scope:

This part of IEC 60815, which is a Technical Specification, is applicable as first approach for the determination of the required d.c. Unified Specific Creepage Distance for insulators with respect to pollution. To avoid excessive over or under design, existing operation experience should be compared and eventually additional appropriate tests may be performed by agreement between supplier and customer.

It is applicable to:

• Glass and porcelain insulators;

• Composite and hybrid insulators with an HTM or non-HTM housing.

This part of IEC 60815 gives specific guidelines and principles to arrive at an informed judgement on the probable behaviour of a given insulator in certain pollution environments.

The structure and approach of this part of IEC 60815 are similar to those explained in Part 1, but adapted for the specific issues encountered with polluted HV d.c. insulation.

The aim of this Technical Specification is to give the user simplified means to:

• Identify issues specific to d.c. applications that can affect the choice and design process;

• Determine the equivalent d.c. Site Pollution Severity (SPS) from measurements, correcting for electrostatic effects, diameter, pollution distribution and composition;

• Determine the reference USCD for different candidate insulating solutions, taking into account materials, dimensions and risk factors; • Evaluate the suitability of different insulator profiles;

• Discuss the appropriate methods to verify the performance of the selected insulators, if required;

This simplified process is intended to be used when comparable operational experience from existing d.c. system is incomplete or not available.

The simplified design approach might result in a solution that exceeds the physical constraints of the project. More refined approaches for such cases, e.g. using a statistical approach, are given in the CIGRE d.c. guidelines [1]. In extreme cases, e.g. for exceptionally severe site conditions, alternative solutions such as changing the line route, relocation of converter stations or using an indoor d.c. yard may need to be considered.