Grounding conductor when using cable shield

[supadupachalupa]

Hi guys. I'm new to the forum and it already seems like there's a lot of good resources here. Hope I can get some help for a question I have.

I'm servicing a 7200-120/240 URD transformer from a switching station with 3-500 kcmil 15kV MV-105 cables but without a grounding conductor. I did the calcs and found that the shield on the conductors will be able to withstand the ground fault current. I tried looking in the NEC but couldn't find anything that will still require me to install the grounding conductor in this setup.

Any ideas or suggestions?

Thanks,
Dennis

 

[bob]

Hi guys. I'm new to the forum and it already seems like there's a lot of good resources here. Hope I can get some help for a question I have.

I'm servicing a 7200-120/240 URD transformer from a switching station with 3-500 kcmil 15kV MV-105 cables but without a grounding conductor. I did the calcs and found that the shield on the conductors will be able to withstand the ground fault current. I tried looking in the NEC but couldn't find anything that will still require me to install the grounding conductor in this setup.

Any ideas or suggestions?

Thanks,
Dennis

you say you are servicing a 7200 - 120/240 volt transformer. That sounds like a single phase transformer. Did you mean single phase or 3 phase? Is the primary wye or delta?

 

[supadupachalupa]

Great question bob! I just took a look at my project again and it actually doesn't have anything to do with the URD transformer. The 3-500 kcmil is between a substation and a primary switch. Voltage is still 7200V. A neutral grounding resistor at the substation transformer limits the fault current at the switch to about 1000 amps. Question is still the same. Do I need a separate grounding conductor if I know that the shield on the 3-500 kcmils can safely carry the fault current for the maximum anticipated fault duration?

Thanks,
Dennis

 

[charlie]

Dennis, I am having trouble understanding your system. You are talking about a 7.2 kV system but that normally denotes a 12.47 kV system with line to neutral loads. If this is true, you are also using an impedance grounding resister so you are prohibited from any line to neutral loads. Also, for fault duty, the minimum wire size is 6 AWG copper or 4 AWG aluminum so your shield wire doesn't cut it.

 

[weressl]

Great question bob! I just took a look at my project again and it actually doesn't have anything to do with the URD transformer. The 3-500 kcmil is between a substation and a primary switch. Voltage is still 7200V. A neutral grounding resistor at the substation transformer limits the fault current at the switch to about 1000 amps. Question is still the same. Do I need a separate grounding conductor if I know that the shield on the 3-500 kcmils can safely carry the fault current for the maximum anticipated fault duration?

Thanks,
Dennis

The grounding resistor ususally limits the fault to 400A or less.
Shielding is not designed nor should it designated to carry ground fault current.

I would like to see a shield that can carry 1000A, as you assert.

You are all over with your statements.....

 

[supadupachalupa]

Thanks for the input guys. Let me try to clarify again. Good catch by charlie but it is indeed a 7200/4160V system with a neutral grounding resistor at the substation transformer. Not sure if this is a weird system but it might have to do with it being a military base? 3-500 kcmils connect the secondary of the substation transformer to a primary switch. No line to neutral loads.

I'm not sure that a neutral grounding resistor can be generalized to limit the fault to 400A or less but I know it decreases it substantially. According to my computer model, it decreased it from ~8000A to ~1000A. By the way, I'm using EDSA Paladin Design Base 2.0 as my modeling software. It seems ok but a little buggy so if anyone knows of a better program please let me know.

I'm not sure whether the shield on a cable is designed purposely to carry fault current or 1000A but I'm fairly confident that it is able to. Please refer to http://ecmweb.com/design_engineering/electric_verifying_groundfault_coordination/. Hopefully the link works ok but if not please google the EC&M magazine for an article titled 'Verifying Ground-Fault Coordination for MV Power Cable Shielding'. It's best to have an actual copy of the magazine since the online version leaves out a very good graph. Basically, it shows the withstand-limit curve of the shield on a 15 kV 4/0 cable and it can carry 1000A for 0.2 seconds.

Fortunately for my case, I won't need to have a single shield carry 1000 amps. The article also shows that the fault current will be split evenly amongst the 3 cables so the withstand-limit on the cable is more like 333 amps for ~1.1 seconds which is well above the operation time of my protective device.

From a calculation standpoint, it seems that grounding the shields of the conductors on both ends (substation and switch) would be sufficient protection from ground faults. But perhaps the NEC or some other authority would still require a separate ground conductor be installed?

Thanks for the input and I would greatly appreciate your guys' help and insight on this.

Dennis

 

[weressl]

Thanks for the input guys. Let me try to clarify again. Good catch by charlie but it is indeed a 7200/4160V system with a neutral grounding resistor at the substation transformer. Not sure if this is a weird system but it might have to do with it being a military base? 3-500 kcmils connect the secondary of the substation transformer to a primary switch. No line to neutral loads.

I'm not sure that a neutral grounding resistor can be generalized to limit the fault to 400A or less but I know it decreases it substantially. According to my computer model, it decreased it from ~8000A to ~1000A. By the way, I'm using EDSA Paladin Design Base 2.0 as my modeling software. It seems ok but a little buggy so if anyone knows of a better program please let me know.

I'm not sure whether the shield on a cable is designed purposely to carry fault current or 1000A but I'm fairly confident that it is able to. Please refer to http://ecmweb.com/design_engineering/electric_verifying_groundfault_coordination/. Hopefully the link works ok but if not please google the EC&M magazine for an article titled 'Verifying Ground-Fault Coordination for MV Power Cable Shielding'. It's best to have an actual copy of the magazine since the online version leaves out a very good graph. Basically, it shows the withstand-limit curve of the shield on a 15 kV 4/0 cable and it can carry 1000A for 0.2 seconds.

Fortunately for my case, I won't need to have a single shield carry 1000 amps. The article also shows that the fault current will be split evenly amongst the 3 cables so the withstand-limit on the cable is more like 333 amps for ~1.1 seconds which is well above the operation time of my protective device.

From a calculation standpoint, it seems that grounding the shields of the conductors on both ends (substation and switch) would be sufficient protection from ground faults. But perhaps the NEC or some other authority would still require a separate ground conductor be installed?

Thanks for the input and I would greatly appreciate your guys' help and insight on this.

Dennis

Refer to IEEE-141, the Red Book, Chapter 7.2.2 for recommended resistance sizing based on fault and system charging currents. High resistance grounding should limit to 5-10A and low resistance to 400A typical and as low as 100A. 2000A is for very large systems with residual ground sensing.

What is the backup protection for the ground fault? The transformer OC protection on the primary side is sufficient to back up the secondary side protection, but when it comes to ground-fault it becomes much trickier. So while many says that shield tape or wire is 'designed' to carry a limited fault current, it is NOT its primary purpose and function. I believe it is a forced issue and when it is allowed to perform that duty it degrades its primary function on the long run. I would use a ground conductor and relie on the tape or wire shield to back up the missing or loose GC.

 

[supadupachalupa]

Thanks weressl for the response. I agree that relying on the shield to carry fault current is sketchy but was somewhat convinced after reading some articles. Glad others feel the same way as me. However, a lot of what we do is driven by money and we just need to explore ways to help the client.

I do have another question regarding the protection on the primary side of the transformer and how it relates to ground fault protection. Could you elaborate on how it's "trickier" for the primary protection to backup the secondary side protection during a ground-fault. IEEE-242, the Buff Book, Chapter 7.1 (3) states that "since ground-fault currents are not transferred through system power transformers that are connected delta-wye or delta-delta, the ground-fault protection for each system voltage level is independent of the protection at other voltage levels."

I'm having a little trouble grasping this statement and would like to know if I'm missing something.

Thanks again,
Dennis

 

[weressl]

Thanks weressl for the response. I agree that relying on the shield to carry fault current is sketchy but was somewhat convinced after reading some articles. Glad others feel the same way as me. However, a lot of what we do is driven by money and we just need to explore ways to help the client.

I do have another question regarding the protection on the primary side of the transformer and how it relates to ground fault protection. Could you elaborate on how it's "trickier" for the primary protection to backup the secondary side protection during a ground-fault. IEEE-242, the Buff Book, Chapter 7.1 (3) states that "since ground-fault currents are not transferred through system power transformers that are connected delta-wye or delta-delta, the ground-fault protection for each system voltage level is independent of the protection at other voltage levels."

I'm having a little trouble grasping this statement and would like to know if I'm missing something.

Thanks again,
Dennis

The question of money? What is cheaper? Installing a ground conductor now or early replacement of a MV cable and the cost of the unscheduled shutdown?

I pointed out that you do not have a backup protection for your ground fault if the device fails. To provide a backup, you go on the next preotective element upstream and coordinate it with the downstream. So if your secondary mains fail to react to the ground fault, the mext awailable point to interrupt the fault is the protective element on the primary side of the transformer. You have to rely on sensors(CT's) installed on the secondary side and zone coordinate them allowing the secondary to trip first. You can use the same set of sensors or an additional set.