Grounding in CENELEC HD 637 S1

[radug]

Dear forum members,

I am considering using a common ground for both LV and HV. The guide of the subject (CENELEC HD 637 S1) in its table 6 says that in order to have a unique common ground for both HV and LV, in case of a TN system in LV, the requirement is:
Ue<=x*Utp where
Ue=ground potential rise
Utp=touch potential
The same publication says that typical x values are x=2 and that values up to 5 are possible.

Does anyone have expirience with this topic? Specifically, how are X values determined? Does X=2 mean that you put connect the transformer secondary neutral (LV) to the substation grid (HV) using 2 cables and to 2 different points of the grid? An with X=3, 3 cables and 3 points and so on?


Thanks.

 

[7anoter4]

I think that CENELEC HD 637 S1 Standard is based upon DIN VDE 0141.
So in the Table no.6 in VDE 0141 for a System is with low-resistance neutral point earthing and if UE>2*(Touch permissible potential) but UE<3000 V and clearance fault time <=0.5 sec. then the earthing is suitable for indoor and outdoor installation.
UE=w*r*I"k*ZE where w= Expectancy factor; r=reduction factor; ZE=Earthing impedance
Reduction factor=1 if no earth wires or cable sheaths are connected.
According to type and number of earth wire reduction factor may be from 0.60 to 0.97.
As an approximation ZE [including Zp parallel earth electrodes as cables, overheadlines, water pipes etc.] would be:
ZE=1/ (1/RA+1/Zp) where RA is Earth Grid [Remote] Resistance.
The touch permissible potential depends [inversely] on the clearing fault time and will be 1000V for 0.07 sec. and decrease up to 65 V for 3 sec.

 

[radug]

Dear 7anoter4,

I do know how to calculate UE and ZE. However, I am not familiar with that "w" parameter you called "expentancy factor". I do not think I have seen anything about it in IEC or IEEE standards. Perhaps it is only in VDE 0141. It would be of no application for my country although I would like to know which are typical values for "w" and how are they calculated.

In Spain, we have regulations concerning earthing in HV and LV which are mainly based in IEC or CENELEC standards, however they are not exactly the same.

For example, in the topic I asked about, spanish regulations say that a global earthing system is admitted only if UE<Maximum allowable touch voltage in case of a TN system or UE<1200V in case of a TT system.

However CENELEC HD 637 S1 says that the requirement for a global earthing system in case of a TN system, is UE<Utouch if neutral is connected to ground only in the substation (the same as spanish regulations) or UE<x*Utouch when the neutral is connected to ground in several points "to limit the neutral voltage".

I need clarification of the "x" parameter, because it is my believe that a global earthing system is preferred whenever possible and for that, the "x" parameter calculation would be vital because it would allow many systems to have a global earthing.

 

[7anoter4]

I'm sorry I can't help you with "x" clarification. The expectancy factor is 1 for rated voltage less then 110 kV and it is about 0.7 for rated equal or more than 110 kV. Never the less we are close to European Standards in Power Station design we prefer IEEE-80 for grounding calculation.