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TRANSFORMERS + SUBSTATIONS
Figure 15: Example of a cable termination where the core crossing is
made below the end of the core screen.
SANS 1332 [6] requires all terminations be done with a top down
principle. In
Figure 16
the strip back dimensions can be seen. If
the top down principle is followed, the screened metallic area is
increased and core crossing can be done easily without any risk of
partial discharge. However with a belted design cable, there is no
metallic screen and core crossing is very risky.
Core crossing for phasing within MV cable boxes
Core crossing for correct phasing withinMV cable boxes is not recom-
mended, however many crossed terminations exist in our networks.
The risk with crossed cores in side unscreened type terminations is
that adequate clearances become reduced, and this leads to increased
electrical stress and partial discharge.
Evolution
of MV Power Cables and
Accessories up to 36 kV:
Part 3
Patrick O’Halloran, City Power Johannesburg
Concluding an interesting discussion – in three parts – on the evolution of MV power cables over the last century.
Figure 14: Illustration
of the correct earthing
for ring type CTs on
each cable core used
for overcurrent and
earth fault detection.
End of core screen
Termination earth
connection to armour or
lead sheath
Armour or
lead sheath
Main earth braid
End of core screen
(screen cut)
Bottom of lug
C
O/C
O/C
O/C
E/F
Electricity+Control
April ‘17
30