pinchedloaf
Electrical
Hi All,
I am working on a low voltage TN-C-S installation with unshielded, unarmoured copper 4C&E or 4x1C + E cables installed on a cable ladder. I need to create an ETAP model of this installation for a client to calculate line-ground faults and I need to input zero sequence impedance data for these cables. We have previously only used simplified short circuit calculation techniques for LV installations, and symmetrical components at HV where this sort of data is available from manufacturers.
Most information available on the web just says get the zero sequence Z info from the cable manufacturer due to influence of cable screen, cable armour etc. I would have thought some conservative assumptions could be made for my scenario above (simplified due to absence of screen/armour) but can't find any supporting evidence on-line or in relevant standards.
An ABB paper I have found (ABB_11_E-03) implies the cable zero sequence R and X can be assumed as follows:
R(0) = R(L) + 3.R(PE)
X = (3.5 ... 4).X(L)
Where R(PE) is the positive sequence resistance of the protective earth conductor within which it is assumed 100% of the zero sequence current will flow. I would think this is reasonable as the impedance of any other path should be much higher than the protective earth conductor which connects to the transformer neutral via a series of earth conductors/earth bars (perhaps a few hundred metres maximum).
Unfortunately I can't find any further information to reference (e.g. standards, papers etc.) which corroborates the assumptions above.
Would appreciate any thoughts/comments on whether the above is reasonable.
Cheers
I am working on a low voltage TN-C-S installation with unshielded, unarmoured copper 4C&E or 4x1C + E cables installed on a cable ladder. I need to create an ETAP model of this installation for a client to calculate line-ground faults and I need to input zero sequence impedance data for these cables. We have previously only used simplified short circuit calculation techniques for LV installations, and symmetrical components at HV where this sort of data is available from manufacturers.
Most information available on the web just says get the zero sequence Z info from the cable manufacturer due to influence of cable screen, cable armour etc. I would have thought some conservative assumptions could be made for my scenario above (simplified due to absence of screen/armour) but can't find any supporting evidence on-line or in relevant standards.
An ABB paper I have found (ABB_11_E-03) implies the cable zero sequence R and X can be assumed as follows:
R(0) = R(L) + 3.R(PE)
X = (3.5 ... 4).X(L)
Where R(PE) is the positive sequence resistance of the protective earth conductor within which it is assumed 100% of the zero sequence current will flow. I would think this is reasonable as the impedance of any other path should be much higher than the protective earth conductor which connects to the transformer neutral via a series of earth conductors/earth bars (perhaps a few hundred metres maximum).
Unfortunately I can't find any further information to reference (e.g. standards, papers etc.) which corroborates the assumptions above.
Would appreciate any thoughts/comments on whether the above is reasonable.
Cheers