Electricity + Control April 2017

DRIVES, MOTORS + SWITCHGEAR T AN FORMERS + SUBS ATIONS

According to depth of lug

According to depth of lug

Figure 18: Example of tri-furcating termina- tion into compact MV switchgear.

Figure 16: Example of screened and belted PILC cable termination prepared from the top down principle.

Figure 19 is a good example of where things have gone wrong in the past. The SF6 insulated ring main unit was installed with additional metering, LV and protection Current Transformers (CTs). This often happens and it is all because the wrong products were ordered, or because end users have not understood the new technologies, or wanted to stay with old technologies. We were able to locate this problem before a failure accorded by using the EA Technologies handle-held UltraTev Plus detectors. These tests are non-intrusive and should be done on line. No interruptions of supply are required.

Figure 17 clearly shows the extra base which needs to be supplied with compact switchgear in order to ensure the correct three core cable height is attained. This would not be the case for three single core cables. The evolution of MV Power cables, switchgear and cable accessories, has made it possible to considerably reduce the size of cable boxes. The bending radius of three core cables has to be considered and in the picture below special removable front covers of the RMU and plinth have been designed to make the jointers life easier. Implementing this will hopefully reduce jointer errors.

Figure 19: Example of incorrect and correct termination into compact MV switchgear with LV CTs.

Figure 17: Example of SF6 RMU with an additional raising plinth and removable front sections.

The installation should have been done with type 4 ter- minations and single core XLPE cables. Instead a type 3 termination was installed and the CTs were installed over the unscreened areas of the termination. This installation would have failed if nothing had been done. PD takes a long time to cause a failure in terminations, but it is guaranteed to fail one day.

Figure 18 shows a clever way of terminating the core XLPE MV power cables into small, compact switch- gear. By performing a tri-furcating termination in the duct or ground, three single core cables is achieved. Terminating single core terminations in such small cable boxes is recommended. This small cable box has two cables terminated in it and there is no risk that a failure could occur. Core crossing is done under the cable box in the duct or ground. Special attention should be paid to using the right single cable clamps and gland plate material.

Testing to ensure reliability Most end-users still use direct current (dc) cable pressure test equipment, which gives no diagnostic results. This type

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