Performance Analysis of Traditional and Improved Transformer Differential Protective Relays

This paper describes a new approach for transformer differential protection that ensures security for external faults, inrush, and overexcitation conditions and provides dependability for internal faults. This approach combines harmonic restraint and blocking methods with a wave shape recognition technique. First, we review the concept of transformer differential protection. We then analyze magnetizing inrush, overexcitation, and current transformer (CT) saturation phenomena as possible causes of relay misoperation. After summarizing the existing methods for discriminating internal faults from inrush and overexcitation conditions, we propose a new approach for transformer differential protection and describe the relay that is based on this approach. Finally, we compare the behavior of some of these methods for real cases of magnetizing inrush conditions. INTRODUCTION Three characteristics generally provide means for detecting transformer internal faults [1]. These characteristics include an increase in phase currents, an increase in the differential current, and gas formation caused by the fault arc [2], [3]. When transformer internal faults occur, immediate disconnection of the faulted transformer is necessary to avoid extensive damage and/or preserve power system stability and power quality. Three types of protection are normally used to detect these faults: overcurrent protection for phase currents, differential protection for differential currents, and gas accumulator or rate-of-pressure-rise protection for arcing faults. Overcurrent protection with fuses or relays provided the first type of transformer fault protection [4]; it continues to be applied in small capacity transformers. Connecting an inverse-time overcurrent relay in the paralleled secondaries of the current transformers introduced the differential principle to transformer protection [4]. The percentage differential principle [5], which was immediately applied to transformer protection [4], [6], [7], provided excellent results in improving the security of differential protection for external faults with CT saturation. This analysis will focus primarily on differential protection. Differential relays are prone to misoperation in the presence of transformer inrush currents, which result from transients in transformer magnetic flux. The first solution to this problem was to introduce an intentional time delay in the differential relay [4], [6]. Another proposal was to desensitize the relay for a given time, to override the inrush condition [6], [7]. Others suggested adding a voltage signal to restrain [4] or to supervise the differential relay [8]. Researchers quickly recognized that the harmonic content of the differential current provided information that helped differentiate faults from inrush conditions. Kennedy and Hayward proposed a differential relay with only harmonic restraint for bus protection [9]. Hayward [10]