Transformer Internal Faults Simulation

Abstract

Abstract —this paper presents a novel method of modeling internal faults in a power transformer. The method leads to a model which is compatible with commercial phasor-based software packages. Consequently; it enables calculation of fault currents in any branch of the network due to a winding fault of a power transformer. These currents can be used for evaluation of protective relays' performance and can lead to better setting of protective functions. Index Terms —Transformers, transformer windings, modeling, short circuit currents, internal fault I. INTRODUCTION Here is an ongoing need to develop analyses and systems for better protection of power systems. Transformers are widely used in the power system and are among the most important and the most expensive equipments. Failure of a power transformer may lead to loss of power in parts of a system, which can be costly and inconvenient for customers. Therefore; the protection of power transformers is of high importance. The internal short circuit currents for a transformer may be larger than its terminal short circuit currents. Accordingly, high current stage protection is used in transformer differential protection in order to protect transformers against such large internal short circuit currents immediately. Therefore, for adequate power transformer protection, an accurate method of calculation of internal faults should be available. There are several researches in the area of simulation of internal faults in a power transformer [1]-[4]. Nevertheless; all of these methods consider that the transformer is separated from the network. However; transformers are always used inside an actual power system. Therefore; the contribution of the network in an internal fault in a transformer should be regarded. Moreover; none of the previous methods is capable of considering the transformer internal faults in short circuit analysis, and none has been addressed in the practical fault analysis standards (such as IEC, IEEE, etc.). Calculation of fault currents due to a winding fault of a power transformer, not only at the terminals of the power transformer but also in any branch of the network, was the motivation for doing this study. This paper presents a method of modeling internal faults of power transformers in phasor-based fault analysis. The model is fully compatible with software packages which are using phasor-based fault analysis. Moreover; it considers the contribution of the network surrounding the transformer, and can calculate the fault currents in any branch of the network. The winding fault can be single phase to ground or two phase to ground or three phase to ground. Although internal two phase to ground fault and three phase to ground fault rarely happen in a power transformer, the model enables calculation of fault currents in these situations too. II. TRANSFORMER MODEL