Please Choose Your Language
Views: 0 Author: Zisheng Electric Technical Engineer Publish Time: 2026-09-07 Origin: Site
Zisheng Electric approaches thermal performance as a design question that must be settled before production release, not as a number to be checked at the end of manufacturing. A transformer temperature rise test demonstrates whether the completed cooling system can remove the heat produced by the agreed loss duty while keeping the top liquid and winding temperatures within the specified limits. For an EPC contractor or utility buyer, the test connects design assumptions, guaranteed losses, cooler configuration, ambient conditions and long-term insulation loading.
The result is useful only when the contract defines the applicable standard, cooling stage, tapping position, loss basis, temperature-rise limits and calculation method. A vague requirement such as “temperature test according to IEC” leaves several decisions unresolved. The purchasing team should convert those decisions into an approved test procedure before the factory acceptance test (FAT).
A valid thermal FAT starts with the completed transformer and the approved cooling configuration.
A liquid-immersed transformer produces no-load loss in the magnetic core and load loss in the windings, leads and structural parts. These losses become heat. The tank, radiators, fans, pumps and surrounding air must remove that heat without creating temperatures that exceed the contractual thermal limits. The test therefore checks the thermal system as a whole rather than one isolated component.
IEC 60076-2:2011 defines temperature-rise limits and temperature-rise test methods for liquid-immersed transformers. The project should state the adopted edition and any purchaser or utility amendments. Where loading and thermal ageing are part of the operating study, IEC 60076-7:2018 provides loading guidance for mineral-oil-immersed transformers. Neither reference removes the need to agree the actual transformer duty, ambient basis and cooling stage.
The FAT normally uses a test method that supplies the transformer losses without requiring an external load bank equal to the full transformer rating. Test connections and calculations depend on the approved method. Purchasers should review the procedure instead of assuming that every laboratory uses the same sequence.
Temperature limits affect conductor sizing, current density, insulation life expectations, radiator area, fan quantity, pump arrangement and enclosure clearances. If the guarantee is changed after design approval, the effect may extend beyond the test sheet. It can require a redesigned cooling system or a different active-part arrangement.
Specification item | Why it matters | Risk if unclear | FAT check |
|---|---|---|---|
Cooling designation and rating | Defines which cooler stage carries each MVA duty | Tested configuration does not represent service | Match nameplate, drawings and fan/pump schedule |
Top-liquid temperature rise | Shows the overall heat-removal capability | Inadequate radiator or airflow margin | Record stabilized liquid and ambient temperatures |
Average winding rise | Connects winding loss to conductor temperature | Unverified winding thermal performance | Use agreed resistance-temperature calculation |
Hot-spot requirement | Addresses the most thermally stressed winding region | Local ageing risk hidden by an average value | Confirm approved calculation or measurement basis |
Reference ambient | Provides the basis for converting measured results | Incomparable or misleading acceptance values | Verify sensors, location and correction method |
Loss duty and tolerance treatment | Determines the heat injected during the test | Under-testing when measured losses differ from design | Reconcile guaranteed and measured losses |
Many power transformers have more than one rating, for example a natural-cooling rating and a higher forced-cooling rating. The datasheet should state which thermal performance is guaranteed at each stage and which fans or pumps operate. Redundant coolers, standby fans and automatic control sequences also need a clear status during the test. A test with every fan running may not demonstrate the agreed N-1 cooler duty.
Check fan rotation, airflow direction, motor current, pump indication, control contacts and alarm logic before the heat run. Small assembly issues can distort the result: a fan installed backwards, a closed valve, a blocked radiator path or an incorrect control setting may reduce cooling without being obvious from the main test connection.
Ambient sensors should represent the air entering the transformer cooling surfaces and should not be heated by the tank, test cables, direct sunlight or unrelated equipment. The procedure should identify the number of sensors, their position, shielding and averaging method. If the ambient record is unstable or unrepresentative, the calculated temperature rise can be disputed even when the transformer itself performs correctly.
For an indoor installation, the room ventilation study remains separate from the factory test. FAT can demonstrate the transformer and its coolers under laboratory conditions; it cannot prove that a future transformer room will remove the rejected heat. The EPC designer must use the guaranteed losses and cooling-air requirement when checking louvers, ducts, fans and room-temperature rise.
The thermal procedure should explain how total losses are established and how the injected duty is controlled. The relationship between measured loss and specified test power matters because a unit with load loss above the design value generates more heat in service. The acceptance method must not quietly substitute a lower design loss for a higher measured loss.
Coordinate the heat run with the transformer no-load loss test and other loss measurements. Confirm the test tap position, frequency, waveform and conductor temperature corrections used for the loss values. If the project calls for multiple cooling ratings, specify whether one continuous sequence can establish the required results or whether additional stabilized conditions are necessary.
Loss records, cooling status and temperature channels must be traceable to one approved test sequence.
The liquid-temperature measurement point and instrument accuracy should be identified in the procedure. Record the temperature throughout the test rather than only at the end. The trend shows whether a stable condition has been reached and helps reviewers distinguish a valid plateau from an early stop. The report should show elapsed time, test power, ambient readings, liquid readings and cooler status on a common timeline.
Average winding temperature is commonly determined from the change in winding resistance. This makes pre-test and shutdown measurements critical. The cold resistance, reference temperature, hot resistance, time after shutdown and extrapolation method must be recorded. Delays occur while the test supply is disconnected and the DC resistance circuit is connected, so the winding begins cooling before the first hot-resistance reading. A single late reading is not enough for a defensible result.
The team should take a planned series of resistance measurements after shutdown and use the agreed extrapolation method. Review measurement leads, bridge range, polarity and tap position. The winding resistance FAT guide explains why temperature, tap selection and stable readings matter independently of the thermal calculation.
Average winding rise does not automatically equal the highest conductor temperature. The hot-spot requirement may be supported by design calculations, thermal models or direct sensors when specified. The contract should say which method applies and what design data must be submitted. Do not infer a hot-spot value by adding an arbitrary allowance that was not part of the approved design method.
A temperature-rise report should identify every temperature channel, power instrument, current transformer, voltage transformer and resistance-measuring device used for the result. Calibration validity and instrument range should be available for witness review. Time synchronization is important because power, ambient and temperature values are compared as trends.
Before energization, perform a channel check: place sensors where shown on the procedure, verify plausible values, confirm units and label each channel consistently. A swapped ambient and top-liquid channel may not be noticed until many hours of testing have been completed. Data backups should be made during long tests so that a software or power interruption does not erase the entire record.
Record test connections and the selected tap position.
List fans and pumps that are operating, isolated or designated as standby.
Log voltage, current, total injected power and power factor as required by the method.
Trend each ambient and liquid temperature channel.
Document shutdown time and every hot-resistance reading.
Retain raw data as well as the summarized acceptance sheet.
If temperature rise is above the limit or the trend is irregular, stop treating the issue as a spreadsheet problem. First verify the test power, sensors, cooling equipment, ambient placement and calculation inputs. Then inspect oil level, valve positions, radiator isolation, fan direction, pump operation and airflow obstructions. Compare the measured losses with the design values and review whether the intended cooling stage was actually in service.
A repeat test should follow an identified cause and approved corrective action. Merely repeating a long heat run without explaining the first result wastes time and may hide an intermittent cooler or instrumentation problem. Any design change—additional radiators, revised fan arrangement, altered control logic or winding modification—should be reflected in drawings, bills of material and final records.
Radiator configuration, airflow and accessory status should match the final service arrangement.
The following example is hypothetical and must not be treated as a real project or a universal requirement.
Field | Example entry |
|---|---|
Transformer | Three-phase liquid-immersed substation transformer |
Rated cooling stages | 20/25 MVA, ONAN/ONAF |
Frequency | 50 Hz |
Test reference | IEC 60076-2 edition stated in the contract |
Thermal limits | Insert project-approved top-liquid, average-winding and hot-spot requirements |
Cooler availability | State all-in-service or defined standby-cooler condition |
Required evidence | Raw trends, loss basis, calibration records, hot-resistance extrapolation and signed report |
This format forces the purchaser to replace placeholders with approved values. It also exposes missing decisions before the FAT date. A useful purchase specification should never copy thermal limits, ambient assumptions or cooling redundancy from an unrelated transformer.
Confirm the approved datasheet, nameplate rating and cooling designation agree.
Review the test method, applied loss duty and tap position.
Verify the project thermal limits and the treatment of measurement tolerances.
Check fan, pump, valve and cooler status against the approved arrangement.
Confirm ambient-sensor locations and instrument calibration.
Review the plan for shutdown resistance readings and extrapolation.
Require raw trends, calculations and deviations in the final FAT package.
Link unresolved results to the nonconformance and release process.
Thermal acceptance should be one defined release condition in the inspection and test plan. The signed report must identify the transformer serial number, final cooling equipment, approved procedure revision and any nonconformance raised during the run. If accessories were substituted, repaired or rewired after the test, engineering should decide whether the change affects the demonstrated thermal configuration. The manufacturing data book should keep the calculation sheet, raw trend file, instrument list, calibration evidence and witness sign-off together so that the result can be reviewed during commissioning or later service investigations.
The transformer temperature rise test is most valuable when it closes the loop between specification, design, manufacture and operation. It does not replace the purchaser’s room-ventilation study, loading study or site ambient assessment. It confirms that the supplied unit meets the agreed thermal duty in the tested configuration.
Zisheng Electric can review requirements for an oil-immersed transformer, a project-specific substation transformer or other transformer configurations. To prepare a technical response, provide the single-line diagram, rated power, voltage ratio, frequency, load profile, ambient data, cooling duty, guaranteed losses, test standard and purchaser FAT requirements. Our engineering team will review the requirements and respond to project inquiries within 24 hours.
