Proper X0 bond and parallel current imbalance 1

[tg103]

During a routine IR scan of a facility's main 4000A switchboard we discovered the following abnormality.

First off, the service entrance is underground 4" PVC duct bank consisting of (10) parallel sets of (4)500 MCM,& 3/0 insulated green equipment grounding conductor for 3PH, 4W, 480/277V. The switchboard is served from a padmount (poco owned) transformer located outdoors approximetly 150' away.

IR scan picked up two 3/0 conductors out of the set of ten that had considerably more heat than the others. We clamped and had around 200A on each of the two in question, the others could not be reached safely. I flagged the problems in the report and strongly recommended further investigation and to coordinate access to the poco's transfomer.

It almost appears somehow that a parallel path of neutral current exists on these 3/0's. The main bonding jumper, and electrode(s) exist at the switchboard. I assume I will find XO is bonded as well in the padmount and also curious why this imbalance exists on these 3/0 conductors. Guys am I on the right track?

Still waiting for the ok to proceed...I'm extremely anxious to see what the other end looks like!!!

 

[waross]

A couple of basic points to be aware of when evaluating on site heating and current unbalances.
This is well known to many of the posters here but may be of interest to the members following this thread to increase their knowlege.
It is very helpful to report both temperature and heating on each cable.
If there is a cable configuration problem affecting the impedance of the cables, the overall current distribution of the current will be affected. The cables with the most current will be the hottest.

Poor connections will cause local overheating. With parallel feeds, the cable with the bad connection will carry less current.
The heating will originate at the poor connection and will conduct through the conductor. The heat profile will be greatest at the point of the poor connection and will drop off the farther away from the connection you go.
The cable with the poor connection will be cooler than the other cables EXCEPT the last few feet where the cable is being heated by conduction from the bad connection.
This looks very simple when described in a post, and is often readily apparent when examining installations in the field. However, there are many factors in the field that may make this effect less than obvious. With lots of space in the switchgear and cables run in an easily accessible tray to another accessible location, it is relatively easy to determine the cause of heating.
In the real world, we find cramped locations, and cables disappearing into inaccessible underground conduits. Access to one end may be restricted for some reason or another. There may be background heat from a transformer or other source. If the bad connection is to a relatively large bus-bar the mass of the bus bar may act as a heat sink and partially mask the heating.
It is not always as easy as it may seem to tell the difference between the two most common causes of heating and unequal currents.
respectfully

 

[wareagle]

"It almost appears somehow that a parallel path of neutral current exists on these 3/0's."

Thats exactly what you have. You do not need the green 3/0
installed form the pad mt to the main. You are only required to have the neutral. The EGC(green conductor) starts at the main.

 

[dpc]

I think wareagle is dead on. In the US, the incoming service entrance cable from the power company to your service entrance equipment has only a neutral conductor, no green wire. This neutral conductor is grounded at the power company's transformer.

The power company neutral is connected to the neutral of your service entrance. In the service entrance, there is one and only one bonding jumper from the neutral point or bus to the ground bus. Your green grounding wires should start from there. From that point on, the neutral should not be connected to the ground wires.

It's hard to give a definitive answer without seeing the installation, but I think you need to disconnect the green ground wires that run from the utility to the service entrance panel. You might just want to verify who owns what and what is considered the service entrance equipment.

 

[waross]

Hello tg103 and wareagle
I understand that the cables are comprised of 10 500 MCM conductors and a 3/0 ground conductor.
If the system is properly connected you should have 10 500 MCM cables in parallel serving the neutral.
That's a lot of copper. Now it appears that the grounds may be connected in parallel with the neutrals.
Ten 500 MCM and ten 3/0 in parallel and two 3/0 are carrying 400 amps. Wow!! If the service is well balanced, that may be the entire neutral current.
It almost appears as if the two 3/0s are carrying the majority of the neutral current.
I think that an inspection of the utilities transformer is in order as soon as possible.
It's almost impossible to say what the exact problem. A couple of thoughts come to mind.
If the two 3/0s are carrying that much current now, in the event of a serious ground fault the source of the problem would almost certainly become the center of a serious explosion.
I visualize a problem starting on the neutral and progressing through heating and heat corrosion until only a few cables have effective connections left.
Your installation with multi conductor cables should effectively minimize reactance. Each cable should have balanced currents. In the event that one conductor in one cable carries significantly more current than the similar conductors in the other cables, this is an unbalanced current in this cable. Increased reactance should tend to limit it. This is not happening and this is an indication of serious connection problems with all the connections.
The situation that was discussed in the other thread was a common occurrence with single conductor cables that are poorly arranged. Your 4x500MCM+3/0 cables should be inherently balanced. If the 500MCM neutral and the 3/0 ground are connected in parallel they should be sharing the current in inverse proportion to their impedances.
My experience with inspection departments with this type of connection is that the grounds must be connected to ground at the source end and then be unconnected and insulated at the load end, but I will defer to wareagle on this point. My experience with inspection departments is not current and I believe his experience is current.
respectfully

 

[tg103]

I agree about the grounding conductors not required, in fact I have only ever see this twice in the ten years I have been doing testing and maintenance. It's the first time I have ever seen current on these "green" grounding conductors though, and this imbalance makes me more curious.

waross - the difference in heat on the IR image we captured is the entire length of the (2) 3/0 conductors in the board right down into the incoming conduits. Fortunately we were able to clamp these two safely enough. I am assuming with the difference in temperature that the other 3/0's have quite a bit less current.

 

[waross]

Thanks for the feedback tq103.
Were you able to measure the current in the neutral cable associated with the "Hot" ground cables? I've been there and understand if it is not possible safely. I am sure that you have serious connection problems. You may also have ground currents, but even so, you still have connection problems. I hope you can inspect the pad-mount soon. I am looking forward to hearing what you find.
respectfully

 

[tg103]

It may have been possible to take additional load readings, and if I was present I have a tendency to spend the extra time but it wasn't in our initial scope of work.

Our company is located within a very competitive marketplace, it's difficult sometimes to spend additional time taking current measurements, voltage drop readings, and researching the recommendations for repair with the time we are allotted.

We make the recommendation to follow up with further investigation where we can dedicate more time to analyze the whole picture. 90% of the time, with an alert status such as this, the customer will have us follow up. Not in this case just yet, it's been a month.

I think the problem hasn't hit the right guy on the ladder yet and/or they have the attitude "it ain't broke, why fix it". "Lights are on, heat and AC is working, who cares". As far as I am concerned a safety hazard and PQ problem exists, I will continue to push it.

I will post back to the group my findings.

Thanks to all for the interest!

 

[waross]

Hello dpc and wareagle and tg103;
dpc is technically correct, but in this instance, it may well be that the only neutral connection left that is dependable is the two green wires. I would strongly recommend that they be left as is until the situation is investigated further. When the 500 MCM neutrals have been verified as properly connected and/or repaired, then by all means disconnect the green wires at the load end as dpc suggests but not until the proper neutrals have been proven.

I know how you feel tg103. I watched a 100 KVA distribution transformer self destroy over a period of a few months because the customer couldn't grasp the severity of the situation. I was well paid to change it (they had to fly me in) but I would have rather have fixed the problem on one of my previous trips and saved the transformer. I recommended urgently to another customer that his 350 KW generator be re-dipped and baked. It spent 4 days in the epicenter of Hurricane Mitch. Not in the budget. A few months later it self-destructed. A rewind was quoted at 2.5 to 3 times the price of a dip and bake. It never was repaired.
I hope your installation doesn't self-destruct before you are allowed to check and repair it. But, we don't always get to say. If it does blow up, there will probably be a nice overtime cheque putting it back together.
respectfully

 

[wareagle]

Waross
You make a good point. It appears that the 3/0 is acting as the neutral rather that the 500's.

TG
You say you can not clip on the 500 neutrals. Is there a location where you can get a total reading? Below is a device you may be able to fish around 1 conductor.

http://us.fluke.com/usen/products/A...lukeUnitedStates&Category=PRCU(FlukeProducts)