A New Digital Dynamic Algorithm for Detection of Magnetizing Inrush Current in Transformers

摘要

This article presents a new method to identify the inrush and fault currents for transformer protection. When a power transformer is energized, the resulting inrush current can cause a false tripping of the protection system. A new concept of using a combined criterion for the detection is presented and implemented. The proposed idea is based on evaluating three factors instead of using the common criterion of the second harmonic. The three factors are the conventional second harmonic content, the DC decaying time constant, and the ratio between the fundamental component and the first peak magnitude. Estimation of the contents of the current signal is achieved by a discrete time-dynamic filter. The basic advantage of the proposed filter is its ability in producing the estimates in a very short time. The algorithm uses sets of digital samples of the distorted waveforms of the inrush and fault currents to extract the features of the signals. A simple expert system is then used to evaluate the three factors in order to differentiate between different situations. The proposed technique has been tested using a simulated power system. The extensive simulation studies indicate that the restrain signal can be issued in less than one-quarter of the cycle; therefore, the method can be used as an effective tool for high-speed digital relaying.