Instrument Transformer Withstand Voltage Testing: Standards, Procedures, and Technical Requirements

Instrument transformer withstand voltage testing is a critical dielectric assessment that verifies the insulation integrity of current transformers, voltage transformers, and combined units under power frequency and impulse stress. Conducted in accordance with IEC 61869 and GB 20840 series standards, these tests protect power systems from catastrophic insulation failure in service. This article explains the test samples, items, methods, acceptance criteria, and practical applications of withstand voltage testing for instrument transformers.

Technical Overview

An instrument transformer withstand voltage test is a high-voltage dielectric test performed on current transformers (CT), voltage transformers (VT), and capacitive voltage transformers (CVT) to confirm that their internal and external insulation can withstand specified overvoltage conditions without breakdown or flashover. Because instrument transformers connect the high-voltage primary system to low-voltage measuring and protection circuits, any insulation failure can compromise both personnel safety and the reliability of the entire substation.

The test family encompasses power frequency withstand tests (dry and wet), lightning impulse withstand tests, induced voltage tests, interturn insulation tests, and partial discharge (PD) measurements. The applicable test voltage levels are derived from the highest voltage for equipment (Um) and the assigned insulation level, as specified in the IEC 61869 series (which superseded the former IEC 60044 series) and the corresponding Chinese national standards GB 20840 and GB/T 16927. High-voltage test techniques, atmospheric correction, and measurement uncertainties are governed by IEC 60060-1, while partial discharge measurement follows IEC 60270.

Withstand voltage tests are divided into routine tests (performed on every manufactured unit), type tests (performed on one representative design sample), and special tests. Routine power frequency tests are the most common quality gate in production, whereas impulse and wet tests validate the external insulation design for outdoor service.

Testing Samples

Current Transformers (CT)

Current transformers with rated primary voltages typically ranging from 0.72 kV to 1100 kV, including wound, bar, window (toroidal), and bushing-type designs, both indoor and outdoor, in air-insulated or gas-insulated (SF6) construction. The insulation between the primary winding, secondary windings, and core/enclosure is the principal object of evaluation.

Voltage Transformers (VT / PT)

Electromagnetic (inductive) voltage transformers, both single-phase and three-phase, with unearthed (line-to-line) and earthed (line-to-neutral) configurations. These units require both applied-voltage withstand tests on the primary insulation and induced-voltage tests to verify winding interturn and section insulation.

Capacitive Voltage Transformers (CVT)

Capacitive voltage transformers comprising a capacitive divider and an electromagnetic unit, used on transmission voltages from 72.5 kV upward. Testing covers the capacitor stack, the intermediate voltage insulation, and the electromagnetic unit separately and in combination.

Combined and Low-Power Instrument Transformers

Combined current-voltage transformers, electronic and low-power instrument transformers (LPIT) with digital outputs, and instrument transformers integrated into switchgear, ring main units, or gas-insulated switchgear (GIS) bays. Sample preparation must follow the manufacturer's mounting instructions so that test conditions represent actual service clearances.

Testing Items

The scope of withstand voltage testing for instrument transformers typically includes the following items, selected according to the applicable product standard and the agreed test schedule:

• Power frequency withstand voltage test on primary insulation (primary winding to all secondary windings, frame, and earth), dry condition, typically 60 s duration.
• Wet power frequency withstand test for outdoor units, verifying external insulation under rain conditions defined in IEC 60060-1.
• Lightning impulse withstand test with standard 1.2/50 μs waveform, positive and negative polarity, verifying insulation against atmospheric overvoltages.
• Induced voltage test for voltage transformers, exciting the primary winding at elevated frequency to verify interturn and section insulation without saturating the core.
• Interturn overvoltage test on secondary windings of current transformers.
• Power frequency withstand test between secondary windings and between each secondary winding and earth, typically at 3 kV for 60 s.
• Partial discharge measurement at specified pre-stress and measurement voltages, with acceptance limits generally expressed in picocoulombs (pC).
• Chopped impulse and switching impulse withstand tests for transmission-class equipment, where specified by the insulation coordination study.

Acceptance criteria are straightforward: no disruptive discharge, no flashover, no breakdown of the test object, and partial discharge magnitudes below the limits stated in the relevant standard or contract specification.

Testing Methods

Power Frequency Withstand Voltage Test

The test is performed with a single-phase AC source at power frequency, with the test voltage applied between the short-circuited primary terminal(s) and all short-circuited secondary windings, frames, and earthed metal parts. The voltage is raised gradually to approximately 75 percent of the full test value, then increased steadily to 100 percent within 5 seconds and held for 60 seconds. Non-destructive diagnostic methods, such as measuring the resistive component of leakage current, may supplement the test. Test voltage values are taken from the insulation level tables of IEC 61869-1; for example, a 12 kV rated unit is typically tested at 28 kV (dry) while a 40.5 kV unit is tested at 80 or 95 kV depending on the assigned insulation level.

Lightning Impulse Withstand Test

A standard 1.2/50 μs lightning impulse, generated by a Marx generator chain, is applied to the primary terminals with all secondary windings, frames, and tank earthed. For type testing, 15 consecutive impulses of each polarity are applied; for routine testing of high-voltage units, a reduced number of negative-polarity impulses is often agreed. The unit passes if no flashover or breakdown occurs on any applied impulse, verified through oscillographic comparison of voltage and current records. Atmospheric correction factors per IEC 60060-1 must be applied to the specified test voltage.

Induced Voltage Test

For voltage transformers, a test voltage is applied to the primary winding by direct excitation at a frequency higher than rated frequency (commonly 100 to 400 Hz) so that the core does not saturate at the elevated flux level. The test duration is shortened in inverse proportion to frequency to limit thermal stress. This method effectively stresses the interturn, interlayer, and winding-section insulation that an applied-voltage test cannot reach.

Partial Discharge Measurement

PD testing is performed using the pulse-current method in accordance with IEC 60270. The test voltage is raised to a specified pre-stress level, held briefly, and then reduced to the measurement voltage, at which the discharge magnitude is recorded. Routine PD testing is mandatory for outdoor instrument transformers at higher system voltages (typically Um of 72.5 kV and above) and is a highly sensitive indicator of voids, sharp edges, or contamination inside cast-resin and oil-paper insulation systems.

Test Equipment and Safety

Typical test setups include an HV test transformer or resonant test system, an impulse voltage generator, capacitive voltage dividers, peak voltmeters, digital oscilloscopes, PD detectors with coupling capacitors, and control systems with programmable voltage ramps. Comprehensive safety measures are essential: interlocked test enclosures or safety clearances calculated per the test voltage, reliable earling of the test area, insulated discharge rods for de-energizing after each test, and operation only by trained, authorized personnel. Atmospheric conditions (temperature, pressure, and humidity) must be recorded so that voltage correction factors can be applied.

Applications

Withstand voltage testing of instrument transformers is applied throughout the product life cycle across the power industry:

• Factory routine testing during manufacturing of current and voltage transformers for quality release of every production unit.
• Type testing of new designs at accredited third-party laboratories or manufacturer high-voltage halls to demonstrate compliance with IEC 61869 and GB 20840 requirements.
• Commissioning and handover testing at new substations, power plants, and industrial facilities before energization.
• Periodic condition assessment and after-repair verification of instrument transformers in transmission and distribution networks.
• Acceptance testing for switchgear and GIS manufacturers who integrate bushing-type CTs and voltage sensors into their assemblies.
• Grid connection verification for renewable energy plants, railway electrification systems, and large industrial loads where accurate, reliable metering and protection depend on healthy instrument transformer insulation.

Test reports issued from these evaluations support product certification, grid operator acceptance, tender compliance, and asset management documentation.

FAQ

What is the purpose of a withstand voltage test on an instrument transformer?

The test verifies that the insulation system between the primary circuit, secondary circuits, and earthed parts can withstand specified overvoltages for a defined duration without breakdown, ensuring safe separation between high-voltage and low-voltage circuits throughout the service life of the unit.

Which standards govern instrument transformer withstand voltage testing?

The primary references are the IEC 61869 series for general and product-specific requirements (current transformers, inductive voltage transformers, and capacitive voltage transformers), the GB 20840 series for the Chinese market, IEC 60060-1 for high-voltage test techniques and atmospheric correction, and IEC 60270 for partial discharge measurement.

How long is a power frequency withstand test held?

The standard duration is 60 seconds at the specified test voltage. Induced voltage tests at elevated frequency are shortened in inverse proportion to the frequency used.

What is the difference between routine and type withstand tests?

Routine tests, such as the dry power frequency test and PD measurement, are applied to every manufactured unit. Type tests, including wet tests, full lightning impulse tests of both polarities, and chopped impulse tests, are performed once on a representative sample of a design to validate its construction.

Why is partial discharge testing performed together with withstand testing?

A unit can pass a short-duration withstand test yet still contain internal voids or sharp electrodes that will degrade insulation over years of operation. PD measurement detects these latent defects quantitatively, making it a powerful complementary diagnostic to pass/fail withstand testing.

Can withstand voltage tests be performed on site?

Yes, field testing is common during commissioning using portable frequency-resonant test sets, typically at reduced voltage levels (for example 80 percent of the factory routine test value) as agreed between the purchaser and the supplier, provided appropriate safety distances, earthing, and environmental controls are in place.

What causes a withstand voltage test failure?

Common causes include moisture ingress in oil-paper or cast-resin insulation, voids from imperfect vacuum casting, inadequate creepage or clearance distances, contaminated bushing surfaces, damaged interturn insulation, and assembly errors such as missing electrostatic shields. Failure investigation should combine visual inspection, PD location techniques, and, where necessary, dissection analysis.