Need help understanding a specific UD fault condition
Need help understanding a specific UD fault condition
(OP)
Hello,
A situation came up at work that I have been tasked to look into; we had some vandals do some damage to our distribution system and it resulted in a cable arcing on the ground until our powerline techs arrived.
It's a 25kV service area, with an overhead feeder line. 3 XLPE cables are connected to the O/H line and protected by a set of fused cutouts. The cables are in ducts (1 duct per phase) and feed a 75kVA pad mounted transformer. A section of 1 U/G cable was removed (while energised) by vandals/thieves and when our powerline techs arrived on site, the fuse on phase B had blown but one end of the phase B cable (still connected to the transformer) was arcing.
I looked at the relaying and fuse protection on the feeder to the customer, and my conclusion thus far is that everything worked as it should and that every reasonable step had been taken to protect the public. I was hoping to get some feedback on my reasoning to see if I have missed something or I am just way off base:
1. the initial cutting into the energised cable would cause the initial fault that tripped the phase B fused cutout.
2. since the other 2 phases remained energised, current was still flowing into the transformer windings from phases A and C.
3. current phasors A and C will remain 120 degrees apart since the transformer is only 75kVA and connected to the robust and much larger utility.
4. this means that the arcing witnessed is likely to be seen as a low current, high impedence fault with magnitude equal to the transformer phase imbalance.
5. the magnitude of this fault cannot be detected by the smallest fuse that coordinates with the transformer internal bayonet fuse.
I should point out no one was hurt and this was assigned to me more for the experience as I am just starting my career in engineering. Any input of any kind would be appreciated.
Thanks! :)
A situation came up at work that I have been tasked to look into; we had some vandals do some damage to our distribution system and it resulted in a cable arcing on the ground until our powerline techs arrived.
It's a 25kV service area, with an overhead feeder line. 3 XLPE cables are connected to the O/H line and protected by a set of fused cutouts. The cables are in ducts (1 duct per phase) and feed a 75kVA pad mounted transformer. A section of 1 U/G cable was removed (while energised) by vandals/thieves and when our powerline techs arrived on site, the fuse on phase B had blown but one end of the phase B cable (still connected to the transformer) was arcing.
I looked at the relaying and fuse protection on the feeder to the customer, and my conclusion thus far is that everything worked as it should and that every reasonable step had been taken to protect the public. I was hoping to get some feedback on my reasoning to see if I have missed something or I am just way off base:
1. the initial cutting into the energised cable would cause the initial fault that tripped the phase B fused cutout.
2. since the other 2 phases remained energised, current was still flowing into the transformer windings from phases A and C.
3. current phasors A and C will remain 120 degrees apart since the transformer is only 75kVA and connected to the robust and much larger utility.
4. this means that the arcing witnessed is likely to be seen as a low current, high impedence fault with magnitude equal to the transformer phase imbalance.
5. the magnitude of this fault cannot be detected by the smallest fuse that coordinates with the transformer internal bayonet fuse.
I should point out no one was hurt and this was assigned to me more for the experience as I am just starting my career in engineering. Any input of any kind would be appreciated.
Thanks! :)






RE: Need help understanding a specific UD fault condition