Research On Influencing Factors And Countermeasures Of Transformer Differential Protection's Mal-operation

Transformer is one of the major electric apparatuses in power system. It plays an important role of power transfer in power system. Differential protection as the main protection of transformer will be influenced by many factors to mal-operate in actual operation, such as inrush current, CT saturation and sympathetic inrush current. Analyzing the reason of differential protection's mal-operation and proposing the corresponding countermeasures has great theoretical value and important significance to improve the reliability of transformer differential protection. Firstly this paper uses Jiles-Atherton theory to simulate CT model, and analyzes the characteristics of CT saturation using this model as the theory basis of the following chapters. On this basis the paper makes an investigation on some mal-operation problems exist in transformer differential protection. Based on deep analysis of the mal-operation reason, new algorithms to solve these problems are provided. The major contributions of this dissertation are as follows:(1) Through combining the J-A(Jiles-Atherton) theory and CT characteristic equations, the CT dynamic analysis model considerating the hysteresis characteristic is established. After that a module development is completed using S-function,Masking Editor and Symbolic Math Toolbox. This module not only has many virtues such as accurate calculation,flexible application and great expansion, but also can realize the development of user-defined model through impression imput, and also support multiform external data imput. Applying this model, this chapter analyzes the saturation characteristics of CT, which provides the theory basis of the following chapters.(2) Aiming at the mal-operation of differential protection during the removal of transformer external faults, this chapter makes a deep analysis of the effect to differential protection of CT saturation and phase compensation. Based on the characteristic of mal-operation, multi-region time delay breaking method is creatively put forward to prevent the mal-operation of differential protection. This method can not only ensure differential protection not to mal-operate during external faults and the removal of external faults, but also can make the quick operation when internal faults occur.(3) According to that transformer differential protection could ont release in time when transferring fault (external fault to internal fault) occurs, this chapter firstly analyzes the characteristics of differential currents during internal and external fault, based on that CT's secondary current has a little linear transmission area while the primary current crossed the zero point. On this basis, this dissertation proposed a method to recognize the transferring fault by checking the dead angle of differential current using oddity point and dead angle detection operators based on mathematic morphology. This method could correctly recognize the transformer internal and external fault under CT saturation, and could also make the right judgement of transformer transferring fault.(4) According to the mal-operation example of inner bridge connected transformer, this chapter respectively analyzes the impaction of inrush current, sympathetic inrush and CT saturation on the transformer differential protection. Study proves that the main reason of mal-operation is caused by CT partial transient saturation induced by non-periodic component. On this basis, a new algorithm based on time differential method to prevent differential protection mal-operation of inner bridge connected transformer is put forward. The new method has many virtues such as simple calculation,rapid,reliable,identification features are significant and so on.(5) During transformer's non-synchronous closing, the phase already closed will induce the other phases to ultra saturation, and further will cause the mal-operation of transformer differential protection. According to the analysis of circulating current in wye-delta connected transformer, two new algorithms to identify transformer's non-synchronous closing are put forward. The new algorithms need not get the actual currents in delta windings, therefore they have good results both for split type transformer and one body transformer. Following that this chapter puts forward a new method of detecting the second harmonic's tendency in differential current. The new method could not only prevent the mal-operation during transformer's non-synchronous closing, but also can prevent the error-lock during teansformer's no-load closing on internal fault.