Transformer Lightning Impulse Test: Standards, Procedures, and Acceptance Criteria

The transformer lightning impulse test is a critical dielectric verification procedure that confirms whether a transformer's internal and external insulation can withstand the steep-fronted transient overvoltages produced by lightning strikes. Governed by IEC 60076-3, GB/T 1094.3, and IEEE C57.12.90, this test applies a standard 1.2/50 µs impulse wave to evaluate the transformer's basic lightning impulse insulation level (BIL). This article explains the test principles, applicable standards, sample types, test items, procedures, failure detection methods, and typical applications in detail.

Technical Overview

A lightning impulse test is a high-voltage dielectric test performed on transformers to verify that their insulation systems can withstand transient overvoltages caused by direct or indirect lightning strikes. When lightning strikes a transmission line, a traveling wave with a steep front propagates along the line toward substations and connected equipment. The voltage stress imposed by such a wave rises to its peak within about one microsecond, which is far more severe on winding insulation than a power-frequency voltage of the same amplitude. The lightning impulse test reproduces this stress in a controlled laboratory environment by applying a standard 1.2/50 µs impulse waveform, where the voltage reaches its peak in approximately 1.2 microseconds and decays to half-value in 50 microseconds.

The withstand capability verified by this test defines the transformer's basic lightning impulse insulation level (BIL), which is a fundamental parameter in system insulation coordination. The test is primarily governed by the following standards:

• IEC 60076-3 – Insulation levels, dielectric tests, and external clearances in air for power transformers
• IEC 60060-1 and IEC 60060-2 – High-voltage test techniques: definitions, requirements, and measuring systems
• IEC 60071 – Insulation coordination principles
• GB/T 1094.3 – Chinese national standard equivalent to IEC 60076-3
• IEEE C57.12.90 and IEEE C57.98 – Test procedures and impulse test guides for liquid-immersed transformers
• IEC 60076-11 – Additional requirements for dry-type transformers

According to IEC 60076-3, the lightning impulse test is a routine test for windings with a highest voltage for equipment (Um) of 72.5 kV and above, while for lower voltage classes it is generally performed as a type test or special test when specified by the purchaser. For the highest voltage classes, typically Um of 245 kV and above, a switching impulse test is additionally required to verify withstand capability against slower-fronted switching overvoltages.

Testing Samples

Oil-Immersed Power Transformers

Generator step-up transformers, interconnection transformers, and converter transformers with ratings ranging from several MVA to more than 1000 MVA and voltage classes from 110 kV up to ultra-high voltage levels of 800 kV and 1000 kV. These units are tested fully assembled, filled with treated insulating oil, and equipped with their service bushings.

Distribution Transformers

Pole-mounted, pad-mounted, and compact substation transformers typically rated up to 36 kV. Although the lightning impulse test may not be routine at these voltage levels, it is widely specified in purchase contracts, especially for units installed in regions with high keraunic activity or on exposed overhead networks.

Dry-Type and Cast-Resin Transformers

Transformers with air or cast-epoxy insulation systems used in commercial buildings, infrastructure, and industrial plants. Because their insulation behaves differently from oil-paper systems under steep impulses, enhanced impulse levels are frequently specified, and testing is conducted per IEC 60076-11 together with IEC 60076-3.

Instrument Transformers, Reactors, and Related Apparatus

Current transformers, voltage transformers, shunt reactors, arc furnace transformers, traction transformers, and transformer bushings may also be subjected to lightning impulse tests in accordance with their respective product standards, such as the IEC 61869 series for instrument transformers and IEC 60137 for bushings.

Testing Items

The lightning impulse test program for a transformer typically includes the following items:

• Full-wave lightning impulse withstand test (LI): application of the standard 1.2/50 µs impulse at the rated BIL for the relevant voltage class
• Chopped-wave lightning impulse test (LIC): impulse chopped on the tail of the wave, typically between 2 µs and 6 µs after the virtual origin, at a level approximately 10 % above the full-wave withstand voltage, imposed to stress the insulation near the line terminals more severely
• Reduced full-wave calibration impulses: applied at 50 % to 75 % of the full test voltage to calibrate the measuring circuit and establish reference oscillograms
• Waveform parameter verification: confirmation that the front time (1.2 µs, tolerance ±30 %), time to half-value (50 µs, tolerance ±20 %), peak value tolerance, and overshoot limits conform to IEC 60060-1
• Failure detection records: simultaneous measurement of the applied voltage waveform, the neutral (earth) current, and where applicable the capacitive transfer current on non-impulsed windings
• Supplementary evaluations: visual inspection, audible discharge monitoring, and dissolved gas analysis (DGA) of the oil before and after the test where specified

Typical rated lightning impulse withstand voltages defined in the standard insulation level tables include 75 kV for Um = 12 kV, 480 kV for Um = 126 kV, 950 kV for Um = 252 kV, and 1550 kV for Um = 550 kV systems.

Testing Methods

Test Circuit and Equipment

The test is performed using a multi-stage impulse generator based on the Marx circuit principle, in which capacitor stages are charged in parallel and discharged in series to produce the required high-voltage impulse. Key elements of the test setup include:

• A multi-stage impulse generator with sufficient voltage and energy capability, often several hundred kilojoules for large power transformers
• A damped capacitive or graded voltage divider for accurate measurement of the impulse waveform
• A high-bandwidth digital impulse recorder compliant with IEC 60060-2 for capturing voltage and current records
• A triggered chopping gap or sphere gap for chopped-wave applications
• Current shunts or measuring resistors for neutral current and transferred surge current measurement
• A low-inductance, single-point grounding system connecting the transformer tank, core, generator base, and measuring references

Preparation and Pre-Test Checks

Before testing, the transformer must be fully assembled and dielectrically prepared. This includes completing oil filling and treatment, observing the required standing time after filling, and confirming acceptable results of insulation resistance measurement, oil breakdown voltage, and baseline DGA. The tap changer is set to the specified position, all non-tested terminals are solidly grounded, and the tank and core are earthed. Ambient temperature, humidity, and atmospheric correction factors are recorded and applied where required.

Test Sequence

The test is normally performed with negative polarity, which reduces the probability of flashover on external insulation without reducing the severity of stress on internal insulation. A typical sequence proceeds as follows:

• Application of reduced full-wave calibration impulses at 50 % to 75 % of the specified test voltage, with adjustment of the generator front and tail resistors until the waveform parameters comply with IEC 60060-1
• Application of the full-wave impulses at 100 % of the rated withstand voltage, with the number of applications defined by the applicable standard and the purchase specification
• For windings of Um of 72.5 kV and above where chopped-wave testing is specified, application of chopped-wave impulses at the elevated level, interleaved with the full-wave applications in the sequence prescribed by the standard
• A time interval of at least one minute between successive impulses to allow thermal and dielectric recovery and to permit review of each recorded waveform

Failure Detection and Acceptance Criteria

Because an internal insulation breakdown may occur without any visible external flashover, failure detection relies primarily on the comparison of impulse records. The voltage oscillogram and the neutral current record obtained at full voltage are superimposed on the corresponding calibration records. The transformer passes the test if the waveforms show no significant differences, no abnormal oscillations or discontinuities in the tail portion, and no reduction in peak amplitude, and if no flashover or puncture occurs. Modern laboratories supplement this classical method with transfer function analysis, in which the frequency-domain relationship between the applied voltage and the neutral current is computed; a shift in resonance frequencies or damping reveals internal winding movement or insulation defects with high sensitivity. Audible noise, gas evolution detected by post-test DGA, and visual inspection provide additional confirmation where discrepancies are suspected.

Applications

Lightning impulse testing is applied throughout the transformer life cycle and across the power industry:

• Factory routine testing of new power transformers as a condition of delivery and grid connection acceptance
• Type testing of new designs to validate winding insulation structures, interleaving arrangements, and bushing and lead clearances
• Certification and compliance testing for utilities, grid operators, and international projects, often conducted at accredited third-party laboratories or witnessed by purchaser representatives
• Verification testing after repair, rewinding, or retrofitting to confirm that insulation integrity has been restored
• Research and development programs, including insulation aging studies, ultra-high voltage insulation research, and validation of numerical electromagnetic transient models
• Forensic analysis of transformers returned from service to determine whether lightning-related overvoltage contributed to insulation failure

FAQ

Below are answers to the most frequently asked questions regarding transformer lightning impulse testing.

Why is the standard waveform 1.2/50 µs?

The 1.2/50 µs waveform was standardized internationally because it closely represents the shape of lightning-induced overvoltages measured on transmission systems. It combines a very steep front, which stresses the winding entrance regions and interleaved insulation, with a decaying tail that stresses the bulk and end insulation.

Why is negative polarity normally used?

Negative polarity is applied because positive impulse voltages cause external flashover across bushings and air clearances at lower amplitudes. Using negative polarity therefore avoids misleading external flashovers while still imposing the full dielectric stress on internal insulation.

What is the difference between a full wave and a chopped wave?

A full wave is the complete standard 1.2/50 µs impulse that decays naturally, whereas a chopped wave is interrupted by a triggered gap on the wave tail, typically within a few microseconds, at a level about 10 % higher than the full-wave withstand voltage. The chopped wave produces a higher and faster stress on the line-end insulation and is specified for higher voltage classes.

Is the lightning impulse test destructive?

The test is designed as a non-destructive withstand test: a healthy transformer with adequate insulation margin should pass without damage. However, if a latent insulation defect exists, the test may deliberately precipitate its failure so that the defect is detected in the factory rather than in service.

How can internal failures be detected without external flashover?

Internal breakdowns are detected by comparing the voltage and neutral current oscillograms at full voltage with the calibration records, and increasingly by transfer function analysis. Any change in wave shape, oscillation pattern, or frequency response indicates an insulation breakdown or winding deformation inside the transformer.

How is the required test voltage determined?

The test voltage is the rated lightning impulse withstand level assigned to the transformer's highest voltage for equipment (Um) according to the insulation level tables in IEC 60076-3 or GB/T 1094.3. Higher standard levels may be selected by agreement for installations in exposed locations or regions with intense lightning activity.

Can a transformer be retested after a questionable result?

If an oscillogram indicates a possible failure, the test is stopped and the records are analyzed. Depending on the findings, limited additional impulses may be applied after investigation and agreement between manufacturer and purchaser, following the retest provisions of the applicable standard.

How long does the test take?

For a typical power transformer, including preparation, calibration, full-wave and chopped-wave applications on all windings, and record evaluation, the lightning impulse test usually requires several hours. Larger units with multiple windings and additional test sequences may require additional time.