The power transformer divided into dry type transformer and oil type transformer. One of the transformer specification parameters is called the connection group label. It has Dyn11 and Yyn0.
Dyn11 has high output voltage quality, neutral point does not drift and good anti-thunder performance. When the three-phase load is unbalanced at the low-voltage side of the box transformer, because the zero sequence current and the third harmonic current can flow in the closed loop of the high-voltage winding. The total zero sequence magnetic potential and the third harmonic magnetic potential on each iron core column are almost equal to zero, so the potential at the low-voltage neutral point does not drift and the voltage quality is high. Since the lightning current can also flow in the closed loop of the high-voltage winding, the total magnetic potential of the lightning current on each core column is almost zero. Eliminate the positive, inverse conversion overvoltage, so lightning protection performance is good. However, the problem of incomplete phase operation can be solved by installing undervoltage protection device in the low-voltage main switch.
Yyn0 wiring, when a phase of the high-voltage fuse breaks, the voltage of one phase will be zero, and the voltage of the other two phases will not change which can reduce the power failure range to 1/3. This situation will not affect the lighting load of single-phase power supply at the low-voltage side. If the low-voltage side is the three-phase power load, it is generally equipped with phase-deficient protection, so it will not cause the power load to burn down due to phase-deficient operation.
In general, the zero-sequence impedance of Dyn11 coupling transformer is much smaller than that of Yyn0 connection transformer, which is conducive to the removal of low-voltage single-phase grounding short-circuit fault. In the selection of transformer connection, it is necessary to choose Dyn11 transformer. Since the insulation strength requirement of Yyn0 connection transformer’s high voltage winding is slightly lower than that of Dyn11 connection transformer, it is not suitable to change Yyn0 connection transformer into Dyn11 connection.