Transfer Switch Phase Differential

[A-1Sparky]

I've got a situation where a transfer switch is being fed from two different sources. The normal source is a 208Y120v source. The emergency source is a 480Y277v source stepped down via a transformer to 208Y120v. When graphing the sine waves of these two sources (both utility sources), there is a phase differential of about 20-25 degrees. What would cause this differential?

 

[ATSman]

I've got a situation where a transfer switch is being fed from two different sources. The normal source is a 208Y120v source. The emergency source is a 480Y277v source stepped down via a transformer to 208Y120v. When graphing the sine waves of these two sources (both utility sources), there is a phase differential of about 20-25 degrees. What would cause this differential?

A certain amount of phase shift in normal thru transformers but this sounds excessive. What type of instrument are you using to measue this phase shift?

 

[A-1Sparky]

I used a Fluke oscilloscope. The differential is significant enough that the in-phase monitor in the transfer switch is not allowing a full transfer upon loss of utility voltage.

 

[jim dungar]

I've got a situation where a transfer switch is being fed from two different sources. The normal source is a 208Y120v source. The emergency source is a 480Y277v source stepped down via a transformer to 208Y120v. When graphing the sine waves of these two sources (both utility sources), there is a phase differential of about 20-25 degrees. What would cause this differential?

This probably comes from the fact that really is no such thing as PHASE A. What you are calling A phase o the 208V system is not the same as what you are calling A phase on the 480V. It seems like you are experiencing a problem with phase rolling and swapping.

You need to supply measurements of:
Normal-Alternate
Va-V'a
Va-V'b
Va-V'c
Vb-V'a
Vb-V'b
Vb-V'c
Vc-V'a
Vc-v'b
Vc-V'c

I am expecting you to find some combinations that are 2V.

 

[A-1Sparky]

Are there any relatively simple solutions that will remedy this problem? Can the in-phase monitor be adjusted to compensate for the differential, and thus allow a complete transfer? Again, what is causing the differential? Is it a function of the transformers involved? Thanks.

 

[A-1Sparky]

Jim, I took measurements between A1 and A2, B1 and B2, and C1 and C2. I had between 10 and 12 volts difference between the corresponding phases for the normal source and the alternate source.

 

[brian john]

Measure from A to A and B to B when they both pass through "0" at the sam time you will have the matching phases and be in-phase, if the transformer shift is not the issue

 

[ron]

A normal delta wye transformer has a phase shift of 30 degrees.

 

[mivey]

You won't get more than a few degrees from the transformer. IMO, unless the tie point on the primary is a couple of hundred miles apart, or you have some serious source impedance issues, you probably have one delta-wye and one wye-wye bank.

 

[jim dungar]

Jim, I took measurements between A1 and A2, B1 and B2, and C1 and C2. I had between 10 and 12 volts difference between the corresponding phases for the normal source and the alternate source.

You need to measure all of the possible voltage combinations.

 

[ohmhead]

Well heres a little more about phase shift its kinda what Ron stated but more into the why .http://www.rycominstruments.com/images/PhaseIdentifier/PhaseAttributes.pdf


The last section is the best part .

 

[charlie b]

I am missing something here, something very critical.

I've got a situation where a transfer switch is being fed from two different sources.

How different? What are the two sources? Is there any reason at all to expect them to have anything in common at all? If you were to trace the path upstream from each source, would the two paths come together at any point?

 

[A-1Sparky]

This particular building is fed with 13kv. The 13kv terminates in a rotary puffer switch, which then feeds two transformers. One transformer steps the 13kv down to 208Y120v for one switchgear cabinet and the other transformer steps the 13kv down to 480Y277v for another switchgear cabinet. The transfer switch's normal power originates from the 208Y120v gear. The transfer switch's alternate source originates from the 480Y277v gear, but is first stepped down via a 480 delta primary 208Y120v secondary transformer. The only variable here seems to be the transformers. So, is it safe to say that the transformers are the culprits as far as the phase differential goes?

 

[gar]

100220-1819 EST

A1-Sparky:

If I have read your response correctly, then ron and mivey already answered your question.

Draw the voltage vector drigrams.

.

 

[dbuckley]

The differential is significant enough that the in-phase monitor in the transfer switch is not allowing a full transfer upon loss of utility voltage.

Thats daft. At the time you need to do a transfer there is only one supply. So although I can see the ATS being tetchy (and possibly righteously so) when it has two supplies to compare, with just one there is no comparison possible....

 

[brian john]

Thats daft. At the time you need to do a transfer there is only one supply. So although I can see the ATS being tetchy (and possibly righteously so) when it has two supplies to compare, with just one there is no comparison possible....

If the ATS is not transferring upon a loss of power you are 1000% right there is another issue and all these post are all chasing the wrong animal.

 

[jghrist]

This particular building is fed with 13kv. The 13kv terminates in a rotary puffer switch, which then feeds two transformers. One transformer steps the 13kv down to 208Y120v for one switchgear cabinet and the other transformer steps the 13kv down to 480Y277v for another switchgear cabinet. The transfer switch's normal power originates from the 208Y120v gear. The transfer switch's alternate source originates from the 480Y277v gear, but is first stepped down via a 480 delta primary 208Y120v secondary transformer. The only variable here seems to be the transformers. So, is it safe to say that the transformers are the culprits as far as the phase differential goes?

If all transformers are delta-wye connected, then there will be a 30? phase shift between the two 208V systems.

 

[gar]

100220-2232 EST

A-1Sparky:

Get a small isolation transformer with a 240 primary, and some reasonable voltage for the secondary. You could use a filament transformer if it had a 208 to 240 primary.

Connect this isolation transformer primary from A to B. Connect your scope common to the 120 neutral. Also connect one side of the isolation transformer to the neutral. Connect one scope channel to line A, and the other to the free end of the isolation transformer secondary. What is your phase shift?

If you do not like this phase shift reverse the isolation transformer primary leads. To see other phase shifts you can put the isolation transformer primary between B and C or C and A. Instead of moving the isolation transformer leads around change the first scope probe to B and then C, with and without the isolation transformer leads reversed.

Be careful and do not short anything.

.

 

[ohmhead]

This particular building is fed with 13kv. The 13kv terminates in a rotary puffer switch, which then feeds two transformers. One transformer steps the 13kv down to 208Y120v for one switchgear cabinet and the other transformer steps the 13kv down to 480Y277v for another switchgear cabinet. The transfer switch's normal power originates from the 208Y120v gear. The transfer switch's alternate source originates from the 480Y277v gear, but is first stepped down via a 480 delta primary 208Y120v secondary transformer. The only variable here seems to be the transformers. So, is it safe to say that the transformers are the culprits as far as the phase differential goes?

Well iam not a expert but you have both transformers online correct and there loaded .

One swbd on one and one swbd on the other the 30 deg phase shift is from primary to secondary internal inside each so i dont see it being a problem.

Your issue is there out of phase after secondary they dont match when you want the emergency power correct and voltage is off and phases dont match correct ?

Transfer phase monitor can be adjusted or bypassed to just test transfer if you want under no load you can do that just to see if transfer switch is working ok .

But this is my thinking if the other transformer is or has issues like overloading PF bad you may have a phase shift between each or the normal source is overloaded it maybe out of phase with the other our these loaded with motors or that kind of loads .

If not i would bypass the phase monitor and turn it on but dont do it with motor loads online .

The only other reason is bad connections in transformers both or one long distance between each meaning power lines feeding transformers .

Last solution would be and your not going to do this but they make a PST transformer which will change phase shift from input to output power companys use these when they have long lines and impedance & capacitance problems on long parallel transmission .


Other than that voltage is a +/_ a few volts does not mean your out of phase just maybe a differance between windings of each and loads on them .


Dont use a meter connect a lite bulb up between a/a b/b/ c/c/ of opposite sources do they work if not its just the meter picking up voltage .

Your scope used on each can pick up capacitance between connection and be off you may not be so far off lastly i think its your transfer switch just getting old or that old puffer switch.

 

[mivey]

So, is it safe to say that the transformers are the culprits as far as the phase differential goes?

Yes. Either change the delta-wye to wye-wye (you will lose some harmonics blocking capability) or add a delta-wye to the other circuit.