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Tap Changers: To Tap or Not To Tap?

Jan 1, 1999 12:00 PM, By Jerry R. Borland, P.E., Technical Editor

In the winter, your facility experiences an extremely high amount of relamping. In the summer, your process line motors take a long time to reach synchronous speed. What's the problem: High voltage, low voltage, or both?

Maybe you've experienced the above symptoms. If so, you might be wondering what to do or who to call. The answer may be in the distribution transformer feeding your facility.

Most of these transformers have de-energized tap changers (DETCs), sometimes called no-load tap changers (NLTCs). Their purpose is to help match the incoming primary voltage to your equipment needs. If the incoming voltage increases or decreases, and remains at this level for an extended period (weeks or months), consider changing the tap setting of your DETC. (Never compensate for a short-term fluctuation in voltage by using your transformer's DETC.)

What's a DETC? In the purest sense, a DETC works by adding or subtracting the number of transformer windings. Usually, the device affects the number of windings on the primary side of the transformer.

Most DETC designs center around a five step switch. The most common practice is to place the nominal system voltage as the middle step. The remaining four steps are split into two groups of taps that will step up or down the voltage usually by 2.5% each step. Most transformers use either the letters A, B, C, D, and E or the numbers 1, 2, 3, 4, and 5 to label the switch positions. The following chart is an example for a 13,200/7620V, 480/277V wye transformer with two 2.5% taps above and below the nominal voltage:

Tap A or 1 — 13,860 V — (+5%)

Tap B or 2 — 13,530 V — (+2.5%)

Tap C or 3 — 13,200 V

Tap D or 4 — 12,870 V — (-2.5%)

Tap E or 5 — 12,540 V — (-5%)

You'll most likely see this arrangement because ANSI (American National Standards Institute) recommends the utility supply a voltage that will not vary by more than 5% from the nominal system voltage.

Let's say repeated measurement of the incoming voltage to your facility reveals a value different from your expected nominal voltage. Here's what you should do.

• First, determine the value of the transformer primary voltage.

• If the value is greater than nominal, move your tap setting up to the next closest setting value.

• If the value is less than nominal, move your tap setting down to the next closest setting value.

If your facility is close to the utility substation, you may need to set your DETC at its highest setting. If it's a long distance from the same substation, you may need to set it at the lowest setting. Nevertheless, always refer to your transformer's nameplate for tap setting values.

A DETC installed on the primary side of your transformer operates in a manner exactly opposite of what you'd expect out of your secondary side voltage. If the incoming voltage is above system nominal, your secondary side voltage will be greater than nominal if your transformer is set at the middle tap. You may need to tap your transformer up one step to match the incoming voltage depending on the incoming voltage value. This will cause the output voltage at the secondary side to approach nominal.

This works the opposite if the incoming voltage is below nominal. The tap must correspond as close as possible to incoming voltage for the secondary voltage to approach nominal.

How do you operate a DETC? Operation of a DETC must happen only under a de-energized condition. This does not mean reducing the load on the transformer. If you operate the DETC while energized, an internal arc can occur that will affect the internal components of the transformer and create a safety hazard for personnel.

If your transformer has a DETC, it requires maintenance. A scheduled shutdown is a good time to check its operation. An unused DETC may become stuck in the current position and impossible to move. Repositioning is the easiest way to check its operation. Remember: Operate these devices in a de-energized state so no current or voltage is present within the transformer. Separate it from both the primary and secondary side sources before operating. Refer to your maintenance guide for exact instructions concerning the maintenance of your DETC.

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© 2012 Penton Business Media, Inc.

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