3 phase Pad-Mounted Transformer

What Are 3 Phase Pad-mounted Transformers?

The 3 phase pad-mounted transformers are basically designed to sit on a pad, thus why it has the name “Pad-mounted Transformer”.

The 3 phase pad-mounted transformer is a transformer designed with a fully enclosed structure. The layout of the heat sink that can directly dissipate heat to the outside makes Three phase pad-mounted transformers have good heat dissipation. The rated power range of the 3 phase pad-mounted transformer is 75~2500KVA. The 3 phase pad-mounted transformer is widely used in parks, mines, stations and other places with high power demand.


3 phase Pad-Mounted Transformer: The Ultimate FAQ Guide

3 Phase Pad-mounted Transformer Specifications

Loop Feed Pad-Mounted Transformer and Radial Feed Pad-Mounted Transformer

In terms of the 3 phase pad-mounted transformer specifications, just like normal transformers, they are capable of doing what typical transformers do such as stepping down voltage levels.

Like the single-phase pad-mounted transformer, 3 phase pad-mounted transformers also have windings inside of them. Like most transformers, 3 phase pad-mounted transformer sizes come in different dimensions as well. There are small ones and large ones available in the market.

The main purpose why pad-mounted transformer manufacturers made different variations in terms of the 3 phase pad-mounted transformer sizes is because to make the 3 phase pad-mounted transformer placeable indoors, or in small spaces outdoors.

However, in terms of the size, you can expect that it is far from the large sizes of big transformers. It is almost the same size as a home cabinet, which is convenient for installment.

An important note when it comes to 3 phase pad-mounted transformer sizes is the voltage rating. The smaller it is, the less kilovolt ampere rating there will be. On the other hand, the bigger it is, then the higher the kilovolt ampere rating.

The 3 phase pad-mounted transformer is divided into two types: Liquid-Filled Loop Feed Pad-Mounted Transformer and Liquid-Filled Radial Feed Pad-Mounted Transformer. 

How To Choose A Distribution Power Transformer?

The following is the specific data.

Width: 1350 mm
Depth:900 mm
Height:1430 mm
Oil Weight: 100kg
Total Weight: 540kg

Width: 1390 mm
Depth:910 mm
Height:1430 mm
Oil Weight: 120kg
Total Weight: 645kg

Width: 1420 mm
Depth:920 mm
Height:1430 mm
Oil Weight: 138kg
Total Weight: 729kg

Width: 1510 mm
Depth:980 mm
Height:1530 mm
Oil Weight: 201kg
Total Weight: 989kg

Width: 1510 mm
Depth:980 mm
Height:1530 mm
Oil Weight: 201kg
Total Weight: 989kg

Width: 1660 mm
Depth:1080 mm
Height:1680 mm
Oil Weight: 260kg
Total Weight: 1415kg

Width: 1810 mm
Depth:1160 mm
Height:1790 mm
Oil Weight: 325kg
Total Weight: 1905kg

Width: 2030 mm
Depth:1300 mm
Height:2030 mm
Oil Weight: 535kg
Total Weight: 2755kg

Width: 1651 mm
Depth:1549 mm
Height:1854 mm
Oil Weight: 650kg
Total Weight: 3235kg

Width: 2210 mm
Depth:1470 mm
Height:2150 mm
Oil Weight: 748kg
Total Weight: 5835kg

Width: 2380mm
Depth:1600 mm
Height:2220 mm
Oil Weight: 950 kg
Total Weight: 6430 kg

Width: 2480mm
Depth:1650 mm
Height:2330 mm
Oil Weight: 1020 kg
Total Weight: 8865 kg

3 Phase Pad-mounted Transformer Features

3 phase pad-mounted transformers
  • 3 Phase Pad-mounted Transformer has strong overload capacity. It allows 2 times of overload for 2 hours. It can even be overloaded 1.6 times for 7 hours without affecting the service life of the 3 Phase Pad-mounted Transformer.
  • 3 Phase Pad-mounted Transformer adopts elbow plug connector. It can be very convenient for the connection of high-voltage incoming cables. Used as a load switch in emergencies. It can be plugged and unplugged under power.
  • The 3 Phase Pad-mounted Transformer is protected by double fuses. The plug-in fuse (BAY-o-net) is a dual sensitive fuse (temperature, current) to protect the short-circuit fault that occurs on the secondary side of the 3 Phase Pad-mounted Transformer. The backup current-limiting protection fuse (ELSP) protects the internal faults of the 3 Phase Pad-mounted Transformer and is used to protect the high-voltage side.
  • The 3 Phase Pad-mounted Transformer can use Fr3 vegetable oil or mineral oil. It can be selected according to its situation.
  • 3 Phase Pad-mounted Transformer adopts a fully enclosed structure design. It places windings, iron cores, fuses, and high-voltage load switches in the oil tank.

What Inside a 3 Phase Pad-mounted Transformer?

inside 3 phase pad–mounted transformer
inside 3 phase pad–mounted transformer

The 3 phase pad-mounted transformer places the windings, iron cores, fuses, and high-voltage load switches in the oil tank. A large amount of transformer oil is stored in the oil tank. These oils will isolate these parts from contact with air, ensuring that they can work stably.

The 3 phase pad-mounted transformer adopts a front and rear distributed layout. The front part is the high-pressure chamber and the low-pressure chamber. The rear part of the heat sink and oil tank.

inside Three phase pad–mounted transformer
inside Three phase pad–mounted transformer

The high-pressure chamber includes a 10 KV load switch, no-load tap switch, plug-in fuse, pressure release valve, oil temperature gauge, oil level gauge, oil filling hole, oil drain valve, etc.

inside 3 phase pad–mounted transformers
inside 3 phase pad–mounted transformers

The high-voltage terminals of the 3 phase pad-mounted transformer are equipped with high-voltage bushings (single-pass or double-pass). It plays the role of connecting the elbow-type cable plug and the elbow-type fully insulated lightning arrester.

Inside Three-phase Pad-mounted transformer
Inside Three-phase Pad-mounted transformer

The 3 phase pad-mounted transformer adopts a plug-in fuse and backup current-limiting fuse for two-stage series protection. The plug-in fuse is responsible for a small range of overcurrent breaking. It can protect the 3-phase pad-mounted transformer in case of overload and high oil temperature.

The backup current-limiting fuse is responsible for a wide range of fault current breaking. If it occurs in the case of a short-circuit fault on the high-voltage side of the transformer. It can provide 3 phase pad-mounted transformer services to quickly cut off faulty transformers and protect high-voltage lines.

The plug-in fuse is easy to replace.

The backup current-limiting fuse needs to be drained before replacing the transformer oil.

Load switches are divided into two positions and four positions.

Two positions are used for terminal type;

Four positions are used for ring network type or dual power supply.

How to Install 3 Phase Pad-mounted Transformer?

Construction of 3 phase pad-mounted transformer grounding device

  • The 3 phase pad-mounted transformer is surrounded by a grounding grid. The grounding grid is connected to the surrounding building grounding grid.
  • The buried depth and welding requirements of the grounding device must meet the design requirements and relevant specifications.
  • After the grounding device is installed, the grounding resistance test can only be carried out in fine weather when the humidity of the ground reaches the specifications. If there is a grounding device with a grounding resistance value that does not meet the requirements, the corresponding grounding electrode and grounding bus must be added as required until the grounding resistance value meets the design requirements.
  • The connection between the grounding device and the equipment should be reliable and beautiful.
Install Pad-mounted Transformer
Install Pad-mounted Transformer

Installation and construction

  • Wiring: After site survey, a reasonable entry point for the 10kV power supply line is determined, and the corresponding power receiving construction plan is drawn up.
  • Pre-embedded: Carry out the foundation construction of the 3-phase pad-mounted transformer, and pre-embed the corresponding components and cable protection steel pipes.
  • Installation: After the foundation reaches more than 70% of the design strength, the 3 phase pad-mounted transformer arrives at the site for inspection first. The equipment can be installed only after the accessories are complete, the equipment is intact, and there is no rust or mechanical damage. In this process, we still need to pay attention to basic maintenance.
  • Inspection: After the construction of the 3 phase pad-mounted transformer is completed, the construction unit first conducts equipment self-adjustment and self-inspection.

How to maintain the 3 Phase Pad-mounted Transformer?

Maintenance procedures for pad-mounted transformers

Whether the foundation is tight, whether the holes are blocked, and whether there is moisture in the cabinet.

Whether the grounding device is complete and well connected. Whether the grounding resistance meets the requirements.

Is there any change in the outdoor environment? Whether it affects the safety of traffic and pedestrians.

Check the load of each feeder. Whether the three-phase load is balanced or overloaded, whether the switching position of the switch, the meter indication is correct, and whether the control device is working normally.

The inside of the dust removal box of the 3-phase pad-mounted transformers is dusted every other year. The cabinet surface and airbox surface of the high-pressure room, the low-pressure room can be wiped with a damp cloth. 3 -phase pad-mounted transformers use air blowing or vacuum cleaner to remove dust.

Daily maintenance and overhaul of the fan. Check the operation of the fan. If it is not running, use the temperature and humidity controller to adjust the temperature below the existing temperature and start the fan to check.

Maintenance and repair of the high-voltage switch and low-voltage switch electric operating mechanism.

8. Auxiliary routine test: Routine test of temperature and humidity controller; routine test of smoke alarm device; tightening and inspection of terminal block: tightening and inspection of standard parts.

9. Terminal block maintenance. Due to thermal expansion and contraction, the terminal block may become loose. For annual maintenance, all terminals in the terminal room should be re-tightened.

Note: Before re-tightening, please make sure that both the primary AC circuit and the secondary control circuit are powered off to avoid electric shock!

Precautions when maintaining the three-phase pad-mounted transformer

  • The three-phase pad-mounted transformer door is equipped with a windproof mechanism. It is a mechanism to ensure that it remains in a fully open state. When closing the cabinet door, be sure to lift up the root of the windproof mechanism. If you close the door, pull it hard to prevent the mechanism or door from deforming. It will affect the normal use of the Three-phase pad-mounted transformer!
  • After the local manual operation of the high-voltage load switch is finished, put the load switch operating handle back on the handle bracket inside the outer door.
  • When the standby circuit of the high-voltage ring main unit is not connected to the cable, the standby circuit should be locked before the ring main unit is powered on, or the cable seat should be blocked with a matching insulating cap to avoid accidents!
  • The dust cap of the three-phase pad-mounted transformer when leaving the factory is not a substitute for the insulating cap!
  • It is not allowed to use any short-circuit plug to be inserted into the test hole during operation. Otherwise, the voltage sensor will be damaged.
  • The low-voltage isolating switch can only be operated when it is in the unlocked position. Don’t pull it hard!

What are the Basics of Transformers?

To begin with, electrical transformers are generally machines or electrical devices that have the ability to transfer electricity from circuit to circuit that involves no change in terms of frequency. 

These electrical devices have numerous variants pertaining to appearance, size, shape, functions, types, etc. In fact, even 3 phase pad-mounted transformer sizes come in variants as well. Imagine the totality of all the different sizes of all types, there are simply countless sizes available in the market.

This is because as time goes on, more and more transformers are developed that are made to serve a unique function while still having the main purpose, which is to increase or “step-up” and decrease or “step down” voltage levels.

Most transformers operate on Alternating Current supply or better known as “A.C” in electrical terms. When it comes to using A.C as a supply, the fluctuation in terms of the supply voltage is affected by the movement of the current. In short, you can consider it as a reciprocal response. Whenever there is an increase in the current, the same goes for the voltage as well. 

Furthermore, transformers back then weren’t only limited to controlling or transferring voltages and currents. They were also designed to enhance its safety it, and as well as its efficiency when it comes to power systems.

This is done by increasing and decreasing voltages whenever there is a need to do so. Moreover, transformers of all types are commonly used in residential applications. For industrial applications, there are many projects it serves as well. Transformers are essential for the distribution of power that is passing through long distances, as well as regulation.

How do Three-Phase Pad-mounted Transformers Operate?

Like most transformers, the 3 phase pad-mounted transformer generally works the same as normal transformers. Even single-phase pad-mounted transformers but you will get to know more about single-phase pad-mounted transformers as we go deeper in the article.

For now, understanding how transformers operate is the main focus. To put things in a simple definition, most transformers follow Faraday’s electromagnetic induction law.

The basic definition of this law is that induced Electromagnetic Force (E.M.F) in a coil or conductor is proportional to the rate of change in flux linkage.

To make these elements easier to understand, most transformers including single-phase pad-mounted transformers and 3 -phase pad-mounted transformers, normally rely on mutual induction when it comes to physical basis.

They normally consist of two windings, which are the primary winding and the secondary winding. Both windings actually share a magnetic core, which is laminated for security purposes. The mutual induction will normally trigger when circuits are operating to import electricity from point A to B.

The amount of linked flux has an impact on the different rates of the change in flux linkage. This is associated with the primary and secondary windings.

The setting of ensuring the flux linkage (maximum) has a lot to do with the reluctance of path in both windings. Basically, the maximum flux that is going through the secondary winding and linking it to the primary is the result of this.

In return, this results in optimum efficiency in working performance, and the core formation of the transformer.

When it comes to the application of alternating voltages to the windings, the primary winding will be responsible for creating an alternating flux in the core. You can expect the linkage between both windings. Afterwards, both windings are able to induce an Electromagnetic Force in the primary and secondary sides.

Electromagnetic Force in the secondary winding generates a current that is commonly known as a “load current”, this only occurs if there is a load that is associated with the secondary section.

Normally, that is how transformers distribute alternating current power from a primary circuit to the secondary circuit by converting the electrical energy from one value to a different value. Again, without disturbing the frequency.

Seven Technical parameters of Pad-mounted Transformers

What are Pad-mounted Transformers?

Single Phase and Three Phase Pad-mounted Transformer From Daelim

Basically, Pad-mounted transformers, from the name itself, are a transformer that is mounted on a pad. But is it large like the DAELIM Distribution Transformers and DAELIM Power Transformers? The simple answer to that question is – No.


In fact, when it comes to appearance, it looks nothing like it, but they do have a similar function such as stepping up or stepping down voltage levels when needed.

Pad-mounted transformer manufacturers specifically designed this type of transformer to be accessible, cost-effective, and not space-consuming unlike other types of transformers. 


When it comes to its maintenance, for a transformer that is not as big as the others, it is expected that it has less maintenance, but this definitely does not mean you should not regularly monitor it.


This brings me to my next point, which is longevity. The same with other transformers, Pad-mounted transformer manufacturers ensured consumers that the longevity of pad-mounted transformers are just as long as the other types of transformers.

This goes the same for both single-phase pad-mounted transformers and 3 phase pad-mounted transformers.

What is a Single-phase Pad-mounted Transformer?

Single Phase pad mounted transformer
Single Phase pad mounted transformer

Now that you know the basics of what a transformer in general is, now is the best time to know what Single-phase pad-mounted transformers are before getting familiar with 3 phase pad-mounted transformers.

The sections are divided this way for you to have a better and simplified understanding of the two pad-mounted transformers.

The sections are divided this way for you to have a better and simplified understanding of the two pad-mounted transformers.

DAELIM Single-Phase Pad Mounted Distribution Transformer

single phase pad-mounted transformer
single phase pad-mounted transformer

The DAELIM Single-phase Pad-mounted Transformer is engineered to improve the performance of standard Single-phase Pad-mounted Transformers. When it comes to performance, the kilovolt ampere rating, lightweight, safety, durability, and compact dimensions of this electrical device are winning factors.

This can be placed indoors and outdoors with no distribution room whatsoever, this can also be placed in the streets with no problems. The green belt ensures the safety of the public in case there is a malfunction or misoperation, which rarely or never happens when properly operated.

The DAELIM Single-phase Pad-mounted Transformer is one of the most commonly used pad-mounted transformers in residential areas, industrial areas, businesses, hospitals, and many more worldwide!

It has a low profile that sports a compartment-type for easy access and it is very suitable for mounting outdoors without any additional protective enclosure. This means that it is cost-effective as well, and it passes the standards of the I.E.C 60076, A.N.S.I / I.E.E.E. C57.12.00, C57.12.20, C57.12.38, C57.12.90 and etc.

Supply Ability: 200 Set/Sets per Month Single-phase Pad-mounted Transformers.

Packaging Details: This can be sealed in a wooden case or depending on what the customer’s request is.

(Europian type pad-mounted 50 kVA / 100 kVA/ 12/4 kV oil-filled power distribution transformers that pass the I.E.C standards for power transformers.).

The requirements of sealing the electrical device are strictly following international standards for optimum security.

Shipping: The shipping will go through either land or water transportation. DAELIM has shipped its electrical devices all over the world without any complaint from its international customers.

DAELIM has a logistic network worldwide for the best transportation option for the customers.

Pad-mounted Transformers vs Overhead Transformers

Pad-mounted Transformers vs Overhead Transformers
Pad-mounted Transformers vs Overhead Transformers

The biggest difference between the two is that pad-mounted transformer manufacturers specifically designed this type of transformer to be mounted on the ground while settling in a pad. On the other hand, Overhead transformers or pole transformers are mounted on a utility pole.

Moreover, most of the components of the overhead transformer are exposed, wherein 3 phase pad-mounted transformers, the unit itself is sealed when its lid is closed.

What is Outside a Pad-mounted Transformer?

Pad mounted transformer manufactuer - Daelim
Pad mounted transformer manufactuer - Daelim

For starters, the outside of a Pad-mounted transformer is secured with a ring lock to unauthorized operations. Most 3 phase pad-mounted transformers have a special socket that allows operators to access the transformer.

If a 3 phase pad-mounted transformer is live, then you will most likely see wires that are connected to it since it is serving a meter or other purposes.

How does 3 phase Pad-mounted Transformer Works?

The way it works is that the primary voltage enters a cable that is capable of withstanding 14,400 Volts. It enters an H1-A bushing and it goes through the transformer, then through the windings. Afterwards, it goes through the side of the Z bars.

From the Z bars, it goes through the connection points that leave the cable to go through the meter or to where it is designated. For houses, this serves around 220 Volts – 240 Volts.

Common 3 phase pad-mounted transformer faults and repair methods

In terms of maintenance, the pad-mounted transformer manufacturers did their best to avoid problems as much as possible. However, like most electrical devices, the 3 phase pad-mounted transformer does have its own maintenance.

The air circuit breaker cannot be closed

  • The control circuit is faulty.
  • The red button on the panel did not reset after the smart trip unit was activated.
  • Energy storage mechanism is not stored

Method of exclusion

  • Check the open circuit point with a multimeter.
  • Find out the cause of the trip and press the reset button after troubleshooting.
  • Manual or electric energy storage.

Molded case circuit breaker cannot be closed

  • After the mechanism is tripped, there is no reset.
  • The circuit breaker has an undervoltage coil and there is no power supply at the incoming line.

Method of exclusion

  • Find out the cause of the trip and reset it after troubleshooting.
  • Make the incoming terminal live, reset the handle, and then close

The circuit breaker trips when it is closed

  • There is a short circuit in the outgoing circuit

Method of exclusion

  • Do not switch on repeatedly for many times, find out the fault, and then switch on after troubleshooting.

Capacitor cabinet cannot be automatically compensated

  • The control circuit power supply disappears.
  • The current signal wire is not connected properly.

Method of exclusion

  • Check the control circuit and restore the power supply.

Final Thoughts on 3-phase Pad-mounted Transformers

Overall,  3 phase pad-mounted transformers are excellent electrical devices that are at the top of their tier when it comes to serving their functions. It easily outperforms single-phase pad-mounted transformers, but this does not mean that single-phase pad-mounted transformers are not worth considering.

Pad-mounted transformer manufacturers specifically designed the two Pad-mounted Transformers with different purposes and depending on what your purpose or project is, the purchase decision or final decision of which one you should get depends on that.

If you have any questions or concerns about which DAELIM pad-mounted transformer is for you, do not hesitate to contact DAELIM’s team of professionals for immediate assistance and professional help.

Custom Pad Mounted 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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