Key Differences Between Factory Acceptance Tests and On-Site Commissioning Tests Using Resonant Test Systems

High-voltage equipment undergoes multiple stages of insulation verification throughout its lifecycle. Two of the most important stages are the Factory Acceptance Test (FAT), performed at the manufacturer's facility before shipment, and the Site Acceptance Test (SAT) or commissioning test, performed after installation at the final location. Both stages frequently employ High Voltage AC Variable Frequency Resonant Test Systems, but the objectives, conditions, procedures, and acceptance criteria differ significantly. Understanding these differences is essential for manufacturers, utilities, and testing service providers to plan effectively and avoid costly surprises. This article examines the key distinctions between FAT and SAT when resonant test systems are used.

Purpose and Timing

The FAT is conducted at the end of the manufacturing process, before the equipment leaves the factory. Its primary purpose is to verify that the equipment meets the design specifications and contractual requirements. Any manufacturing defects, material inconsistencies, or assembly errors should be identified and corrected at this stage, when rework is relatively easy and inexpensive. The FAT also provides a baseline record of the equipment's insulation characteristics that can be referenced during future maintenance.

The SAT is conducted after the equipment has been transported to the site, installed, and connected to the surrounding system. Its purpose is to confirm that the equipment survived transportation and installation without damage, that the installation work was performed correctly, and that the equipment is ready for energization. The SAT is the final gate before the equipment is placed into service, and a failure at this stage can delay a project by weeks or months.

Environmental Conditions

The factory environment is controlled: temperature and humidity are regulated, electromagnetic interference is low, and a stable power supply is available. This allows the resonant test system to operate at its best performance, with low partial discharge background noise and precise voltage control. The test bay is typically shielded and grounded to a dedicated earth electrode.

The site environment is unpredictable. Temperature and humidity may be extreme, the power supply may be weak or unstable, and electromagnetic interference from nearby equipment may be significant. Partial discharge background noise at the site is often higher than in the factory, which can mask low-level defects. The resonant test system must be robust enough to cope with these conditions, and the test procedure must account for them. For example, the background noise should be measured and documented before the test, and the acceptance criteria may be adjusted if the noise floor is unavoidably high.

Test Object Condition

In the FAT, the test object is clean, dry, and at ambient temperature. It has not been exposed to the rigors of transportation. The capacitance and dissipation factor are likely to be close to the design values.

On site, the test object may have been exposed to moisture, dust, and mechanical stress during transport and installation. Cable terminations may have been assembled on site, introducing variability in workmanship. Transformers may have absorbed moisture if they were not properly sealed. The resonant test system must be capable of testing the equipment in its actual installed condition, and the results must be interpreted with these factors in mind. A higher partial discharge level at site than at the factory does not necessarily indicate a defect; it may simply reflect the different environmental conditions.

Test Procedures and Standards

The FAT follows a well-defined procedure specified in the contract and relevant standards. For example, a power transformer FAT may include applied voltage tests, induced voltage tests, and partial discharge measurements according to IEC 60076-3. The test sequence is planned in advance, and all equipment and personnel are dedicated to the test. The resonant test system is set up permanently in the test bay and is calibrated regularly.

The SAT procedure is often more compressed and must be coordinated with other construction activities. The test may be performed at night or during a narrow window when the grid is available. The resonant test system must be transported to the site, assembled, and commissioned before the test can begin. The test procedure may be adapted to the site conditions, and some tests that were performed at the factory may be omitted if they are not practical on site. For example, a full induced voltage test on a large transformer may be impractical at site due to power supply limitations, so a reduced test may be substituted with the agreement of the client.

Acceptance Criteria

The acceptance criteria for the FAT are typically strict and based on the design specifications. A partial discharge level above a specified threshold, such as 100 pC for a transformer, results in rejection and rework. The voltage withstand test must be passed without flashover or breakdown.

At site, the acceptance criteria may be slightly relaxed to account for environmental factors. For example, the permissible partial discharge level may be increased if the background noise is high, provided that the increase is documented and justified. The focus is on detecting gross defects that would jeopardize safe operation, rather than on achieving laboratory-grade precision. Nevertheless, the criteria should be agreed upon in advance and should be consistent with relevant standards and the criticality of the equipment.

Documentation and Reporting

The FAT report is a formal document that includes the test object identification, test conditions, measured values, calibration certificates, and a pass/fail statement. It is often reviewed by the client's engineers before shipment is authorized. The report becomes part of the equipment's permanent record.

The SAT report serves a different purpose. It documents that the equipment was installed correctly and is ready for service. It may include additional information such as installation inspection records, grounding resistance measurements, and a list of any deficiencies found and corrected. The SAT report is often required by the project owner and may be used to release payment to the contractor. Both reports should be stored in a common database to enable comparison of factory and site results over time.

Logistics and Resource Requirements

The FAT is performed in a fixed location with permanent test facilities. The resonant test system is already installed and calibrated. Personnel are available and experienced. The test object is brought to the test bay.

The SAT requires mobilizing the resonant test system to the site. This involves transportation, rigging, assembly, and connection to a suitable power supply. The test team must travel to the site and may need to work in confined or remote locations. Logistics can be complex, especially for offshore or mountainous sites. The resonant test system must be designed for portability and quick setup to minimize site time. Modular reactors, lightweight control units, and pre-assembled cable sets reduce setup time from days to hours.

Case Study: Transformer FAT and SAT Comparison

A 230 kV, 400 MVA power transformer was tested at the factory and again on site. At the factory, the partial discharge level during the induced voltage test was 45 pC, well below the 100 pC limit. The resonant test system in the factory had a background noise level of 2 pC. On site, the same transformer was tested with a portable resonant system. The background noise was 15 pC due to nearby construction equipment. The measured partial discharge level was 60 pC, which was within the adjusted acceptance criterion of 80 pC for the site conditions. The higher reading was attributed to the increased background noise and the effects of transportation, not to a defect. The transformer was energized successfully and has operated without issues. This case illustrates the importance of understanding the environmental differences between FAT and SAT when interpreting test results.

Conclusion

Factory acceptance tests and site acceptance tests serve complementary but distinct purposes in the lifecycle of high-voltage equipment. The FAT verifies manufacturing quality under controlled conditions, while the SAT confirms installation integrity under real-world conditions. Both rely on High Voltage AC Variable Frequency Resonant Test Systems, but the system requirements, test procedures, and acceptance criteria differ. By planning for these differences and documenting the conditions and results of each test, manufacturers and utilities can ensure a smooth transition from factory to site and a reliable, long-lived installation.