A Fast Discrimination Method of Inrush Current based on Transformer Magnetizing Characteristics

Abstract

Recently customers who need electricity of higher quality have been installing co-generation facilities. They can avoid voltage sags and other distribution system related disturbances by supplying electricity to important load from their generators. For another example, FRIENDS, highly reliable distribution system using semiconductor switches or storage devices based on power electronics technology, is proposed. These examples illustrates that the request for high reliability in distribution system is increasing. In order to realize these systems, fast relaying algorithms are indispensable. The authors propose the necessity of pro-tection devices that implements the fast relaying algorithms for distribution system. In designing higy speed protection system, fast discrimination of magnetizing inrush current is also indispensable. Conventional method of inrush current detection for transformer protection uses the ratio of second order components of differential current. The authors propose a new method of detecting inrush current based on transformer magnetizing characteristics. Inrush current occurs when transformer core becomes saturated. The proposed method estimates transformer core saturation by the linear relation between differential current and the integral of voltage. The propose method can be applied for single-phase transformer and also for three-phase transformer by considering subsidy effect. The proposed method is verified using experimental data using single-phase transformer and three-phase transformer and the proposed method is proved to be effective.