Electricity + Control April 2017

TRANSFORMERS + SUBSTATIONS

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.

End of core screen

Bottom of lug

O/C

O/C O/C

C

E/F

End of core screen (screen cut)

Termination earth connection to armour or lead sheath

Main earth braid

Armour or lead sheath

Figure 14: Illustration of the correct earthing for ring type CTs on each cable core used for overcurrent and earth fault detection.

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.

Electricity+Control April ‘17

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