The final report of CIRED Working Group 2017-1 offered important recommendations for testing screen connections for medium and high voltage land and offshore cables. Nonetheless, application of this test procedure on different screen connections and evaluation of test results has still been problematic and requires further investigation.
In the future, the screen connection will become more and more difficult for all kinds of MV and HV cables due to increasing conductor cross-sections and resulting increased screen current. For example, maximum current in the screen can reach 250 A for a 630 mm² MV cable with copper conductor in flat formation.
Modern MV cables increasingly use a laminated aluminium screen instead of a copper wire or copper tape screen. Use of 66 kV offshore inter array cables at wind farms are currently highly in demand due to the increasing power of wind turbines. In modern submarine cables, use of laminated aluminum in combination with copper wires as screen in place of traditional lead sheath as water barrier is growing. While this makes a cable more economical, the screen connection in cable accessories represents a challenge, especially so for application in direct grounding systems where circulating currents during normal operation as well as abnormal operation (such as short circuit) must be considered.
Tests and test recommendations for cable screen connections are growing in importance. In the past, there were no major problems with the screen of medium voltage cables and their connections in joints and terminations.
But in the future the problem of cable screen connections will become of increased importace for reliability of medium and high voltage cable systems. This is due to:
• Increasing cross-section of cables;
• Increasing maximum conductor current and therefore increased screen current;
• Increasing failure rate of joints and terminations.
Use of full ampacity of cables in combination with increased cable cross-section leads to higher conductor and screen currents. This is the reason for a growing incidence of problems such as shown in Figs. 1 and 2.
outer conductive layer by insufficient screen contact.
To avoid a rising number of joint failures by cable screen connections, CIRED Working Group WG 2017-1 has proposed procedures for the test and evaluation for medium voltage power cables. But this recommendation is close to connector standard IEC 61238 -1-3 and only applies for cable with copper wire screen and a direct connection of screen wires, as shown in Fig. 3. Moreover, more than 95% of cable screen connections are not as presented in Fig. 3. Even for cables with copper wire screens, use of modern all-in-one joints with incorporated screen contact systems (see Fig 4) requires a different screen contact solution.
For cables without a copper wire screen (i.e. over 50% of MV cables) a special screen connection device is always required. Moreover, increasingly medium voltage cables come with a laminated aluminum screen. For this cable type, the screen connection is far more complex (see Fig. 5).
Plan to participate in the upcoming 2025 INMR WORLD CONGRESS in Panama. Prof. Klaus-Dieter Haim from Germany, an international expert in cable accessory design and testing, will explain the test procedure proposed by this CIRED WG as well as test results for 4 different cable screen connection designs of 66 kV separable plug-in terminations for offshore array cables. He will then present results of tests on 5 of the most-commonly-used screen connection designs for standard 20 kV medium voltage cables. Based of these results, he will propose recommendations for modifying CIRED acceptance criteria.
