Weird UPS Situation

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George Stolz

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Windsor, CO NEC: 2017
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Consider the following (highly simplified) one line. There is a transformer inside the UPS, it takes in 480 and outputs 208/120V. There is a second feeder, by way of a second transformer outside the room, that serves the bypass inputs of the UPS.

View attachment UPS Setup.pdf

I will call the two feeders 1 and 2. Feeder 1 is created inside the UPS, and Feeder 2 is from the outside transformer to the bypass of the UPS. All feeders are from solidly grounded wye systems.

Would these voltages be normal, or would you be concerned?

Feeder 1:
A - N = 120V
B - N = 120V
C - N = 120V
A - B = 208V
B - C = 208V
C - A = 208V

Feeder 2:
A - N = 120V
B - N = 120V
C - N = 120V
A - B = 208V
B - C = 208V
C - A = 208V

Then we measured between sources: "f1A" below means "Feeder 1, Phase A".

f1A - f2A = 120V
f1A - f2B = 120V
f1A - f2C = 240V
f1B - f1A = 120V
f1B - f2B = 240V
f1B - f2C = 120V
f1C - f2A = 240V
f1C - f2B = 120V
f1C - f2C = 120V

Now, these readings were unexpected to us, and we all had varying theories on why we were seeing what we saw.

We expected to see a zero voltage between phases, indicating that the two phases were the same phase, even if f1A were zero volts to f2C - we expected to find three zeroes in there somewhere.

My feeling is that, despite the two transformers being derived from the same 480V source, and having interconnections via grounding conductors, that they were two independent sources and that readings between the sources were irrelevant and meaningless.

Another electrician felt that there had to be a winding open somewhere, a motor load connected inside the UPS somehow, causing an oddly high voltage between phases of the two systems.

In theory, only one source will serve the load panel at a time, so we decided to roll with what we had.

Thoughts?
 

hurk27

Senior Member
George.

Since you most likely have a common X0 bond on both transformers, the 120 volt reading is what would be a 0 volt reading, this is because you are reading the half of the Y winding to the X0, the only thing I can say about the 240 volt reading is this must be a reading of two windings in series between a phase and X0, like from F1a to F1's X0 through the grounding to F2 X0 out to F2c, the same goes for F1b to F2b? are the transformers phased exactly the same as I would expect if so, that you would have a 120 or 240 volt reading all the way around?

after drawing out two Y transformer windings this is about all I can think thats happening????

Edited to add the same question for the primary phase out, are the same phases on each primary, never seen a problem like this with correctly wired transformers and could be a problem if you have a make before break transfer?

Try placing a load across the 120 volt reading to see if it is a true voltage and not a ghost voltage
 
Last edited:

__dan

Senior Member
Consider the following (highly simplified) one line. There is a transformer inside the UPS, it takes in 480 and outputs 208/120V. There is a second feeder, by way of a second transformer outside the room, that serves the bypass inputs of the UPS.

View attachment 7413

I will call the two feeders 1 and 2. Feeder 1 is created inside the UPS, and Feeder 2 is from the outside transformer to the bypass of the UPS. All feeders are from solidly grounded wye systems.

Would these voltages be normal, or would you be concerned?

Feeder 1:
A - N = 120V
B - N = 120V
C - N = 120V
A - B = 208V
B - C = 208V
C - A = 208V

Feeder 2:
A - N = 120V
B - N = 120V
C - N = 120V
A - B = 208V
B - C = 208V
C - A = 208V

Then we measured between sources: "f1A" below means "Feeder 1, Phase A".

f1A - f2A = 120V
f1A - f2B = 120V
f1A - f2C = 240V
f1B - f1A = 120V
f1B - f2B = 240V
f1B - f2C = 120V
f1C - f2A = 240V
f1C - f2B = 120V
f1C - f2C = 120V

Now, these readings were unexpected to us, and we all had varying theories on why we were seeing what we saw.

We expected to see a zero voltage between phases, indicating that the two phases were the same phase, even if f1A were zero volts to f2C - we expected to find three zeroes in there somewhere.

My feeling is that, despite the two transformers being derived from the same 480V source, and having interconnections via grounding conductors, that they were two independent sources and that readings between the sources were irrelevant and meaningless.

Another electrician felt that there had to be a winding open somewhere, a motor load connected inside the UPS somehow, causing an oddly high voltage between phases of the two systems.

In theory, only one source will serve the load panel at a time, so we decided to roll with what we had.

Thoughts?

I'm not exactly clear on what you have described. The one line diagram is confusing unless the bypass is inside the UPS. I am assuming feeder 1 is the load side of the UPS, feeder 2 is load side of the external transformer, and the bypass is load side of both at 120/ 208 Y.

Matching phases, A to A, and finding voltage differences indicates a phase shift. Most likely, the bypass will not like it and could fail somehow. If the bypass is a manual mechanical switch, it could be closed transition and designed to keep the load up during transition. Closing both sources into a 120 V voltage difference is the same as a line to line fault.

If the bypass is a fast electronic static switch, it could do open transition in 1/4 cycle but again will not like a phase shift between matching phases. It's monitoring circuit could be smart enough to refuse to transfer (out of synch condition) but not smart enough to avoid damaging itself.

I would start at square one and see if the arrangement is in compliance with the manufacturer's requirements. Consult the UPS factory engineer to determine exactly what is going on. The UPS and bypass should be as specified by the factory.

I have no guess as to what's causing the phase shift, but I would not guess it is not caused by the load. I would assume the sources are out of synch enough to read the 120 V between matching phases. I would guess that the factory already knows the answer.
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
There is a 60 degree phase shift between the two transformers.
That could be verified with a phase rotation meter. Two delta wye transformers connected to the same source would have a 60 degree phase shift if primary connection rotation is not the same.
 

ron

Senior Member
If you make similar measurements on the input side, do you get the same odd results?

Sometimes people get creative in trying to mitigate harmonics and buy varied phase shifting transformers, so it is worth it to check as sugested in the earlier post.

From inverter output to bypass, that the client would normally like to have a seemless transfer, so the two feeders should be syncronized.

Look for a weird zig zag transfromer somewhere from input to output.
 

George Stolz

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Location
Windsor, CO NEC: 2017
Occupation
Service Manager
That could be verified with a phase rotation meter. Two delta wye transformers connected to the same source would have a 60 degree phase shift if primary connection rotation is not the same.

Phase rotation was checked on the secondary side of the feeder, and matched. If the secondary is in rotation then the primary must be as well, right?
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
If you make similar measurements on the input side, do you get the same odd results?

Sometimes people get creative in trying to mitigate harmonics and buy varied phase shifting transformers, so it is worth it to check as sugested in the earlier post.

From inverter output to bypass, that the client would normally like to have a seemless transfer, so the two feeders should be syncronized.

Look for a weird zig zag transfromer somewhere from input to output.

They claim it is a delta-wye transformer inside the UPS. I have to confess that I am unfamiliar with zig zag transformers. Is 208/120 a common output for a zig zag transformer?
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Wayne, I used a Lo-Z tester to confirm the tech's readings.

We originally landed the UPS with reservations, then the tech arrived for energize and had reservations, and I wound up removing the 208 bypass feeder while he investigated the setup. The factory then fully confirmed the configuration and I relanded the 208 conductors.

One UPS seems to have no issue with the setup, the other has an intermittent complaint about a fault.

Since the GC has all trades out of order, then the UPSes have been shut down until the dust-creators finish the room they are in. In the meantime, I am trying to get up to speed on what's going on for my own piece of mind. I am pretty sure that the feeders are in fact paralleled during a bypass, but I don't think anybody had the guts to try during commissioning. :D
 

George Stolz

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Location
Windsor, CO NEC: 2017
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I'm not exactly clear on what you have described. The one line diagram is confusing unless the bypass is inside the UPS. I am assuming feeder 1 is the load side of the UPS, feeder 2 is load side of the external transformer, and the bypass is load side of both at 120/ 208 Y.

Click here to see the UPS. If this isn't the exact one, it's really close.

The "utility" 208V feeder is routed to a maintenance bypass panel on it's way to the UPS. I omitted it earlier, because I thought it might distract from the main issue of the question - different voltages between two SDSs. Click here to download the manual for the panel.

Here is the inside of the UPS:
ups.jpg

The diagram for the Maintenance Bypass Panel indicates that as switch Q003 is closed, the panelboard load is paralleled between the UPS output and the utility bypass input - but the UPS does monitor the state of switch Q003, so that might not be the case.

mbp.jpg

I'm starting with minimal information, because once it's all laid out it gets a little unwieldy. :)
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
120915-1007 EDT

George:

SG-1 provided the answer for you.

Draw two identical vector diagrams. The differences between these two are what you expected.

Rotate one vector diagram by 60 degrees. Now what are the voltage differences? Just what you measured.

What is a UPS? This probably stands for Uninterruptible Power Supply. Further in your application it is probably a 60 Hz 3 phase unit with 208/120 output using a battery to supply the uninterrupted energy. The internal transformer is probably the supply to charge the battery. Note you said the internal transformer was delta-wye. What does that mean in terms of phase angles?

For the battery charger portion of the UPS no one cares about its phase angle or frequency, and even if its source of energy is from the same source as the normal supply.

.
 

ELA

Senior Member
Occupation
Electrical Test Engineer
My feeling is that, despite the two transformers being derived from the same 480V source, and having interconnections via grounding conductors, that they were two independent sources and that readings between the sources were irrelevant and meaningless.


Thoughts?

I did not see where the neutrals between the two systems were common? If they are not (USP's often having floating output neutral, or where is it bonded?) then I vote with your statement above.
 

__dan

Senior Member
Click here to see the UPS. If this isn't the exact one, it's really close.

The "utility" 208V feeder is routed to a maintenance bypass panel on it's way to the UPS. I omitted it earlier, because I thought it might distract from the main issue of the question - different voltages between two SDSs. Click here to download the manual for the panel.

Here is the inside of the UPS:
View attachment 7414

The diagram for the Maintenance Bypass Panel indicates that as switch Q003 is closed, the panelboard load is paralleled between the UPS output and the utility bypass input - but the UPS does monitor the state of switch Q003, so that might not be the case.

View attachment 7415

I'm starting with minimal information, because once it's all laid out it gets a little unwieldy. :)

APC does build some hokey equipment. I would be careful, it will not be *their* fault. The UPS document in the link above shows a unit that is 208 volt Y input with the bypass internal and your one line drawing shows a unit that is 480 volt (delta?) input. You are saying the bypass is external? The bypass document link above does not open. The UPS above is double conversion, inverter is always online.

The UPS would be Y transformer output but it is synching to the input. The input at the rectifier likely includes line reactors and capacitor filters. The configuration could be anything (knowing APC) and may not be Y. The UPS is synching to that input, at the rectifier.

You or I do not have enough information. It is safe to say the bypass is doing, or capable of doing, closed transition switching. So, the bypass inputs must be matched exactly ( 0 v between matching phases).
 
Draw two identical vector diagrams. The differences between these two are what you expected.

Rotate one vector diagram by 60 degrees. Now what are the voltage differences? Just what you measured.

Running with that-- if the UPS sync's the inverter to the battery charger input instead of the bypass input, you'll see this phase shift. Have the UPS tech the inverter's sync/timing source, it should sync to the bypass (although will free-run if the bypass is not connected). I've see this sort of problem, although it wasn't with APC.

It would be interesting to confirm the shift with a dual-input isolated scope. The UPS tech should have one if you don't.

Do you get a "System Not Synchronized to Bypass." error?
 
Last edited:

ron

Senior Member
Running with that-- if the UPS sync's the inverter to the battery charger input instead of the bypass input, you'll see this phase shift. Have the UPS tech the inverter's sync/timing source, it should sync to the bypass (although will free-run if the bypass is not connected). I've see this sort of problem, although it wasn't with APC.

Agreed, the inverter (UPS) output is typically sync'ed to the (static) bypass since it has to be able to switch over in <1 cycle.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
120915-2347 EDT

My reading of the original post was that the voltage measurements were made between the output of the internal transformer, before the rectifiers, and with the bypass feeder.

I did not read into the original statement that the UPS output was being compared with the bypass supply, but possibly that is what was implied.

.
 
My reading of the original post was that the voltage measurements were made between the output of the internal transformer, before the rectifiers, and with the bypass feeder.

You might want to check again, George said:
Feeder 1 is created inside the UPS, and Feeder 2 is from the outside transformer to the bypass of the UPS. All feeders are from solidly grounded wye systems.

I'm kind of guessing without a one-line of the UPS, but my assumption is that the transformer mentioned inside the UPS would be it's output transformer (after the inverter), and is a grounded Y secondary. The secondary of the battery charger/input transformer would be irrelevant, and is most likely delta-connected.
 

brian john

Senior Member
Location
Leesburg, VA
Often the UPS manufactures use transformers with a phase shift, your bypass transformer needs to match theirs and often is sold as part of the UPS sale.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
120916-1030 EDT

zbang:

From the first post:
Consider the following (highly simplified) one line. There is a transformer inside the UPS, it takes in 480 and outputs 208/120V. There is a second feeder, by way of a second transformer outside the room, that serves the bypass inputs of the UPS.

The one line drawing shows a 480 input to the UPS. From the above quote it says that the internal transformer takes in 480. This seems to imply that the transformer is at the input of the UPS. But I don't really know and your analysis is equally likely.

More detail is needed.

.
 
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