transformer for power supply
The transformer is wired as follows:







simulate this circuit &ndash; Schematic created using CircuitLab

Notice that the two 15V wires are 180° out of phase, resulting in a 30V(AC) between the outer two windings and 15V relative to the middle wire.

You mustn't cut the transformer's insulation as that is a safety hazard. The two 15V wires share a common ground (0V) wire, you cannot change that. You do have several choices how to use them, but to answer that you have to elaborate in your question on what your goal is.