Medium Voltage Equipment Question

[Sparky2791]

Hello-



Have not done much design work with 15KV equipment.



Voltage is 12470/7200V



Have a primary service coming from a vacuum circuit recloser (supplied by power company) to a metal enclosed switchgear with fused switches. Feeding a liquid filled 500KVA transformer with one of the switches. My question is regarding Neutral and ground. Do I need a neutral wire? Instinct tells me I only need (3) 15 KV MV-90 EPR cables. #2 is the minimum size cable at this voltage per table 300.106(A) which works because fuse size is only 50A, I use 200% for primary side protection, and according to 310.60(C)(77) #2 is good for 145A. I guess I do not understand the 7200V. Is that the line-to-line voltage, which does not make sense to me?

Finally, if no neutral is required (?) a ground certainly would be. How is the size of the ground determined? Circular mills of the #2 cable? Ampacity of the fuse? I would think circular mils of the cable otherwise the ground size would only be a #10 which does not seem correct to me at all.



Any articles I can read to help me understand are always appreciated.



Thanks for your reply

 

[paulengr]

Hello-



Have not done much design work with 15KV equipment.



Voltage is 12470/7200V



Have a primary service coming from a vacuum circuit recloser (supplied by power company) to a metal enclosed switchgear with fused switches. Feeding a liquid filled 500KVA transformer with one of the switches. My question is regarding Neutral and ground. Do I need a neutral wire? Instinct tells me I only need (3) 15 KV MV-90 EPR cables. #2 is the minimum size cable at this voltage per table 300.106(A) which works because fuse size is only 50A, I use 200% for primary side protection, and according to 310.60(C)(77) #2 is good for 145A. I guess I do not understand the 7200V. Is that the line-to-line voltage, which does not make sense to me?

Finally, if no neutral is required (?) a ground certainly would be. How is the size of the ground determined? Circular mills of the #2 cable? Ampacity of the fuse? I would think circular mils of the cable otherwise the ground size would only be a #10 which does not seem correct to me at all.



Any articles I can read to help me understand are always appreciated.



Thanks for your reply

Use MV-105. Same price, higher ampacity. It’s not MV-105 EPR by the way it’s just MV-105. The insulation is part of the spec. You MUST use termination kits here. If you have no experience installing them get someone who is. At those voltages even accidentally slicing something or crimping the lug with say an indent crimper or not filing edges can cause major issues (failures). Lots of little things not in the 2-3 pages of instructions. Call a larger local motor shop if you don’t know someone. They do these all the time.

No neutral assuming transformer is delta primary. If not delta-wye for most customers that’s mistake #1, 2, and 3. If you go wye-wye every ground fault on the utility side is also a ground fault on the secondary side. The utility generally doesn’t supply a neutral.

Size ground sane as all service entrances. No change there. The only thing new here are the shield drains. Tie back to ground. They aren’t very big and just drain capacitive charges. Currents are under 1 A. They have to be there but size is no critical.

 

[Sparky2791]

Use MV-105. Same price, higher ampacity. It’s not MV-105 EPR by the way it’s just MV-105. The insulation is part of the spec. You MUST use termination kits here. If you have no experience installing them get someone who is. At those voltages even accidentally slicing something or crimping the lug with say an indent crimper or not filing edges can cause major issues (failures). Lots of little things not in the 2-3 pages of instructions. Call a larger local motor shop if you don’t know someone. They do these all the time.

No neutral assuming transformer is delta primary. If not delta-wye for most customers that’s mistake #1, 2, and 3. If you go wye-wye every ground fault on the utility side is also a ground fault on the secondary side. The utility generally doesn’t supply a neutral.

Size ground sane as all service entrances. No change there. The only thing new here are the shield drains. Tie back to ground. They aren’t very big and just drain capacitive charges. Currents are under 1 A. They have to be there but size is no critical.

Thank You for your reply. I am an electrical designer so Ill leave the installation details up to the professional installers for sure. However, I am the type of designer who likes to fully understand what I place on my drawings as if I had to install it myself. I've always said I would like every contractor to spend 1 year working as an engineer and every engineer to spend 1 year working as a contractor. Imagine the harmony in the world of construction & design.
Yes - Delta Primary - Wye secondary on the LFPM transformer.

Makes sense on the ground size as it is a service entrance , 250.66 it is.

 

[Julius Right]

The grounding wire cross section area it depends on the short-circuit current level and the fault clearance time.
If we take the maximum short-circuit as per IEC standard 60076-5 Power Transformer Ability to withstand short-circuit Table 2 Short-circuit apparent power of the system, for up to 24 kV it is 500 MVA. So, for 12.47 kV Ik=500/sqrt(3)/12.47=23kA.
If we take 0.1 sec the fault clearance time and use the formula 38 from IEEE 80/2013 :
I=5.07/10^3*Akcmil*sqrt(Tcap/tc/αr/ρr*ln((Ko+Tm)/(Ko+Ta)) kA
or:
Akcmil=Ik*10^3/5.07/sqrt(Tcap/tc/αr/ρr*ln((Ko+Tm)/(Ko+Ta))
According to IEEE 80/2013 Table 1 —Material constant for copper conductor , annealed, soft-drawn: Tcap=3.4;αr=0.003 93;ρr=1.72;Ko=234
Ta=40oC Tm=1084[for bare copper far from cable] but only 250oC for EPR or XLPE insulation.
Akcmil=Ik*11.659*sqrt(tc)
If Ik=23 kA and tc=0.1 sec then Akcmil=84.8 [approx.1/0 awg]

 

[Julius Right]

I forgot to mention the short circuit current-if the system neutral is not grounded as MV usually, then the short-circuit will be the phase-to ground-to phase fault.