Design and Quality Control 1 General requirements

The quality of the manufactured instrument transformers is already determined to a great extent during the design work. Experience shows that bad design is an important cause of failures. The design engineering work should be carried out by qualified and experienced personnel to ensure that design, tests, approval and issue of design documents are executed in a satisfactory manner. Development work should also be based on theoretical knowledge and field experience, as well as on full control of the techniques used. Design review should be carried out with the participation of qualified manufacturer's personnel or independent engineers, who can influence and critically examine the selected technical solutions.

In the event of changes in design, the same rules should be applied as to the original design work. All modifications should be examined and approved by qualified personnel, and validated by appropriate tests.

During design of all types of ITs, special precautions should be taken in order to ensure operational safety. Examples of special points that should be taken into account as part of design control are:

− compatibility between materials, especially between oil and rubber (gaskets), enamel, varnish, paint, for the whole temperature range of the IT,

− shielding of the secondary winding against high frequency influences and fault current to earth inside the unit, − coordination of withstand strength between different parts of the HV insulation system especially during HV surges

or bursts,

− low and high temperature behaviour of the oil expansion system i.e. metal bellows and gaskets, − mechanical, thermal and seismic stresses,

− accuracy requirements during operation and transient conditions, for all specified burdens, − special constructions for transportation and storage.

The routine, type and special tests are carried out on the first manufactured units in order to verify the correctness of the selected design. The result of these tests should be recorded in written reports. The documentation should also define the technical requirements to be met in production, inspection, and packaging of the products.

2.2.2 Manufacturing documents

• Drawings and technical notes:

These documents should describe the instructions and requirements for each activity concerned. The allocation of responsibility for formulation, review, approval and issue of documents must be established:

− the documents must be available at the right place at the right time,

− all changes are examined and approved by the responsible engineer who improved the original design.

• Process control documents for the manufacturing process:

Documents must be established specifying method, sequence, instruments, gauges, etc, to be used for inspection, as well as the measurements with their appropriate tolerances. They must be completed by the worker and checked by the quality control operator.

2.2.3 Purchasing of components and materials

The quality of an instrument transformer is greatly influenced by the quality of components and materials used such as enamel-insulated wires, magnetic material, insulating paper, plastic films, insulating oil, SF6 gas, resin hardener and filler, insulators (porcelain or composite), gaskets, etc.... Relevant instructions, type and routine tests should ensure that purchasing is properly planned and carried out. However this presumes that goods and services are purchased from suppliers who can provide the desired quality.

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The necessary requirements regarding the quality system of suppliers are determined after an evaluation and classification of products obtained. IEC and ISO standards can be used to specify the quality and the tests to be performed on various components and materials: type, routine or incoming tests. The manufacturer may also add specific requirements or tests to the International Standards, dependent on the IT design, the type of ITs or the service conditions: compatibility, ageing, thermal behaviour, etc. The quality of external components has to comply with the environmental requirements. Special attention has to be paid to the compatibility of the components with liquid or gaseous insulating materials.

Examples of components which may critically affect the quality of an instrument transformer are listed together with their most critical properties:

− oil: electrical, physical and chemical properties,

− paper or film: dimensions, electrical, mechanical and chemical properties,

− insulators: appearance, dimensions, porosity (porcelain), ageing due to environment (synthetic), electrical and

mechanical strength,

− bellows: material quality, tightness, withstand pressure, volume, thermal behaviour,

− gaskets: material quality, appearance, dimensions, compatibility, temperature range,

− core material: magnetic properties, dimensions and appearance,

− housing: dimensions, appearance, porosities in cast housings, mechanical strength, surface treatment (against

corrosion), tightness (take care that the type of internal treatment or paint will not cause chemical reaction resulting in any gas development),

− resin mixture: weathering properties. water diffusion, erosion resistance,

− SF6 gas: purity of gas, according to IEC 376.

Supplier evaluation should be made when a new supplier is contacted or in the case of special requirements. The evaluation will include financial, commercial and technical assessments.

The purchase technical documentation normally contains the following: − Procedures to test and to control the quality of products supplied. − Procedures for the dealing with non-conforming products or components. − The right to monitor quality and test procedures of the supplier.

− Requirements for packaging and storage.

2.2.4 Process control

Control of activities which are part of the production and assembly process should be carried out in accordance with a documented system. The manufacturing specification is an important document of the production department during the manufacturing process. The specification contains details of the sequence of operations, production methods, materials and special tools required. All special requirements, for cleanness, ventilation, temperature, air humidity,... should be stated in the instructions; an example of these requirements is the cleanness of rooms for the production of capacitors or SF6 gas insulated ITs.

Special tools should be tested for functioning at fixed intervals. If a tool is found to be defective whilst in use, the work should immediately be interrupted and the tool adjusted, repaired or replaced.

All checklists must be completed by the workers and checked by the quality control operator.

2.2.5 Product identification and traceability

A system must be used which ensures that components are easily identified in the production documentation at all stages. Requirements for the identification marking of purchased items are specified in the purchase documents. The receiving department is responsible for identifying incoming goods:

− Insulators are identified by drawing reference on the first shed. − Oil, paper, resin, etc, are identified by supplier references. − All mechanical pieces are identified by the drawing number.

− During the manufacturing process, each IT and each document are identified by the order number.

2.2.6 Quality records

A system should be applied to control the handling and filing of quality records. "Quality Records" means written reports, issued in connection with the implementation of the quality system as well as test reports, which demonstrate that the products supplied fulfill the stated quality requirements. Quality records include results of inspections and tests, materials certificates and any reports on non-conformity, audit reports, procedures and testing reports. Documents issued in connection with tests must include the date of the test, the name of the test engineer, the results of the test and information on how the applicable requirements have been satisfied. If the test results deviate from the applicable requirements, the non-conformances are described and accompanied by a note on the action required to rectify them. The quality records should not be limited to the final electrical tests, but extended to all the other technical departments; as for example:

− Research and Development: design review of new special processes and apparatus. − Purchasing: homologation of the suppliers.

− Resources and Human: recording of the training of the personnel.

− Quality Assurance: rejection and rework notes, corrective actions files, internal quality audits, calibration certificates of measuring instruments, statistical results, follow-up of special processes, quality plans.

− Technical-commercial: manufacturing cards with the endorsements, orders files.

2.3 Main manufacturing processes