The connection groups of distribution transformers are commonly used in two wiring forms, Dyn11 and Yyn0.
Dyn11 wiring method: the short-circuit currents of single-phase short-circuit and three-phase short-circuit are similar in magnitude. Since zero-sequence current and third harmonic current can circulate in the closed-loop of high-voltage winding delta wiring, they cannot circulate in star wiring.
Therefore, when the low-voltage side three-phase load is unbalanced, the total magnetic potential of the total zero sequence current and the third harmonic current on each iron core column is almost equal to zero, so the low-voltage neutral point potential does not drift, and the output voltage qualification rate of each phase high.
When the one-phase fuse on the high-voltage side is blown(three phase voltage regulator), the other two-phase loads can still operate normally, but an Undervoltage protection device is installed in the low-voltage main switch to solve the problem of non-full-phase operation.
Yyn0 wiring method: When one phase of the high-voltage fuse is blown, 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 have an impact on the low-voltage side of the single-phase power supply lighting load, but the low-voltage side is a three-phase power supply power load that needs to be equipped with phase loss protection, so as not to cause the power load to burn due to phase loss operation.
Only core transformers with a capacity of 1600kV•A and below are allowed to use the Yyn0 connection because the third harmonic magnetic flux will generate eddy current loss when passing through the iron component.
The neutral current of the Yyn0 connection transformer should not be too large, because the high-voltage winding of the Yyn0 connection transformer has lower insulation strength requirements than the high-voltage winding of the Dyn11 connection transformer, so it is difficult to change the Yyn0 connection transformer to Dyn11 connection.
Taking into account the high-order harmonics of the distribution transformer windings and other reasons, the two-frame three-leg iron-core distribution transformer should be connected as Dyn11 as possible to improve the voltage waveform of the high and low voltage windings of the distribution transformer and reduce the impact of harmonics on the power grid, Improve the voltage qualification rate and power supply reliability.