How a Distribution Transformer Is Born

When you buy a distribution transformer, you see a steel tank, some bushings, and a nameplate. What you do not see is the long chain of precise, repeated, and strictly controlled processes that turn raw materials into a reliable piece of equipment that will run for decades.

Two transformers with the same ratings can differ greatly in loss, temperature rise, noise, and service life. The difference almost always comes down to one thing: how well the production process is controlled.

At Jiangsu Zhongmeng Electric, we focus on distribution products below 35kV—especially distribution transformers and compact substations. In this article, we open our factory doors and walk you through every major step of transformer production, so you can see exactly what goes into the equipment you buy.

A transformer is not assembled from a few parts. It is built through a sequence of interdependent processes. Each step affects the next:

Poor silicon steel cutting increases no-load loss.

Loose winding increases load loss and temperature rise.

Incomplete drying leaves moisture in insulation, leading to partial discharge.

Improper oil treatment reduces insulation strength and cooling performance.

In other words, quality is not inspected into a transformer—it is built into it, step by step.

2.1 Incoming Material Inspection and Preparation

Everything starts with the materials.

We inspect and prepare:

Silicon steel sheets – for core lamination

Copper or aluminum conductors – for windings

Insulation materials – paper, pressboard, insulation cylinders

Transformer oil – for insulation and cooling

Steel plates – for the tank

Each batch is checked for thickness, conductivity, insulation strength, and other key parameters. A transformer can only be as good as the materials that go into it.

2.2 Core Manufacturing

The core is the magnetic circuit of the transformer.

Key steps include:

Cutting the silicon steel sheets to the required shape and size

Laminating the sheets in a precise sequence

Clamping the core to keep it tight and stable

Insulating the core to prevent short circuits between laminations

The quality of the core directly affects no-load loss and noise level. A well-made core runs cooler and quieter.

2.3 Winding Production

The windings are the electrical heart of the transformer.

We produce:

High-voltage windings

Low-voltage windings

Depending on the design, windings may be cylindrical, disc-type, or foil-type. During winding, we control:

Winding tension

Turn arrangement

Insulation layers between turns and layers

Dimensional accuracy

Winding quality affects load loss, temperature rise, and short-circuit withstand capability.

2.4 Core and Coil Assembly

Once the core and windings are ready, they are assembled into the transformer body.

This stage includes:

Fitting the windings onto the core limbs

Installing insulation barriers and spacers

Connecting leads and taps

Positioning the assembly for the next steps

Every clearance and every insulation distance matters. A small deviation can lead to partial discharge or even failure during operation.

2.5 Drying and Vacuum Treatment

Moisture is one of the biggest enemies of transformer insulation.

To remove moisture, we use:

Heating

Vacuum drying

The transformer body is placed in a vacuum chamber and dried under controlled temperature and vacuum conditions. This step is critical for:

Insulation resistance

Dielectric strength

Partial discharge performance

Long-term reliability

2.6 Oil Treatment and Vacuum Oil Filling

Transformer oil provides both insulation and cooling.

Before filling, the oil is:

Filtered

Degassed

Dehydrated

Then the oil is filled into the tank under vacuum. This prevents air bubbles and moisture from being trapped inside the insulation system. Clean, dry oil is essential for safe operation.

2.7 Final Assembly and Sealing

After oil filling, the transformer is assembled with its external components:

Tank and cover

Bushings

Tap changer

Pressure relief valve

Temperature indicators

Nameplate

All sealing surfaces are carefully checked. We perform leak tests to ensure there is no oil leakage under normal and abnormal conditions.

2.8 Factory Testing

No transformer leaves our factory without passing strict tests.

Routine tests include:

Ratio test

DC resistance measurement

Insulation resistance test

Dielectric withstand test

No-load loss and load loss measurement

Impedance voltage test

Depending on customer requirements, we also perform:

Temperature rise test

Partial discharge test

Impulse voltage test

Only after all tests are passed and the data is reviewed does the transformer receive its factory test report and get released for delivery.

Among all the processes, several are especially critical:

These are not just technical steps. They are the points where experience, discipline, and standards make the real difference.

Once, we produced a batch of S14 600 kVA 10/0.4 kV oil-immersed transformers for a customer. During the factory ratio test, we found that one transformer had a ratio test error that exceeded the standard, and the no-load current was too high.

After checking the problem, we found the cause: the number of turns was wound incorrectly. This problem not only increased the cost, but also delayed the delivery time.

After that, we strengthened the monitoring process during winding. This helped us prevent the same problem from happening again.

So every production step of a transformer is very important. We must control the quality of every step.

A distribution transformer is not simply a product. It is the result of dozens of processes, hundreds of checks, and years of manufacturing experience.

At Jiangsu Unita Electric, we believe that a transparent production process is the foundation of customer trust. That is why we welcome customers to visit our factory, see the production lines, and check the test reports for themselves.

When you choose a transformer, you are not just choosing a specification. You are choosing the process behind it.