The connection method of transformer Dyn11: delta on high voltage side, star on low voltage side with neutral line, and high voltage and low voltage have a 30-degree phase difference.
The connection method of transformer Yyn0: star on high voltage side, star on low voltage side with neutral line, and there is no phase difference between high voltage and low voltage. Yyn0: means high voltage Y connection, that is, star connection, low voltage is also y connection, low voltage neutral point leads out, the phase angle of high and low voltage side is 0 o'clock above the clock.
Dy11: it means high voltage D connection, that is, delta connection, low voltage is also y connection, the phase angle of high and low voltage side is the angle of 11 o'clock above the clock. "11" means that the line voltage on the secondary side of the transformer lags the line voltage on the primary side by 330 degrees (or 30 degrees ahead).
In addition, the following tips are added: there are three connection methods for high and low voltage transformers: star, delta and zigzag connection. The high voltage windings are represented by symbols Y, D, and Z (uppercase), respectively; the medium and low voltage windings are represented by y, d, and z (lowercase), respectively. When there is a neutral point, YN, ZN (high voltage neutral point) and yn, zn (low voltage neutral point) are used respectively.
The numbers use clock notation to represent the phase relationship between the primary and secondary side line voltages. The primary side line voltage phasor is used as the minute hand, which is fixed at the 12 o'clock position of the clock, and the secondary side line voltage phasor is used as the hour hand.
Of the two windings of the autotransformer having a common part, the one with the lower rated voltage is indicated by the symbol a. For instance, the amorphous alloy dry type transformer is combined in the order in which the high voltage and low voltage windings are connected to form the connection group of the windings.
For example: high voltage is Y, low voltage is yn connection, then the winding connection group is Yyn. Coupled with the clock method, the phasor relationship between the high and low voltage sides is the connection group.
The two windings of the transformer are combined to form four wiring groups: "Yy", "Dy", "Yd" and "Dd". Since there are two types of Y connection with neutral line and without neutral line: without neutral line, no symbol is added, and with neutral line, the letter n is added after the letter Y. "n" means that the neutral point has a lead wire. For Yn0 wiring group, UAB and uab coincide, and the hour and minute hands both point to 12. "12" is represented by "0" in the new wiring group.
The three commonly used connection groups have different characteristics: 1. Y connection: the star connection is characterized by the most economical high voltage winding. The star connection allows to reduce the insulation at the midpoint, allowing the high voltage winding of transformer to adopt the graded insulation, thus reducing the cost. The star-connected winding voltage is equal to 1/√3 of the line voltage, and the winding current is equal to the line current. The star-connected neutral point lead wire can also be used for direct grounding, or grounding through the arc suppression coil. If it is not grounded, it can also be connected to a lightning arrester to prevent operating overvoltage and lightning overvoltage. In addition, the neutral point leads to grounding, which can also be used to realize four-wire power supply. The main disadvantage of this connection is that there is no circulation loop for the third harmonic current.
2. D connection: delta connection is the most economical for low voltage windings of transformers with low voltage and high current. The delta wiring must adopt equal insulation but not the graded insulation. As a result, for low voltage windings, because the voltage is low, this will not have any effect. The current in each phase winding of the delta connection is only 1/√3 of the lead-out current. The delta connection method can flow the zero-sequence current and the third harmonic current in the winding, and generate the anti-magnetic flux that suppresses the zero-sequence magnetic flux and the third harmonic, so that the induced electromotive force of the transformer can be prevented from being distorted.
3. Z connection: Z connection has the advantages of Y connection, the number of turns is 15.5% more than that of Y connection, and the cost is higher. Its zero-sequence impedance is small and suitable for making grounding transformers. Since it can reduce the neutral point displacement, it is suitable for the manufacture of low voltage windings of small and medium-capacity transformers that require three-phase unbalanced loads. In addition, it also has a certain effect on preventing lightning overvoltage.
The difference between Dyn11 and Yyn0 connection:
Since the high voltage side is connected in a delta, when zero-sequence magnetic flux or third-harmonic flux appears in the transformer core, zero-sequence electromotive force or third-harmonic electromotive force is induced in the delta winding. Because this induced electromotive force is three-phase in phase, it is superimposed in series in the delta winding, and produces the corresponding current. This zero-sequence (third harmonic) circulating current generates a reverse magnetic flux that minimizes the zero-sequence (third harmonic) magnetic flux in the iron core, thereby reducing the displacement of neutral point voltage of the star winding at low voltage side.
At the same time, it also minimizes the zero-sequence (third harmonic) induced electromotive force in the high voltage side winding, thereby avoiding the pollution of the voltage waveform in the high voltage side power grid caused by the distortion of the low voltage side load current waveform. Therefore, the transformer with delta connection can prevent the influence of the third harmonic or zero-sequence magnetic flux on the voltage waveform of the high voltage side power supply. In addition, this connection method also has a good effect on preventing the overvoltage of the lightning intrusion wave. Because the three-phase lightning intrusion waves are also in the same direction, this situation is similar to the zero-sequence current. The circulating current generated in the delta winding also inhibits the magnetic flux generated by the lightning current in the three-phase iron core of the transformer.
GB/T13499-2002 "Application Guidelines for Power Transformers" stipulates that since the zero-sequence impedance of the 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 the Dyn transformer is only 0.9 times the positive-sequence short-circuit impedance, and the neutral line can carry rated current.
However, if a drop-out fuse is used for overload and short-circuit protection on the power supply side of the transformer such as our three phase oil immersed transformer connected by Dyn11, and one-phase fuse is blown, that is, single-phase disconnection on the high voltage side, then at this time, the low voltage side has two phases with the phase voltage value that is half of the rated voltage, that is, from 220V to 110V, and household appliances such as refrigerators will burn out because they cannot be started. At this time, the low voltage side of the transformer connected by Dyn11 must be equipped with reliable low voltage protection.
In this connection combination, the low voltage side has a neutral line, 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 iron core is a three-phase three-column, the magnetic flux generated by the zero-sequence current and the third harmonic current in the winding cannot form a closed loop in the iron core (because the third harmonic current flux and the zero sequence flux in the three-phase iron core column are in the same direction), and it can only go out of the iron core and return to the iron core through the transformer insulating medium (transformer oil) and the iron metal of the box. Due to the large magnetic resistance of the insulating medium outside the iron core, the zero-sequence magnetic flux and the third harmonic magnetic flux are small. However, the zero-sequence electromotive force and the third harmonic electromotive force generated by its induction are superimposed on the phase voltage, making the three-phase voltage asymmetrical, causing the displacement of neutral point, and causing that some phase voltages increase, and some phase voltages decrease.
In order to prevent the serious asymmetry of the three-phase phase voltage and affect the normal power consumption of users, the china transformer manufacturer industry standard SD-292-1988 "Overhead Distribution Line and Equipment Operation Regulations (Trial)" stipulates that the unbalance degree of the three-phase load should not be greater than 15% . A small amount of single-phase load is allowed in the three-phase transformer, but the neutral current should not exceed 25% of the rated current. Therefore, this provision is made to limit the displacement voltage of neutral point to about 5%.
If the Yyn0-connected transformer has a three-phase five-column structure, the zero-sequence (or third harmonic) magnetic flux in the core can flow through the side columns, which is much smaller than the magnetic resistance when it flows through the box. Therefore, the zero-sequence (or third harmonic) magnetic flux is much larger than that of a three-phase three-column transformer, and the zero-sequence (or third harmonic) electromotive force induced in the windings is much larger, giving rise to a serious voltage displacement of the neutral point, and causing that the unbalance degree of three-phase voltage increases. Therefore, the transformer of Yyn0 connection group does not adopt the three-phase five-column structure or the connection mode of three single-phase transformers.