The instrument transformer temperature rise test is a type test that verifies whether current transformers, voltage transformers, and combined units can operate within safe thermal limits under rated continuous load conditions. Performed in accordance with the IEC 61869 series, the GB 20840 series, and IEEE C57.13, the test measures the temperature rise of windings, cores, insulation oil, and enclosure surfaces to validate thermal design and long-term insulation life. This article explains the test principle, sample requirements, measurement methods, acceptance criteria, and typical applications in accredited third-party laboratories.
Technical Overview
The temperature rise test is one of the most important type tests for instrument transformers, including current transformers (CT), inductive voltage transformers (VT), and capacitor voltage transformers (CVT). Its purpose is to demonstrate that, when the transformer carries its rated continuous thermal current or operates at its rated continuous thermal voltage with rated burden connected, the temperature of the windings, magnetic core, insulating liquid, and accessible surfaces does not exceed the limits assigned to the insulation system. Excessive temperature accelerates insulation aging, degrades measurement accuracy, softens cast resin, and can create burn hazards on terminals and enclosures.
The test is primarily governed by IEC 61869-1 (general requirements for instrument transformers) together with the product-specific parts IEC 61869-2 (current transformers), IEC 61869-3 (inductive voltage transformers), and IEC 61869-5 (capacitor voltage transformers). Equivalent Chinese national standards are the GB 20840 series, while IEEE C57.13 covers the North American practice. Insulation thermal classes and their limiting temperatures follow IEC 60085. Temperature rise limits are expressed in kelvin (K) above the ambient temperature and depend on the insulation class and the cooling medium.
- Insulation class A (105 °C): maximum winding temperature rise of 60 K
- Insulation class E (120 °C): maximum winding temperature rise of 75 K
- Insulation class B (130 °C): maximum winding temperature rise of 85 K
- Insulation class F (155 °C): maximum winding temperature rise of 110 K
- Insulation class H (180 °C): maximum winding temperature rise of 135 K
- Liquid-immersed designs: top-oil temperature rise typically limited to 60 K, with winding limits verified by the resistance method
The exact limit applicable to a given product is defined by its insulation system class, cooling method, and the specific clause of the governing standard, and the test report must state the criterion used for the verdict.
Testing Samples
Current Transformers
Low-voltage, medium-voltage, and high-voltage current transformers in resin-cast (epoxy), dry-type, oil-immersed, or gas-insulated construction. Samples range from compact molded CTs for switchgear to outdoor porcelain-housed units with rated primary currents from a few amperes to several thousand amperes, including multi-secondary and multi-ratio designs.
Voltage Transformers
Inductive (electromagnetic) voltage transformers and capacitor voltage transformers for metering and protection, in single-phase or three-phase arrangements, indoor or outdoor execution, with resin, oil, or gas insulation. Grounded (earthed) and ungrounded designs are both covered.
Combined and Special Units
Combined instrument transformers integrating a CT and a VT in one tank, summation current transformers, and transformers intended for gas-insulated switchgear (GIS) or ring main units. For a family of similar designs, one representative sample may be tested, provided it embodies the least favorable thermal characteristics of the family.
Testing Items
During a temperature rise test, the following quantities are measured and compared against the applicable limits:
- Primary winding temperature rise, determined by the variation-of-resistance method
- Secondary winding temperature rise for each secondary winding, by the resistance method
- Magnetic core temperature rise, measured with thermocouples attached to the core surface
- Top-oil and bottom-oil temperature rise for liquid-immersed transformers
- Tank, enclosure, and cover surface temperature rise, including hottest accessible points
- Primary and secondary terminal and connection temperatures
- Ambient air temperature, recorded continuously as the reference for all rise calculations
- Verification that thermal steady state has been reached before final readings are taken
For multi-secondary transformers, all secondary windings are loaded with their rated burdens simultaneously, because the total losses, not a single circuit, determine the thermal stress.
Testing Methods
Test Circuit and Loading Conditions
For current transformers, rated frequency is applied and the primary circuit is loaded with the rated continuous thermal current, which is at least the rated primary current and may be higher if specified. Each secondary winding is connected to its rated burden at the rated power factor, typically 0.8 lagging for measuring cores. For voltage transformers, a continuous voltage of 1.2 times the rated primary voltage is applied with rated burden connected, reflecting the maximum continuous operating condition defined by the rated voltage factor. Circulating currents from adjacent phases, forced cooling, and abnormal air flow are excluded so that the sample is evaluated under free convection conditions representative of service.
Ambient Temperature Measurement
The ambient temperature is measured with lagged thermometers or thermocouples placed at approximately one to two meters from the test object, at half its height, shielded from drafts, radiation, and magnetic influence. The ambient during the test should normally remain within the range of 10 °C to 40 °C; if readings drift, the mean value over the final measurement period is used as the reference. Where the ambient deviates significantly from the reference cooling temperature, measured rises are corrected according to the rules of the applicable standard.
Winding Temperature by the Resistance Method
The average winding temperature is determined from the change in winding resistance. The cold resistance R1 is measured after the sample has soaked at a stable ambient temperature, and the hot resistance R2 is measured immediately after the test supply is disconnected at steady state. For copper windings, the winding temperature at the end of the test is calculated as θ2 = R2/R1 × (θ1 + 235.5) − 235.5, where θ1 is the winding temperature when R1 was measured; the constant 225 is used for aluminum conductors. The temperature rise is then θ2 minus the ambient temperature at switch-off. Because resistance falls rapidly after disconnection, extrapolation techniques or fast sampling recorders are used, and readings may be taken on both primary and secondary windings.
Surface and Internal Temperatures by Thermocouples
Thermocouples are bonded to the core laminations, the inner and outer resin surfaces, tank walls, covers, and terminals to capture hot-spot locations that the resistance method cannot reveal. Oil temperature is measured with sensors immersed near the top and bottom of the liquid. Infrared thermography is often used as a supplementary tool to identify the hottest accessible surface before positioning contact sensors.
Steady-State Criteria and Test Duration
The test continues until thermal steady state is reached, defined as a rate of temperature change of no more than 1 K per hour as observed over the final hours of the test. In practice, dry-type and resin-cast units typically stabilize within six to ten hours, while oil-immersed transformers may require considerably longer due to the thermal inertia of the liquid. Final resistance, thermocouple, and ambient readings are recorded at switch-off, and all rises are computed against the mean ambient of the concluding period.
Applications
Temperature rise test reports from accredited third-party laboratories are used across the entire life cycle of instrument transformer products:
- Type testing of new designs before series production and market launch
- Compliance demonstration for grid utilities, switchgear manufacturers, and EPC contractors during bid qualification
- Support for product certification and regulatory market access based on IEC, GB, EN, or IEEE standards
- Design validation after a change of insulation materials, conductor sizing, core material, or enclosure geometry
- Thermal verification of transformers integrated into switchgear, ring main units, and gas-insulated switchgear assemblies
- Failure analysis and quality investigation when overheating, resin cracking, or accuracy drift is reported in service
- Research and development benchmarking of competing insulation systems and cooling constructions
FAQ
Is the temperature rise test a type test or a routine test? It is a type test performed on a representative sample of a design family. Routine tests in production cover items such as power-frequency withstand, partial discharge, and accuracy verification, but not full thermal loading.
Which standards specify the test? The governing documents are IEC 61869-1 with parts 2, 3, and 5, the GB 20840 series for the Chinese market, and IEEE C57.13 for North American practice, with insulation classes referenced to IEC 60085.
How is the winding temperature measured if no sensor is inside the winding? The average winding temperature is calculated from the change in DC winding resistance between the cold state and the hot state at the end of the test, using the temperature coefficient of the conductor material.
How long does the test take? The test runs until steady state is reached, normally when the temperature rise changes by less than 1 K per hour. Depending on construction and insulation medium, this typically means six to ten hours for dry and resin-cast units and longer for oil-immersed transformers.
What happens if the measured rise exceeds the limit? The sample fails the type test. The manufacturer must revise the thermal design, for example by enlarging conductors, improving the insulation system class, or enhancing heat dissipation, and a new sample must be tested.
Can the test be combined with other verifications? Yes, it is common practice to verify error and accuracy characteristics immediately after the temperature rise test while the transformer is still hot, confirming that measurement performance is maintained at operating temperature.