The Study Of The Transient Simulation Of The Current Transformer For Rapid Digital Protection

In recent years, the Chinese electric power industry develops rapidly. Many large scale ultra-voltage transformer are using in the electric power system. It is required that the electric power system must run very stable. However, the accuracy of protection for the large scale transformer is not so high at this moment. It can not follow the requirements of the practical electric power system.The current transformer is an important device in the electric power system. With a non-linear model for the saturation of magnetic core of the current transformer, the transient equation for magnetic flux can be constructed. Consequently, the influence of CT transient status on the digital protective device can be analysed. With the simulation of the transient performance of the current transformer, some important parameters can be decided for the differential protection of the transformer. According to the principle of electromagnetic, the characteristics of the time changing field can be obtained by solving Maxwell equation. This method can be used to analyse the saturation of the magnetic core, hysteresis loss, the eddy current dissipation of the magnetic core, etc. However, this simulation is very expensive. It takes a lot of computer resources. This paper proposes a new way to analyse the transient performance of the current transformer. Firstly, the finite element model for ANSYS simulation is constructed and the distribution of the magnetic field in the transformer is calculated with different primary current. From the results, the inductance of the primary ring, the inductance of the leaking magnetic flux, the resistance of the eddy current can be obtained. Of course, these parameters are the functions of the primary current if the transformer is saturated. The leaking inductance is also influenced by the saturation of the magnetic core. Because the leaking magnetic flux is generated by the primary ring and the magnetic core together, the variation of the leaking inductance is not the same as that of the main inductance.The resistance of the eddy current, the main inductance, the leaking inductance and other non-linear parameters are obtained in this paper. A numerical method is used to solve the transient equation. The transient performance of the current transformer is also analysed in this paper. We also study the wave forms of the primary current and the second current with different loads and different saturation status. From the waveform of the second current, the influence of the saturation of the magnetic core on the current transformer can be seen.This paper constructs an accurate model for the saturation of the current transformer. With this model, the saturation process of the magnetic core is simulated precisely. This paper proposes a new method to analyse the transient performance of the current transformer. In this method, the computation of the magnetic field is combined with the numerical simulation of the circuit formula. From the simulation results, it can be seen that the transient performance of the current transformer can be analysed accurately and rapidly with the method proposed in this paper. From this analysis, the criterion of the differential protection can be predicted. As results, the performance of the transformer protection can be improved.