In order to show the phase relation of the coils voltage on both sides , the connection mode of the three-phase transformer is divided into several groups which are called the connection group. Among the Three phase power transformer, the phase voltage ratio of the primary and secondary power transformers is fixed and equal to the ratio of the number of turns. According to the way that the coil is connected, it can be divided into twelve kinds of connection groups: 1--12 points.
The connection group of three-phase transformer is divided into 12 kinds, including 6 kinds of single arrays and 6 double arrays. When primary side coil and second side coil connects differently, it is a single array, including Y/Δ-1, 3, 5, 7, 9, 11. When primary side coil and the second side coil connection is consistent, such as Y-Y, Δ-Δ, it's a double array, including: Y/y-2,4,6,8,10,12. In China, the connections of Y shape to Y shape are 12 point wiring. The connection of Y shape to Δ is 11 points wiring, except other special cases.
The groups of different transformers generally adopt the clock representation method. Because the phase difference between the line voltage corresponding to the one and two side of the transformer is integer multiples of 30 degrees. It coincides with the angle among the clock hours. The clockwise moves exactly 30 degrees an hour. Therefore, it can be a very vivid description of this problem with clock representation. In the method, use the primary side line voltage vector as the long hand of a clock and fix it on the 12 points. And then use the line voltage vector corresponding to the two side as the short hand of a clock, and take the point which is pointed by the short hand as the group number of wiring group.