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Don't Be Left Short by Current Transformers

Oct 1, 1999 12:00 PM, By C. Sankaran, P.E.

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Don’t get fooled by current transformer ratings and applications.

Although a seemingly simplistic device, current transformers (CTs) are quite interesting. First let’s discuss their basic design.

A CT is an inductor placed around a current-carrying device. As the device carries more current, the CT produces more current. However, due to the nature of the CT, the current leaving the CT is usually within a 0A to 5A range. That’s why CT ratings consist of a ratio. For example, a 400-to-5 ratio means the CT will output 5A with a 400A flow in the measured primary conductor.

CTs perform two basic functions:

• They transform line current to values suitable for standard instruments (normally operating on a maximum of 5A); and

• They isolate instruments and meters from line voltage. To make this protection complete for both instruments and operators, you must ground the secondary circuit. (See IEEE 52, Application Guide for Grounding of Instrument Transformer Secondary Circuits and Cases.)

CT types. There are three types of CTs. Here are the details:

Window. This CT has a secondary winding completely insulated and permanently assembled on the core, but has no primary winding. You commonly would see this type of construction on 600V class current transformers. You might hear this CT design referred to as a “donut,” as it has that appearance. The monitored conductor actually passes through the middle.

Bar. This CT is the same as the window type except for the insertion of a primary bar into the window opening. You can permanently fix it into position or remove it. This CT usually bolts into the circuit.

Wound (Wound-primary). This CT has the primary and secondary windings insulated and permanently assembled on the core. The primary is usually a multi-turn winding. There is no window for the primary conductor to pass through. You insert this type of CT into series with the circuit you wish to monitor. This type of design is most prevalent at higher voltages (above 600V).

CT accuracy. The type of installation dictates the accuracy rating of the CT you’re using. The accuracy of a CT is the percent difference between the actual secondary current and the rated secondary current. The transformation error (primary-to-secondary values) consists of a shift in the ratios and a shift in the phase angle.

The most prevalent uses of CT involve metering and relaying, yielding the same accuracy classes. Metering applications limit the error in the ratio and phase angle, whereas the relaying applications limit only the error in the ratio.

So what does this all mean? Referring to the Figure (on page 62 of original article), you can see the vertical axis contains a ratio correction factor (RCF) and the horizontal axis contains the phase angle (in minutes). For a CT to classify as a 0.3 % metering accuracy class, its measured RCF and phase angle at 100% rated current must fall within this parallelogram. In addition, at 10% rated current, the RCF and phase angle must fall within this parallelogram.

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