Enhancements in Transformer Unit Protection using IEEE C57.109 withstand capability curves

Transformers are one of the critical assets in the substation. In general, transformers are protected with differential or unit protection and the protection zone is defined by the location of CTs. Through faults or external faults are categorized as faults outside the differential protection zones and excessive currents due to an external fault is not detected by the differential protection. The life expectancy for liquid-immersed power transformers are largely influenced by the number of “through-fault” operations. To detect the through faults, it is essential to coordinate overcurrent protective devices with the transformer short-circuit withstand capability. In general, majority of through fault algorithms uses cumulative I
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t to calculate the transformer remaining withstand capability. However, many practical cases are identified, where the conventional algorithms fail to accurately predict the maintenance alert. IEEE Std C57. 109-2018 provides transformer damage withstand capability curves based on the transformer category, exposure to frequent/in-frequent faults, duration and magnitude of the symmetrical fault currents subjected to the winding. This paper discusses and compares the conventional I
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t algorithm with algorithm based on IEEE Std C57.109™ -2018. The transformer through fault scenarios are simulated using EMTP software and results are analyzed. A comparative analysis of transformer maintenance scheduling with different faults are presented with graphical results.