Explain delta-wye (delta-star) transformation and when it is useful.

Delta-wye transformation converts between delta (triangle) and wye (star) resistor configurations when neither series nor parallel simplification applies. For delta to wye: Ry = (Ra*Rb)/(Ra+Rb+Rc) where Ra and Rb are adjacent delta resistors. For wye to delta: Rd = (R1*R2 + R2*R3 + R3*R1)/R_opposite. This technique simplifies bridge circuits, three-phase network analysis, and complex ladder networks.