Transformer Dyn11 connection: high-voltage side triangle, low-voltage side star and neutral, high-voltage and low-voltage have a phase difference of 30 degrees
Transformer Yyn0 connection: HV side star, LV side star and neutral, no phase difference between HV and LV
Dyn11
As the high-voltage side of the triangular wiring, when the transformer core zero-sequence flux or the third harmonic flux, in the triangular winding inductively generated zero-sequence electromotive force or the third harmonic electromotive force, due to this induced electromotive force for the three identical phases, in the triangular winding superposition of the series, and produce the corresponding current. This zero-sequence (third harmonic) circulating current generates an antimagnetic flux, which weakens the zero-sequence (third harmonic) flux in the iron core to a minimum, and thus reduces the neutral voltage displacement of the star-wired winding on the low-voltage side. At the same time also make the high-voltage side of the winding in the zero sequence (third harmonic) induced electromotive force is reduced to a minimum, thus avoiding the low-voltage side of the load current waveform distortion caused by the high-voltage side of the power supply grid voltage waveform is contaminated. Therefore, a transformer with delta wiring prevents the influence of the third harmonic or zero sequence flux on the voltage waveform of the high-voltage side power supply. In addition, this connection method is also good for preventing the overvoltage of lightning incursion wave. Because the three-phase lightning intrusion wave is also in the same direction, this condition is similar to the zero-sequence current, the circulating current generated in the triangular winding, the magnetic flux generated by the lightning current in the three-phase core of the transformer also has a suppression effect.
GB/T13499-2002 "Power Transformer Application Guidelines" stipulates that: as the zero sequence impedance of Yyn transformer is as high as 60%, the neutral current should not be greater than 10% of the rated current; the zero sequence impedance of Dyn transformer is only 0.9 times of the positive sequence short-circuit impedance, and the neutral can be with the rated current.
However, Dyn11 connection transformer power supply side such as falling fuse as overload and short-circuit protection, there is a phase fuse blown, that is, the high-voltage side of the single-phase disconnection, then the low-voltage side of the two-phase phase voltage of the value of 1/2 of the rated voltage, that is, from 220V to 110V, then, refrigerators and other household appliances will be unable to start due to the cause of burnout. In this case, the low voltage side of the transformer with Dyn11 connection must be equipped with reliable low-voltage protection.
Yyn0
In this type of connection combination, there is a neutral line on the low-voltage side, and the third harmonic current in the low-voltage load and the zero-sequence current in the three-phase unbalanced load can circulate in the transformer winding through the neutral line. If the core for three-phase three columns, there are zero-sequence current and third harmonic current generated in the winding flux can not form a closed loop in the core (because the three-phase core columns in the third harmonic current flux and zero-sequence flux is in the same direction), can only be out of the core through the transformer insulating medium (transformer oil) and the case of ferrous metal and so on and then back to the core. Because of the core outside the insulation medium magnetoresistance is larger, so the zero sequence magnetic flux and the third harmonic magnetic flux is smaller. But the zero sequence electromotive force and the third harmonic electromotive force generated by its induction, superimposed on the phase voltage, so that the three-phase voltage asymmetry, causing the neutral point displacement, some phase voltage increases, some phase voltage decreases. In order to prevent serious asymmetry of three-phase phase voltage, affecting the normal use of electricity, the industry standard SD-292-1988 "overhead distribution lines and equipment operation regulations (for trial implementation)" on the three-phase load imbalance should not be greater than 15%. Allow a small number of single-phase loads in three-phase transformers, but the neutral current should not exceed 25% of the rated current. So this provision is made to limit the neutral displacement voltage to about 5 per cent.
If Yyn0 linkage transformer, its core for three-phase five-pillar structure, then the core of the zero sequence (or the third harmonic) flux can be circulated through the side columns, than through the box circulation of the magnetoresistance to be much smaller, therefore, the zero sequence (or the third harmonic) flux than the three-phase three-pillar transformer is much larger than the zero sequence (or the third harmonic) electromotive force generated by the induction in the windings is much larger, so that the neutral point voltage displacement is serious, three-phase voltage imbalance increases, therefore, Yyn0 linkage transformer is not used in three-phase five-pillar structure or three single-phase transformer linkage. Phase voltage imbalance increases, therefore, Yyn0 linkage group of transformers do not use three-phase five-pillar structure or three single-phase transformer linkage.
