The power frequency dry withstand voltage test is a fundamental dielectric verification procedure used to confirm the insulation integrity of current transformers, voltage transformers, and combined instrument transformers under dry ambient conditions. Performed in accordance with the IEC 61869 series, GB 20840 series, and IEEE C57.13 standards, this test applies a specified AC voltage at rated frequency to detect insulation defects before units enter service. This article explains the purpose, sample types, test items, procedures, and acceptance criteria of the power frequency dry withstand test for instrument transformers.
Technical Overview
The power frequency dry withstand voltage test (also referred to as the short-duration power frequency withstand test) is one of the most important dielectric tests for instrument transformers. Its purpose is to verify that the insulation system between the primary winding, secondary windings, core, and frame can withstand a specified alternating voltage at rated power frequency (50 Hz or 60 Hz) for a defined duration without flashover or breakdown, under normal dry atmospheric conditions.
The test serves two complementary objectives. First, as a type test, it demonstrates the design margin of a new transformer family against the required insulation level defined by insulation coordination standards. Second, as a routine test, it is applied to every manufactured unit to screen out workmanship defects such as voids in cast resin, damaged enamel insulation, winding displacement, or contaminated oil. The test is typically followed by partial discharge measurement to provide additional evidence of insulation quality.
The principal standards governing this test include:
- IEC 61869-1 — Instrument transformers, Part 1: General requirements
- IEC 61869-2 — Additional requirements for current transformers
- IEC 61869-3 — Additional requirements for inductive voltage transformers
- IEC 61869-5 — Additional requirements for capacitor voltage transformers
- IEC 60060-1 — High-voltage test techniques (voltage application and atmospheric correction)
- IEC 60071 — Insulation coordination (definition of withstand voltage levels)
- GB 20840.1, GB 20840.2, and GB 20840.3 — Chinese national standards corresponding to the IEC 61869 series
- IEEE C57.13 — Requirements for instrument transformers in North American practice
Testing Samples
Current Transformers (CTs)
Both indoor and outdoor current transformers, including cast-resin, oil-impregnated, gas-insulated (SF6), and porcelain-housed designs, are subjected to this test. For indoor units and routine tests, the test is performed dry; outdoor units additionally undergo a wet withstand test as part of type testing.
Inductive Voltage Transformers (VTs / PTs)
Single-phase and three-phase electromagnetic voltage transformers are tested to verify the withstand capability of the primary winding insulation relative to the secondaries and the tank or frame.
Capacitive Voltage Transformers (CVTs)
For CVTs, the power frequency dry withstand test is applied to the capacitive divider assembly, with test levels determined by the highest voltage for equipment and the specified insulation level.
Combined Instrument Transformers
Units integrating a current transformer and a voltage transformer in a common housing are tested with appropriate voltage application to each primary circuit while all other circuits and the frame are grounded.
Testing Items
Depending on the transformer construction and rated insulation level, the power frequency dry withstand test covers the following dielectric verification items:
- Primary-to-earth withstand: voltage applied between the short-circuited primary winding and all short-circuited secondary windings connected together with the frame, tank, and core (where accessible) grounded
- Primary section withstand: for primary windings designed in two or more insulated sections, voltage applied between adjacent sections to verify inter-section insulation
- Secondary-to-earth withstand: a lower-level test voltage (commonly 3 kV rms for 60 s, or 2 kV where specified) applied between all short-circuited secondary windings and the frame
- Secondary-to-secondary withstand: verification of insulation between independent secondary windings intended to be isolated from one another
- Interrupted-core or undetached-core designs: special circuit arrangements as specified by the relevant product standard to avoid unrealistic stressing of insulation gaps
The magnitude of the test voltage for the primary insulation is selected from standard insulation levels according to the highest voltage for equipment (Um). Typical values include 28 kV rms at Um = 12 kV, 50 kV rms at Um = 24 kV, and 95 kV rms at Um = 40.5 kV. For equipment rated Um of 300 kV and above, the short-duration power frequency test is generally replaced by switching impulse testing, although partial-load or sectionalized power frequency tests may still apply to specific components.
Testing Methods
Test Setup and Circuit
The test is performed with a power frequency test transformer energized between the defined test terminals. The primary terminals are short-circuited, all non-tested windings are short-circuited and connected to ground together with the frame, and the transformer stands clean and dry in the laboratory. The test voltage waveform shall be substantially sinusoidal, and the peak-to-rms ratio shall remain within the tolerance stated in IEC 60060-1.
Atmospheric and Conditioning Requirements
As a dry test, no artificial rain, fog, or humidity conditioning is applied. The ambient temperature is normally maintained between approximately 10 °C and 40 °C, and the insulation surfaces must be clean and free of condensation. Atmospheric correction factors are applied when the test is performed at altitudes or atmospheric conditions differing significantly from standard reference conditions, in accordance with IEC 60060-1.
Voltage Application Procedure
The voltage is raised gradually from a low value (typically below one third of the full test value) to the specified withstand level, held constant for 60 seconds for type tests, and then reduced smoothly to zero without sudden interruption. For routine tests, manufacturers and purchasers may agree to shorten the duration (for example to 10 seconds) or to apply an adjusted voltage level as permitted by the relevant standard, provided equivalent dielectric stress is demonstrated.
Acceptance Criteria
The transformer is considered to have passed the power frequency dry withstand test if no disruptive discharge, external flashover, or internal breakdown occurs during the voltage application. The test current and partial discharge behavior are monitored during the procedure; any sudden increase in leakage current, visible arcing, or audible discharge inside the unit constitutes failure. Where partial discharge measurement follows the withstand test, the measured discharge magnitude must not exceed the limits specified in IEC 61869 (for example, 10 pC for gas and solid insulation, or 100 pC for liquid-immersed insulation, at the specified measurement voltage).
Applications
The power frequency dry withstand test is applied throughout the life cycle of instrument transformers across multiple industries and use cases:
- Manufacturer quality control: routine dielectric testing of every current and voltage transformer prior to dispatch from the factory
- New product type verification: confirmation of insulation design during development and type testing of new transformer families
- Utility acceptance testing: on-site or laboratory verification by power transmission and distribution utilities before installation in substations
- Switchgear and panel integration: verification of transformers installed in metal-clad switchgear, gas-insulated switchgear, and distribution panels
- After-repair assessment: dielectric requalification of refurbished or rewound instrument transformers
- Export and regulatory compliance: testing in accredited third-party laboratories to support conformity assessment against IEC, GB, EN, and IEEE requirements
- Metering and protection systems: assurance of insulation reliability for transformers feeding revenue metering and protective relaying circuits
FAQ
What is the difference between a dry withstand test and a wet withstand test? The dry test evaluates insulation strength under normal indoor atmospheric conditions with clean, dry surfaces, while the wet test simulates outdoor service by spraying the external insulation with water of specified conductivity and precipitation rate. Outdoor transformers require both as type tests, whereas routine production testing is performed dry.
Why is the test duration typically 60 seconds? A one-minute duration has been established by international standards as sufficient to reveal insulation weaknesses from manufacturing defects while avoiding unnecessary thermal or aging stress on sound insulation. Shorter routine durations may be used by agreement with equivalent or increased voltage stress.
How is the test voltage determined? The voltage is selected from the rated insulation level corresponding to the highest voltage for equipment (Um) of the transformer, as tabulated in IEC 61869 and IEC 60071 or in the equivalent GB and IEEE standards. It substantially exceeds normal operating voltage to verify design margin.
Does the test damage a healthy transformer? No. When performed correctly at the specified voltage and duration, the test stresses the insulation well below its capability limit and does not degrade a sound insulation system. Overvoltage or prolonged application beyond standard requirements, however, should be avoided.
What happens if a unit fails the test? Any flashover or breakdown constitutes a failure. The failed unit must be withdrawn, inspected to identify the insulation defect, repaired or redesigned as appropriate, and retested successfully before release.
Is partial discharge testing performed together with this test? In most standard testing sequences, partial discharge measurement follows immediately after the power frequency withstand test on the same test circuit, since withstand testing may initiate or enlarge discharge sites that the PD measurement will then detect.