Research On Digital Differential Protection For Power Transformer

Along with the development of the modern power system, the requirements of digital differential protection of transformer are becoming higher and higher. After the analysis on the problems, which influence the differential protection, this paper makes the following research which includes:Based on the peaked wave characteristic this paper proposes two novel methods to identify magnetic inrush. In the method one, internal fault current can be discriminated from inrush current by comparing the similarity between the actual wave and two reference waves under two different frequency conditions per half cycle. The second method can discriminate fault currents from inrush currents by comparing two phasors. The former is formed by shifting the fundamental phasor evaluated from the current data window. The latter is a phasor evaluated from the next data window. The dynamic test shows the high performance and practicality of the two methods.In this paper the principle of transformer magnetizing inrush currents is analyzed. Base on the transformer equivalent circuit, the recurrence calculation approach of decay inrush currents is derived. By curves fitting, the practical formula for the dependence of intermission angle and inrush current amplitude as well as time is presented. The simulation test shows that the precision of the formula satisfies the requirements of practice. It provides a tool of analysis and calculation for the research on differential protection and buck-up protection.Base on the transformer equivalent circuit and the flux variety, in this paper the principle and characteristic of sympathetic interaction is studied in depth firstly. Then, the effecting parameters are investigated such as system resistance, parallel or series of transformers, and the load of connected transformer. Finally, based on the simulation and combined with the characteristics of sympathetic interaction, the infection to the differential protection, back-up protection and other relative protection is emphatically studied in the parallel connection and the series connection, and the corresponding solutions are given.Based on the magnetic restraint principle to detect current transformer saturation, an improved criterion for detecting TA saturation is put forward in which the second current of TA is only used. This method effectively resolves the influence of the residual flux and the load resistance on the magnetic restraint principle. And the proposed method is easy to be carried out and has good practicality.To satisfy the requirement of the quick development of protection scheme, a digital protection developing and testing platform is put forward. Based on the experience of developing protection devices, the rapid developing and testing of the scheme are realized by using the modern software engineering and database technology. By the platform, the developing cycle of the protection products can be shortened greatly and the cost can be reduced...