections used for power transformer, this thesis proposed an novel time difference based criterion improved by means of enhanced half-wave Fourier pre-filtering in order to improve the operation security of transformer longitudinal differential protection based on time difference criterion. Combined with the second harmonic blocking criterion and fundamental proportional differential criterion, a novel comprehensive main protection scheme for power transformer was established. This scheme was suitable for both general HV transformers and EHV converter transformers, which provided a new approach towards improving operation reliability of longitudinal differential protections of various HV/UHV transformers under various types of complex faults, disturbances and special operation conditions. In order to avoid the potential mal-operation risk of conventional zero-sequence differential protection without additional blocking criterion due to external fault removal inrush, this thesis proposed an additional blocking criterion based on waveform matching criterion implemented with standard Hausdorff graphical similarity matching algorithm, which works with conventional zero-sequence differential protection criterion. By this means, the mal-operation risk caused by fault removal can be avoided to a great extent without losing existing performance of conventional zero-sequence differential protection. This work provided a new method to enhance the operation performance of zero-sequence differential protection of power transformer. In order to enhance the withstanding capability of differential protection for dual-converter-transformer equipped for UHV converter station on the symmetrical inrush, this thesis designed a new additional blocking criterion based on Hausdorff distance algorithm, which is used to enhance the operation security of the differential protection for UHV dual-converter-transformer. By this means, a variety of inrush (including symmetrical inrush) and differential current resulting from internal faults (even those fault currents accompanied by inrush) can be identified clearly, which provided a technical solution towards improving the operation performance of differential protection for dual-converter-transformer equipped for UHV converter station. |
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