Three Phase Transformer Winding Configurations and Differential Relay Compensation

Most engineers have some familiarity with two commonly known delta connections that give
either a +30
 or - 30
 phase shift of positive sequence voltages and currents, and just these two configurations seem to cause extensive confusion. There are actually many other ways to configure a wye or delta that give other phase shifts, and to further complicate matters, there is the occasional zigzag winding application and the additional confusion over what occurs when CTs are connected in delta. These alternate transformer winding configurations are sometimes referred to by terms such as Dy# or Yy#, or Yd#, Dz#, and Yz#, and where the # can be, seemingly, almost any hour of the clock, hence the term “around the clock” phase shifting is sometimes heard. The paper will review the variety of possible winding configuration and give examples of the nomenclature that is used with them and how these various phase shifts are  created.

The paper will also show how a transformer differential relay compensates for the effects of the various transformer winding configurations, as well as account for delta CT configurations. Many papers and instruction manuals refer to compensation in terms of phase shifting. This leads engineers to have a vague and misleading understanding that the relay is somehow phase shifting currents to compensate for the transformer phase shift. While a “sequence component differential” relay might be able to work this way, most transformer differential relays work outside of the sequence component domain and do some form of current balance calculation in the ABC domain.

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