Star connected circuit, which has an isolated neutral there can be no zero sequence components. Since the zero sequence components are by definition in time phase with another their sum can not be zero at the junction point as per kirchoff’s law. It follows that there are limitations upon the phase loading of a bank of transformers connected in star – star unless the neutral points are connected to the source of power in such a manner that the zero sequence components of current have a return path or unless the transformer are provided with tertiary winding.