? on an Open Delta System

Status
Not open for further replies.
J

jimmyglen

Senior Member
Location
St. Louis, Missouri
One of our commerical electricians at our shop (I am mainly an estimator / resi electrician myself)
came to me with this

We have an "open delta" 4160 primary with a 480 secondary

one leg will sometimes go to ground

so the readings are phase A = 480V to ground
phase B = 480V to ground
phase C = sometimes ZERO to ground - sometimes 360 - sometimes 280

we are thinking that something is causing this

this is a large old car plant in St Louis - i think its the old corvette plant and its now just a storage warehouse

we cant isolate where the problem is - we had CH come out and megger the transformer and it checked out Ok

thanks in advance for any ideas
Jim
 
It sounds like either C-phase should be grounded, and the bond is intermittent, or the system should be floating, and shouldn't be grounded, and there's an intermittent ground fault.


Look for an intentional bonding of the C phase in the main disconnect. It should look similar to neutral bonding, and fused switches should have a dummy fuse in the C phase, too.
 
since your voltage to ground is varying, I'd say you have an intermediate or "poor" fault to ground. It might not necessarily be from one specific location. Since it varies, it might be a motor winding.
You could convert it to a corner grounded system and the fault should take out an overcurrent device, or you could install ground fault indicators (which would be required today by 250.21{B}) and try to isolate the faults as they occur.
 
Augie - your response baffles me a bit.

augie47 said:
--- or you could install ground fault indicators (which would be required today by 250.21{B}) and try to isolate the faults as they occur.
Click to expand...
Sounds like you think the system is ungrounded 480D. If so:
1. Why aren't the A and B phase readings to ground varying when the C phase varies?

2. How would you isolate the fault using a ground fault detector?

augie47 said:
--- You could convert it to a corner grounded system and the fault should take out an overcurrent device, ---.
Click to expand...
So, your intent here is to intentionally ground a phase and see where the system explodes? The assumption being that point of energy emision is where the intermittent fault is?

cf
 
Cold Fusion said:
Augie - your response baffles me a bit.


Sounds like you think the system is ungrounded 480D. If so:
1. Why aren't the A and B phase readings to ground varying when the C phase varies?
Click to expand...

I agree that's an assumption, but what other system gives you these readings.
"so the readings are phase A = 480V to ground
phase B = 480V to ground
phase C = sometimes ZERO to ground - sometimes 360 - sometimes 280"

I assumed he's reading A & B -480 while C=0
Again, I can't visualize anything other than an ungrounded delta..I, of course, could be wrong
Cold Fusion said:
2. How would you isolate the fault using a ground fault detector?
Click to expand...
Turn off various feeders/branch circuits until the fault clears.
Cold Fusion said:
So, your intent here is to intentionally ground a phase and see where the system explodes? The assumption being that point of energy emision is where the intermittent fault is?

cf
Click to expand...
"explode" might be a little drastic :grin: Hopefully the fault would cause the nearest overcurrent device to open as it would had it been designed that way.
 
augie47 said:
I agree that's an assumption, but what other system gives you these readings.
"so the readings are phase A = 480V to ground
phase B = 480V to ground
phase C = sometimes ZERO to ground - sometimes 360 - sometimes 280
Click to expand...
It may help to know which phase is the "open" winding...

but once again, having only two windings, a problem in the transformer windings is going to affect more than one line voltage. Need a three phase voltage monitor to take simultaneous readings... or connect a grounded wye load to the system if it is ungrounded.
 
Last edited:
jimmyglen said:
...We have an "open delta" 4160 primary with a 480 secondary

one leg will sometimes go to ground

so the readings are phase A = 480V to ground
phase B = 480V to ground
phase C = sometimes ZERO to ground - sometimes 360 - sometimes 280

we are thinking that something is causing this...
Click to expand...

Jim the open delta is a 3 phase system that uses only 2 transformers [windings] rather than 3. Additionally it can be a grounded or ungrounded sysetm meaning one of the transfromers are intentionally grounded, or none of the transformers are grounded for an ungrounded system.

Your case sounds like the "C" phase is grounded, normally the "B" phase is grounded. The grounded phase is still a "phase" in how it functions but becasue it's grounded it will have zero referance to ground (no voltage) while the other 2 phases will. Your readings from "C" to ground seem to point at an intermittent bonding connection, this is the point where the phase in intentionally connected to ground.
 
Well ill go with a open delta ungrounded .

Bad connection to ground !

Question when testing voltage are you loaded or unloaded in that plant ?

Yur open delta is only two windings you may have just a simple imbalance it may increase with load swings meaning voltage goes up or down as this type of connection is not good for heavy loads .

Meaning 3 phase or single phase i dont see a plant using just a open delta ?

A standard ungrounded open delta the voltage on any leg to ground is indefinate meaning can be any voltage or = none if loaded .

Open deltas with mismatched transformers are famous for voltage floats during mismatched loads .

But iam not a engineer just a electrician .

And iam a Ford man myself !!!
 
Last edited:
OK - i am going over this with the electrician on the job

Smart$ and Cold Fusion - I think you are onto something

all legs are flucutating - i had this wrong
it seems to be going from an un grounded system to a grounded system (?)


When the system shows to be an open system the
A leg = apprx 240v to ground
B leg = 280v to ground
C leg = 360V to ground

when the problem happens
A leg = 480V to ground
B leg = 480V to ground
C leg = 0 volt to ground

CH has megger it out and told us that it is an un grounded delta system coming off the transformer

we know something is going to ground - possibly a motor - transformer or ???

there are 5 buildings in the complex and thats the problem when it comes to isolating the problem / problems

we are thinking to ground a leg and convert it to a grounded system - what do you think about this idea?

thanks again for the help!

Jim
 
Yes, a ground fault somewhere in the system sounds likely.

Going to a grounded system in the future is plausible... but you still need to find (and correct) this problem. Being an intermittent fault makes it harder to track down. The simplest method, yet most undesirable for an up and operating facility, is to shutdown distribution paths during a fault event, and use the process of elimination to track it down. Otherwise, I believe you will have to monitor your system to see how much the current changes during a fault event. Use this change in current profile to then monitor the distribution paths for the same change. Upon finding the path, move the monitoring one level at a time towards the culprit...
 
jimmyglen said:
OK - i am going over this with the electrician on the job

Smart$ and Cold Fusion - I think you are onto something

all legs are flucutating - i had this wrong
it seems to be going from an un grounded system to a grounded system (?)...
Click to expand...

Are either of the two secondary windings connected to ground?
 
Status
Not open for further replies.
Top