stevesummers
Electrical
- Nov 2, 2015
- 26
Hi All,
i am trying to size some surge arresters for a buried 33kV underground cable system using the IEC 60071 and 60099 standards, and i am getting a little lost - can anyone help shed some light / check my working.
The system is a 33kV network, which is impedance earthed through zig/zag and NER. There is a long cable run of 4km, 1c x500mm2, aluminium, configured in two parallel circuits from the source substation to our substation.
For various reasons, i am not worried about lightning, but mainly switching surges, for bringing in/out the cable circuits due to any charging current. So according to the IEC 60099 standard, I assume that a type SH, SM or SL arrester is needed, as the distribution range (DH, DM, DL) don't protect against switching?
I am looking at the Siemens MV Surge arrester catalogue, as it seems reasonably detailed - but ho preference to use them.
Voltage selection, based on impedance earthing
Uc = Us = 33kV and round up to 36kV
Ur = 1.25 * Us = 41.25kV and round up to 45kV
Surge Impedance Calculation:
Cable L = 334uH /km
Cable C = 0.298uF / km
Surge impedance (Z) = sqrt(L/C)
Z = 33.5
Charge Transfer
Using IEC 60099-5 equation 8a (also appears in quite a lot of application guides)
Qs = (Urp-Ures)/ Z *2 * l/c
Where
Urp = maximum switching voltage (I assume this is the maximum normal system voltage so 36kV)
Ures = residual voltage (not sure how to get this ?)
Z = surge impedance (as calculated above)
l = cable length (in km?)
c = speed of light
Its at this point i am a bit lost, as I am not entirely clear how you get Ures - do i pick a likely surge arrester, look at its value for residual voltage, and then use this to calculate Qs
This seems to tie back to line discharge class - which i can't also quite get my head around. I assume that as its a relatively low voltage and short length the line discharge class is 1 and the Peak currents would be under 125 and 500A.
So if I take a 3EJ3 045 - 0 C J 4 1 - 4 (page 44 of the Siemens manual) - the resiudal value for a 125 30/60uS is shown as 79.8kV, but if i then drop that back in my original formula for Qs i get a negative value, which cant be correct?!
All help much appreciated
i am trying to size some surge arresters for a buried 33kV underground cable system using the IEC 60071 and 60099 standards, and i am getting a little lost - can anyone help shed some light / check my working.
The system is a 33kV network, which is impedance earthed through zig/zag and NER. There is a long cable run of 4km, 1c x500mm2, aluminium, configured in two parallel circuits from the source substation to our substation.
For various reasons, i am not worried about lightning, but mainly switching surges, for bringing in/out the cable circuits due to any charging current. So according to the IEC 60099 standard, I assume that a type SH, SM or SL arrester is needed, as the distribution range (DH, DM, DL) don't protect against switching?
I am looking at the Siemens MV Surge arrester catalogue, as it seems reasonably detailed - but ho preference to use them.
Voltage selection, based on impedance earthing
Uc = Us = 33kV and round up to 36kV
Ur = 1.25 * Us = 41.25kV and round up to 45kV
Surge Impedance Calculation:
Cable L = 334uH /km
Cable C = 0.298uF / km
Surge impedance (Z) = sqrt(L/C)
Z = 33.5
Charge Transfer
Using IEC 60099-5 equation 8a (also appears in quite a lot of application guides)
Qs = (Urp-Ures)/ Z *2 * l/c
Where
Urp = maximum switching voltage (I assume this is the maximum normal system voltage so 36kV)
Ures = residual voltage (not sure how to get this ?)
Z = surge impedance (as calculated above)
l = cable length (in km?)
c = speed of light
Its at this point i am a bit lost, as I am not entirely clear how you get Ures - do i pick a likely surge arrester, look at its value for residual voltage, and then use this to calculate Qs
This seems to tie back to line discharge class - which i can't also quite get my head around. I assume that as its a relatively low voltage and short length the line discharge class is 1 and the Peak currents would be under 125 and 500A.
So if I take a 3EJ3 045 - 0 C J 4 1 - 4 (page 44 of the Siemens manual) - the resiudal value for a 125 30/60uS is shown as 79.8kV, but if i then drop that back in my original formula for Qs i get a negative value, which cant be correct?!
All help much appreciated