CSneider
Member
- Location
- Maumee, OH
Hello,
Long time follower, first time poster on the forum.
Lately, I've been running into more and more questions on the grounding of shields on medium voltage cables, specifically in regards to whether or not we should tie the shields together and ground them at both ends or one end. Been trying to get more opinions on the matter.
Some background:
We do a lot of work in the oil & gas industry, specifically at pumping stations along the pipelines. Because of the size of the motors at these stations (5kV, 2500HP and greater) and the size of the transformer required to provide the necessary capacity, it is quite common to see fault currents around 20kA.
If we choose to ground the shields at both ends, this will cause some circulating current and lower the cable ampacity; however, this is not what we are concerned about. When we ground at both ends, we allow the shield to become a path for ground fault current. Granted, it is a significantly higher impedance path than the EGC ran with (or inside) the cables, so it will see a small portion of the ground fault current. But a small portion of 20,000 can still be 200-2000 amps for a given amount of time, which can certainly be enough to melt the copper tape shield on these cables, especially on our service entrance conductors that do not have overcurrent protection (aside from the transformer primary fuses, and through-fault clearing times on these can be seconds, which is an eternity in the electrical world).
We, as a company, have seen cable shields melt during faults leading to a complete cable replacement even though the cable itself was perfectly fine.
What we prefer to do, is to tape back the cable shields at one end. This will cause induced voltages on the shield, but these cable runs are short (<400 feet typically), so the induced voltage will be less than 25V (Okonite's recommended maximum allowed induced voltage on shields when grounding at one end).
This ensures that we do not have to worry about fault currents that the shield may or may not be able to handle, and since the induced voltage is so low, we do not have to worry about potential personnel harm.
Recently, this approach has been questioned more than normal. Even Okonite and Southwire will always recommend grounding the shields at both ends. But we are not comfortable taking the risk that we may melt a cable shield. These stations are critical for pipeline operations and downtime is difficult to obtain, so having to do an emergency cable replacement is not ideal. Especially because finding several hundred feet of medium voltage cable and getting it on a truck and delivered within a day is nearly impossible sometimes.
Sure, we can do an analysis on all of the possible GFRPs and get an approximation for shield fault current and duration of the fault, but even this is still just an approximation, I would need a large margin of error before I felt comfortable saying there is no way we could melt the shields.
Has anyone else had any experience with this and can shed some light on your perspective/experiences?
Thanks much
.
Long time follower, first time poster on the forum.
Lately, I've been running into more and more questions on the grounding of shields on medium voltage cables, specifically in regards to whether or not we should tie the shields together and ground them at both ends or one end. Been trying to get more opinions on the matter.
Some background:
We do a lot of work in the oil & gas industry, specifically at pumping stations along the pipelines. Because of the size of the motors at these stations (5kV, 2500HP and greater) and the size of the transformer required to provide the necessary capacity, it is quite common to see fault currents around 20kA.
If we choose to ground the shields at both ends, this will cause some circulating current and lower the cable ampacity; however, this is not what we are concerned about. When we ground at both ends, we allow the shield to become a path for ground fault current. Granted, it is a significantly higher impedance path than the EGC ran with (or inside) the cables, so it will see a small portion of the ground fault current. But a small portion of 20,000 can still be 200-2000 amps for a given amount of time, which can certainly be enough to melt the copper tape shield on these cables, especially on our service entrance conductors that do not have overcurrent protection (aside from the transformer primary fuses, and through-fault clearing times on these can be seconds, which is an eternity in the electrical world).
We, as a company, have seen cable shields melt during faults leading to a complete cable replacement even though the cable itself was perfectly fine.
What we prefer to do, is to tape back the cable shields at one end. This will cause induced voltages on the shield, but these cable runs are short (<400 feet typically), so the induced voltage will be less than 25V (Okonite's recommended maximum allowed induced voltage on shields when grounding at one end).
This ensures that we do not have to worry about fault currents that the shield may or may not be able to handle, and since the induced voltage is so low, we do not have to worry about potential personnel harm.
Recently, this approach has been questioned more than normal. Even Okonite and Southwire will always recommend grounding the shields at both ends. But we are not comfortable taking the risk that we may melt a cable shield. These stations are critical for pipeline operations and downtime is difficult to obtain, so having to do an emergency cable replacement is not ideal. Especially because finding several hundred feet of medium voltage cable and getting it on a truck and delivered within a day is nearly impossible sometimes.
Sure, we can do an analysis on all of the possible GFRPs and get an approximation for shield fault current and duration of the fault, but even this is still just an approximation, I would need a large margin of error before I felt comfortable saying there is no way we could melt the shields.
Has anyone else had any experience with this and can shed some light on your perspective/experiences?
Thanks much
.