480 VAC, 3∅ Distribution Question

[Bretzel]

The physical plant I manage buys power from the POCO at primary. We have our own 3Mw substation. The beast was built c. 1935. It is, unfortunately, an operating electical museum. The secondary is 480v 3∅ Wye, 3 Wire, local grounds. We have a number of stepdown transformers running our buildings. We have a number of rotating loads up to 100 hp. Recently, we have observed some irregular voltage readings. We're seeing 460 to 495 between the three legs. Two legs fairly consistently measure 460 and 480 to ground. The third leg is 1.75 volts to ground. All rotating loads appear to be operating normally. Some transformers, depending on the legs they are connected across, are 120v on one side and 95 - 102 on the other. My Electricians are not certain of the source of the problem. I don't want them to waste a lot of time with hit or miss solutions. We know the system is out of balance from observed current draws. We know our local grounds vary in conductivity as the soil dries from a wet condition in the spring to dry in mid-summer. We are not sure if the balance issue is a cause or a symptom of the problem. Thoughts?

 

[ptonsparky]

Others will jump in but you either have a corner grounded system by design or an ungrounded by design that is now grounded by accident. The ungrounded system by design should have a series of lights that will indicate which phase is faulted. This allowed time for a planned shut down of production in older facilities. The first fault was not a major problem, but the second was more spectacular from what I understand. I’ve only seen two of these systems that I can remember. Corner grounds are fast becoming a thing of the past as well

Do not use earth (rods) for a return path to the transformer. You end up with unstable voltages and a PP path for faults.

 

[electrofelon]

you either have a corner grounded system by design or an ungrounded by design that is now grounded by accident.

:thumbsup:

 

[infinity]

As stated it is now (or always was) a corner grounded delta system. Unless it was installed improperly it is not a WYE system.

 

[__dan]

Line to line voltage imbalance should never exceed 1%. Motor losses can go up over 25%. The L to L voltage imbalance causes a current imbalance in the motor field windings and the rotating magnetic field is no longer rotating perfectly smoothly.

First call would be to the utility for a voltage complaint. You want to know if the line to line voltage imbalance is coming in on the utility or is happening after that on your secondary distribution system (because one leg is faulted to ground, that fault is drawing a lot of current, and the current draw is causing a voltage drop on the characteristic system supply impedance.

I've seen or found exactly what you describe and it could be a risk of death scenario. Without a contiguous safety ground bonding system, the equipment grounding conductor, there may exist hazardous voltage differences between all of the exposed metal of the plant and site (because one leg is faulted to ground but there is no effective fault clearing path back to the source which would limit voltage to ground, on the fault path that does exist and is flowing ...).

You do want your electricians aware of this and wasting their time finding the source of the voltage imbalance.

The main transformer can be Y secondary with nothing connected to the neutral and only three wires from there to the load as you describe. In the old days all the loads would be delta connected motors and step down transformer primaries.

You want to bring in a ghostbuster who can immediately and intuitively understand exactly what is happening, discover the specifics, and convey this to you. The risk of accident and catastrophic injury from your equipment is at the highest level.

Most likely you will need to convert to three phase three wire plus the contiguous equipment grounding conductor, so you will need four conductors from the source to the loads, and additional frame and structure bonding to everything else.

 

[kwired]

The physical plant I manage buys power from the POCO at primary. We have our own 3Mw substation. The beast was built c. 1935. It is, unfortunately, an operating electical museum. The secondary is 480v 3∅ Wye, 3 Wire, local grounds. We have a number of stepdown transformers running our buildings. We have a number of rotating loads up to 100 hp. Recently, we have observed some irregular voltage readings. We're seeing 460 to 495 between the three legs. Two legs fairly consistently measure 460 and 480 to ground. The third leg is 1.75 volts to ground. All rotating loads appear to be operating normally. Some transformers, depending on the legs they are connected across, are 120v on one side and 95 - 102 on the other. My Electricians are not certain of the source of the problem. I don't want them to waste a lot of time with hit or miss solutions. We know the system is out of balance from observed current draws. We know our local grounds vary in conductivity as the soil dries from a wet condition in the spring to dry in mid-summer. We are not sure if the balance issue is a cause or a symptom of the problem. Thoughts?

Are you certain this is a wye system? What (if any of the source) conductors are grounded?

If none of source conductors are grounded - as mentioned you at least need to have ground fault indication system, and as mentioned having nominal voltage to ground on two lines and near nothing on the third line tells us the third line is grounded whether it is intentional or not.

 

[Jraef]

You might have a rare ungrounded Wye system, which exhibits similar symptoms as an ungrounded delta, which is why others have speculated on that issue. You can read about it here:
http://forums.mikeholt.com/showthread.php?t=163092

It could also be that you HAD a solidly grounded wye, but some tweaker cut off your GEC (Grounding Electrode Conductor). That happened a lot in the copper price crisis because stupid tweakers would think that since it was an exposed bare copper wire, it was “safe” to cut it and steal it.

Regardless, you have something going on that needs a serious investigation by someone with experience in power distribution systems. I’m not casting aspersions at your electricians, but lots of maintenance and construction electricians can go through their entire careers without ever seeing an unusual distribution configuration.

 

[Wire-Smith]

shut down anything you can to see if it goes away, if so that's where your problem is (go as far up the system as you can with disconnects), its not necessarily going to be a big obvious ground fault, it could be a #12 on your ground fault indicator system. when you get an idea where it is isolate the suspect and get out the megger. if you can't shut anything down get a ground fault locator like a dynatel 573A you can get a used one off ebay or rent one somewhere, find the ground fault and have it fixed. i think there are some videos on youtube how to use it.

 

[Bretzel]

Are you certain this is a wye system? What (if any of the source) conductors are grounded?

The primary is 13,200 Wye, the secondary is 480 Wye. (Switchable to 600, 575, 535, 480, 460, 230 with the turn of a bakelite knob.) The primary is grounded at the transformers but the secondary is not and appears never to have been grounded. The entire distribution system is three wire. The older (1920s patent dates) oil-filled transformers are a mix of single and three phase installtions with local grounds to earth. This construction appears to be original. Newer dry transformers are installed in a similar manor. There is a mix of Wye and Delta transformers on the system.

We have a number of ancient line surge arresters which appear to be of the carbon-filled ceramic canister type. I have to assume these require at least periodic maintenance or replacement. If one of them had a lightning strike at some point it could easily be the source of our ground fault.

Apparently, no one knows exactly when the voltage issues arose but with the addition of some newer equipment with PLC and VFD controls it has become much more obvious.

My plan at this point: While I find a consulting electrical engineer to bring in, I'll have my electricians start checking the grounding of the newer transformers and downstream equipment working backward in installation time looking for the lower voltage leg to be grounded at some location. We'll also check the very few underground runs we have for ground faults.

 

[Wire-Smith]

460, 480 and 1.75, to ground & 460-495 b/w legs sounds like wye to me. i take it you don't have any kind of ground fault indicators toward the service of this system? (three light bulbs on a panel). good luck with the engineer thing, i've seen even old engineers come in and freak out about un-grounded systems.

 

[Bretzel]

Are you certain this is a wye system? What (if any of the source) conductors are grounded?

The primary is 13,200 Wye, the secondary is 480 Wye. (Switchable to 600, 575, 535, 480, 460, 230 with the turn of a bakelite knob.) The primary is grounded at the transformers but the secondary is not and appears never to have been grounded. The entire distribution system is three wire. The older (1920s patent dates) oil-filled transformers are a mix of single and three phase installtions with local grounds to earth. This construction appears to be original. Newer dry transformers are installed in a similar manor. There is a mix of Wye and Delta transformers on the system.

Apparently, no one knows exactly when the voltage issues arose but with the addition of some newer equipment with PLC and VFD controls it has become much more obvious.

My plan at this point: While I find a consulting electrical engineer to bring in, I'll have my electricians start checking the grounding of the newer transformers and downstream equipment working backward in installation time looking for the lower voltage leg to be grounded at some location. We'll also check the very few underground runs we have for ground faults.

We have a number of ancient line surge arresters which appear to be of the carbon-filled ceramic canister type. I have to assume these require at least periodic maintenance or replacement. If one of them had a lightning strike at some point it could easily be the source of our ground fault.

 

[Bretzel]

460, 480 and 1.75, to ground & 460-495 b/w legs sounds like wye to me. i take it you don't have any kind of ground fault indicators toward the service of this system? (three light bulbs on a panel). good luck with the engineer thing, i've seen even old engineers come in and freak out about un-grounded systems.

No ground fault indicators are present in the system.

I know it may take a while to find an old EE who doesn't show up wearing a belt AND suspenders!

 

[Wire-Smith]

i think your unintentional ground is on your secondary not your primary arrestors. it's on your 480, the leg that is 1.75 to ground

 

[ptonsparky]

No ground fault indicators are present in the system.

I know it may take a while to find an old EE who doesn't show up wearing a belt AND suspenders!

Hey, I’m sure not all old EE wear belts and suspenders, but that may be the one you need.

 

[Bretzel]

i think your unintentional ground is on your secondary not your primary arrestors. it's on your 480, the leg that is 1.75 to ground

I agree. If the arresters are the problem, they would be in the 480 side. If they were on the primary, I don't think we would have any power at the secondary or wild have highly irregular readings.

 

[Wire-Smith]

have you measured that suspect leg in different locations. toward service, midstream in plant and furthest area from service. 1.75 makes me think it might be quite a ways downstream.(impedance of the wire before it connects to ground)

if 1.75 is close to the secondary of the 13,5 to 480 transformer, and you measure a lower voltage way downstream it probably is closer to where you're measuring the lower voltage

 

[LarryFine]

460, 480 and 1.75, to ground & 460-495 b/w legs sounds like wye to me.

Sounds like corner-grounded Delta to me.

 

[Wire-Smith]

Sounds like corner-grounded Delta to me.

true with the fault it looks like it might be corner grounded delta, but i mean what it was intentionally installed to be. i should've been more clear

 

[LarryFine]

true with the fault it looks like it might be corner grounded delta, but i mean what it was intentionally installed to be. i should've been more clear

If it was a wye, there would be a neutral, likely grounded, leading to a fault.

 

[Wire-Smith]

If it was a wye, there would be a neutral, likely grounded, leading to a fault.

doesn't have to be a neutral

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