MV Cables Shield Grounding

[Rakan Aljawabreh]

Good Day..

In wind farm power plant, at 33/132KV step up substation portion with short circuit level of 25ka the following MV cables (XLPE/AL-PE/MDPE) are there:

1- 33KV cables (1/phase)from wind turbine to 33kv switchgear incomer (length=1000mtr)
2- 33KV cables (3/phase)from 33KV swicthgear to step up 33/132KV power transformer (170mtr)

accordingly we have a confusion which sides of cable must be grounded (sending or receiving end) at both cases and based on which international standard.

Thanks in advance.

 

[dpc]

It is safer to ground both ends. There can be some issues with circulating current in the shield, but for runs this short, I would ground both ends. No idea about IEC standards.

 

[7anoter4]

I agree with dpc. Since the cable construction is as per BS 7870-4.10[in my opinion] then for- let's
say- 1000 MCM[500 sq.mm] and screen resistance 35 sqr.mm [wire+tape] the total losses
will be only 4% more in multiply point grounding case.
In cable duct bank of 3 ducts 3 cable per duct [NEC Table 310.60(C)(78) Det 2] 405 A and in
single point grounding 416 A[calculated].

 

[Marmite]

Has the cable rating study been based on single point bonding? If it has then solidly bonding the installation would potentially derate the cables. For your wind turbine feeder there is likely to be less of an effect because the current must be reasonably low if you are only using 1 cable per phase. For the connection between the transformer and 33kV switchgear, in my experience this is usually single point bonded. Over short distances the standing voltage on the ungrounded end of the cable is low.Which end you bond is down to Company preference. One Company I worked for bonded at the switchgear end on the basis that there was a higher likelihood of staff being preent in proximity. Another Company I worked for bonded at the source (Transformer) end by preference. As far as i know there is no standard.

Regards
Marmite

 

[7anoter4]

I agree with Marmite.However, according to BS 7430/2011
9.12 Earthing of sheath and/or armour of cables
9.12.2 Single-core cables
There are 3 method to connect to Earth :
a)End point bonding[ one end bonding] if the build-up induced voltage it is less than 65 V.
b)Mid-point bonding
c)For longer cable runs, cross-bonding has no restriction on permissible length between terminations.
In this case both ends may be bonded without increased losses.