This chapter discusses a single-phase transformer or one phase of a 3-phase transformer with uncoupled phases. Furthermore, the windings have been without internal interconnections. It applies the theory to more complicated transformers, where the windings can be interconnected within a single phase. When the second 3-phase core is added, both cores are analyzed individually, as discussed up to this point. Initially, the two cores are assumed to be uncoupled before interconnections between the cores are made. The zigzag connection is often used for grounding purposes. It affords a low impedance to zero sequence currents but a high impedance to balanced currents. The multiterminal circuit analysis method presents a systematic way of modeling transformers, especially the ones with complicated winding interconnections. The terminal equation could be integrated into a larger system for power flow analysis, for example. As part of a larger network, only the terminal quantities of the transformer are of interest.