The Complete Guide to 3-phase Distribution Transformer Connection

The phase difference between the high and low voltage winding line potentials of a three-phase transformer varies depending on the method of 3-phase distribution transformer connection.

The connection of three-phase transformer windings is not only a matter of forming a circuit system, but also a matter of harmonics in the transformer electromagnetic quantities, as well as operational problems such as parallel operation.

For this reason, it is necessary to correctly identify the coupling group of the three-phase winding.

The analysis of 3-phase transformer connections has always been a key point of emphasis and difficulty in the course “Fundamentals of Electrical Machines and Dragging”. Three-phase transformer winding coupling group usually includes two aspects of the problem.

3-phase transformer connection

First, according to the winding wiring diagram to draw the corresponding phase diagram, determine the linkage group.

The second is to draw the phase volume diagram and wiring diagram according to the coupling group. Three-phase transformer coupling group is commonly used to describe the phase relationship between the corresponding line potential on the high and low voltage side.

High and low voltage winding for the star connection, with the symbol “Y (or y)”, the three first ends of the winding A, B, C (or a, b, c) led outward, the end of X, Y, Z (or x, y, z) together to become the neutral point, with N (or n) said.

When doing the triangle connection, the symbol “D (or d)” is used, so that the first end of one of the three phases is connected to the end of the other phase.

Because the three-phase winding can be used in different connections, so that the line voltage in the primary and secondary windings of the three-phase transformer appears different phase differences, so the transformer winding connection is divided into various connection groups according to the phase relationship between the primary and secondary line voltage.

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Table of Contents

What are the standard connections of a 3-phase transformer?

3-phase distribution transformer connection

There are many possible connection groups for 3-phase transformers, but for the convenience of manufacturing and parallel operation, the industry standard specifies the following five as the standard connection groups: Y, yn, 0; Y, d11; YN, d11; Y, z11; D, z0. The symbol z indicates a zigzag connection.

The Y, yn, 0 coupling group has the secondary side of the lead center line, so that it becomes a three-phase four-wire system, this connection is commonly used in distribution transformers for power and lighting.

Y, d11 coupling group is used on lines where the voltage on the secondary side exceeds 400 V. The angular coupling on the secondary side of the transformer facilitates operation.

YN, d11 coupling group is mainly used in high-voltage transmission lines; transformers with z-shaped coupling are used on distribution transformers with high lightning protection performance.

What is the 3-phase transformer connection?

Because the three-phase system is more economical, efficiency and higher than the same capacity of single-phase transformers, so almost all countries in the world’s power systems are using three-phase system, so the application of three-phase transformers is extremely widespread.

Three-phase transformers operate under symmetrical conditions, the voltage, current and flux of each phase are of the same size, and the phases lag behind 120° in turn, so the analysis of three-phase transformers can be analyzed as long as a certain phase is taken.

Three-phase transformers can be divided into two types according to their core structure: three-phase group transformers consisting of three independent single-phase transformers, or called three-phase transformer groups.

The other is a three-phase core type transformer with a core common to all three phases.

Three-phase transformers are also available in both core and shell types.

The connections of three-phase transformer windings are not only a matter of composing the circuit system, but also relate to the harmonics in the transformer electromagnetic quantities, as well as operational issues such as parallel operation.

The winding of three-phase power transformer generally adopts two types of coupling, star and triangle, and the regulations for the first and end mark of the winding are shown in Table 1.

Name of winding (coil)

Top

Ends

 Neutral Point

 

High-voltage winding (coil)

A、B、C

 X、Y、Z

N

Low-voltage winding (coil)

a、b、c

 x、y、z

n

Representation of three-phase transformer connections

3-phase transformer connections use two letters plus a number of clock representation, where the first letter indicates the connection method of the original side, with capital letters, the second letter indicates the connection method of the secondary side, with lowercase letters, Y or y indicates star-shaped connection, D or d indicates triangular connection, followed by a number indicating the phase difference of the transformer original and secondary side line potential.

The latter number indicates the phase difference of the transformer original and vice side line electric potential, that is, the original side line electric potential vector as the long hand of the clock, and fixed on the “12”, vice side line electric potential vector as the short hand of the clock, the number of clock points that is the number of the coupling group.

The phase difference of the original secondary line potential is only possible for an integer multiple of 30°.

Factors affecting Three phase transformer connections

There are three factors that affect the Three phase transformer connections: the way the original and secondary windings are connected, the same name end and the phase shift.

The effect of the change of winding coupling on the coupling group

There are two types of winding connections: star and triangle.

Star coupling is the connection of three ends of a three-phase winding together, with the three first ends leading out, and sometimes with the three ends leading through the center line.

The triangular connection is to connect the end of one phase winding with the first end of another phase winding, in order to form a closed circuit, and then lead out from the first end, and the triangular connection is divided into two types: cis-link and inverse-link.

The relationship between the line potential and the phase potential of the winding is different depending on the way the winding is connected.

If the winding connection is changed from star to triangle with no change in phase potential, the line potential will lag behind the original line potential by 30°.

If the winding connection is changed from star to triangle, the line potential will exceed the original line potential by 30°.

If the winding connection is changed from a triangular connection to a triangular inverse connection, the line potential will exceed the original line potential by 60°.

Through the above relationship between the phase change of electric potential under different connection methods, it is not difficult to derive the effect of the change in the connection method of the three-phase transformer on the connection group.

In the case of other conditions do not change, if the secondary side from the star linkage to change the triangle paralleling, the linkage group number plus 1 (the secondary side line voltage lagged 30 ° is equivalent to a clockwise rotation of the short hand on the clock a clock point number).

If the secondary side is changed from a star connection to a triangular inverse connection, the linkage group number is reduced by 1. If the secondary side is changed from a triangular forward connection to a triangular inverse connection, the linkage group number is reduced by 2.

If the above changes occur on the original side or in the reverse direction, the linkage group number is changed in the opposite direction, i.e., plus becomes minus and minus becomes plus, while the amount of change remains the same.

The impact of the change of the same name end of the winding on the joint group

transformer connection types

The eponymous end of the transformer refers to the two terminals of the original and vice windings in the same core column with the same potential polarity at a certain instant.

The eponymous end depends on the winding direction of the two windings on the same core column.

When the winding direction of one of the windings changes, the eponymous end will change, and vice versa, if the eponymous end changes, it means that the winding direction of one of the two windings on the same core column has changed.

In other words, when the eponymous end changes, the winding direction of one winding in the original secondary winding changes, the phase potential of the winding will be reversed, and the corresponding line potential will also be reversed, and the line potential has a phase change of 180° before and after the change.

Through the above analysis, we can get the influence law of the same name end change on the junction group: for each change of the same name end of the three-phase transformer, the junction group number is plus 6 or minus 6.

Effect of winding shift on the coupling group

distribution transformer connection

In a three-phase transformer, the windings belonging to the same phase in the original and vice windings are not necessarily installed in the same core column, the original and vice windings only need to ensure the respective phase sequence relationship on the line, the original and vice windings in different core columns of this position change is called phase shift.

If the U-phase winding is shifted to the V-phase winding, the V-phase winding is shifted to the W-phase winding, and the W-phase winding is shifted to the U-phase winding, this phase shift is called phase sequence shift.

Conversely, if the U-phase winding is shifted to the W-phase winding, the W-phase winding is shifted to the V-phase winding, and the V-phase winding is shifted to the U-phase winding, the phase shift is called reverse phase sequence.

When the secondary winding of the three-phase transformer is shifted once in phase sequence, the phase potential in each phase winding will be 120° lagged than the original phase potential, then the line potential in the secondary winding will also be lagged 120° phase change before and after the phase shift, if this phase change is reflected to the change of the coupling group number, that is plus 4.

Through the above analysis, we can obtain the effect law of winding shifting to the junction group.

When the secondary winding is shifted once in phase sequence, the joint group number is increased by 4, and when it is shifted once in reverse phase sequence, the joint group number is decreased by 4.

If the original winding is shifted once in phase sequence, the joint group number will be reduced by 4, and if it is shifted once in reverse phase sequence, the joint group number will be increased by 4.

Quick identification of 3-phase distribution transformer connection

After mastering the law of 3-phase distribution transformer connection, the same name end and winding shift to the connection group, we only need to remember the connection diagram of a specific connection group, then we can look at the connection method, the same name end, the phase shift situation, and make a simple addition or subtraction to the group number of the specific connection group according to the change law. The group number of the linkage group to be identified can be obtained by adding or subtracting the numbers according to the change rule.

The specific coupling group can choose some easy to remember, such as Y, y0, the original vice side of this coupling group is the same star coupling, the original vice side of the same phase winding in the same core column, and the same name end is the same as the first end.

Rapid identification method of inverse application

The above quick identification method can not only be used for the identification of 3-phase distribution transformer connection, but also can quickly get the corresponding linkage diagram according to the linkage group.

The specific method is as follows.

Firstly, remember the coupling diagram of a specific coupling group and make corresponding changes in the coupling diagram of the specific coupling group to obtain the coupling diagram of the target coupling group.

secondly to compare the difference between the letters of two linkage groups, to change the linkage of the original secondary edge to match the correspondence between the letters and the linkage patterns, and to make a quick identification of the new linkage group after the change of the linkage pattern.

Finally, compare the difference between the group number between the new coupling group and the target coupling group, convert this difference into a combination of addition and subtraction in the form of 2, 4 and 6, and make corresponding adjustment to the original sub-winding according to the changes corresponding to this combination of numbers to obtain the coupling diagram of the target coupling group.

For example, the coupling diagram of the Y, y0 coupling group can be quickly obtained from the coupling diagram of the Y, d5 coupling group, where the two coupling groups are connected in different ways.

First, change the sub-side from star to triangle, add 1 to the coupling group number, and get the coupling diagram of Y, d1, which differs from Y, d5 by 4.

It is also possible to change the vice side from star linkage to triangle inverse linkage to obtain the linkage diagram of Y, d11, which differs from Y, d11 to Y, d5 in the linkage group number by 6, and the linkage diagram of Y, d5 can be obtained by changing the same name once.

It can be seen that the same linkage group corresponds to the same linkage mode is not unique.

What is the Y,yn0 transformer connection?

Y,yn0 transformer connection

1, Y,yn0 transformer connection of the internal winding of the primary side is connected to the star, the secondary side is also connected to the star, the primary and secondary side of the line voltage in the same phase.

The winding wire filling factor is large, the mechanical strength is high, and the insulation consumption is low, so the three-phase four-wire system can be used to supply low-voltage power and lighting loads.

2, Y,yn0 coupling group of distribution transformers, the original winding can not pass the excitation current in the third harmonic component, so that the main flux becomes a flat top wave, that is, in the main flux contains the third harmonic component.

The third harmonic flux component in the core can not form a circuit, only through the oil tank and other iron parts as a circuit, thus increasing the magnetic resistance, so that the loss increased sharply, resulting in local heat temperature rise increased, operating efficiency decreased.

3, Y, yn0 wiring three-phase group transformer can not take single-phase load; Y, yn0 wiring three-phase heart transformer can take a small single-phase load.

Y,yn0 transformer connection to control the secondary side of the three-phase load symmetry. General Y,yn0 connection group of distribution transformer secondary load for power and lighting mixed load, that is, in a distribution network, both three-phase load, but also mixed with single-phase load, this situation is very easy to appear three-phase load asymmetry.

When the three-phase load is not symmetrical, there will be zero sequence current in the low-voltage winding.

The zero-sequence flux induced by the zero-sequence flux is superimposed on the voltage of each phase, resulting in a shift of the neutral point of the three-phase voltage.

As a result, the voltage of the heavily loaded phase falls and the voltage of the lightly loaded phase rises, which is detrimental to the low voltage side of the appliance. The size of the zero-sequence flux depends on the size of the zero-sequence current.

Therefore, the relevant regulations stipulate that the secondary neutral line current of the Y,yn0transformer connection must not exceed 25% of the rated current of the low-voltage winding. When the neutral line current does not exceed this value, the offset voltage at the neutral point is about 5% of the phase voltage, the effect on the three-phase voltage is not significant and can still be regarded as basically symmetrical.

4.Y,yn0 transformer connectionWhen the high-voltage fuse fuses a phase, there will be a phase voltage of zero, the other two phases of the voltage did not change, can make the outage range reduced to 1/3.

This situation does not affect lighting loads with a single-phase supply on the low-voltage side. If the low-voltage side is a three-phase power supply, generally configured with phase loss protection, so it will not cause the power load to run out of phase and burn up.

5. As the insulation strength of the primary winding of the Y,yn0 transformer connection is slightly lower than that of the D,yn11 transformer connection, the manufacturing cost is slightly lower than that of the D,yn11 transformer connection.

Therefore, in TN and TT systems, the neutral line current caused by single-phase unbalanced load does not exceed 25% of the rated current of the low-voltage winding, and the current of one phase does not exceed the rated value at full load, it can still be used.

What is the Y,d11 transformer connection?

Y,d11 transformer connection

1、The internal winding of the Y,d11 transformer connection is connected as a star on the primary side and a triangle on the secondary side, with the secondary line voltage lagging behind the primary line voltage by 3300.

2, Y,d11 transformer connection third harmonic current can be circulated, eliminating the third harmonic voltage. The neutral point is not led out, commonly used in the neutral point is not dead ground, the secondary voltage is higher than 400V on the medium and large step-down transformer.

3, Y,d11 transformer connection in normal operation, the secondary side has a good sinusoidal waveform, high power quality, Y,d11 transformer connection with load is not limited.

What is the D,yn11 transformer connection?

D,yn11 transformer connection

1、The internal winding of D,yn11 transformer connection is connected as a triangle on the primary side and a star on the secondary side, with the secondary line voltage lagging behind the primary line voltage 3300.

2, D,yn11 transformer connection has high output voltage quality, neutral point does not drift, good lightning protection performance and so on.

When the three-phase load on the low-voltage side is unbalanced, the total zero-sequence magnetic potential and the third harmonic potential on each core column are almost equal to zero because the zero-sequence current and the third harmonic current can circulate in the closed circuit of the high-voltage winding, so the low-voltage neutral potential does not drift and the voltage of each phase is of high quality.

Similarly, as the lightning current can also circulate in the closed circuit of the high-voltage winding, the total magnetic potential of the lightning current on each core column is almost equal to zero, eliminating the positive and negative overvoltage, so the lightning protection performance is good.

However, there is a problem of non-full-phase operation, which can be adopted by adding an undervoltage protection device to the low-voltage main switch.

3, D,yn11 transformer connection, its 3n (n for positive integer) harmonic excitation current in its triangular wiring of the primary winding to form a loop, not injected into the public grid, which is more conducive to suppressing high harmonic currents than the primary winding connected to the star-shaped wiring of the Y,yn0 connection group.

4, D, yn11 connection group distribution transformer, the main flux becomes sinusoidal, the induction potential in the secondary winding is also sinusoidal, thus improving the quality of the output voltage waveform, that is, to improve the quality of power supply.

5, D, yn11 wiring transformer than Y, yn0 wiring transformer zero sequence impedance is much smaller, conducive to single-phase grounding short circuit fault removal.

6, when connected to a single-phase unbalanced load, Y,yn0 wiring transformers require neutral line current does not exceed 25% of the rated current of the low-voltage winding, limiting the capacity of the single-phase load, affecting the full use of transformer equipment capacity.

The D,yn11 transformer connection allows the neutral line current to reach more than 75% of the phase current, and its ability to withstand single-phase unbalanced currents is much greater than that of the Y,yn0 transformer connection. connection.

This makes it all the more necessary to promote the use of the Dyn11 transformer connection in modern power supply systems where single-phase loads are increasing dramatically.

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