A power transformer should not be released for shipment simply because its assembly has been completed. Before leaving the factory, the transformer must be checked against the approved technical specification to verify its electrical performance, insulation condition, losses, impedance, connections, accessories, control functions, and mechanical condition.
This process is generally organized through a Factory Acceptance Test, commonly known as a transformer FAT.
A power transformer FAT is not a single electrical measurement. It combines routine testing of the completed transformer with mechanical inspections, functional checks, document review, and any required type-test or special-test evidence.
For 35kV and 110kV power transformer projects, the FAT scope should be agreed before production begins. HONLE coordinates transformer manufacturing and factory testing according to the approved technical data, drawings, applicable standards, Inspection and Test Plan, and project-specific acceptance requirements.
Quick Answer: What Does a Power Transformer FAT Include?
A power transformer FAT normally covers three main test categories:
- Routine tests verify the actual transformer being manufactured and delivered.
- Type tests verify important performance characteristics of a transformer design.
- Special tests address additional project, utility, operating, or technical requirements.
These test categories serve different purposes and should not be treated as interchangeable.
For example, a routine impedance measurement confirms the impedance of the completed transformer, but it does not replace short-circuit withstand verification. Similarly, a routine test report does not automatically replace temperature-rise, sound-level, or other design-related evidence when such information is required by the project.
Table 1: Routine, Type and Special Transformer Tests
| Test Category | Main Purpose | Typical Application |
| Routine tests | Verify the completed transformer being delivered | Performed on each transformer as required by the applicable standard and contract |
| Type tests | Verify the performance of a transformer design | Conducted on a representative transformer or supported by technically applicable test evidence |
| Special tests | Verify additional project-specific requirements | Conducted when specified in the technical agreement, utility standard, or project specification |
The final FAT classification and test scope should follow the applicable standard edition and the signed technical agreement.
What Is a Power Transformer FAT?
A Factory Acceptance Test is the final organized verification process carried out before a transformer is approved for packing and shipment.
It normally connects three sets of information:
- The original project specification;
- The approved transformer drawings and technical documents;
- The measured and inspected condition of the completed transformer.
The FAT therefore begins before the transformer reaches the test area. The project requirements must first be converted into a clear testing and inspection schedule.
A typical FAT may include:
- Review of approved drawings and technical data;
- Routine electrical tests;
- Mechanical and dimensional inspection;
- Tap-changer and accessory checks;
- Cooling and control-system testing;
- Alarm and trip-contact verification;
- Review of type-test evidence;
- Project-specific special tests;
- Witnessed or remote inspection;
- Final document review;
- Release for packing and shipment.
The FAT scope varies according to transformer capacity, voltage class, insulation level, cooling method, tap-changing arrangement, accessories, installation environment, and project standard.
What Should Be Confirmed Before FAT?
Testing requirements should be clarified during the technical confirmation stage rather than after transformer production has been completed.
HONLE normally reviews the following information before manufacturing and FAT preparation:
- Rated capacity;
- Rated voltage and voltage ratio;
- Frequency;
- Number of phases;
- Vector group;
- Tapping range and tap positions;
- Percentage impedance;
- Guaranteed no-load and load losses;
- Cooling method;
- Insulation level;
- Temperature-rise limits;
- Short-circuit level;
- Sound requirement, when applicable;
- Tap-changer type;
- Bushings and terminal arrangement;
- Protection and monitoring devices;
- Control voltage;
- Applicable IEC, national, utility, or project standards;
- Required routine, type, and special tests;
- Witness and hold points;
- Required report format and document language.
The approved General Arrangement drawing should also be available during FAT. It provides the reference for checking the transformer dimensions, terminal positions, bushings, radiators, conservator, control cabinet, valves, lifting points, wheels, cable boxes, and other project interfaces.
Table 2: Key Documents Used During Transformer FAT
| Document | Main FAT Purpose |
| Approved technical data sheet | Confirm electrical and performance requirements |
| General Arrangement drawing | Check dimensions and equipment arrangement |
| Nameplate drawing | Verify ratings and identification |
| Terminal drawing | Confirm phase, neutral, and earthing connections |
| Control schematic | Check auxiliary, alarm, trip, and cooling circuits |
| Inspection and Test Plan | Define test sequence and acceptance points |
| Guaranteed-loss schedule | Compare measured and guaranteed losses |
| Accessory list | Confirm completeness of supply |
| Applicable standards list | Confirm the required test basis |
| Packing specification | Confirm shipment and storage protection |
Early confirmation is particularly important when the project requires temperature-rise testing, impulse testing, sound measurement, third-party inspection, or other special arrangements that may influence cost and production scheduling.
Routine Tests for the Transformer Being Delivered
Routine tests are performed on the completed transformer according to the applicable standard and agreed project requirements. They verify the characteristics of the actual unit prepared for delivery.
The exact test schedule depends on the transformer design and voltage class, but the following measurements are commonly included for oil-immersed power transformers.
Winding resistance measurement
Winding resistance testing helps confirm winding continuity, conductor connections, terminal joints, phase consistency, and tap-changer circuit condition.
Because resistance changes with temperature, the test report should record the winding or oil temperature and identify the reference temperature used for result correction.
Unexpected differences between phases or tap positions should be reviewed before the transformer is released.
Voltage ratio and vector-group verification
The voltage-ratio test confirms that the relationship between the high-voltage and low-voltage windings matches the approved design.
Testing across the required tap positions can also verify:
- Correct turns ratio;
- Tap-changer connections;
- Winding polarity;
- Phase sequence;
- Vector group;
- Angular displacement.
This test provides direct confirmation that the internal winding connections correspond to the approved transformer data.
Short-circuit impedance and load-loss measurement
Transformer impedance affects fault current, voltage regulation, protection settings, switchgear selection, motor starting, and parallel operation.
The measured impedance should be compared with the approved technical value and the applicable tolerance.
Load-loss measurements are corrected to the agreed reference temperature and compared with the guaranteed value. For transformers intended for parallel operation, impedance consistency deserves particular attention because significant differences may result in unequal load sharing.
No-load loss and no-load current measurement
No-load testing evaluates the transformer core while the secondary winding remains open and the specified voltage and frequency are applied.
The test provides information about:
- Core-loss performance;
- Excitation current;
- Core assembly condition;
- Magnetic-joint condition;
- Possible core or insulation abnormalities.
No-load loss is also commercially important because it remains present whenever the transformer is energized, even when the connected load is low.
Routine dielectric tests
Dielectric tests verify the ability of the transformer insulation system to withstand the specified electrical stress.
The required test voltage, duration, terminal connections, insulation level, and acceptance criteria depend on the transformer voltage class, insulation arrangement, applicable IEC 60076 requirements, and signed project specification.
Different dielectric tests verify different parts of the insulation system, so the FAT schedule should identify the specific test method rather than using only the general description “high-voltage test.”
Mechanical, Accessory and Functional Inspections
Electrical testing is only one part of transformer acceptance. The completed transformer also includes a tank, bushings, valves, radiators, protection devices, indicators, control wiring, and transport components that must be inspected before shipment.
Visual and dimensional inspection
The final inspection may cover:
- Overall transformer dimensions;
- Bushing and terminal arrangement;
- Phase markings;
- Nameplate information;
- Tank and cover condition;
- Welded joints;
- Paint finish;
- Radiator arrangement;
- Conservator installation;
- Earthing terminals;
- Lifting and jacking points;
- Base structure or wheels;
- Control cabinet;
- Cable-entry arrangement;
- Warning labels;
- Loose supplied accessories.
Dimensions and interfaces should be checked against the approved drawing rather than evaluated only by visual appearance.
Tank and oil-system checks
For an oil-immersed transformer, the factory inspection may include:
- Leakage inspection;
- Oil level verification;
- Valve-position checks;
- Conservator condition;
- Breather installation;
- Buchholz relay arrangement, when supplied;
- Pressure-relief device;
- Oil and winding temperature indicators;
- Radiator and pipe connections;
- Oil sampling and drain valves.
The specific pressure, vacuum, and leakage checks depend on the transformer design and agreed testing procedure.
Tap-changer functional checks
For an off-circuit tap changer, FAT should confirm the mechanical position, electrical continuity, and correspondence between the tap indicator and winding connection.
Where an on-load tap changer is included, the functional scope may also cover:
- Local and remote operation;
- Raise and lower commands;
- Tap-position indication;
- Limit switches;
- Electrical interlocks;
- Motor-drive operation;
- Alarm contacts;
- Manual operating functions.
Cooling, protection and control checks
For power transformers equipped with fans, pumps, temperature controllers, or automatic cooling stages, the FAT can verify:
- Manual and automatic operation;
- Fan or pump direction;
- Cooling-stage sequence;
- Start and stop settings;
- Local indication;
- Remote signals;
- Alarm contacts;
- Trip contacts;
- Auxiliary supply compatibility;
- Failure alarms.
Installing a cooling device does not by itself confirm that the complete control logic works correctly. Functional testing is therefore an important part of final acceptance.
When Are Type Tests and Special Tests Required?
Routine tests verify the completed transformer, while type and special tests address broader design or project requirements.
Temperature-rise testing
A temperature-rise test evaluates whether the transformer can dissipate the heat produced by its losses under the specified cooling and loading conditions.
The assessment may include:
- Top-oil temperature rise;
- Average winding temperature rise;
- Cooling-stage operation;
- Oil circulation;
- Radiator performance;
- Fan or pump performance.
Where existing type-test evidence is proposed, the tested transformer and offered transformer should be compared in terms of capacity, voltage class, losses, winding design, cooling method, tank arrangement, insulation system, and temperature-rise limits.
A report from a substantially different power transformer should not automatically be treated as representative.
Short-circuit withstand verification
Short-circuit withstand verification addresses the ability of a transformer to resist the thermal and dynamic forces produced by an external fault.
It is different from the routine impedance test.
Depending on the project, verification may be supported through design calculations, technically applicable test evidence, design review, or a special short-circuit test.
The required method should be agreed together with the system fault level and project standard.
Other project-specific tests
Additional tests may include:
- FRA or SFRA baseline measurement;
- Sound-level measurement;
- Zero-sequence impedance;
- Partial-discharge measurement;
- Capacitance and dielectric dissipation factor;
- Insulating-oil laboratory analysis;
- Harmonic measurement of no-load current;
- Additional digital-monitoring checks;
- Transport-impact monitoring.
Not every power transformer requires every special test. Each additional test should have a clear technical purpose.
Depending on the transformer rating and project specification, certain special tests may be completed at the factory, coordinated with a qualified external laboratory, or supported by technically applicable test evidence. The test location, witnessing arrangement, cost, and additional lead time should be confirmed before production.
Witnessed and Remote Transformer FAT
Some projects accept certified factory test reports, while others require customer representatives, EPC engineers, utilities, or third-party inspection companies to witness the FAT.
The Inspection and Test Plan should clearly identify:
- Review points;
- Witness points;
- Hold points;
- Notification period;
- Responsible parties;
- Test sequence;
- Acceptance criteria;
- Instrument and calibration requirements;
- Required signatures;
- Nonconformity handling.
Before a witnessed FAT begins, HONLE confirms that the transformer identification, approved drawings, test procedure, test connections, and required instruments are ready.
Remote FAT can also be arranged when agreed by the project. It may include live video, test-instrument displays, document sharing, photographs, and recorded evidence.
The remote arrangement should be confirmed in advance so that the camera view and transmitted information allow the required test results and transformer details to be verified clearly.
How Are FAT Results Reviewed?
A test report should provide more than a collection of measured values.
The results should be compared with:
- The approved project data;
- The applicable standard;
- The agreed tolerance;
- The guaranteed performance values;
- Related phase and tap-position results.
Loss measurements should identify the measured value, test temperature, correction method, reference temperature, corrected result, guaranteed value, and applicable tolerance.
For power transformers with multiple taps, the report should clearly identify the principal, maximum, minimum, and other tested tap positions.
When a result falls outside the agreed acceptance range, the test setup, instrument connection, temperature correction, and technical cause should be reviewed. Any corrective action and retest should remain traceable in the final FAT documentation.
What Documents Are Provided After FAT?
Factory test records are valuable not only before shipment but also during receiving inspection, installation, commissioning, maintenance, warranty review, and future condition assessment.
Depending on the agreed document scope, the final transformer record may include:
- Approved technical data sheet;
- Final transformer drawings;
- Routine test report;
- Applicable type-test evidence;
- Special test reports;
- Oil test records, when required;
- FAT inspection checklist;
- Accessory documentation;
- Nonconformity and corrective-action records;
- Packing list;
- Installation manual;
- Operation and maintenance manual;
- Spare-parts list;
- Warranty documents.
Photographs of the nameplate, overall assembly, terminals, control cabinet, accessories, internal transport protection, final packing, and container loading may also be included when specified.
The required document language, format, number of copies, and submission schedule should be confirmed before manufacturing begins.
How HONLE Supports Power Transformer FAT
HONLE manufactures oil-immersed power transformers for industrial distribution systems, substations, renewable-energy projects, utilities, and other project-based applications.
For 35kV and 110kV power transformer projects, the final FAT arrangement is developed around the approved transformer configuration rather than a fixed universal checklist.
Before production, HONLE reviews the project’s:
- Rated capacity and voltage ratio;
- Frequency and vector group;
- Impedance and guaranteed losses;
- Insulation level;
- Cooling method;
- Tap-changing arrangement;
- System fault level;
- Accessories and monitoring devices;
- Applicable standards;
- Required test reports;
- Witnessing requirements;
- Packing and delivery conditions.
These requirements are then connected with transformer manufacturing, routine testing, functional inspection, report preparation, and shipment release.
Where type-test evidence or special testing is required, the technical applicability, acceptance basis, test location, witnessing arrangement, commercial scope, and additional scheduling requirements should be confirmed before production starts.
This approach helps avoid late changes while ensuring that the transformer supplied corresponds to the approved technical agreement.
Frequently Asked Questions
Does every power transformer require a witnessed FAT?
No. Routine factory tests are carried out according to the applicable standard and contract, but customer attendance is only required when witnessing is specified. Projects may accept certified reports, arrange remote FAT, appoint a third-party inspector, or send their own representatives.
Are routine tests sufficient for transformer acceptance?
Routine tests verify important characteristics of the completed transformer. However, temperature-rise performance, sound level, short-circuit withstand, and other design-related requirements may require applicable type-test evidence, special testing, or design verification.
Can a type-test report from another power transformer be used?
It may be acceptable when the tested transformer is technically representative of the offered design and the project specification permits equivalent evidence. Capacity, voltage, winding design, losses, cooling arrangement, insulation system, impedance, and tank configuration should be reviewed.
Is an impedance test the same as a short-circuit withstand test?
No. An impedance test measures transformer impedance and load loss under controlled factory conditions. Short-circuit withstand verification evaluates whether the transformer can resist the thermal and mechanical forces produced by a severe external fault.
When should FAT requirements be confirmed?
FAT requirements should be confirmed during technical clarification and before production. Special tests, witnessed inspections, third-party testing, and additional document requirements can affect cost, production planning, and delivery time.
Conclusion
A power transformer FAT is a complete verification process rather than one final electrical test.
Routine tests confirm the characteristics of the transformer being delivered. Type tests provide evidence of design performance. Special tests address additional project and operating requirements. Mechanical inspection, functional testing, document review, and shipment release bring these activities together into a complete factory acceptance process.
For a clear FAT proposal, customers can provide HONLE with the transformer capacity, voltage ratio, frequency, vector group, impedance, insulation level, cooling method, system fault level, required accessories, applicable standards, witnessing arrangement, quantity, and delivery destination.
Contact HONLE to discuss the appropriate power transformer FAT and documentation scope for your project.