Sparking while installing neutral cable 1

[bacon4life]

I recently heard about some arcing that puzzled me and I am hoping someone might have some idea of a plausible explanation for the arcing.

Situation:
A crew was replacing substation getaway #1, consisting of 150 ft of underground 15 kV cable between the substation switchgear the overhead termination. The installation consisted of three - 350 kcmil 15 kV cables with tape shields plus a bare 4/0 neutral. When the crew touched the 4/0 neutral to the metal beam switchgear enclosure, there was loud and arcing. The arcing continued as the bare conductor was moved across the surface of the beam. I do not know the grounded/isolated status of the opposite end of the cables at the overhead termination. The arcing did not result in significant burning/melting/charring.

The distribution system is wye grounded with a multi-grounded neutral. The substation was in service, with other getaways carrying load when the arcing occurred. The phases on the in- service getaway are somewhat imbalanced, causing flow on the multi-grounded neutral. I am guessing maybe 600 amps of load with 100 amps of neutral current.

Possible explanations include:
1)The imbalance on the in-service getaways was causing flow on the multi-grounded neutral. When the bare copper neutral on getaway #1 contacted the switchgear beam, it formed a parallel path. As the bare conductor moved across the painted surface of the switchgear beam, the path was intermittently interrupted causing arcing. Is there any easy way to estimate likelihood this could generate enough voltage to cause noticeable arcing? A simplistic calculation of voltage resulting from 100 amps flowing through a single cable is 100 amps * 0.256 ohm/1000 ft * 150ft = 4 volts. The system is multi-grounded, so I would expect that only a portion of the imbalance current actually flowed on 4/0 cable installed in the conduit with the 350 kcmil insulated cables.

2)The neutral cable in getaway #1 was energized by induction by sharing a duct bank with other getaways. I assumed not much voltage would be induced by running in parallel for less than 100 ft. I remember doing this sort of calculation by hand in college, but I am hoping someone might have an online calculator I could do quick sanity check.

3)The bare neutral could have picked up capacitive coupling from the overhead 15 kV or distribution line or the 115 kV transmission line. However, the conduit riser going up the pole is vertical, so the cable was mostly perpendicular to the distribution and transmission lines.

4)Anything else?

 

[Parchie]

All of the above. There is a reason why most people work on dead wires so things don't go wrong. I'd bet those things did not happen if you shut down the substation prior to doing the job/s.

 

[FacEngrPE]

Current flow will be divided between all possible paths. The difference in available voltage with your new connection making contact and not making contact is likely small, but is comprised of enough components it is pointless to calculate.

I would be concerned with the life safety aspect of possibly a person being inserted into this circuit. Recommend you consider this a near miss, and reconsider the work practice, as the neutral bonding system is obviously carrying current in a manner that was not expected.

 

[DBL-E]

Do the 115 kV also have shield wires or is there any portion of the 115 and 15 kV that have a shared corridor? Induction is a pretty common issue for work that supposed to be de-energized.

You can measure the existing current on the neutral 4/0 with a clamp on meter.

You can also use a fall of potential style measurement to see the substation ground grid voltage, but you just need to voltage probe to determine if the grid is at an elevated potential relative to 'remote earth'.

Are there any air-core reactors at the station, as those can induce into the ground grid if it is installed too close?

 

[stevenal]

I do not know the grounded/isolated status of the opposite end of the cables at the overhead termination.

I'm guessing the neutral was made up, and connected to the in service feeder loads. Unbalance current would have a parallel path back to the source.

I'm with FacEngrPE on this. Work rules were likely violated. Incident investigation should be performed to find root cause(s) and to avoid reoccurrence.

 

[bacon4life]

I am surprised folks jumped straight to discussion work practices. Utility workers in the USA routinely work on live equipment using appropriate insulated tools and techniques. I was hoping to start by understanding the fundamental explanation for the arcing in this case. This case appeared to be significantly different than arcing I have looked at due to induction/capacitive coupling on overhead transmission lines.

Can 2 volts AC cause noticeable arcing when making/splitting a parallel path? Maybe my assumption about a 2 volt arc being tiny is wrong?

DBL-E -Unfortunately this occurred in the past, and I am just curious rather than directly involved. There were no air core reactors at the station. Although the 115 kV and overhead 15 kV lines do share a pole line in the street, the getaway is perpendicular to the street. The getaway cables are rather short at around 150 ft.

 

[stevenal]

Since safety is paramount among engineers, I don't find the concern surprising.
Touching a neutral to an enclosure does not sound like an appropriate technique, since as FacEngrPE pointed out, a worker could be in the path.

 

[Kiribanda]

You have mentioned that the NEUTRAL of your 15kV WYE distribution system is MULTI GROUNDED?
Would you mind letting us know why the NEUTRAL is multi grounded? Is it a code requirement in the country where the installation is or otherwise?
Do you agree to remove the multi grounding and to use single point grounding at the WYE point only?

 

[stevenal]

Kiribanda,
Multi-grounded neutral utility distribution systems are very common in North America. Simply removing grounds is not possible, the system would need to be completely re-configured.

 

[prc]

I used to caution substation engineers not to remove or tighten the transformer body grounding wire to the substation earthing mat while the transformer is alive. . There will be a current always in the tank grounding lead, and I have seen a current as high as 800 A in the case of a high current rectifier transformer.
There are cases of death from arc flash while miscreants tried to steal grounding copper in live substations.

 

[bacon4life]

Kiribanda-Multi-grounded neutral distribution is extremely common throughout my region of the USA. I don't know we do it this way, but switching safely to a different grounding system would be an enormous challenge. (Although plenty of copper thieves would be happy to convert the system in an unsafe manor by stealing all the pole grounds)

prc- There was a substation not to far from my house where someone tried to steal the grounding leads on a transformer, which resulted in serious damage to both the person and the equipment.

stevenal-At some point during the installation the bare neutral cable has to be bolted to neutral bus was already bolted to the enclosure. I have not directly seen the site, so I assumed the bare cable touched the enclosure similar to how the shield wires in this photo touch the wall.

comparison photo from

https://www.powerandcables.com/partial-discharge-monitoring-location/

 

[stevenal]

bacon4life,

I would propose doing the connections you depict first. The connection to the overhead system neutral at the other end would be done last with a hot stick. This way the workers are never in the circuit.

 

[Kiribanda]

If the neutral is grounded at multiple locations then it is obvious that unbalanced or and ground fault currents can flow through those multiple paths.
That means the moment you open one of them you should see an arc depending on the current your are breaking.

 

[bacon4life]

Stevenal-I had assumed that the practice of using insulated tools and appropriate PPE was required due to the possibility of fault currents rather than due normal neutral currents. The fault currents could be 2 orders of magnitude higher than the normal neutral current. Is there any reason for arcing would different depending which end of the neutral was made up first?

Kiribanda-Yes, it is obvious that some amount of arcing would take place. Line workers are used to substantial amounts of arcing when they apply grounds on parallel HV transmission lines, so I assumed the arcing on this neutral installation would have had to have been non-trivial to be noteworthy enough to mention as unusual. Based on the discussion, I may under estimated how low of a voltage can cause noticeable arcing. So far I have not found the right search terms for arcing at voltages in the range of 1-2 volts.

 

[stevenal]

bacon,

Required, I agree. Sounds like it may not have been done.
The arcing current when the last connection is made would be the same, but the stick ensures that last connection occurs away from the worker while keeping the worker outside the path. The rule I know is always connect ground end first, system last. All of this would be gone over during the tailboard job briefing.

I believe your estimate of 1- 2 volts might be low. According to Alstom, Ra=28710*La/Ia. Ra is arc resistance in ohms, La is arc length in meters, and Ia is arcing current.

 

[davidbeach]

Back in the day, maybe 25 years ago or so, I spent a day with an electrician crew at a facility that had primary service and their own site distribution. The primary service was deenergized and an emergency generator was powering the lighting it supplied. One step in the work was the disconnect the neutral-ground jumper in a some 480V switchgear, the point at which that separately derived system was bonded to ground. That drew an impressive arc. Apparently the neutrals in dual sourced luminaires weren't all kept separate and it was a much lower impedance path back to the generator via the normal system neutral and grounds. So, some relatively low voltages and currents and still an attention grabbing arc.

I’ll see your silver lining and raise you two black clouds. - Protection Operations