26.4 : 정격 회전수 비율이 낮은 변압기

Per-unit quantity models simplify transformer analysis compared to actual quantity models. Ideally, transformer winding is eliminated when voltage base ratios match the windings' voltage ratings.

However, selecting voltage bases is challenging for parallel transformers with different ratings.

For such cases, per-unit models with off-nominal turn ratios are developed, where selected base voltages are not proportional to the transformer's voltage ratings.

Consider a transformer with rated voltages related by the term a.

If chosen voltage bases satisfy a relationship with term b, term c is their ratio and substituted in the rated voltage relationship.

This setup can be modeled by two transformers in series: one representing the standard per-unit model with losses, and the other an ideal transformer. In this model, shunt-exciting branches are neglected for simplicity.

This model may not suit computer programs that do not accommodate ideal transformer windings.

An alternative involves nodal equations, providing admittance parameters representing off-nominal turn ratio transformers in computer programs.

For real c, a pi circuit network can be modeled.