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Transformer tap changers & voltage adjustment taps

Transformer voltage taps and tap changers allow you to adjust the input voltage rating of the transformer to maintain the correct secondary voltage.

June 9, 2023

A transformer voltage adjustment tap changers on a loop feed padmount transformer

What are transformer voltage adjustment taps?

Transformer voltage adjustment taps are physical connection points on transformer windings which allow incremental adjustments to the input voltage rating of the transformer to maintain the correct secondary voltage when the primary is higher or lower than expected.

Transformer coil with voltage adjustment taps

In a perfect world, an electrical transformer would not need taps, but sometimes the primary power fed into a transformer from the utility is higher or lower than what is intended. This can occur for a variety of reasons such as voltage drop across a long run of distribution power lines or excessive power consumption from one or more consumers in a network–creating a dip in the supply voltage for other end users downwind.

Since voltage fluctuation issues are typically isolated to specific portions of a larger distribution network, it is not practical to make adjustments to the whole network, but only at the problem points. Transformer taps solve this issue by providing utilities and end users the ability to control the input voltage rating of the transformer at the point of power consumption. This makes it easier to regulate the voltage supplied to all consumers across an entire distribution network, and prevents the chance for further voltage distortion by making the adjustment directly at the point of consumption.

What happens if you feed a transformer with a higher or lower voltage?

If the primary voltage feeding a transformer is slightly higher or lower than the transformer’s rated input, the output voltage will follow suit also being higher or lower as well. Slight increases or decreases in the output voltage coming from a transformer may be acceptable, but a large enough discrepancy can cause significant problems for any load–especially those with very sensitive equipment designed with little to no tolerance. Voltage adjustment taps remedy this issue by providing end users the means to adjust the voltage supplied to their equipment.

Transformer taps are adjusted at the coils with either a tap changer device or by manually moving a jumper lead cable between tapping positions. Below, we will explain how each tapping configuration works with examples in specific transformer designs.

Transformer Tap Changer

A transformer tap changer is a rotary style device that allows adjustments to the number of turns in the primary coil to create the correct secondary voltage.

Transformer tap changer before installation

On most standard utility grade padmount transformers, tap changers are located in the primary HV cabinet of the transformer. These devices are designed to be operated only when the transformer is de-energized and typically come with some type of locking capability to prevent accidental operation while the unit is under load. For this reason, Maddox furnishes its off-load tap changing devices with pad-locking provisions. Since tap changers require off-load operation, de-energized tap changers (DETCs) are not ideal for applications with frequent voltage fluctuations. For such instances, on-load tap changing devices or voltage regulators are typically used.

Voltage adjustments with tap changer

The tap changer settings are outlined on each transformer nameplate and show the number of taps and the voltage adjustments above or below the nominal nameplate rating. The available taps will be listed from the top down with the highest voltage configuration in tap position 1 (or A), the one below it being position 2 (B), then 3 (C), and so on. You will notice that for a typical medium voltage transformer, like the one represented above, the taps are on the higher voltage winding.

Tap changer connected with leads to the coil

Internal view of a padmount tap changer connected with leads to the coil.

Transformer tap leads are installed during the winding process at the factory. Once the transformer core and coil assembly is installed in the tank, the tap leads are then routed to the correct position on the tap changing device.

Graphic of tap changer connected with leads to coil

How do voltage adjustment tap changers work?

The proper use of tap changers on a transformer requires selecting the tap setting which is closest to the actual value of the incoming voltage supply. This will ensure the output voltage feeding the load remains at the desired nameplate value.

Let’s assume the incoming voltage at the transformer is too low. To fix this, the tap setting is adjusted by rotating the tap changer in the HV cabinet to a lower voltage setting. When the tap changer is moved to a lower voltage position below the nominal tap, small sections of the primary winding are disengaged, which alters the ratio of primary to secondary windings. By lowering the amount of turns on the primary coil and leaving the secondary coil the same, the resulting new ratio yields a higher voltage on the secondary side, bringing the sagging voltage level back up to where it should be.

A transformer tap changer engaging and disengaging different sections of coil

Figure 1 below shows how the rotation of the tap changer engages and disengages different sections of the primary coil. Most larger transformer coils have the tap adjustments at the center of the coils (Figure 1), but we’ve also included Figure 2 with the taps located at the end of the coil (more common on smaller transformers) to help simplify the concept of tap adjustment.

Graphic showing taps located at end of coil

With the taps at the end of the coil like in Figure 2, it is easier to see how tapping above or below the nominal (100%) rating will add or remove sections of the coil on the primary winding.

Jumper Lead Cable Voltage Adjustment

For dry type transformers, taps can be adjusted manually at the windings by moving a jumper lead cable (or tap jumper) between the tap connections. Since the tap connections are physically accessible in dry type transformers, there is no need to run tap leads to an external tap changing device. Taps can only be adjusted when the transformer is off.

Voltage taps with jumper lead on a dry type transformer coil

A transformer core and coil with jumper lead cables

Tap Configurations

5 Position

Most distribution transformers are configured with 5 tap positions–two taps at 2.5% increments above and below the nominal nameplate voltage rating. These taps are called full capacity taps because they are able to be used at the transformer’s full kVA rating. The most common configuration is notated as FCAN (full capacity above nominal) and FCBN (full capacity below nominal).

While the most common 5 position tap configuration is to have two taps above and below the nominal rating (allowing for instances where the supply is either higher or lower than the transformer’s nominal rating), some transformers will configure the nominal rating in the top tap (or tap position one) and have the remaining adjustment taps below–allowing more flexibility for areas where the incoming utility voltage may be significantly lower.

7 Position

Some tap changing configurations include seven taps instead of five. Many standard designs for smaller 600 volt dry-type transformers include two taps above and four taps below the nominal rating–each 2.5% apart from one another. In total this allows for a supply voltage which is within 5% above and 10% below the nominal voltage rating.

Other tap configurations are also available which provide multiple common voltage connections rather than smaller incremental adjustments. For example: 14400, 13800, 13200, 12870, 12470, 12000, 11400.

A 5 position and 7 position tap changer

Conclusion

Transformer voltage adjustment taps and tap changers allow you more flexibility to create the desired output voltage you need to power on your project. Maddox stocks various sizes and configurations of transformers with taps. If you are looking for a transformer with specific voltage adjustment taps or want to know more about this topic, please fill out the form below.

Maddox padmount transformer loaded on truck

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