Every transformer, regardless of type, comprises primary and secondary windings and a core. When a normal voltage is applied to the transformer’s primary 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. It is vital because it maintains a magnetic field within the core and is largely independent of the secondary load.

The excitation current consists of two main components:

1. Core Loss Current Component
2. Magnetizing Current Component

Core Loss Current

The 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: Due to eddy currents, these are the most significant.
• Minor Dielectric Losses
• Copper Losses: From the excitation current.

These losses are influenced by factors like frequency, maximum flux density, and the magnetic circuit’s characteristics. Typically, core loss values are provided by the transformer manufacturer upon delivery, which aids in substation maintenance planning.

Magnetizing Current

The magnetizing current lags the applied voltage by 90 degrees, and its magnitude depends on:

• The number of turns in the primary winding.
• The transformer’s saturation curve.
• 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. Monitoring magnetizing current is a critical aspect of high voltage testing during substation maintenance.

 

For simplicity, their vectors can be graphed like this

The total excitation current can be calculated using the core loss and magnetizing current components. Typically, core loss current levels are around 1% of the full-load current. The magnetizing current varies in the 0.25%–5% range of full-load current but can be as high as 10% in some niche application transformers.

Any issues with the core, such as increased reluctance in the magnetic circuit, will lead 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 exceeds expected or published values, it indicates potential problems requiring further inspection—a critical part of substation maintenance.

Importance in Substation Maintenance

Routine diagnostic tests and acceptance tests for current transformers should include measurements of excitation current and no-load loss. This is especially important after extreme physical stress, such as a failure on the line, circuit breaker operation, or surge events. Utilizing a primary injection test set and conducting high voltage testing can help identify these issues early.

Magnetization curve of Current Transformers

Understanding the magnetization curve helps assess the transformer’s health and operational quality. It identifies a safe and reliable zone where the ratio is stable, the core is not saturated, and the output is undisputed

And it can easily be evaluated on the graph below.

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 Definition: The intersection of the curve with a 45-degree tangent line.
• IEC Definition: The intersection of straight lines extended from non-saturated and saturated parts of the excitation curve.

In practical terms, the knee point is where a 10% increase in applied voltage causes a 50% increase in excitation current. This evaluation rule allows technicians to measure primary and secondary values using tools like a relay tester to determine if the transformer is saturated—a key step in substation maintenance.

Conclusion

Understanding transformer excitation current is vital for maintaining the health and efficiency of power systems. By incorporating regular circuit breaker testing, utilizing advanced primary injection test sets, performing high voltage testing, and employing precise relay testers, you can ensure your transformers operate reliably and avoid costly downtime.

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