The Difference and Principle of Wiring Between Transformer Dyn11 and Yyn0

Transformer Dyn11 connection method: high-voltage side triangle, low-voltage side star-shaped with neutral line, high voltage and low voltage have a 30-degree phase difference

Transformer Yyn0 connection method: high-voltage side star, low-voltage side star with neutral line, there is no phase difference between high voltage and low voltage

Yyn0: It means high voltage Y connection, that is, star connection, low voltage is Y connection, the low voltage neutral point is drawn out, and the phase angle of the high and low voltage sides is 0 on the clock.

Dyn11: It means high voltage D connection, that is, triangle connection, low voltage is also Y connection. The phase angle of high and low voltage sides is the angle at 11 o'clock on 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).

There are three connection methods for high and low voltage transformers: star, triangle and zigzag connection. The high-voltage windings are represented by the symbols Y, D, and Z (uppercase) respectively; the medium-voltage and low-voltage windings are represented by y, d, and z (lowercase) respectively. When there is a neutral point lead out, it is represented by YN, ZN (high voltage neutral point) and yn, zn (low voltage neutral point) 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, fixed at the 12 o'clock position of the clock, and the secondary side line voltage phasor is used as the hour hand.

The one with the lower rated voltage of the two windings in the common part of the autotransformer is represented by the symbol a. The transformer is combined in the order in which the high-voltage and low-voltage windings are connected to form a connected group of windings.

For example: the high voltage is Y and the low voltage is yn connection, then the winding connection group is Yyn. Adding the clock method to express the phasor relationship between the high and low voltage sides is the connection group.

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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 connections: with neutral wire and without neutral wire, without neutral wire, no symbol is added, and with neutral wire, the letter n is added after the letter Y. n means that the neutral point has a lead-out line. Yn0 wiring group, UAB and uab coincide with each other, and the hour and minute hands point to 12. "12" is represented by "0" in the new wiring group.

Three commonly used linkage groups have different characteristics:

1. Y connection: The characteristic of star connection is that the high voltage winding is the most economical. Star connection allows the insulation at the midpoint to be reduced, allowing graded insulation of the high voltage windings of the transformer, thereby reducing costs. The star connection winding voltage is equal to 1/√3 of the line voltage, and the winding current is equal to the line current. The neutral point lead wire of the star connection can also be used for direct grounding or grounding through an 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 most economical for low voltage windings of transformers with low voltage and high current. Because the delta connection must be made into equal insulation and cannot be made into graded insulation, for low-voltage windings, this will not have any impact because the voltage is lower. The current in each phase winding of delta connection is only 1/√3 of the lead wire current. The delta connection method can flow zero-sequence current and third-harmonic current in the winding, generate counter-magnetic flux that suppresses the zero-sequence magnetic flux and third harmonic, and prevents distortion of the induced electromotive force of the transformer.

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 manufacturing grounding transformers. Because it can reduce the neutral point displacement, it is suitable for manufacturing low-voltage windings of small and medium-capacity transformers that require three-phase unbalanced loads. In addition, it also plays a certain role in preventing lightning overvoltage.

The difference between Dyn11 and Yyn0 connections:

Dyn11

Since the high-voltage side has a delta connection, when zero-sequence magnetic flux or third-harmonic magnetic flux appears in the transformer core, zero-sequence electromotive force or third-harmonic electromotive force is induced in the delta winding. Since this induced electromotive force has three identical phases, The triangular windings are superimposed in series and produce corresponding currents. This zero-sequence (third harmonic) circulating current generates a reverse magnetic flux, which weakens the zero-sequence (third harmonic) magnetic flux in the iron core to a minimum, thus reducing the neutral point voltage of the low-voltage side star-connected winding. Displacement. At the same time, it also minimizes the zero-sequence (third harmonic) induced electromotive force in the high-voltage side winding, thereby avoiding contamination of the voltage waveform in the high-voltage side power grid caused by low-voltage side load current waveform distortion. Therefore, a transformer with delta connection can prevent the third harmonic or zero sequence flux from affecting the high-voltage side power supply voltage waveform. In addition, this connection method also has a good effect on preventing overvoltage from lightning intrusion waves. 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 triangle winding also inhibits the magnetic flux generated by the lightning current in the three-phase core of the transformer.

GB/T13499-2002 "Power Transformer Application Guidelines" stipulates that since the Yyn transformer's zero-sequence impedance is as high as 60%, the neutral line current should not be greater than 10% of the rated current; the Dyn transformer's zero-sequence impedance is only 0.9 times the positive sequence short-circuit impedance. , the neutral wire can carry rated current.

  However, if a drop-out fuse is used as overload and short-circuit protection on the power side of the transformer with Dyn11 connection method, one phase fuse will blow out, that is, the single phase on the high voltage side will be disconnected. At this time, there will be two phase voltages on the low voltage side, and the value will be the rated value. 1/2 of the voltage, that is, reduced from 220V to 110V. At this time, household appliances such as refrigerators will fail to start and cause burnout. At this time, reliable low-voltage protection must be installed on the low-voltage side of the transformer with Dyn11 connection method.

Yyn0

  In this connection combination, there is a neutral line on the low-voltage side. The third harmonic current in the low-voltage load and the zero-sequence current in the three-phase unbalanced load can flow through the neutral line in the transformer winding. If the iron core is three-phase and three-legged, 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 in the three-phase iron core leg and The zero-sequence magnetic flux is in the same direction) and can only cross the iron core through the transformer insulation medium (transformer oil) and the ferrous metal of the box and then return to the iron core. Since the magnetic resistance of the insulating medium outside the iron core is large, the zero-sequence magnetic flux and third harmonic magnetic flux are small. However, the zero-sequence electromotive force and third harmonic electromotive force generated by its induction are superimposed on the phase voltage, making the three-phase voltage asymmetric, causing the neutral point to shift, and some phase voltages increase and some phase voltages decrease. In order to prevent serious asymmetry of three-phase voltages and affect users' normal power consumption, the industry standard SD-292-1988 "Operation Regulations for Overhead Distribution Lines and Equipment (Trial)" stipulates that the unbalanced degree of three-phase loads should not be greater than 15% . A small amount of single-phase load is allowed in a three-phase transformer, but the neutral line current should not exceed 25% of the rated current. Therefore, this provision is made to limit the neutral point displacement voltage to about 5%.

  If the Yyn0 connection type transformer has a three-phase five-column structure in its core, then the zero-sequence (or third harmonic) magnetic flux in the core can flow through the side columns, and the magnetic resistance is smaller than when it flows through the box. Therefore, the zero-sequence (or third harmonic) magnetic flux is much larger than that of the three-phase three-column transformer, and the zero-sequence (or third harmonic) electromotive force induced in the winding is much larger, causing the neutral point voltage to shift. Seriously, the three-phase voltage imbalance increases. Therefore, the transformer in the Yyn0 connection group does not adopt a three-phase five-column structure or a connection method of three single-phase transformers.

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