IEEE C57.12.70:2020 pdf free download – IEEE Standard for Standard TerminalMarkings and Connections for Distribution and Power Transformers
7.1.1 Phase sequence markings
The markings shall be so applied that if the phase sequence of voltage on the highest voltage winding is inthe time order H, H,H, it shall be in the time order of Xi,X，, X, and Yi，Y2,Y3,etc,on the otherwindings.
7.1.2 Phase relationship markings
In order that the markings of terminals shall indicatc definite phase relations，they shall be made inaccordance with one of the three-phase groups shown in Figure 14. The angular displacement between theH winding and the X winding is the angle in each of the voltage phasor diagrams (see Figure 14), betweenthe lines passing from its neutral point through Hj and Xj, respectively.
7.1.3 Phasor group designation for three-phase transformers
The phasor grouping of a wye, delta or zigzag connected three-phase transformer shall be indicated by theuppercase letters Y,D, or Z for the high-voltage windings and lower case letter of y, d, or z for the low orintermediatc voltage windings. The phasor grouping shall be listed from high voltage to low voltage.If aneutral point is externally available,the phasor grouping for the voltage set shall end with an uppercase Nor lowercase n as appropriatc for the winding voltage level.For an auto connected transformer, the symbolof the lower voltage winding is replaced with the letter “a”for auto. If a transformer is specified with areconfigurable winding connection (as either dual voltage or dual phase displacement) then the initialconnection for service shall be the main phasor group, with the alternative connection (voltage and/or phasedisplacement) noted in parenthesis.
The phasor grouping can be further described by designating the angular displacement. The angulardisplacement of a polyphase transformer is the phase angle expressed in degrees between the line-to-neutralvoltage of the reference identified high-voltage terminal H and the line-to-neutral voltage of thecorresponding identified low-voltage terminal X. The angular displacement may be from 0° to 330° insteps of 30°. The angular displacement number refers to the phase angle, as viewed on a 12 hour clock. Forexample,one o’clock is 30° , two o’clock is 60° , etc. This is sometimes referred to as“clock numbernotation”; and is intended to be the same format as used in IEC 60076-1.The phasor group designation(sometimes referred to as“vector group”) is the combination of the phasor grouping and the angulardisplacement.
Many transformer product standards specify that the angular displacement between high-voltage and low-voltage phase voltages of three-phase transformers with A-d or Y-Y connections shall be zero degrees asshown in Figure 14,Group l. The corresponding phasor group designation would be Dd0 or Yy0,respectively.
lf the angular displacement between high-voltage and low-voltage phase voltages of three-phasetransformers with A-Y or Y-A connections shall be 30″ , with the low voltage lagging the high voltage asshown in Figure 14,Group 2, then the corresponding phasor group designation would be Dyl or Yd1,respectively.
7.5.1 Station type transformers
For station type transformers, and others with high-voltage and low-voltage terminals on opposite sides ofthe case, the H] terminal shall be on the right-hand side of the case when facing the highest voltage side.The H and Hs terminals shall be arranged in numerical order reading from right to left when facing thehighest voltage side. The Ho terminal, if present，shall be located on the right of the H terminal whenfacing the highest voltage side of the case(see Figure 15,Group 1 and Group 2).
The Xy terminal shall be on the left-hand side of the case when facing the low-voltage side. The Xz and X3terminals shall be arranged in numerical order, reading from left to right, when facing the X winding sideof the case. The Xo terminal, if present, shall be located to the left of the Xterminal when facing the Xwinding side of the case (see Figure 15,Group 1 and Group 2).