Instrument Transformer No-Load Current Testing

Instrument transformer no-load current testing, also referred to as excitation current testing, is a core diagnostic procedure used to verify core integrity, detect inter-turn short circuits, and confirm the magnetization characteristics of current transformers and voltage transformers. Conducted in accordance with international standards such as the IEC 61869 series, IEEE C57.13, and the GB 20840 series, this test is an essential element of factory routine testing, commissioning, and preventive maintenance programs across power systems from low voltage up to extra-high voltage.

Technical Overview

No-load current, also known as excitation current, is the current drawn by the magnetizing branch of an instrument transformer when rated voltage at rated frequency is applied to one winding while all other windings are open-circuited. It is composed of a reactive magnetizing component that establishes the core flux and a small active component that supplies the iron losses (hysteresis and eddy-current losses). Because instrument transformers are designed to operate with their cores in the linear region of the magnetization curve, the no-load current is normally very small, typically ranging from a few milliamperes for medium-voltage units to tens of milliamperes for large extra-high-voltage transformers.

For voltage transformers (VTs), the no-load current is measured by energizing the winding at rated voltage with all secondary windings open. For current transformers (CTs), the equivalent measurement is the excitation (magnetization) curve test, in which a variable AC voltage is applied to the secondary winding while the primary winding is open-circuited, allowing the full saturation characteristic of the core to be recorded. In both cases, the magnitude and shape of the exciting current are extremely sensitive indicators of internal condition.

A no-load current test can effectively reveal the following defects:

• Inter-turn short circuits within the primary or secondary winding
• Damaged lamination insulation or degraded magnetic core material
• Poorly assembled core joints, air gaps, or mechanical core damage
• Incorrect number of turns or winding configuration errors
• Core saturation, over-fluxing history, or excessive remanent flux after system faults or DC testing

The test is governed by IEC 61869-1, IEC 61869-2, IEC 61869-3, and IEC 61869-5 (which replaced the former IEC 60044 series), IEEE C57.13, and the Chinese national standards GB 20840 series (formerly GB 1207 and GB 1208). It is performed by manufacturers as a routine test and by accredited third-party laboratories for independent verification.

Testing Samples

Current Transformers (CTs)

Measuring CTs of accuracy classes 0.2, 0.5, 0.2S, and 0.5S; protection CTs of classes 5P, 10P, PR, PX, and transient classes TPS, TPX, TPY, and TPZ. Constructions include wound-type, bar-type, window (toroidal) type, and bushing CTs, covering voltage levels from low voltage up to 1100 kV. Each magnetic core of a multi-core CT is tested individually.

Inductive Voltage Transformers (VTs / PTs)

Single-phase and three-phase voltage transformers, both earthed-neutral and unearthed-neutral designs, with single or multiple secondary windings and, where applicable, residual voltage (open-delta) windings used for earth-fault protection.

Capacitor Voltage Transformers (CVTs)

For CVTs, the electromagnetic unit consisting of the intermediate voltage transformer and compensation reactor is subjected to excitation characteristic verification. These results also support ferroresonance suppression assessment and transient response evaluation.

Combined Units and GIS-Embedded Transformers

Combined CT/VT units, instrument transformers integrated within gas-insulated switchgear (GIS), and low-power instrument transformers (LPIT) where magnetization verification is applicable under IEC 61869-6 and related parts.

Testing Items

Depending on the product type and the applicable standard, a complete no-load test program typically includes the following items:

• No-load (excitation) current at rated voltage and rated frequency, measured as a true RMS value
• No-load loss (iron loss) in watts, determined with a power analyzer or precision wattmeter
• Excitation (magnetization) curve: exciting current versus applied secondary voltage, recorded and plotted on linear or log-log scales
• Knee-point voltage and knee-point exciting current, including the limiting secondary excitation voltage for PX-class cores per IEC definitions
• Exciting current readings at specified check points, for example at defined percentages of the knee-point voltage, for trend and phase-to-phase comparison
• Remanence-related parameters, such as the remanent flux factor, for TPY and TPZ class cores
• Waveform quality and harmonic content of the exciting current as an indicator of core non-linearity
• Verification of complete demagnetization of the core prior to measurement, particularly after DC tests such as winding resistance measurement

Testing Methods

No-Load Test for Voltage Transformers

A regulated, low-distortion AC voltage source is connected to the primary (or secondary) winding with all other windings open-circuited. The voltage is raised gradually from zero to the rated value while the current and input power are recorded; measurements at 80% and 110% of rated voltage may also be taken to characterize the magnetization behavior. The waveform of the applied voltage must remain substantially sinusoidal, with harmonic distortion and form-factor deviation kept within the limits specified by the relevant standards, since core flux waveform distortion directly affects the measured current. After the test, the voltage is reduced smoothly to zero to demagnetize the core.

Excitation Curve Test for Current Transformers

The primary winding is open-circuited and a variable AC voltage is applied to the secondary winding. The voltage is increased stepwise while the corresponding exciting current is recorded, continuing until the knee point of saturation is reached but without exceeding the thermal withstand capability of the winding. The knee point is determined using the IEC criterion, defined as the point at which a 10% increase in secondary exciting voltage produces a 50% increase in exciting current, or alternatively by the tangent method defined in IEEE standards. The measured curve is compared against the manufacturer's declared characteristic, the factory test record, and the readings of similar cores on the same or adjacent phases.

Instrumentation and Accuracy

Typical equipment includes a stepless variable voltage source or step-up transformer capable of delivering the required saturation voltage (which may reach several kilovolts for high-ratio protection cores), a true RMS ammeter of class 0.2 or better, RMS and mean-reading voltmeters for form-factor verification, and a power analyzer for loss measurement. Ambient temperature and the complete test circuit should be recorded to ensure reproducibility and valid trend comparison.

Safety Precautions

Although the excitation test deliberately operates the CT with its primary open-circuited, this is only acceptable because the winding is supplied from a controlled, energy-limited test source rather than a live power system. The primary terminals must be isolated, guarded, and treated as potentially hazardous due to induced voltages during core saturation. All equipment should be properly grounded, windings must be discharged after testing, and the core must be demagnetized if DC tests have previously been performed. Only qualified personnel following established electrical safety procedures should carry out the test.

Applications

No-load current testing is applied throughout the entire life cycle of an instrument transformer, from product development to in-service diagnostics:

• Factory routine and type testing of new instrument transformers in accordance with the IEC 61869 and GB 20840 series
• Acceptance testing and pre-energization commissioning verification in substations, power plants, and industrial facilities
• Preventive maintenance and condition-based monitoring programs, using baseline comparison to detect gradual core degradation
• Post-event diagnostics following through-fault currents, ferroresonance incidents, lightning or switching overvoltages, and suspected insulation failures
• Protection engineering support, verifying CT saturation characteristics required for relay coordination and differential protection stability studies
• Independent product certification and conformity verification at accredited third-party laboratories

FAQ

What is the difference between no-load current and excitation current?

The two terms describe the same physical quantity: the current flowing in the magnetizing branch of the transformer. For voltage transformers the term no-load current is customary because the measurement is made with the secondary unloaded, while for current transformers the same characteristic, driven from the secondary side, is called the exciting current.

Why is an abnormally high no-load current a warning sign?

A significant increase in exciting current, or a shifted saturation curve, indicates problems such as shorted turns, damaged core lamination insulation, poor core joints, or an incorrect number of turns. Left undetected, these defects cause increased ratio and phase errors, inaccurate metering and billing, incorrect protection relay operation, and potential overheating of the transformer.

What acceptance criteria are typically applied?

There is no universal limit fixed by the standards; the acceptance values are defined by the manufacturer's design data and the purchase specification. In practice, the measured no-load current should agree with the type-test value within the manufacturer's stated tolerance, commonly on the order of 10 to 30 percent, and the excitation curve must fall within the specified characteristic band, with the knee-point voltage no lower than the declared value.

Is it dangerous to test a current transformer with an open-circuited winding?

In normal service an open-circuited CT secondary is extremely dangerous because the fully saturated core allows dangerously high voltages to develop. During the excitation test, however, the secondary is excited from a controlled, energy-limited AC source, and the primary circuit is isolated and guarded, so the test is safe when performed correctly by trained personnel.

Why is demagnetization necessary before the measurement?

DC tests such as winding resistance or insulation resistance measurement leave remanent flux in the magnetic core, which distorts the excitation characteristic and can lead to false conclusions. The core must be demagnetized, typically by gradually raising and then smoothly lowering an AC excitation voltage, before recording the magnetization curve.

How often should the test be repeated?

The test is performed on every unit as a factory routine test, repeated during commissioning to establish a site baseline, and then carried out periodically according to the maintenance cycle, commonly every three to six years, or immediately after fault events, ferroresonance occurrences, or other abnormal operating conditions.