What Is a Tap Changer in a Transformer? Types, Working Principle, and Applications

In an actual power system, voltage is not a fixed value. It may rise or drop slightly as loads change and the power grid fluctuates. That’s when the star of this article comes into play: the tap changer. It is the key component on a transformer that fine-tunes voltage. Let’s learn more about the important component together.

What Is a Tap Changer in a Transformer？

The definition of tap changer in Wikipedia is as follows：A tap changer is a mechanism in transformers that allows for variable turn ratios to be selected in distinct steps. This is done by connecting to a number of access points, known as taps along either the primary or secondary windings.
In brief, a tap changer is a device that adjusts a transformer’s output voltage by switching between different connection points on the winding.
Here are two concepts that we often confuse:

Tap
A tap is a preset connection point on the transformer winding.

Tap Changer
It is the mechanical or electrical device that switches between those taps.
It does not change the transformer’s power rating. Instead, it changes the voltage by adjusting the number of winding turns in operation.

Why Do Transformers Need Tap Changers？

Ideally, the input voltage of transformers should be fixed and so is the output voltage. But the reality is far more complicated than that. For instance, if the load on the power grid suddenly increases, then the voltage will drop. During long-distance power transmission, line losses will result in a lower voltage levels. During peak and off-peak electricity usage periods, the voltage levels are naturally different.

If the output voltage remains consistently too high or too low for a long time, it will cause many problems. For instance, the equipment operates with instability, the motor gets hot, efficiency declines, and the lifespan of electrical equipment shortens, etc. The function of tap changer in transformers is to enable the transformer to “adapt to changes” and keep the output voltage within a reasonable range.

Basic Working Principle of a Tap Changer

One of the most basic rules of transformers is:
The output voltage is proportional to the number of winding turns.

Here’s a simple example:
If you are using a winding of 100 turns, and the voltage is 10 kV.
Then, when you increase to 105 turns, the voltage will increase slightly.
When you reduce to 95 turns, and the voltage will decrease.
The task of a tap changer is to switch between the connection points corresponding to these different numbers of turns ——this is how it works in transformers.

Which Side Is the Tap Changer Installed On?

In actual projects, they are usually installed on the high-voltage winding. The reason is quite simple:
The current on the high-voltage side is relatively small, so the arcing and switching losses are easier to control, the size and cost of the equipment are more reasonable. That’s why, in most oil-immersed transformers, you’ll find it on the high-voltage side.

Main Types of Tap Changers

The most mainstream classification method is by by load operation condition. In this way we divide them into off load and on load tap changer.

1. Off Load Tap Changer

It is also known as de-energized tap changer (DETC) or no load tap changer.
It has distinct features.
It can only be operated when the transformer is de-energized. Simple in structure and low in cost, it is not suitable for frequent adjustments.

Typical applications:
It is commonly used in distribution transformers and scenarios where the voltage remains stable for a long time.
Usually, during installation or maintenance, the tap changer will be adjusted according to the local grid voltage conditions, and then it is rarely touched again.

2. On Load Tap Changer (OLTC)

This is the more advanced option.
Its biggest advantage is that the taps can be changed while the transformer is still under load, so power supply won’t be interrupted.
OLTC tap changers use more complex switching arrangements such as transition resistors or reactors to avoid short circuits or open circuits during tap changes.

Typical applications:

– Substation main transformers
– Transmission systems
– Power grids with frequent voltage fluctuations
Of course, it has its drawbacks as well – it comes with higher costs, a more complex structure, and stricter maintenance requirements.

How Are Tap Changers Controlled?

Here are the common control methods:

Manual control
Tap position is adjusted by operators as needed.
Automatic control
Voltage sensing devices automatically decide when to change taps.
Remote control
They are Integrated with SCADA or automation systems and we can operate from a control center.
In modern power grids, OLTCs are usually completely automatic.

Tap Positions and Voltage Regulation Range

Tap changers do not provide continuous adjustment.
They regulate voltage in fixed steps.
Common configurations include:
– ±2 × 2.5%
– ±5%
– Multiple tap positions
Each tap change will adjust the output voltage up or down by a fixed percentage.
The more taps there are, the finer the voltage regulation can be, but the more complex the structure will become.
OMEK can provide customized products according to your needs. Our maximum voltage regulation range is±10%. If you are interested in our customized service, please contact us.

What Are the Benefits of a Tap Changer?

Keeps output voltage Stable

It can help keeping the user-side voltage within the normal range, regardless of grid fluctuations.

Improves system reliability

It can reduce the impact of abnormal voltage on equipment and make the system run more smoothly.

Prolongs Equipment Life

With this component, we can avoid long-term overvoltage or undervoltage of the equipment, reducing wear and damage.

Typical Applications

As long as there are requirements for power supply quality, the tap changer is practically a necessary component.
This component is widely used in:

– Distribution networks
– Substation power transformers
– Industrial power systems
– Renewable energy systems such as wind and solar
– Grids in areas with large voltage variations

Limitations of Tap Changers

Of course, tap changers are not perfect.
You need to know that OLTCs are complex in structure and quite expensive. Frequent operation will cause mechanical wear, and regular inspection and maintenance are needed.
So, whether to install a tap changer and which type to choose must be determined based on the actual application scenario.

Conclusion

After reading this blog, I’m sure you have realized the importance of tap changers.
They are used to regulate transformer output voltage. They work by switching between different winding tap points. They are mainly divided into off-load and on-load types. If a transformer is the “heart” of the power system, then the tap changer is the regulator that helps it adapt to changing conditions.

At OMEK, we offer both OCTC and OLTC options to meet different requirements. Our products are of high quality and provide wide voltage regulation range. If you have any questions, feel free to contact us. Our expert team will give you rational suggestions based on your transformer ratings, applications and other requirements.