Research On Problems About Residual Flux Of Current Transformer Magnetic Core

Electromagnetic current transformer(CT) is widely used as standard protection and measurement equipment in power system, but there is a great deal of residual flux in the core of CT after cutting off short-circuit current. Therefore, the degree of accuracy of CT, the reliability of power system operating, economic benefit of power consumers, and fairness of market will be influenced directly by the residual flux. This paper presents the comprehensive research on the problems induced by residual flux of CT core in power system, and the main contents are as follows.1) The systematic studies of the influences of several factors on residual flux and the characteristics of residual flux occurrence in CT have been performed by computer simulation. The CT model is established with the use of the electromagnetic transient program ATP, and the productions of the residual flux under the respective action of the several factors, i.e. short-circuit current breaking time, short-circuit current and its aperiodic component, primary side time constant, power factor, and impedance of secondary-side load, are systematically simulated. The characteristics of residual flux occurrence obtained by this work provides theoretical basis for inhibiting the residual flux.2) A new magnetization model of CT core on the basis of stack operating has been presented. The magnetization characteristics of the core are summarized, the classification for existed various magnetization curves is made, and the category of curves is determined by the magnetic field intensity H and by stack operating. Corresponding to various categories of BIH curves, the systematic expressions of calculating magnetic flux density B are given. The comparison between the B values calculated by using the present model and the experiments shows the reliability of the present model. It should be emphasized that using the present model, the magnetization process of core with residual flux can be simulated and the residual flux of core can be calculated after cutting off magnetic field. Therefore, the present model is more perfect for reliable and comprehensive simulations of the magnetization of core. Besides, CT model is constructed by means of searching solution algorithm for the coupling of the magnetization model and electro-magnetic induction equation of CT. Then protective relaying and energy measurement influenced by residual flux is analyzed on the basis of changing of secondary current wave simulated by the CT model under different residual flux.3) Based on the principles of residual flux occurrence and taking into account the factors of influencing residual flux, steps of eliminating residual flux are suggested. These steps are to change material and structure of core, to limit short-circuit current, and to decrease secondary load of CT. Further, for the method of inhibiting residual flux by connecting the element of stabilizing voltage in parallel at secondary side of CT, its effectiveness is examined by simulations. This method doesn't change physical design and normal operation of CT, has no requirement for the circuit to control and detect signal, and can start and recover well by itself. Due to the above characteristics and its high reliability and low cost, this method may be widely applied to power system.