Research On Transient Mechanism And New Protection Principle Of Large-scale Transformer

Along with rapid development of power system in China, long-distance transmission and power network interconnection throughout the country are further strengthened. So the requirement for digital transformer protection is becoming higher and higher, and it has great significance to study and research new and high-performance theory for transformer protection. But under the condition of complex power network, transformer protection is influenced by multiple factors and its correct operation rate is relatively low. So this dissertation researches several key problems about transformer protection, and carries out the following research which includes: transient mechanism and restraint method of energizing inrush and sympathetic inrush, the speciality and differential protection configuration of UHV transformer, protective device development for large-scale transformer.The dissertation analyzes the complexity of transformer inrush's transient process, deeply studies generation mechanism, influence facts and wave characteristic of energizing inrush and sympathetic inrush, and some conclusions are drawn. Firstly, the decrease of large-scale transformer's saturation magnetic flux makes second harmonic ratio of energizing inrush decrease, but a special phase relation is kept between second harmonic and fundamental component of inrush, which provides theory basis for introducing phase characteristic criterion in second harmonic restraint method. Secondly, considering the influence of the circulating current in three-phase transformer's triangle side, the wave characteristic of excitation current, differential current, phase current when the transformer is energized are discussed. Finally, the generation mechanism, influence facts of sympathetic inrush are analyzed using the way basing on the change of transformer flux linkage, which can provide theory basis for researching new protection method.Considering some problems of the traditional second harmonic restraint method during the application on field, the dissertation studies that the phase ratio of fundamental component and second harmonic of energizing inrush is fixedly close 0°or 180°, and an improved second harmonic restraint method is proposed, which is validated through kinds of dynamic simulation experiment. The new method synthetically uses the magnitude and phase relationship between fundamental component and second harmonic of differential current to distinguish inrush, which can improve inrush restraint performance and make differential protection operation fast and sensibly when internal faults happen. Considering some mal-operation accidents of transformer differential protection for sympathetic inrush, the dissertation researches the mal-operation reason through dynamic simulation experiment and analysis for field malfunction accidents, and the conclusion is drawn that second harmonic content of sympathetic inrush is so high that the second harmonic restraint method can effectively block differential protection for a long time, and actually mal-operations often happen when sympathetic inrush makes TA saturation. So a new method to identify sympathetic inrush basing on the fundamental component increment of differential current is proposed, which can identify sympathetic inrush fast and reliably before TA saturation, and accordingly adopt effective measure to avoid differential protection mal-operation.According to 1000kV UHV AC transmission project in China, the speciality of 1000kV UHV transformer protection are discussed, and basing on building the dynamic simulation experiment system of UHV transmission, the sensitivity and reliability of differential protections for UHV transformer are studied when kinds of internal faults happen and transformer is no-loaded switched on, so the protection configure scheme of UHV transformer are given out, and the following conclusion are obtained: The main winding's differential protections is sensitive to main winding's internal faults, but can't operate sensibly when the voltage regulating winding or the voltage compensating winding inter-turn faults happen, and the differential protection of the voltage regulating winding and the voltage compensating winding are needed respectively.Finally, according to different requirements for large-scale transformer protection system in various field conditions, a set of universal protection system for large-scale transformer is developed. By integrated universality consideration, perfect protection configuration, combination of new technology and mature theory, high performance hardware platform and modular software structure, the universal protection system can meet actually application requirements of kinds of large-scale transformer in various field conditions. The dynamic simulation experiment results show that protective performance can meet the design necessity and agree with prospective design goal.