Electrical Strength of the material –

and of the insulation system

It follows from the manufacturing process that the pressboard sheet has maximum dielectric strength in the direction perpendicular to the sheet.

The electrical strength is tested according to a standard method. A minimum figure is guaranteed. The specific strength in kV per mm of thickness decreases slowly with the thickness of the sheet – see curves under the catalogue tabs of the different grades of material.

The electrical strength of the insulation system against a dielectric field component parallel to the surface of a sheet is not a matter of puncture inside the pressboard. Inception of a discharge along the pressboard depends on conditions at the interface surface between the solid material and the surrounding oil.

This is an immensely complex problem. Transformer manufacturers have proprietary rules, based on experience. One fact is certain, however: The success of the insulation system in test and service depends very much on the cleanliness during all steps of manufacture and assembly of the transformer.

When designing complex insulation labyrinths of high-voltage  winding combinations, it is a well-known principle to avoid high dielectric field gradients along interface surfaces as much as possible. The sheets, cylinders, or caps shall be oriented as much as possible  along equipotential surfaces of the electrical field of the assembly.

Other chemical and electrical properties

Other chemical and electrical characteristics which are checked, as a matter of continuous production quality control, are ash content and properties of the aqueous extract. The ash content indicates the purity of the cellulose pulp that is used. The aqueous extract is an indicator of the capability of the board production process and the quality of cellulose pulp used.

Mechanical characteristics

Apparent density

The mechanical strength naturally depends on fibre quality, but is also related to the degree of pre-compression during manufacture. This pre-compression is immediately indicated by the apparent density measured on samples cut from sheet material.

Low-density sheet has 0.75-0.90 g/cm³, and high-density pre-compressed sheet has 1.00-1.30 g/cm3 (values rising with the thickness of the sheet).

These properties are measured on strips cut from sheets – in machine direction (MD) and perpendicular thereto: cross machine direction (CMD). The difference between these two samples is an indicator of the anisotropy of the material (also showing up in shrinkage properties). As an example, for material with 3 mm thickness, the guaranteed minimum values of tensile strength. N/mm², and for elongation before fracture, in %, are:

Low-density material tensile strength MD 50N/mm²

Low-density material elongation MD 7%

Low-density material tensile strength CMD 40N/mm²

Low-density material elongation CMD 8%

High-density material tensile strength MD 110N/mm²

High-density material elongation MD 3%

High-density material tensile strength CMD 85N/mm²

High-density material elongation CMD 4% 

Laminated block material is characterized in a different way, namely with a guarantee of flexural strength. This is more relevant with regard to the actual application of the material – as structural members for the support of windings etc.

The Flexural strength is indicated both in MD and in CMD. See the tables for the different grades of laminated material under their respective tabs.

Compressibility

Compressibility is a vital property for pieces subjected to high pressure during the manufacture of the transformer and in service. The parameter is submitted for HD material and for laminates.

There is a standard procedure for the determination of the figure of compressibility. A stack of samples is put in a press, and the height is measured under a low bedding pressure. The pressure is then raised, and the compression measured. Part of this compression, however, is a permanent deformation, and the rest is reversible compression. This is determined by lowering the pressure down to the original bedding pressure again and recording the remaining set.