What is transformer excitation current?

Take any type of transformer. It has a primary and secondary sides, and a core of course. When a normal voltage is applied to a transformer's terminals with the secondary circuit open, a small current flows in the primary winding. This current, known as the transformer excitation current, flows continuously during the transformer's operation. Why it´s so important? The excitation current is necessary to maintain a magnetic field within the core and is largely independent of the secondary load.

It consists of 2 main parts - Core loss current component and Magnetizing current component. Let´s have a better look at them.

Core Loss Current

Core loss current represents the resistive losses in the transformer's core and is in phase with the applied voltage. It determines the no-load losses of the transformer, which include iron losses, minor dielectric losses, and copper losses from the excitation current. Of these, the iron losses due to eddy currents are the most significant. These losses are influenced by the frequency, maximum flux density, and the magnetic circuit's characteristics. Typically, core loss values are provided by the transformer manufacturer upon delivery.

Magnetizing Current

The magnetizing current lags the applied voltage by 90 degrees and its magnitude depends on factors such as the number of turns in the primary winding, the transformer's saturation curve, and the maximum flux density for which the transformer was designed. This current is purely reactive and does not directly contribute to no-load losses. However, reactive magnetizing power (VARs) is necessary for the transformer's operation, and this current must be supplied by the power source.

For simplicity, their vectors can be graphed like this

So the total excitation current can be calculated by the formula

Core loss current levels are usually around 1% of the full load current. For magnetizing current situation varies in the 0.25%-5% range of full load curren, but can be as high as 10% in some types of niche application transformers.

Any issue with the core will increase the reluctance of the magnetic circuit, leading to a higher primary current. A short between coil turns will cause additional current flow in the transformer, resulting in a higher-than-expected excitation current. If the excitation current is higher than expected or published values, it indicates potential problems with the transformer that may require further inspection.

That's why any routine diagnostic tests and acceptance current transformer testing, as well as tests following extreme physical stress (for example failure on the line, CB operation, surge operation), should include measurements of excitation current and no-load loss.

Magnetization curve of Current Transformers:

Now, that we know about magnetization and how it reflects the health and quality of transformer operation, it would be great to locate a safe and reliable zone. A range where the ratio is stable, the core is not saturated and output is undisputed.

And it can easily be evaluated on the graph below.

Transformer saturation with ankle and knee point

The curve is typically displayed with secondary voltage on the y-axis and the excitation current, measured in the secondary winding, on the x-axis. By examining this curve, one can readily determine the magnitude of the exciting current necessary to generate a specified secondary voltage in a current transformer (CT).

The magnetization or excitation curve is divided into four distinct regions:

1. From the origin to the ankle point

2. From the ankle point to the knee point

3. The knee point area

4. The saturation region

Knee Point Voltage of Current Transformers:

ANSI/IEEE describes it as the intersection of the curve with a 45-degree tangent line

IEC defines the knee point as the intersection of straight lines extended from non saturated and saturated parts of the excitation curve.

If you compare both - IEC knee is higher than ANSI - ANSI more conservative.

In general, the simple rule to describe and understand it from the operational point of view is this:

the point where 10% increase in applied voltage will cause 50% increase in excitation current

With this evaluation rule, you can always measure the primary and secondary values of the CT you are testing and understand if it´s saturated or not.

Modern equipment even has automated testing procedures that will locate the saturation point and demagnetize the transformer after the test.

If you wonder how to find the knee point of transformer units like Raptor can provide you with a complete report with numerical and graphical values to evaluate and release the assessment

CT saturation curve with knee point detected

So now you know what, in fact, is the excitation current, how to understand if your transformer is saturated, and how to find the Knee point.

Any questions? Let´s discuss your procedures and experience in the comments!