With XLPE technology, the reliability of cables – especially those from manufacturers with strict quality control – has become very high. As a result, a large proportion of cable failures in recent years seem to have involved problems with accessories, such as terminations, rather than with the cables themselves. Tests of accessories according to published standards are carried out with samples taken at random from the production line in order to ensure that findings represent an evaluation of a ‘typical’ product. This way, they also help confirm that both the design and production process are satisfactory. Moreover, since cable accessories are subject to possible installation errors or other problems in the field, the whole cable system is sometimes later subject to on-site commissioning testing – something that becomes increasingly important at higher voltage levels. In order to maximize customer acceptance of associated test reports and documents, design and type tests of this sort are often performed at independent facilities, such as laboratories accredited according to ISO/IEC 17025. These laboratories offer the advantage of meeting all required standards in quality management and use of a common interpretation of the standards.
The following article, contributed by Heiko Jahn of CESI Group’s FGH Engineering & Test Laboratory in Germany, reviews some test requirements for cable accessories.
Fig. 1 depicts various IEC and CENELEC standards for testing medium and high voltage cable accessories. While these standards define the minimum requirements to satisfy the quality demands of customers, certain large power utilities sometimes issue their own additional specifications, based on these standards but placing higher stress on the accessories to be tested.
As shown in Fig. 2, some tests (e.g. the thermal cycling test) are similar across all applicable voltage levels. By contrast, short-time current and peak withstand current tests are requested mostly for MV cable accessories. Pre-qualification tests are generally required only in the case of HV accessories.
Tests on Cable Accessories
Humidity & Salt Fog Testing of Terminations
Given the importance of reliable service performance, even under demanding service conditions, non-ceramic insulation has become the state-of-the-art for terminations in most MV cable systems. Moreover, such insulation is now increasingly found in HV cable terminations as well. Nonetheless, there is always some concern that the surfaces of such polymeric accessories might be adversely affected by tracking and erosion. In the case of MV terminations, for example, these are usually installed with a significant mechanical strain on the cable. In the event that tracking affects a termination’s edge or that any surface erosion becomes too deep, the material could conceivably rupture. This, in turn, could lead to moisture ingress and the eventual need to replace the termination.
Given this, a humidity test for indoor terminations as well as a salt fog test for outdoor terminations have been introduced into the standards with the goal of assuring that the required performance could be met in both types of applications. The main differences between the two are in test duration as well as in the conductivity of the salt water being used.
A corrosion-proof test chamber with inclined roof is essential to avoid any influence from water that might drip down onto the test samples and affect test results. Moreover, the feeding circuit has to provide low impedance so that there is only a small voltage drop under high test currents. Testing three-phase cable accessories with spreader caps may require significant additional preparation to build up the appropriate test circuit.