Control Practices Contribute To Premature Transformer Failures

This paper focuses on tapchanger control practices and settings that may cause excessive or untimely tapchange operations. Recent case study evaluations of premature LTC (Load Tap Changer) transformer failures on utility systems identified the tap changers as a major contributing factor. Common practices discussed include basic voltage control ranges and setting effects, timing options available, LDC (line drop compensation) misapplications, first house protection methods and various paralleling techniques. The transformer applications considered include transmission tie transformers as well as transmission-distribution interface transformers. Attention is drawn to unusual network conditions as well as emergency or contingency system operations. These conditions and operations are continuously affected by automatic operations on the transmission and distribution systems, such as protective relay operations or load management techniques. Also detailed is common, in-field, paralleling commissioning practices that can create conditions for tapchanger "hunting." In many applications, a control function will result in different actions depending on the system configuration and parameters.