The 30th year anniversary of the first high-voltage cables prequalification test

In January 1992, BEWAG , the then-electrical utility of Berlin , had launched an initiative to investigate the feasibility of setting tip an urban bulk power connection using 400 kV XLPE cables .

The project, which would have changed the cable market for years to come, aimed to replace the traditional oil filled cables around the city of Berlin with XLPE cables, due to lower installation cost, lower dielectric losses and thermal resistance, and because this type of cable was considered maintenance-free once put into operation .

The tender was awarded to CESI , which obtained the project thanks to its already established expertise and, above all, thanks to its independence . What CESI embarked on had been a very challenging task: at the time of the tender and the subsequent tests (from 1992 to 1995), there was no experience regarding the life and the reliability of XLPE cables when used so extensively. Furthermore, there were no international standards over the years those tests were carried out by CESI.

However, CESI was not only able to overcome such challenges, as these tests paved the way to modern cable market as well as to the definition of the IEC 62067. As a matter of fact, the first publication of IEC 62067 occurred in 2001, inspired by the tests CESI carried out for BEWAG .

In particular, 144 measurement were performed along cables and accessories, and 100 additional measurements were performed in the filling material surrounding the cables : the soil, for the first test, and also the concrete of the tunnel walls, in the second test, to which the cables were braced with special shelves and clamps. The temperature readings were on site converted in digital form and then transmitted to a remote computer. A suitable software was used for elaboration and graphical representation of the most important quantities of each cable. These quantities were transmitted in real time via modem to Berlin enabling the BEWAG engineers to monitor continuously the test evolution .

One measurement every minute during the heating cycle, and one every five minutes during the cooling period were performed. Additional temperature measurements along the cables were performed by means fiber optic systems incorporated in each cable kit sake of assessing the reliability of this innovative technology. Oscillating switching impulse were performed according to the test program, using a mobile generator located on site.

After an actual test duration (high voltage switched on) of 8,760 hours with 183 heating/cooling cycles (16/32 hours) and the final lightning impulse test , only one cable out of six has passed the prequalification test.

By the end of the first test, it had become apparent that the objective of testing whether 400-kV XLPE insulated cables and accessories are suitable for operation in large buildings could not be achieved by long-term testing if all actual laying conditions , including any prefabricated service structures, were not fully simulated . However, the first test reasonably demonstrated that a pre-qualification test was urgently needed if no service experience was available. Therefore, the tests results underlined that the test method: heating of cable conductor by means of heating and cooling periods plus voltage in monophase configurations both continuously applied,  and conditions: cables and accessories directly buried first only in soil and then also laid in pipes and tunnel  were suitable for testing the long-term behavior of cables and accessories.

Furthermore, considered that at the end of the first test at least one cable system survived matching all the requirements, this had clearly showed that the electrical requirements were not too difficult to meet in terms of dielectric performance.

The failure statistic clearly showed that the cable companies master the production of cables and terminations. It was to be expected in the preliminary stages of the test that the development of solid-insulated joints might not be finished. Analyzing the failure reasons of the joints showed that the use of taped joints could not withstand the prequalification test.

Further development of joint technology showed a clear trend towards prefabricated straight joints, which could be tested before mounting. Silicon rubber, EPDM respectively epoxy resin were used as insulation materials. Development of new types of joints was already well advanced, so BEWAG decided to carry out a second long-time test of 400-kV-cables and accessories at CESI, in Milan . The test construction was finished in July 1995, so the second long-time test could start in August 1995.