Unsolved Voltage Drop

Status
Not open for further replies.

Mohammed

Member
Location
Morocco
Hello to everyone,

I'm dealing with a voltage drop issue that I can't explain. I would be grateful if anyone could provide elements of solution.
First step was to measure voltages on the load side of a 630 kVA transformer. Even under load, measured voltages are good and quite well balanced : 220V between neutral and phases, 380V between each couple of phases. I made these measurements on the general breaker A.
According to this, my first conclusion is that there's not a voltage drop in the line side as nominal phase to phase voltage is 380V in this area.
General breaker feeds a subpanel which has its own breaker B.
Length of the cables are about 200 ft. Each phase is connected by a 3*150 mm2 copper cable. That corresponds to 3*AWG300. For each of the 3 cables of each phase there's a load of 130A so a total of about 400A per phase.
According to theses values, I should have a voltage drop of only some volts.
However when measuring voltages on breaker B under load, I find only 203-204 V neutral to phase and about 350 V phase to phase. Voltages are also well balanced.
As I can't explain this voltage drop based on cables losses, I don't see how I should consider this issue and how to solve it.

Thank you all for your time and help.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
where are you measuring the voltage drop at breaker B? Line or load?

did you measure the actual current in each of the 9 conductors going out to breaker B?

My guess would be that either you have a break in one of the cables somewhere or someone installed smaller cables than what you think. could be they are 150 mm^2 at both ends and something else in the middle.
 

Mohammed

Member
Location
Morocco
where are you measuring the voltage drop at breaker B? Line or load?

did you measure the actual current in each of the 9 conductors going out to breaker B?

My guess would be that either you have a break in one of the cables somewhere or someone installed smaller cables than what you think. could be they are 150 mm^2 at both ends and something else in the middle.

I measured the same voltage drop at breaker B on both load and line sides. Installation was running well since several months post installation so I guess, i could not have been cheated otherwise I would have seen the voltage drop from the beginning. I don't feel any heat touching cables at both ends.

Is it possible that a break in one of the cables causes a "balanced voltage drop" ?

I was thinking to measure amps at both ends simultaneously with 2 network analyzers and compare data. Do you think it could be useful ?

Thank you very much for your comments.
 

Mohammed

Member
Location
Morocco
Are you certain that the parallel runs are exactly, or close to exactly as possible.

Hello Denis. Yes I'm certain. What is astonishing is that installation was running well since several months and this voltage drop happened suddenly.
I was wondering if magnetic field created by amps circulation could create a voltage drop of this magnitude.

Thank you.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Hello Mohammed and welcome to the forum. Are you saying that the voltage at breaker "A" is fine under load but at Breaker "B" you have a loss? Not sure how the loss could show up at breaker "B" and not "A"
 

kwired

Electron manager
Location
NE Nebraska
Hello Mohammed and welcome to the forum. Are you saying that the voltage at breaker "A" is fine under load but at Breaker "B" you have a loss? Not sure how the loss could show up at breaker "B" and not "A"

Unless I am misunderstanding, breaker "A" is in the supply side panel, breaker "B" is in the subpanel, possibly the "main" in that panel, there is 3 parallel 300MCM (approximate metric equivelent) conductors per phase between the two and they are about 200 feet long with a load near 400 amps.

I did not run any VD calc yet but sounds like a lot of drop for the conditions just off the top of my head.


Mohammed, you indicate the paralleled conductors are same length, next question - is one conductor of each phase plus neutral in each raceway or cable, and if not are they in steel raceways or other ferrous enclosure?

If you were to say run all three phase A conductors in one steel raceway, all of B in another and C in another - you develop an inductive choke with the steel raceways, you need to run all three phases plus neutral in each raceway or cable, or all conductors in the same raceway or cable so magnetic fields around each individual conductor all cancel one another out.

If not in steel raceways you still need to run each raceway near the others of same circuit to get similar canceling effect from the magnetic fields.
 

Mohammed

Member
Location
Morocco
Hello Mohammed and welcome to the forum. Are you saying that the voltage at breaker "A" is fine under load but at Breaker "B" you have a loss? Not sure how the loss could show up at breaker "B" and not "A"

Thank you Dennis for your welcome. I think you are assuming that breakers "A" and "B" are in parallel. B is under A. I have not been clear on this point.
 

Mohammed

Member
Location
Morocco
Unless I am misunderstanding, breaker "A" is in the supply side panel, breaker "B" is in the subpanel, possibly the "main" in that panel, there is 3 parallel 300MCM (approximate metric equivelent) conductors per phase between the two and they are about 200 feet long with a load near 400 amps.

I did not run any VD calc yet but sounds like a lot of drop for the conditions just off the top of my head.


Mohammed, you indicate the paralleled conductors are same length, next question - is one conductor of each phase plus neutral in each raceway or cable, and if not are they in steel raceways or other ferrous enclosure?

If you were to say run all three phase A conductors in one steel raceway, all of B in another and C in another - you develop an inductive choke with the steel raceways, you need to run all three phases plus neutral in each raceway or cable, or all conductors in the same raceway or cable so magnetic fields around each individual conductor all cancel one another out.

If not in steel raceways you still need to run each raceway near the others of same circuit to get similar canceling effect from the magnetic fields.

Hello and thank you for your time.

What you describe is exactly what I have. Regarding the second part, I don't know If I well understood. For each phase, the three conductors are inside a same cable but each has its own conduit. I think it's better to provide an idea of the cable used (U1000RVFV) :

http://www.nexans.ma/eservice/Morocco-en/navigate_20681/U_1000_RVFV_industrial_armoured_.html

Then these 3 "big" cables (one for each phase) plus neutral are running in a same raceway.

Thank you.
 
What you describe is exactly what I have. Regarding the second part, I don't know If I well understood. For each phase, the three conductors are inside a same cable but each has its own conduit. I think it's better to provide an idea of the cable used (U1000RVFV) :

Then these 3 "big" cables (one for each phase) plus neutral are running in a same raceway.

If I'm reading everything correctly, each steel-armored cable (with three insulated conductors) carries a single phase on all three conductors, then each cable is in one conduit, and all the conduits are in one raceway.

Conduit 1 contains Cable (X X X)
Conduit 2 contains Cable (Y Y Y)
Conduit 3 contains Cable (Z Z Z)
also assuming that the steel armor tape is earthed.

You can remove the outer conduits and raceways from the discussion because you still have all of one phase's conductors together in a single cable. As Kwired said, you need to put one of each phase in each armored cable:
Conduit 1 contains Cable (X Y Z)
Conduit 2 contains Cable (X Y Z)
Conduit 3 contains Cable (X Y Z)
 

Mohammed

Member
Location
Morocco
If I'm reading everything correctly, each steel-armored cable (with three insulated conductors) carries a single phase on all three conductors, then each cable is in one conduit, and all the conduits are in one raceway.

Conduit 1 contains Cable (X X X)
Conduit 2 contains Cable (Y Y Y)
Conduit 3 contains Cable (Z Z Z)
also assuming that the steel armor tape is earthed.

You can remove the outer conduits and raceways from the discussion because you still have all of one phase's conductors together in a single cable. As Kwired said, you need to put one of each phase in each armored cable:
Conduit 1 contains Cable (X Y Z)
Conduit 2 contains Cable (X Y Z)
Conduit 3 contains Cable (X Y Z)

Sorry, I was not very accurate. Your description is almost perfect but there are no conduits. Each steel-armored cable (with three insulated conductors) carries a single phase on all three conductors, then each cable (3) are in one raceway.

Do I have as you mentioned to put one of each phase in each armored cable ?

Thank you Zbang.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
From the point of view of induced magnetic heating and increased AC impedance, the steel armored/shielded cable will act pretty much the same as a ferrous raceway, and all of the associated rules about not separating phases should apply.
 

SG-1

Senior Member
How does the neutral get from breaker A to breaker B ?

Between what two points are you making the Line to Neutral reading with the voltmeter at each location ?

The phase separation should also cause a lot of watts to disappear between breaker A and breaker B.
 
Last edited:

Mohammed

Member
Location
Morocco
How does the neutral get from breaker A to breaker B ?

Between what two points are you making the Line to Neutral reading with the voltmeter at each location ?

The phase separation should also cause a lot of watts to disappear between breaker A and breaker B.

So, I have to avoid phase separation or the contrary as discussed previously.
One armed cable per phase or each phase distributed on the three armed cables ?
Neutral is 1*90mm2 cable. It's running with the 3 armed cables in a same raceway.

Thank you.
 
One armed cable per phase or each phase distributed on the three armed cables ?
Neutral is 1*90mm2 cable. It's running with the 3 armed cables in a same raceway.

Think of each armored cable as a separate steel conduit for this purpose, so ALL phases in EVERY cable (see my previous message). The neutral conductor should also be in the armored cable or conduit but that's difficult if there are only three conductors.
 

kwired

Electron manager
Location
NE Nebraska
By not having all conductors of the circuit in each metal sheathed cable - especially if a ferrous metal - you have no cancellation of the fields around the conductors because of opposing polarities. this effectively makes an inductor out of each cable assembly, but with the "coil" inside the "core" instead of the other usual way around.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Think of each armored cable as a separate steel conduit for this purpose, so ALL phases in EVERY cable (see my previous message). The neutral conductor should also be in the armored cable or conduit but that's difficult if there are only three conductors.
And if the three phase load is balanced, or close to it, the neutral current will be low and the impedance will not be a critical problem for VD.
 
Status
Not open for further replies.
Top