Transformer Protection

5/29/2015 Schweitzer Engineering Laboratories1 Copyright SEL 2015 IEEE SF Power and Energy SocietyMay 29, 2015 Transformer ProtectionAli Kazemi, PERegional Technical MangerSchweitzer Engineering LaboratoriesIrvine, CASources of Transformer Stresses Thermal cycling Vibration Local heating due to magnetic flux Impact due to through-fault current Heating due to overload or inadequate coolingSource: IEEE Std. , IEEE Guide for Protecting Power Transformers5/29/2015 Schweitzer Engineering Laboratories2 Transformer Failure Statistics1983 1988 Winding failures37% Tap changer failures22% Bushing failures11% Terminal board failures3% Core failures1% Miscellaneous failures26%Source: IEEE Std. , IEEE Guide for Protecting Power TransformersTransformer Protection Differential Protection current Transformer Performance Through Fault Protection Mechanical Protection5/29/2015 Schweitzer Engineering Laboratories3 Design Considerations for Transformer Differential Protection CT ratio and CT voltage class selection CT connections current phase shifts across Transformer Inrush detection Differential pickup settings Zero-sequence currents Slope High excitation currentsDesign Considerations for Transformer Backup Protection Overcurrent Directional overcurrent External faults Sudden pressure Hot spots Loss of coolers5/29/2015 Schweitzer Engineering Laboratories4 Differential Protection Overcurrent50 Protected EquipmentIOP= 0 CTCTN ormal LoadBalanced CT

Current Transformer (CT) Principle • CT isolates relay from the HV system • Drastically reduces current Ideally: is = ip / Ns. 5/29/2015 Schweitzer Engineering Laboratories 9 Core and Secondary Winding Example The Current Transformer Equivalent ... • Operation conditions

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  Operations, Principles, Current, Transformers, Current transformer

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