Everything You Should Know About 3-phase Transformer Connections

Even though you might have never seen one before, you most likely know what a transformer is. Transformers are those oddly shaped metal devices that are usually found near power lines, in old buildings, or on top of utility poles.

In the course of their work, transformers change the voltage from the low voltage distribution lines to a higher voltage for long distance transmission of electricity. And as if changing voltages isn’t hard enough, there are also different types of transformer connections to deal with.

One of those connections is a three-phase transformer connection.Whether this term sounds familiar or foreign, we are here to unravel it. Read on to the end of this blog.

Understanding transformer connections

Do you know how a transformer works? If not, here is a brief explanation. Transformers change the voltage of the electricity that flows through them. They do this by using two or more coils of wire called windings. One winding is called the primary winding, and it is connected to one end of the power line, while the other winding is called secondary and it’s connected to the other end of the power line. When electricity passes through these windings, it creates an electromagnetic field around it thereby creating voltage on its own separate from that of its power source. The voltage created by this electromagnetic field is what powers your appliances in your home or business.

So, what does transformer connection imply?

Transformers have different connections that allow the electricity to flow through them in various ways. These connections are categorized into two:

Single Phase Transformers

These transformers have only one winding. They are connected to the power line in such a way that they produce either a positive or negative voltage. This means they can power only one appliance at a time.

Three Phase Transformers

These transformers have three windings, hence the name three phase transformer connection. They produce three separate voltages, each of 120 degrees apart from each other, and these voltages can power more than one appliance at a time (depending on how many phases there are).

The focus on this article is three-phase transformer connections.

What is a three-phase transformer connection?

A three-phase transformer connection is a method by which three separate wires are attached to a transformer. Each of these wires carries one third of the electricity, hence the name three-phase. This allows the transformer to power more than one appliance at a time.

In any given three-phase transformer, there will be a phase difference of 120 degrees between each phase. The three phases are usually named A, B and C. Although the labeling can be altered depending on your geographical region.

What is the purpose of a three-phase transformer connection?

The purpose of a three-phase transformer connection is to allow more than one appliance to be powered at a time, by the same source of electricity. The electrical power from the source comes into the transformer in one wire, and then one of the three phases (A,B or C) will carry 1/3rd of this power out to an appliance.

If an appliance is connected to say phase A and it draws more than 1/3rd of the current passing through that phase, that will cause another phase (B or C) to also carry some current. This can be seen as an advantage because it allows more than one appliance to be powered at any given time.

However if this happens, it will cause what’s known as voltage drop or voltage sag in that phase which can cause problems with some appliances. For instance, some applications cannot run on low voltage.

Types of three-phase transformer connections

At this point you may be wondering if there is a single universal configuration of the three-phase transformer connection. 3-phase transformers come in different connections namely:

-Star-star connection

-Star-delta

-Delta-star

-Open-delta

-Scott connection

We are going to look at each of these connections and their respective application areas.

Star-star connection

This type of 3-phase transformer connection is commonly used in motor-starting applications.

In a star-star connection, the 3-phase windings of the transformer are connected to form a star with each of the 3-phase windings being connected to each other at their ends.

This connection  is only used in applications where the load is balanced. That is the input is balanced with the output load. The line loads on both sides of the transformers (primary and secondary) are properly balanced.

This type of transformer connection also features minimal insulation.

Star delta connection (Wye-delta)

This type of connection is also known as the Wye-Delta.

In a star delta connection, the 3 phase windings are connected in a way that forms 2 separate stars with each phase winding forming one of the stars and the end of one phase winding connected to a point on another phase winding.

This type of transformer connection is used in  drop-down transformers mostly in power substations where the voltage shifts from high to low.

What about the angular difference? Well star-delta connections feature a 30 degree angle difference between the lines on both sides (primary and secondary) of the transformer.

Delta-star connection

This type of 3-phase transformer connection is also known as Delta-Wye connection. As the name suggests, in this type of connection, the primary winding  is connected in the delta while the secondary winding is connected in the star. The connection features a neutral wire that is grounded.

Delta- star connection is commonly used for the step-up transformers. This directly means that you are likely to find it right at the beginning  of power stations, where there will be a need to increase the supply voltage.

What about the angle? Delta-Wye  transformer connections feature a 30 degree angular difference between the lines.

Open-delta connection

This type of transformer connection is also referred to as V-V connection. In this type of connection, the primary winding  is in the delta-delta connection which is technically disabled but the service has to continue until the faulty transformer is repaired.

Open delta transformer connections are mostly used for small loads or transformers that don’t carry significant loads. Do not confuse this connection to the delta-delta transformer connection. It carries almost 60% less voltage than that of V-V connection.

Scott connection (T-T)

This type of connection is used between two power transformers. It is also termed as T-Wye or T-Y connection.

The primary winding of the first transformer is connected in delta while the secondary winding is connected in wye. The neutral wire of the second transformer will be grounded.

Scott connection (T-T) can be used to achieve a voltage increase for the second transformer and a voltage decrease for the first transformer but this does not mean that it can only be used for voltage increase and reduction. It can be used for both purposes too. It achieves this by tapping off some of the energy from one phase to feed another phase using the delta phase transfer method.

The most common use for Scott connections are at AC power stations where there are multiple generators that have different voltages and frequencies and you need to combine them into one single generator with a different voltage, frequency, or both from one another.

Three-phase transformer construction

Now that we have seen the different connections used in 3-phase transformers, let’s now have a brief look at how these transformers are constructed in relation to their respective connections.

A three-phase transformer is usually constructed using three identical coils that are placed in a line. These coils are wound in the same direction and are connected end to end across the secondary winding of the transformer.

The primary winding of the transformer is connected to one of these three coils; this coil will be called the first (primary) winding. The second and third winds of the transformer are then connected to their respective secondary windings respectively.

The first (primary) winding is connected directly to one phase of the AC power source, while the second and third windings are connected in delta connection fashion to two other phases which will be called “A” and “B”.

The construction of the three-phase will be based on the different connections that we have just discussed. So, next time you are buying a transformer, whether it is a step-up or step-down, it will be viable to inquire  about the type of connection that has been used in its construction.

Difference between delta  and Wye connection

What is the Y connection in a 3-phase transformer? Since we have covered the different types of 3-phase transformer connections, I won’t be surprised when you confuse Delta vs Wye connection. Figuratively, these connections appear  similar. They are technically different.

What is a delta connection? This is a type of connection  where the three phases  are arranged to form a triangular-like shape. The supply line ends up being divided into three terminals and each terminal is connected to one of the three phases. This is a very simple and effective way of winding the three phases.

What is a wye connection?  This is a type of 3-phase transformer connection where all the three terminals are connected to a single neutral point. The neutral conductor is grounded. The connection ends up forming a Y-shape.

There are several differences between Delta vs Wye connections. One of them is in terms of  the configuration. Delta connection is a simple and effective way of winding the three phases. Wye connection is more complicated than that. It requires a much more complex layout.

Delta vs Wye connections are also different in terms of the number of turns required for each phase. The number of turns required for each phase in the delta connection is 3, while it’s 4 in the wye connection.

Another difference is based on the load. For the delta connection, the load is concentrated on a single phase while on the Wye connection, the load is distributed on three different phases of the transformer line.

How do I know whether I have a Delta or Wye connection?

You are  about to buy a 3-phase transformer line and not sure on how to differentiate between Delta vs Wye connections. You can differentiate them based on their physical outlooks.

Their shapes will give you a hint. Delta has a triangular shape while Wye is Y-shaped.

Still not satisfied? Well you can simply count the number of lines used. Delta transformer connection has three  hot lines and a single ground line. The Y connection on the other hand has three hot lines connected to the neutral line.

Which type of 3-phase transformer connection should I choose?

At this point, we have looked at all the different types of 3-phase transformer connections. Whether it is Star-Delta, Delta-Star, among others. Which one should I choose? Well, that depends on the load, the line length and the amount of power you have to move.

It depends on the purpose of your project. For example, if you are moving a large amount of power for commercial purposes, then a Star-Delta connection might be a better option than a Delta-Star connection.

But if you are moving an industrial machine or device that requires a lot of power and also requires heavy duty transformers, then it might be better to go for Delta-Star connection.

Whether it is an industrial or commercial project, it is important that you choose the best 3-phase transformer connection for your needs and requirements.

Conclusion

I hope you learned something from this article, whether it is about 3-phase transformer connections or about different types of transformers, their applications, and other important things related to transformers.

And if you have any queries, let ICRFQ help you find the answers. We are a trusted supplier for transformers in China.

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