Stress Cones on 5kV cables 2

[rockman7892]

I have recently been involved with a discussion regarding the use of stress cones on 5kV motor leads for a particular motor.

The first part of the discussion regarded weather or not stress cones were needed on 5kV cable. I was always under the impression that these kits were indeed needed on 5kV cables and was looking for a standard or reference to back me up since he was arguing otherwise. Are these stress cones required on all 5kV shielded cables? Is it legal or a good practice to use non-shielded MV cable for a 5kV cable run?

The second part of the discussion regarded how motor leads were terminated onto a 5kV motor. Because this motor does not have a surge caps or arrestors there is not bus connection at the motor to terminate the motor leads or motor feeders on. Therefore a Motor Lead Pigtail Splice kit was used with the motor feeders and motor leads bolted together. This kit does not have a ground around the shield at the end of the motor feeder cable to tie to ground at the motor. I was under the impression that you indeed needed to tie the shield to ground at "BOTH" ends of the cable to ensure uniform charge distribution at both ends, and dont present a path for charge flux to go out the end of the cable. The contractor is arguing that as long as shield is grounded at the source end then this is o.k.

Also as one last point, this motor happens to be a 6-lead motor that is connected to our source in a wye configuration. When connecting the T4,T5,T6 leads together for the wye configuration there was no splie kits or any other kits used and it was just bolted and taped. I know that if everything is blanced there will be no voltage at this wye point, but for an unbalanced condition there may be a voltage with respect to ground at this point. Does this junction need some kind of kit as well?

I would appreciate any insight into this application.

 

[dpc]

It used to be fairly common (or not uncommon) for some industrial sites to use unshielded 5 kV cable. But in US, the NEC now requires shielded cable at 5 kV and has for many years.

Shield grounding has been discussed before in this forum. There are technical advantages and disadvantages to both methods (one end or both ends).

I believe the Okonite website has some technical info on this issue. We always ground at both ends.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[rockman7892]

I checked out the Oakanite website as mentioned and found the following article on shielding.

http://www.okonite.com/engineering/shielding.html

The do state that shielding should be used on anything 2000V and above, and as dpc mentioned, it appears that this is required by code here in the U.S. I'm not sure about the stress cones however.

The article also states that the cable should be grounded at both ends to prevent voltage buildup on the cable which would damage the cable and be a hazardous shock potential. It does point out, that by doing this you could be setting up for circulating currents in the shield that may cause heating, and therefore decrease the overall heat carrying capacity of the cable.

The bottome of the article gives a table recommending the distance between grounded points on the cable. I interpert this, that in a case such as mine were we are only dealing with 250ft or so and cannot ground at the other end due to the splice kit it would be o.k. since this distance falls under that recommended by the table. Just my interpertation, however I'm open to others opinions or expertise.

 

[dpc]

If you have shielded cable, you must have some type of stress relief, I believe, to handle the transition from shielding to no shielding. Cable must be installed in accordance with manufacturer's requirements, and that will include stress cones.

With very short runs of cable, grounding at one end should probably not pose any safety concerns.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[alehman]

The National Electrical Code requires shielding for all permanent installations 2000 volts, except for series lighting circuits for airfields. It also requires "stress reduction means" at all terminations of factory applied shielding.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[esee135]

Shielded is the preferred method these days. Older installations (1920's) were unshielded, 1940's also had non-shielded cable leads into generators, and the use of 11kV non-shielded instrument transformer connections. With good line to line, line to phase clearances, you could make anything work in the unshielded world.

The 'pigtail' to the stator leads is performed unshielded up to the phase outlet terminals, all splices made of similar material to the generator ground wall insulation system. Past the phase outlet continues with today's standards of shielded power cables.

For bare leads of the stator to customer connection, shielded cable should not be used since you will bring ground potential into the stator, hence the 'phase outlet' design common across all the manufacturers.

I agree with dpc, once you commit to a shielded cable, you're in bed with the termination kit. I've dealed with a few termination kit failures, and the tracking voltage literally splits the kit, semi-cons and dielectrics open during a line to line/line to ground failure of the HV kit. Unfortunately with shielded cables, post failure, you have to become very creative during the repair since the semi-con and dielectric layers can't be reused after a failure, and large cables are not readily available. In some ways dealing with unshielded cables is nicer post failure. Good luck finding 15kV unshielded cables off the shelf with high MCM diameter.

 

[rockman7892]

So it sounds like that here in the U.S., for any voltage 2000V and above, a shielded cable needs to be used and for any shielded cable that has manufacture installed shielding a stres relief needs to be used on the ends.

Attached for reference is the instruction sheet for the pigtail splice kit that was used.

As far as grounding the shielding to eliminate dangerous potentials even if you are grouding the cable at the motor end, aren't you still esentially grounding the shielding to the EGC, which would have the same distance as the cable run? Is it due to the fact that long distance of the shielding might have a higher resistance in comparison with the EGC?


esee135

Can you please elaborate further on your comments:

"The 'pigtail' to the stator leads is performed unshielded up to the phase outlet terminals, all splices made of similar material to the generator ground wall insulation system. Past the phase outlet continues with today's standards of shielded power cables.

For bare leads of the stator to customer connection, shielded cable should not be used since you will bring ground potential into the stator, hence the 'phase outlet' design common across all the manufacturers."


In this case where you are bringing a shielded cable to pigtaile splice with an unshielded stator lead are you saying that you do not want to connect the shielding to the stator lead to bring the ground potential into the stator? How does this differ when you are not using a splice kit but rather are connecting cable to a bus bar in a motor termial box on which the stator leads are connected to? Does this effect the grounding at the cable/motor lead interface?

 

[rcw retired EE]

Voltage impressed on the shield is the governing factor for distance between MV cable shield grounding points. The load current in the cable induces a voltage in the shield. If the shield is grounded at both ends, current flows in the shields adding some temperature rise to the cable. If one end is left ungrounded, there is no current flow but the exposed end of the shield wires at the non-grounded end could develop a hazardous voltage.

The Okonite tables and similar guides limit the distance to keep the impressed voltage below some arbitrary "safe" limit. Note that transposing the shields on long runs is used to limit the shield voltages and currents.

If your circuit has an EGC wire, connecting the shields to that earthed conductor is a good idea.

 

[rockman7892]

Looks like there is a plus and minus to grounding both ends of the shield. For my particular case we are talking about a 500ft cable run with about 35A of operating current on a #1/0 cable. It looks like this would be o.k. for for only having grounding at one point. It sounds like as stated above that these distances are determined by the amount of voltage that would be presnet on the cable under certain conditions.

If grounded at both ends will there be current circulating in the shield due to the fact that there will be a difference in ground potential at both ends and thus a voltage difference for current to flow. Why would currents not circulate in the shield if only grounded at the one end. Is it the EGC or grounding path that completes the circuit for current to flow?

For long runs of shielding without grounding, how do these voltage potentials build up? Is this because the only path is the capacitance coupling to ground and thus all the voltage is between shield and ground where as if grounded at both ends the volage is all dropped along the series resistance of the shield and ground combination?

 

[dpc]

Have you looked at the discussion on the Okonite website:

http://www.okonite.com/engineering/shielding.html

It does a good job explaining the issues related to shield grounding.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[esee135]

In reference to the phase outlets, large generators (hydro-gen) usually have the entire stator winding up to and past the parallel circuit leads unshielded in mica wrapped bus (whether by rect bar, round tubing, solid round bus) aka circuit ring bus, with taps to the phase outlet terminal connections. This are of the stator enclosure facilitates electrical clearances either by GPO3, G11 insulation with lashing or proper insulators and bushings to allow for a good bus or cable interconnection to 'customer' switchgear.

Looking at your PDF, if you did a proper design with good clearances you could technically bring an XLPE cable with a termination kit into the stator enclosure, I haven't seen this in hydro gen. It seems in the rewinds I've been on that there's usually a hard line where unshielded installation ends, and shielded installation begins, usually at the phase outlet location which serves as a bushing area or actual wall bushing exiting the stator enclosure.

 

[JBUDA54]

http://www.eng-tips.com/viewthread.cfm?qid=229637&page=1