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Voltage drop plus current on neutral and ground

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whbaxter912

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Location
Houston, Tx
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Electrician
We have a 600ft run of 3 parallel sets of 500kcmil AL. When we put a load bank at the far end we see 7v of vdrop on phase 3 ONLY plus ~30a of current on the neutral and ~30a of current on the ground with about 1v between neutral and ground. When we put the load bank at the service there is no vdrop(thinking impedence coming from xformer) but there is current on the neutral and ground. The measurements were taken with a Hioki PQA. We have toned out the neutral and ground looking for a floating neutral and there is none. We have meggered all the phase conductors. We have thermal imaged all the terminations and found nothing outstanding. We have not toned out the phase conductors as we assumed crossing one of the 3 conductors per phase would cause an immediate breaker trip. Thoughts as to why only L3 would have the vdrop and to why the current on the neutral and ground?
 
Joethemechanic

Joethemechanic

Senior Member
Location
Hazleton Pa
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Electro-Mechanical Technician. Industrial machinery
Not sure how your conductors are laid out, but maybe capacitive coupling? What's the voltage? Do you have any way to tell if maybe this current is just VAR?
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
If you haven't already done it, I suggest measuring the current on the L3 conductor of each set to see how well they match each other when the load bank is powered up.
 
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whbaxter912

Member
Location
Houston, Tx
Occupation
Electrician
Joethemechanic said:
Not sure how your conductors are laid out, but maybe capacitive coupling? What's the voltage? Do you have any way to tell if maybe this current is just VAR?
Click to expand...
VAR was a distant thought. All 9 conductors plus N/G are snake tray for most of the run. Phases are then split into separate conduits coming up/down the walls at load centers/POI
 
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whbaxter912

Member
Location
Houston, Tx
Occupation
Electrician
synchro said:
If you haven't already done it, I suggest measuring the current on the L3 conductor of each set to see how well they match each other when the load bank is powered up.
Click to expand...
When measured current is lower by 3A with 100A expected on the effected L3 phase with vdrop.
 
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whbaxter912

Member
Location
Houston, Tx
Occupation
Electrician
ptonsparky said:
What is the size of the load bank? What voltage?
What is the current on each conductor of all phases?
Size of Neutrals? Equipment Grounds?
Click to expand...
100kw load bank.
current is within 3% across the phases when at the far end of the conductor run. L1=98.7/123.25, L2=100.15/125.28, L3=97.85/118.33
Native voltage is 124/125 L to N
Neutral and grounds on the combined run are 350kcmil I think maybe 250kcmil. The POI has 400a fuses.
 
Joethemechanic

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
I've never heard of this happening due to the secondary, It's usually due to a long underground primary, but ferroresonance? I know it's a long shot. No funny sounds right?
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
whbaxter912 said:
We have a 600ft run of 3 parallel sets of 500kcmil AL. When we put a load bank at the far end we see 7v of vdrop on phase 3 ONLY plus ~30a of current on the neutral and ~30a of current on the ground with about 1v between neutral and ground. When we put the load bank at the service there is no vdrop(thinking impedence coming from xformer) but there is current on the neutral and ground.
Click to expand...

Was the 30A on neutral and ground measured on each of the three sets, or is this the total current from all three sets in parallel?

If you can do it, put a clamp around both neutral and ground conductors to see whether their currents will add together, subtract, or have a different result.

Perhaps the voltage drop you are seeing on phase 3 is not primarily on the phase 3 conductors themselves, but on the neutral and ground conductors If the voltage drop on N or G is in-phase with the phase 3 voltage, then the L3-N voltage would be reduced and the L2-N and L1-N voltages increased by a smaller amount (since they are at 120 degrees from L3).
Just for testing, you might consider using a ~600 ft length of #14 or #12 tied to neutral or ground at the source end to use as a reference when making voltage measurements at the load end. That way you could see how much of the voltage drop is on the line conductors and how much is on the neutral and ground.
 
W

whbaxter912

Member
Location
Houston, Tx
Occupation
Electrician
whbaxter912 said:
We have a 600ft run of 3 parallel sets of 500kcmil AL. When we put a load bank at the far end we see 7v of vdrop on phase 3 ONLY plus ~30a of current on the neutral and ~30a of current on the ground with about 1v between neutral and ground. When we put the load bank at the service there is no vdrop(thinking impedence coming from xformer) but there is current on the neutral and ground. The measurements were taken with a Hioki PQA. We have toned out the neutral and ground looking for a floating neutral and there is none. We have meggered all the phase conductors. We have thermal imaged all the terminations and found nothing outstanding. We have not toned out the phase conductors as we assumed crossing one of the 3 conductors per phase would cause an immediate breaker trip. Thoughts as to why only L3 would have the vdrop and to why the current on the neutral and ground?
Click to expand...
We resolved the issue. Was a case of inductance due to multiple conductors of the same phase in the same conduit. We caught it on the second site visit. Thank you everyone.
 
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