WCA November 2023

Technical Article

like a single type test report commonly used. All these critical parameters should be closely observed and regularly tested to ensure compliance and the quality level are maintained. In addition, the same materials should be used as the first tested sample, which is a standard requirement to validate the performed type test, by a third party like BASEC, through regular factory audits, including supply chain and production records, to ensure that the approved materials are used. Likewise, BASEC carries out material fingerprinting using thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) analysis, which is the only way to confirm the standard requirements of the materials, ensuring that the cables are tested and approved. Relying on a manufacturer’s declaration of using the same material will keep the quality level of the cables at risk, affecting the entire project’s safety, quality and performance. BASEC routinely selects more samples than other certified bodies, to ensure that all cables are regularly re-tested. During a BASEC audit, samples are selected independently and sent to the BASEC laboratory for testing. This ongoing testing highlights deviations from the relevant standards and gives the manufacturer regular opportunities to confirm the conformity of their cables. Once a manufacturer receives product approval for a cable, they can mark it with the BASEC Quality Mark. When placed on the market, this gives all stakeholders (utilities, consultants, contractors and so on) confidence in the ongoing quality and conformity of the used approved cables. BASEC has invested £1.7mn in a new state-of-the-art laboratory in Dubai for low and medium voltage cable testing, to increase test capacity, technology and global reach. The laboratory investment significantly enhances BASEC’s service proposition. The first specialised laboratory for cable testing in the Middle East and Africa will cover MEAF, India and Asia markets. The Dubai lab will enhance BASEC Group’s testing capacity and regional market capabilities, and will expand on 50 years of being the trusted reassurance of cable quality worldwide.

Accordingly, the partial discharge test is vital to check the quality of insulation of medium voltage cables. Tan delta measurement : tan delta (δ) is known as loss angle or dissipation factor. This test is used to determine the quality of the cable insulation. The measured value should be low and not exceed the specified values in the standards at the specified voltage or temperature. This test can be repeated on cables to assess life expectancy and to plan or predict maintenance or replacement cycles.

Vector diagram to explain Tan delta (δ)

The heating cycle test is a pivotal test to evaluate how the complete cable withstands the thermal stress during the operating time of the cable. It simulates the actual life scenarios in which the cable will be subjected to heating (when the cable is in service) and cooling (when the load is disconnected for any reason) during the cable lifetime. This is done by induction heating: the conductor is heated to higher temperatures than the rated/designated temperature, which generally is 90°C. The cable should withstand 20 cycles (each cycle includes heating and natural cooling), and once the test is evaluated, the tested sample should be subjected to another partial discharge test. Impulse test : as the cable is a part of the electric network, it should withstand the lightning transients; therefore, the impulse test is critical for medium voltage cables to withstand. The impulse test will multiply the applied voltage. Through this test, BASEC applies high voltage levels to reach 194kV for a 33kV cable, and this test should be carried out at a temperature between 95°C and 100°C, which means the cable must be pre-heated and should be stable for two hours prior and throughout the test. To evaluate the test, the tested sample should be subjected to a power frequency voltage test. Adherence of screens at short circuit temperature test is required within some standards, as the cable may be subjected to short circuit conditions in which the temperature can reach 250°C. The cable should withstand these conditions even if it is for a few seconds. This test will evaluate that, following application of a short circuit current for three seconds, when a subsequent partial discharge test is carried out, the discharge magnitude is compliant to the requirement of the standard, and that no deterioration of performance of the insulation system has occurred following the exposure to short circuit temperature. The role of independent third party and accredited body, certification and comprehensive testing organisations such as BASEC is to ensure that the required quality level is maintained and the testing regime is in place to complement the standard, and all the critical parameters have been tested. It is obvious that all the highlighted tests are critical, and the cable performance is tested with an independent ongoing surveillance scheme that continuously checks the cables’ conformity to ensure the quality has not dropped from the first set of initial type testing – and not only at one point in time,

BASEC (British Approvals Service for Cables) Presley House, Presley Way, Crownhill, Milton Keynes, MK8 0ES United Kingdom

Tel: +44 1908 267300 Email: mail@basec.org.uk Website: www.basec.org.uk

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November 2023