Validated Techniques for Modeling Shell-Form EHV Transformers

It is very common for power transformers to have at least one dominant natural frequency in the 8-to 20-kHz range and minor resonances at a multitude of other frequencies. It is important for the transformer designer to known how a transformer will respond to impulses that may excite these natural frequencies and lead to conditions which are commonly referred to as part-winding resonances in the literature. The utility engineer applying the transformer may also have a need to be aware of the transformer characteristics because there may be transient events on the power system that would excite the natural frequencies. The phenomenon of part-winding resonances has been recognized for many years. However, transformer failures attributed to part-winding resonance continue to occur. The problem persists for mainly two reasons: 1. Lack of validated models and adequate computer tools for designers. 2. System conditions that generate oscillatory transients of a frequency to which the transformer is vulnerable. This paper primarily addresses the progress made toward providing designers with validated models and computer tools. However, the models should also be of benefit to systems analysts to address the latter issue. The power system analyst would use the models, or the results of modeling, to identify potentially dangerous system conditions. Up to the time of the efforts reported here, determining the model inductances and capacitances was largely an art. The task was delegated to a few specialists, the number of which has been dramatically reduced by the economic downturn in the power transformer industry.