List of the possible vector groups for the transformer from which that required may be set. Values in the enumeration list follow a standard international code where the first letter describes how the primary windings are connected, the second letter describes how the secondary windings are connected, and the numbers describe the rotation of voltages and currents from the primary to the secondary side in multiples of 30 degrees. D: means that the windings are delta-connected. Y: means that the windings are star-connected. Z: means that the windings are zig-zag connected (a special start-connected providing low reactance of the transformer). The connectivity is only relevant for three-phase transformers.