Substation transformers have been around for decades and the most common type of them is the distribution substation transformer. In this article, DAELIM, one of the top substation transformer manufacturers in the world, will provide you details about substation transformer ratings, substation transformer diagrams, substation transformer capacity, and many more.

This is crucial for consumers to learn these elements in order to make a clear purchase decision on which substation transformer they should choose from. There are several characteristics you should learn first before proceeding with what substation transformers are.

For instance, since you will learn about what distribution substation transformers are, it is vital that you understand first what distribution transformers are for you to easily understand the characteristics and components of distribution substation transformers.

630 kVa Mini Substation and Miniature Substation

Pad-mounted Transformer

We can provide you single phase and three phase pad mounted transformer

Dry-type Transformer

Type:Cast resin; Rated Capacity: Up to 25MVA; Rated Voltage: Up to 36KV;

Pole Transformer

TypeCSP type Frequency: 50/60Hz; Rated Power: 5~167kva

Dry-type Transformer

Frequency: 50/60Hz Rated voltage:10kv, 20kv,30kv Rated Power: 400~2500kva


What is a Distribution Transformer?

A distribution transformer is basically a service transformer that provides electricity to houses and other buildings by providing the final voltage transformation in the electric power distribution system. Distribution transformers also have the ability to step down voltages that are used in distribution lines. The purpose of this is to meet consumer needs.

Of course, it comes with several other features as well which is why it is not surprising to see distribution transformers at transmission systems for power distribution purposes since it has the highest power as well or volt-ampere ratings. 

In terms of the substation transformer rating, this is determined through what type of cooling medium the substation transformer is using. This is either pressurized air (forced air) or transformer oil (mineral oil).

Transformers that use pressurized air or forced air are called Dry-type transformers. On the other hand, transformers that utilize oil or mineral are called oil-immersed transformers. Each of which has its own advantages and disadvantages.

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Dry-type Transformer
Dry-type Transformer
oil-immersed transformers
oil-immersed transformers

How do Distribution Transformers Work?

The working principle of distribution transformers is similar to the substation transformer diagram. The main working principle operation of a transformer relies on mutual inductance that is between two circuits, in which, it is linked by a magnetic flux. 

In the case that the second coil circuit is closed, You can expect that current will still flow in it, and this makes electrical energy transferred through the laws of magnetism from the first coil to its secondary coil.

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What are the Common Applications of Distribution Transformers?

Generally, distribution transformers like distribution substation transformers have the ability to transform high voltage level electricity to low voltage electricity in a matter of seconds for consumer use (i.e., like houses and other establishments.)

Distribution transformers have no problem distributing electrical energy to large industries with less voltage of under 33 kV and 440 V to 220 V for common domestic purposes.

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What are the Different Types of Distribution Transformers?

Before you learn what a distribution substation transformer is, it is highly recommended that you familiarize other distribution transformers first. This is another amendment to make things easier for you to understand. 

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Single-phase Distribution Transformers

Single-phase transformers are primarily used in applications that do not need a three-phase source. This type of transformer is commonly used for repairing overhead distribution powers. The most common scenario would be residential cases.

This type of transformer is also used in commercial loads, power applications, industrial lighting, and many more. However, for applications that have a three-phase source, a three-phase distribution transformer is needed for a successful operation.

Electric Pole Transformer | Single Phase Transformer

Pole-mounted Transformers
Pole-mounted Transformers

Three-phase Distribution Transformers

Speaking of three-phase distribution transformers, these are basically transformers that keep electrical energy from basic distribution networks to a distribution user that uses it less. This means that the transformer passes the current to a secondary distribution case that decreases the voltage primary component.

This type of transformer with a three-phase utility decreases the voltage source for the primary network to meet the user’s requirements or needs.

The voltage involved in this is often modified. Moreover, it can be different for consumers of residential and commercial areas since these electrical devices operate based on the level of ratings of frequency and voltage. This is according to the standards of various countries.

So one of the main differences between single-phase distribution transformers and three-phase distribution transformers is the employed residential application, and the pad used in primary underground networks.

Three-Winding Transformer | Three-Phase Transformer

Mounted Transformers

There are two common types of pad-mounted transformers that are commonly seen in public, and that is the ground-mounted transformer and the pole-mounted transformer. These two are famous transformers that are safe to be exposed in public. However, they do not have the same functions.

But in general, pad-mounted transformers usually consist of metal housing, and they are set at a location where there is not enough space. For pole-mounted transformers, this simply is not the case since they can be mounted onto utility poles.

Basically, these devices can be employed with electric energy distribution lines at an overhead electrical path for lowering the primary voltage for the purpose of providing the consumers’ electricity,

One of these mounted transformers can actually serve multiple houses or buildings. Supplying electricity to a single building or house is no problem. But for large establishments like industrial buildings or factories, then a transformer of another type is best for that situation.

These transformers can be constructed and secured from corrosive elements that can pose a threat to ordinary transformers. Since they are exposed to the public, this means that outside elements like harsh weather, animals, pedestrians, etc. can easily interfere with the transformer’s operations.


What is a Substation Transformer?

Now that you have met some of the different types of distribution transformers, getting to know and understanding what substation transformers are will no longer be a difficult topic for you to understand.

For starters, substation transformers are commonly used with outdoor substations. Moreover, these are highly rated, usually about 750 to 5,000 kVA or Kilovolt Amperes (for single-phase devices) and 750 to 25,000 kVA for three-phase devices.

High voltage transformers can also step down voltages. For instance, 220kv stepped down or decreased to 10kV.


Substation Transformer Diagram

When it comes to the substation transformer diagram, substation transformers basically follow the rule of thumb of transformers in general as well. Transformers are usually categorized as a passive electrical device that transfers electricity or electrical energy from circuit to circuit through the process of electromagnetic induction.

The substation transformer diagram is commonly used to increase or decrease the voltage levels between the circuits.

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Winding of 10 kv transformer
Winding of 10 kv transformer

Substation Transformer Ratings

For substation transformer ratings, there is a variety of rating out there since nowadays there are a lot of substation transformers available with different ratings. This could depend on the substation transformer manufacturers if they add or decrease the traditional substation transformer ratings.

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Substation Transformer Capacity

This also goes the same for the substation transformer capacity because substation transformer manufacturers can nerf or tweak this aspect of the substation transformer. A typical substation transformer would normally be rated somewhere between 69-13 kV and 20 MVA.

This means that the primary voltage is usually around 69 kV. On the other hand, the secondary voltage would be around 13 kV. The substation transformer capacity has a power rating of 20 MVA or 20,000 kVA.

Selection and Capacity Calculation of Transformers Used in Substations


The connections of substation transformers indicate how three phases of transformer winding are associated with one another. There are basically two connections of the windings, the Delta which is where the end of each phase winding is linked to the start of the following phase, and the Star that connects two ends of each phase are connected with one another.

Normally, substation transformers are connected with a delta formation at the high-voltage aspect and wye for the low voltage aspect. The delta connection also isolates two systems when it comes to giving the harmonics space. 

However, these are not preferred by the system. The wye connection on the other had established a convenient neutral point for the purpose of connecting it to the ground.

Connection Mode of Working Winding of Three-phase Transformer

Voltage Regulation

This indicates that the transformer is capable of changing the low voltage side voltage in order to maintain nominal voltage at customer service points. The voltage at customer service points can fluctuate as a result of either primary system voltage fluctuation or excessive voltage drop due to the high load current.

Transformer taps effectively change the transformation ratio and allow voltage regulation of 10–15% in steps of 1.75–2.5% per tap. Transformer tap changing can be manual or automatic.

However, only under-load type tap changers can operate automatically.

What is a Distribution Substation Transformer?

Now that you know the fundamentals of substation transformers, it is time to move on to what are distribution substation transformers. 

For starters, distribution substation transformers are transformers that usually operate at 2.4 – 34.5 kV voltage levels. Moreover, distribution substation transformers deliver electric energy or electricity directly to residential consumers. For industrial consumers, this is also possible as well.

Distribution feeders also transport power from the distribution substations for the purpose of delivering it to the homes of the consumers. Basically, these feeders are capable of serving multiple buildings or establishments.

In the case of the consumer’s premises, the distribution substation transformer will be responsible for transforming the voltage to the service level voltage that is directly used in households and industrial plants that are usually rated from 110 volts to 600 volts.

Substation Transformer
Substation Transformer

Supply Line

When it comes to the distribution substation transformer’s supply line, the distribution substation transformer is basically connected to a sub-transmission system with one supply line. This is referred to as the “primary feeder”.

However, keep in mind that it is normal for distribution substation transformers to be supplied with two or more supply lines. The purpose of this is to increase the reliability of the power supply in the case that a supply line is accidentally disconnected.

These supply lines are connected to the substation transformer through high voltage disconnecting switches. This is to prevent isolated lines from the substation in order to do maintenance or any repair work.

Power Rating

substation transformer ratings are usually expressed through kilovolt amperes or kVA. But they can be expressed in MVA as well, which means megavolt ampers. This basically indicates the amount of power there can be when there is a transferring process through the transformer.

substation transformers usually range from 3 kVA to 25 MVA.

Voltage Rating

In terms of the voltage rating, it is a different aspect from the power rating, and it is governed by the sub-transmission and distribution voltage levels in which the transformer is connected. Furthermore, there are voltages that are governed by applicable standards as well.

This is one of the reasons why the voltage rating is showcased by the substation transformer manufacturers

The voltage rating is what dictates the construction and insulation of the requirements of the transformer. This is for the transformer to be able to withstand the rated voltage or higher when the system is in operation.

Cooling Medium

In terms of the cooling medium, as mentioned there can be two possible cooling mediums used by transformers, which are mineral oil and pressurized air.

The cooling medium basically dictates what the transformer’s power rating is capable of, as well as inspecting the level of temperature it can withstand during peak demands. Transformer rating involves self-cooling ratings at specified temperatures during operations.

Final Thoughts

Substation transformers are excellent electrical devices that are a combination of both a substation and a distribution transformer. Basically, they are capable of doing what substations and distribution do. 

This means that if your purpose or project requires distribution operations, then the substation transformer is a great option to choose from.

However, if you have any questions or clarifications about substation transformers, it is best to contact DAELIM’s team of professionals first before making a purchase decision. This is to ensure that you are getting the right type of transformer for your project or usage.

About Bin Dong

Hello, I am Bin, General manager of Daelim which is a leading transformer manufacturer. As a expert, I am glad to share my experience in transformer to you. I believe our professional team can always provide you with high quality service and we are ready to provide you with one stop solution. If you have problems when you are looking for the equipment, what you need to do is tell us.

Custom Substation Transformer

When you need to find more than just existing transformers, Daelim’s Transformer Service Center can help you design and produce distribution transformers that meet your unique needs.

We have our own factory and a professional team of engineers, which can design and modify application requirements that meet all your conditions.

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