TESTING AND QUALITY ASSURANCE (QA) DURING MANUFACTURE
- Unlike many items of electrical power plant (for example, switchgear and motors) most transformers are still virtually handmade, little or no mass production is employed in manufacture and each is produced very much as a one-off.
- This means that the user cannot rely on extensive type testing of pre-production prototypes to satisfy himself that the design and manufacture renders the transformer fit for service, but must have such proving as is considered necessary carried out on the transformer itself.
- From a series of works tests, which might at most be spread over a few days, it is necessary to ascertain that the transformer will be suitable for 30 years or more in service.
- It is therefore logical that this testing should be complemented by a system of QA procedures which operate on each individual unit and throughout the whole design and manufacturing process.
- The final tests, with which this chapter mainly deals, are checks on all QA procedures carried out throughout the period of manufacture.
- The stringency and thoroughness of these tests are of vital importance. This chapter gives a detailed description of the various methods employed.
- To obtain accurate results it is essential that low power factor wattmeters, precision grade ammeters, voltmeters, and class 0.1 (see BS 3938 and 3941) current and voltage transformers are used.
- These instruments should be checked at intervals not exceeding 12 months to ensure that the requisite accuracy is maintained.
- The above comments might be less true for small distribution transformers where a degree of standardisation, automation and mass production technology is tending to appear in some production areas, notably in the manufacture of cores, insulation components and resin-encapsulated windings for dry types.
- Distribution transformers and dry-type transformers and larger transformers which are still manufactured by ‘conventional’ methods.
- Details of operation of QA systems are beyond the scope of this volume and are covered adequately elsewhere, for example by BS 5750, Quality Systems, but it must be pointed out that testing alone will not demonstrate that the transformer is fully compliant with all the requirements which may be placed upon it.
- Many factors which will have a strong bearing on the service life of a large high-voltage transformer are very dependent on attention to detail in the design and manufacture and the need for a high standard of QA, and a culture of quality consciousness in the manufacturer’s works cannot be emphasised too strongly
