What is the difference between transformer “Type testing”and “Factory acceptance testing”(FAT)?

In the field of power engineering and electrical equipment, type testing and factory acceptance testing of transformers play a crucial role in ensuring their performance and quality. These two tests serve as precise benchmarks, evaluating transformers from different perspectives.

A thorough examination of the differences between their testing standards not only enables transformer manufacturers to effectively control the production process, ensuring product compliance and reliability, but also helps industry professionals and technical personnel fully grasp the fundamentals of the transformer quality assessment system.

In this article, we will analyze the differences between transformer type testing and transformer factory acceptance testing in terms of standard basis, test items, parameter requirements, and criteria for passing judgment. This analysis aims to provide valuable references and guidelines for the design, manufacture, acceptance, operation, and maintenance of transformers.

1.Testing purposes

Type test:

The main purpose of type test is to verify whether the design of the transformer meets the requirements of relevant standards and technical specifications. It is a comprehensive performance test of the transformer, used to determine whether the electrical performance, mechanical performance and thermal performance of the transformer reaches the specified index.

For example, after the development of a new transformer product is completed, or when there are significant changes in transformer design, materials, processes, etc., a type test is required to prove that the type of transformer can meet the expected requirements for use.

Factory Acceptance Test (FAT):

FAT is mainly designed to check and accept the transformer by the user (or user’s representative) before the transformer is shipped out of the factory to ensure that the delivered transformer meets the contract requirements in terms of quality and performance.

It focuses on checking the quality control of the transformer during the manufacturing process and whether there are any manufacturing defects, etc. For example, the user can use the FAT to check whether the appearance and dimensions of the transformer meet the requirements, whether the various components are installed correctly, and so on.

2.Testing contents

Type test:

Including but not limited to short-circuit withstand capacity test, no-load loss and no-load current measurement, load loss and short-circuit impedance measurement, insulation characteristics test (such as insulation resistance measurement, dielectric loss factor measurement, partial discharge test, etc.), temperature rise test and so on.

Take the short-circuit withstand ability test as an example, it is to check the transformer’s ability to withstand short-circuit current impact during short-circuit fault, which is crucial for the safe operation of the transformer in the power grid.

Factory Acceptance Test (FAT):

It usually includes appearance inspection, size inspection, preliminary test of electrical performance (such as winding DC resistance measurement, ratio measurement, insulation resistance measurement, etc.), and inspection of auxiliary equipment (such as cooling system, on-load voltage regulator, etc.). For example, in the winding DC resistance measurement, it is possible to check whether the connection of the winding is good or not, and whether there is any short-circuit between turns.

3.The timing of the testing

Type test:

Generally carried out when the new product design is finalized, the old product is transferred to the factory for production, and the design, material or process of the product is changed significantly. In other words, it is not a test for every transformer, but a test carried out to verify the overall design performance of the transformer under specific circumstances.

Factory Acceptance Test (FAT):

It is a test carried out before each transformer is manufactured and shipped out of the factory. It is the last quality check before the transformer is delivered to the user.

4.Testing Personnel

Type test:

Usually executed by the professional test equipment and experienced technicians, in accordance with national and international standards for fair and objective test. This can ensure the authority and fairness of the test results and provide reliable data support for the design verification of the transformer.

Factory acceptance test (FAT):

Mainly by the transformer manufacturer in their own factory, at the same time the user (or user representative) can participate in the test process, the test results for supervision and confirmation. This allows the user to directly understand the quality of the transformer, before leaving the factory to find and solve any problems that may exist.

5. Different Standard Basis

Type – Testing: It mainly refers to national standards, international standards and industry – wide standards and other general specifications. For example, the type – testing of power transformers needs to follow IEC 60076 and other IEC standards. These standards provide comprehensive and general requirements for various performance indicators and test methods of transformers.

Factory Acceptance Testing: In addition to referring to some of the general requirements in the above – mentioned standards, it also needs to be based on specific contract requirements, special user requirements and more detailed and strict factory – specific standards formulated by the enterprise itself. For example, if the user has special requirements for the noise level and partial discharge amount of the transformer, these will serve as an important basis for factory acceptance testing.

6. Testing Items

Type – Testing:

• Insulation Tests: It includes impulse withstand voltage test, power – frequency withstand voltage test, etc. These tests must be carried out strictly in accordance with the parameters such as voltage waveform, amplitude and duration specified in the standards. For example, the standard waveform of the full – wave impulse test is 1.2/50μs, and the impulse voltage amplitude that transformers of different voltage levels can withstand is different.
• Temperature – Rise Test: It is used to verify whether the temperature rise of the transformer under rated working conditions meets the standards. For oil – immersed transformers, the top – oil temperature rise generally should not exceed 55K, and the average winding temperature rise should not exceed 65K.
• Short – Circuit Withstanding Capacity Test: It simulates the most severe short – circuit faults to assess the dynamic and thermal stability performance of transformer windings, tap – changers and other components. After the test, the transformer should have no obvious deformation or damage, and all performance indicators should still meet the requirements.
• No – Load Current Harmonic Measurement: It is used to check the saturation degree of the iron core. The content of each harmonic current should be within the specified range. The specific standard values are determined according to factors such as transformer capacity and voltage level.
• Zero – Sequence Impedance Measurement: It is used to measure the impedance of the windings of a three – phase transformer under zero – sequence current, providing a basis for the parameter setting of relay protection devices. The measurement results should meet the design and use requirements of the transformer.
• Winding Deformation Test: Frequency response analysis method and other methods are used to detect whether the winding is deformed. If the frequency – response curve changes beyond a certain range, it is considered that the winding may be deformed.

Factory Acceptance Testing:

• Visual Inspection: Check whether the appearance of the transformer has damage, deformation, rust and other defects, whether the paint is uniform, and whether the nameplate identification is clear and accurate.
• Dimensional Inspection: Check whether the external dimensions and installation dimensions of the transformer meet the design drawings and contract requirements.
• Preliminary Electrical Performance Tests: For example, in the measurement of the DC resistance of the winding, the unbalance rate between phases should be less than 2%, and the unbalance rate between lines should be less than 1%. The deviation of the transformation ratio measurement should meet the regulations. The deviation of the rated tap – changer transformation ratio is ±0.5%, and that of other taps is generally no more than ±1%.
• Insulation Resistance Measurement: Use a megohmmeter of the specified voltage level to measure the insulation resistance of the winding together with the bushing. Generally, it should not be less than 70% of the factory test value. At the same time, measure the insulation resistance of the iron core and clamping parts to the ground, which should meet the corresponding standards.
• Auxiliary Equipment Inspection: Check auxiliary equipment such as the cooling system and on – load tap – changer to ensure their normal functions. For example, check whether the fans and oil pumps of the cooling system operate smoothly and whether there is abnormal noise, and check whether the operation of the on – load tap – changer is flexible and reliable and whether the gear indication is accurate.

7. Testing Parameter Requirements

Type – Testing:

All test parameter requirements must be strictly in accordance with the general standards to ensure the consistency of product design performance and quality. For example, the peak voltage, front – time and half – peak – time and other parameters of the lightning impulse test have clear and fixed standard ranges.

Factory Acceptance Testing:

Some of the test parameters are derived from the general standards, but for some non – critical performance indicators, the parameter requirements may be relatively lenient. For the key indicators clearly specified in the contract, such as when the user has more stringent requirements for the partial discharge amount of the transformer, the acceptance must be carried out according to the contract – specified parameters.

8.Qualification Judgment Standards

Type – Testing:

All test items must strictly meet the requirements of relevant standards, and all performance indicators must be within the specified range. Failure to meet any one item will result in the failure of type – testing.

Factory Acceptance Testing:

The main judgment basis is whether the product meets the technical requirements specified in the contract and the special needs of the user. For some minor defects that do not affect the normal operation and performance of the transformer, with the user’s approval, they can be judged as qualified after negotiation for rectification or other compensatory measures.