Cables and cable accessories provide the vital connections between substations, transformers and end users and must provide reliable service for many years. Because cable systems are so widely used, it is very important to have a means of evaluating the quality level of each of the system components individually and in combination, prior to selection and installation. This can only be accomplished by performing type tests on the cables and the various cable/accessory combinations. Various national and international standards have been developed that specify the procedures for each of the type tests to be performed.
Various standards are applicable to the type testing of cables and cable accessories. The type test sequence specified by each standard include initial electrical tests, followed by a load cycle test that may last for several days or even weeks. The load cycle test is then followed by electrical tests to determine if the electrical and mechanical properties of the cable have changed since the initial tests were performed. Cables are then subjected to additional tests to evaluate the non-electrical properties of the conductor, insulation screens, armoring and outer sheath.
Our experience with Type Testing
DNV KEMA has been type testing cables and cable accessories for many years. We started with oil and paper insulated cables and have actively worked on the use and testing of XLPE materials and composite materials.
Our experience with cable and cable accessory testing indicates that the number of failures experienced during type testing has not decreased during the last decade. The figure shows the failures per cable and per type of accessory measured over a 9-year period.
Joints are obviously the most crucial component but cables and terminations have a similar failure rate. The failure rate is high – nearly 20% of the cables do not pass the international standard – and in spite of the availability of better materials and more experience, the failures during type testing have not decreased in the last decade.
Ultra High-Voltage cables and accessories
IEC 62067 is the applicable standard for ultra high-voltage cables of extruded insulation material rated 170 kV and above. This standard was first introduced in 2001 and has been widely adopted by all manufacturers and utilities around the world. A special feature of this standard is the differentiation between the Type Test Sequence and a Prequalification test. The type test sequence applies to the cable and individual accessories, while the prequalification test is intended to test the long-term behavior of the system when installed in normal conditions – buried in the ground and / or in a cable gallery.
For oil-filled cables and accessories, other standards like IEC 60137 are applicable. Oil-filled cables are used less often today. Repairs or extensions of oil-filled cable systems are made with XLPE cable, and special transition joints are then required to separate the oil from the oil-sensitive XLPE.
High-voltage cables and accessories
IEC 60840 is one of the standards that applies to the voltage class between 36 kV and 170 kV. Cables and accessories in this voltage class are very popular nowadays. The standard includes a regular type test sequence for cables and a separate sequence for the cable accessories. The test sequence for the combination of cables and accessories is not as integrated as it is in IEC 62067 for ultra high-voltage cables.
Medium-voltage cables and accessories
The IEC 60502-series of standards applies to the medium-voltage class of cables and accessories. Separate standards are available for medium-voltage cables (IEC 60502-2) and for medium- voltage accessories (IEC 60502-4). Although a certified cable must be used when testing cable accessories, this standard is not considered to be an integrated test sequence for both cables and accessories.
Low-voltage cables and accessories
Various standards are applicable to low-voltage power cables, but IEC 60502-1 is the most commonly used. Although a full electrical test sequence with load cycles is included for low-voltage cable accessories (mainly joints), the electrical test sequence for cables is limited with most attention devoted to material properties.
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