The Basics of Instrument Transformers — Part 1

Understanding the potential transformer (PT), a conventional, single-phase transformer that has its primary and secondary windings on a common core

With any switchboard or switchgear lineup, you can order ammeters, voltmeters, frequency meters, etc. These meters come with factory-installed instrument transformers, which are needed to convert the measured parameter to that suitable for the meter itself. Knowing what these transformers are and how they work will help you when specifying and/or installing them in existing equipment.

In this first part of this multi-part series, we'll talk about the potential transformer, or PT, as it's more commonly known.

What's a PT?

Basically, a PT is a conventional, single-phase transformer that has its primary and secondary windings on a common core. The most popular type of PT provides 120V at the secondary when nameplate-rated voltage is applied to its primary. It's important to note that a PT is designed so that its secondary voltage has a fixed relationship, or ratio, with its primary voltage.

Table 1 (click here to see Table 1) lists commercially available Pts for application with 100% of rated primary voltage across the primary winding when connected line-to-line or line-to-ground. Table 2 (click here to see Table 2) lists commercially available Pts for line-to-line service that may be applied line-to-ground or line-to-neutral at a winding equal to the primary voltage rating divided by √3. Most new installations have a single voltmeter connected to a selector switch, which is used to select the specific phase-to-phase measurement (e.g. A-B, B-C, and A-C).

Pts are capable of continuous and accurate operation when the voltage applied across the primary is within ±10% of rated primary voltage. Pts are normally identified for polarity by marking the primary terminal H1 and the secondary terminal X1. These points may also be identified by distinctive color markings.

According to the IEEE Buff Book (ANSI/IEEE Standard 242), standard accuracy classifications of Pts range from 0.3 to 1.2, representing the percent ratio corrections to obtain a true ratio. These accuracies are high enough so that any standard PT will be adequate for protective relaying purposes as long as it is applied within its open-air thermal and voltage limits. Table 3 (click here to see Table 3) lists standard burdens for Pts with a secondary voltage of 120V.

In normal practice, you should not exceed the thermal burden limits specified by the PT manufacturer, if you want to avoid questionable PT accuracy and reduced life. Per the Buff Book, thermal burdens are given in volt-amperes and may be arithmetically calculated by adding the volt-ampere burdens of the devices connected to the PT. If the sum is within the rated thermal burden, the PT will perform satisfactorily over the range of voltages from 0% to 110% of nameplate voltage.

Choosing a PT

Suppose you've been asked to install a voltmeter and selector switch on an existing medium-voltage switch to measure current and voltage on a 4,160V, 3-phase power line feeding a large industrial motor. You would take the following steps in determining what's required:

  1. Calculate the PT ratio

    We know the phase-to-phase voltage is 4,160V. Most voltmeters are self contained and come with scales ranging from 150V to 750V. When you need to measure higher voltages, the PT reduces the measured voltage to that suitable for the voltmeter. The scale is then proportioned to read the actual higher voltage being measured. Let's say you decide on a voltmeter having a 150V full-scale indication. You can determine the PT ratio by dividing the phase-to-phase voltage, which is 4,160V, by the voltmeter full-scale range, which is 150V. This simple calculation results in a value of 27.7, which you would interpret as an approximate 28:1 ratio. The next higher standard PT ratio (see Table 2) is 40:1, and you would choose this ratio PT.

  2. Verify the selection

    To make sure you've chosen the PT with the correct ratio, you should calculate its secondary voltage. This is done by dividing the phase-to-phase voltage by the ratio (4,160V ÷ 40), which yields 104V. This is well within the limits of the chosen voltmeter.

  3. Connect the PT and voltmeter

    You would connect the PT and voltmeter as shown in the Figure (click here to see Figure) above. Where balanced system load and, therefore, balance voltage are anticipated, you would normally connect the Pts in open delta. Where line-to-neutral loading is expected, you would more often connect them wye-wye, particularly where metering is required. If you have an ungrounded power system, you can use Pts for ground detection by connecting them wye-broken delta. Note the PT's primary fusing, which protects the power system by de-energizing a failed PT. Usually, a current-limiting fuse (or its equivalent) is used in the primary connection to an ungrounded power system leg or conductor.

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