Finding the Earth Fault at 11kV load side

[Edelma_1]

Hello Everyone,

Please how do you find the earth fault at the load side where you need to connect an RMU only at the load side?, the Resistance of the combined HV/LV earthing system(TN-C) can be taken as ≤10 Ohms. The available data I have from the source substation are:

1. 11kV Fault level - Three phase fault (6777A)
Earth Fault (987A)

2. Earth Resistance - Calculated(0.16 Ohms)
Measured (0.13 Ohms)

3. Cables connecting source to load side - 3c 0.3in² PILCSWA (Al) @0.965km
3 x 1c 150mm² EPR (16mm² screen) @0.122km
3 x 1c 70mm² EPR (12mm² screen) @0.402km

 

[7anoter4]

The IEC 60364 standard [art.11.2] it covers:
a) circuits supplied at nominal voltages up to and including 1 000 V a.c. or 1 500 V d.c.;
So, if the system is 11 kV, TN-C cannot be applied.
Usually,11 kV system has not a neutral but only earthing conductors for earthing [grounding] connection to earthing grid or as equipment earthing connection.
For installations above 1 kV it is another standard: EN 50522 [BS EN 50522/2010] Earthing of power installations exceeding 1 kV a.c. There are many features in common with the IEEE 80/2013 standard.
An RMU is-at least- a group of 3 switchgears-2 feeders and one transformer.

 

[Edelma_1]

what if the site uses/consumes 11kV? then there would not be a need for a transformer yeah?

 

[7anoter4]

In my opinion, that is the possible single line connection diagram.

 

[Edelma_1]

The TN-C is for the sheeath of the cables, they have earthings that surround the conductors. the can be aluminium or copper type.

 

[7anoter4]

The screen -and lead sheath for PILC cable- was calculated as follows:
Screen resistance calculation:
Screen [CU wire] resistance=length[m]/58/area[mm^2]
Lead sheath res.=length[m]/4.5/area[mm^2]
The PILCswa 3*185 mm^2 Al overall diameter 69 mm lead area 550 mm^2
Cable CblA PILCSWA sheath resistance 0.39 ohm
Cable CblB screen resistance 0.131466 ohm
Cable CblC screen resistance 0.5776 ohm
FeederA and B of RMU two sets of 3 single core cables [in underground]
of 400 mm^2 copper EPR [or XLPE] 35 mm^2 screen 0.2463-ohm[1000m].