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Figure 18: Example of
tri-furcating termina-
tion into compact MV
switchgear.
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.
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.
Testing to ensure reliability
Most end-users still use direct current (dc) cable pressure
test equipment, which gives no diagnostic results. This type
Figure 16: Example of screened and belted PILC cable termination
prepared from the top down principle.
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 17: Example of SF6 RMU with an additional raising plinth and
removable front sections.
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.
DRIVES, MOTORS + SWITCHGEAR
T AN FORMERS + SUBS ATIONS
Figure 19: Example of incorrect and correct termination into compact
MV switchgear with LV CTs.
According to
depth of lug
According to
depth of lug
31
April ‘17
Electricity+Control