An Identification Method Based on Mathematical Morphology for Sympathetic Inrush

Sympathetic interaction between transformers is a quite normal phenomenon in power systems. For the purpose of preventing transformer differential protection relays from malfunction, this paper proposes a morphological method for the identification of sympathetic inrush, which is the first time when mathematical morphology is applied in this field. Since the waveform of differential current is symmetrical in an internal fault case while asymmetrical in a sympathetic inrush case, the proposed method uses a morphological operator to extract the peaks and valleys of the differential current to distinguish sympathetic inrush. Considering the possible current-transformer (CT) saturation conditions, this paper combines a morphological gradient with a weighted mathematical morphological operator to improve the effectiveness of the proposed method. The proposed method is evaluated on data collected from simulation cases established in PSCAD/EMTDC and from laboratory experiments, respectively. Identification results have verified that by comparing with the traditional second harmonic restrain method, the proposed method can distinguish sympathetic inrush from an internal fault current more accurately and more effectively, even when the CT is fully saturated.

I. Introduction

Transformer is one of the most important primary equipments and its stability and security have great influence on the operational conditions of a power grid. Differential protection has been the primary protective method of transformers for a long time. However, malfunction of differential protection may happen when inrush current appears in a transformer. In the past decades, scientists focused mainly on the discrimination between inrush currents and internal fault currents. A number of identification and restraining methods for inrush currents based on various mathematical principles have been proposed in [1]– [5]. Nevertheless, these methods assumed that there was only one transformer in the power grid, ignoring the fact that there were always other transformers already connected. In fact, sympathetic interaction between transformers occurs when energizing a no-load transformer in parallel or in series with transformers already connected [6]. As a result, offset magnetizing currents with high magnitude, called sympathetic inrush, are generated in those transformers already in operation. Obviously, this phenomenon would influence the accuracy of the transformer differential protection relay [7]. Moreover, sympathetic inrush may cause power quality problems such as voltage digs [8]. Therefore, accurate identification of sympathetic inrush is essential for the reliability of transformer differential protection.