Studies On Transformer Longitudinal Tapped Admittance Protection

Power transformer is the pillar of the national economy of modern society, the ef-ficiency and safe operation of the power system is closely linked to the national welfare and the people’s livelihood. Power transformer is the most important equipment of the power system, the safe operation of it will affect the stability of the power system. With the increase of the voltage level, and the prosperity of the smart grid, the price of the transformer becomes more and more expensive, and the demand for the transformer pro-tection becomes higher either. Therefore, finding a reliable and sensitive transformer protection method is always a hot topic.Currently, the transformer differential protection is still the main protection method for power transformer. But there is no direct electrical connection between the two side of transformer, the current from two side do not conform to the Kirchhoff’s current law strictly. Therefore the transformer differential protection still has the problem of the inrush current discrimination, which affects the speed and the accuracy of protection. This paper firstly proposed a unified transformer equivalent circuit, and proposed the longitudinal tapped admittance (LTA) method in transformer protection. This method doesn’t base on differential protection and is not affected by the inrush current. It has the feature with big margin of protection and low demand for parameters measuring accuracy which is fit in with engineering applications.This paper studied the basical principle of differential protection and inrush cur-rent phenomenon. Concluded the essential reason of the inrush current discrimination problem of transformer differential protection, in the aspect of transformer equivalent circuit. This paper proposed a brand new unified transformer equivalent circuit for dif-ferent transformer condition includes inrush no fault and internal fault, which is based on amout of research of equivalent circuit. The proposed equivalent circuit has the classic T shape circuit structure. Based on the unified equivalent circuit, this paper proposd the longitudinal tapped impedance, and discussed the tapped impedance tra- jectories in various circumstances. Although the tapped impedance trajectories can be used to discriminate the fault and healthy condition of transformer in many different circumstances, the trajectories are too complicated and the range is too large, which is not fit in the engineering application. This paper determined the longitudinal tapped admittance (LTA) parameter as the key parameter of the protection method by studing the exciting branch of the equivalent circuit. And the method of protection settings has been given.Theoretical analysis shows that the inrush current only affect the susceptance of the LTA, while the internal fault only affect the conductance of the LTA. Therefore, the LTA parameter is most suitable for reflecting the condition of transformer and not affected by inrush current.To validate the effectiveness of the LTA method, a total of 635 cases of simulation experiments have been carried out. And various types of experiment have been car-ried out in the physical-simulation experiments of Shandong University. All the results prove the validity of the theoretical analysis. This method can escape the impact of inrush current and performs well in any kind of transformer fault, especially in the sce-nario that slight turn-to-turn fault and inrush current exists simultaneously, comparing with the traditional transformer differential current protection with sencond harmonic restraint method.