What is a no-load current?

This current may seem simple, but in reality it is a powerful indicator of:

• the magnetic health of the core
• possible winding defects
• asymmetry between phases
• saturation behaviour
• manufacturing issues

The no-load current shows exactly how the transformer behaves magnetically when it is “at rest”. This is precisely why the no-load current measurement (no-load test, magnetization measurement, magnetization analysis, or V–I magnetization curve measurement) is so valuable in maintenance, overhaul, and quality control.

Why is the no-load current so important?

A healthy transformer exhibits predictable magnetic behaviour. The no-load current is mainly determined by:

• the material and structure of the core
• the winding ratio and geometry
• manufacturing quality
• any damage or short circuits

An increased no-load current may indicate:

• core saturation
• loose or damaged laminations
• shorted turns (a turn-to-turn short often gives a clearly higher no-load current in that phase, while the winding resistance changes only slightly)
• mechanical deformation due to transport
• errors in the connection scheme

The insulation itself is assessed with an insulation resistance or tan δ measurement. An insulation fault only shows up in the no-load current once it leads to a turn-to-turn short.

Because the secondary side is open, deviations are often quickly reflected in the core and windings. However, note that residual magnetism, tap position, connection method, and supply quality can also influence the measured values.

The phase angle between voltage and no-load current provides insight into the relationship between:

1. Inductive component – responsible for magnetizing the core – lags the voltage by nearly 90°
2. Active component – related to core losses (hysteresis + eddy currents) – in phase with the voltage

The no-load phase angle is generally strongly inductive (large phase angle, low cos φ). As core losses increase or (incipient) saturation occurs, the phase angle becomes smaller (the phase angle here refers to the lag angle between voltage and current; a higher active component increases cos φ and thus reduces the lag angle).

A smaller phase angle indicates:

• higher core losses
• saturation
• damaged core laminations

For voltage transformers, the no-load current is highly sensitive and crucial for the accuracy class.

Deviations in no-load current directly lead to:

• phase errors (with impact on protection relays)
• deviating voltage ratios
• measurement errors within classes 0.2, 0.5, 1.0

Therefore, a no-load current measurement is essential for VT/PTs during:

• acceptance testing
• periodic maintenance
• fault analysis

A small issue in the core can quickly become a major problem in the measurement chain.

In large transformers, the comparison between phases is particularly important.

What should you look for?

• No-load current per phase
• Phase angle per phase
• Symmetry between phases
• V–I magnetization curve

A deviation in just one phase may indicate:

• core damage due to short-circuit currents
• manufacturing deviations
• transport-related deformation

Note: inherent phase asymmetry: in a three-phase transformer with core-type construction (E-I core), the centre limb has a shorter magnetic path than the outer limbs. The no-load current of the centre phase is therefore structurally lower. This is a design characteristic, not a defect. Always compare phase against phase over time (trending) rather than relying solely on absolute values between phases.

For overhaul companies and grid operators, this measurement shows problems at an early stage.

How do you measure no-load current and phase angle?

With a system such as the HighTest TRAN-203, you can:

• measure no-load (excitation) current per phase at a test voltage of up to 250 V AC
• determine the phase angle difference between primary and secondary
• check the magnetic balance of the core (asymmetry)
• detect manufacturing defects without load

Normative framework

The no-load test is a routine test as specified in IEC 60076-1 clause 11.5 (power transformers). For voltage transformers (VT/PT), the IEC 61869 series applies (successor to IEC 60044-2). IEEE C57.12.90 covers the equivalent test procedure for the North American context. Acceptance criteria for no-load current and no-load losses are established in the factory test report in accordance with these standards; deviations are evaluated against design values and previous measurement series.

Compare no-load current only at the same tap position, the same nominal frequency, and with a supply voltage that is as sinusoidal as possible. Deviations in waveform or frequency significantly affect the measured no-load current and no-load losses.

Interpretation: what does it mean in practice?

Healthy transformer

• low no-load current
• large inductive phase angle
• three nearly identical phases
• smooth magnetization curve

Deviating transformer

• increasing no-load current
• decreasing phase angle
• clear phase imbalance
• kinks or jumps in the curve

These patterns indicate early stages of:

• winding faults
• core problems
• mechanical damage

Conclusion

A no-load current measurement (magnetization measurement / no-load test / magnetization analysis)
provides a lot of information, but is often underestimated as a diagnostic measurement for voltage and power transformers.

The combination of no-load current + phase angle provides a good picture of:

• core quality
• winding condition
• symmetry
• saturation
• mechanical integrity

For engineers, overhaul companies, and grid operators, this forms an essential foundation for a reliable power supply.

The TRAN-203 Series transformer analyser by HighTest, available for rent from IONIO, measures the no-load (excitation) current per phase during the ratio test, together with ratio error, polarity and phase angle difference. The ratio test voltage is up to 250 V AC; winding resistance is measured with up to 20 A DC. Contact IONIO for advice on your measurement application.