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Chapter 18. Power Transformer Protection

Chapter 18. Power Transformer Protection - 18 Power...

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18 Power Transformer Protection Arman do Guzma ´ n Hector J. Altuve Gabriel Benmou yal Schweitzer Engineering Laboratorie s, Inc. 18.1 Introduction ..................................................................... 18 -1 18.2 Transformer Differential Protection .............................. 18 -2 18.3 Magnetizing Inrush, Overexcitation, and CT Saturation ........................................................... 18 -4 Inrush Currents . Transformer Overexcitation . CT Saturation 18.4 Methods for Discriminating Internal Faults from Inrush and Overexcitation Conditions ................ 18 -7 Harmonic-Based Methods . Wave-Shape Recognition Methods 18.5 An Improved Approach for Transformer Protection ....................................................................... 18 -14 Even-Harmonic Restraint . Fifth-Harmonic Blocking . DC Blocking 18.6 Current Differential Relay ............................................. 18 -16 Data Acquisition, Filtering, Scaling, and Compensation . Restraint-Differential Element . DC Filtering and Blocking Logic . Relay Blocking Logic 18.7 Differential-Element Performance during Inrush Conditions ......................................................... 18 -20 Case 1 . Case 2 . Case 3 18.8 Conclusions .................................................................... 18 -26 18.1 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 disconnec- tion of the faulted transformer is necessar y to avoid extensive damage and = or preser ve power system stability and power quality. Three t ypes of protection are normally used to detect these faults: over- current protection for phase currents, differential protection for differential currents, and gas accumu- lator or rate-of-pressure-rise protection for arcing faults. Overcurrent protection wi th fuses or relays provided the first t ype of transformer fault protection [4], and it continues to be applied in small-capacity transformers. The differential principle for transformer protection was introduced by connecting an inverse-time overcurrent relay in the paralleled secondaries of the current transformers (CT) [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. ß 2006 by Taylor & Francis Group, LLC.
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The analysis presented here focuses 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] and Mathews [11] further developed this method by adding percentage-differential restraint for
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