odd voltage

[ceb58]

This was posted on another forum and I am curious about the voltage. This concerns a 75Kw 120/208 3 phase generator. The utility voltage in the ATS is 125v A-N 125v B-N and 250v C-N. A-B 208v A-C 208v B-C 375v. The generator voltage is as should be 120v Phase to Neutral on all 3 and 208 Phase to Phase. I know why they moved the high leg to the C phase because the ATS relies on voltage from A and B phase for the controls and solenoid and they are single phase. Its the 375v that has me confused? What type system would this be?

 

[gar]

150226-2358 EST

Looks like a concocked problem with voltages that are not quite correct.

As an approximation I assumed that C-N is about 180 deg out of phase with B-N. So 125 + 250 = 375. The angle between B-N and A-N is not quite 120 deg. Using 125, 125, and 208 as sides of a triangle I get 48.05 added to 125 to form a right triangle. Next 208 and 125 + 48.05 forms a right triangle with the other side equal to 115.40. The remaining right triangle formed from two legs of 115.4 and (250 - 48.05) has an hyp of 232.6. This is above the 208 stated in your post.

If the 125 and 250 were not quite 180 deg out of phase, then the 232 could be squished to 208 without a great affect on the 375 reading. So accuracy of the readings has a great affect on actually how the vectors position themselves.

Graphically you could more simply solve the problem, but precise voltage measurements are needed.

Check my math.

.

 

[don_resqcapt19]

Does everything work ok when connected to the utility? What is the neural to ground voltage, what are the phase to ground voltages?

 

[iceworm]

... a 75Kw 120/208 3 phase generator. The utility voltage in the ATS is 125v A-N 125v B-N and 250v C-N. A-B 208v A-C 208v B-C 375v. The generator voltage is as should be 120v Phase to Neutral on all 3 and 208 Phase to Phase. ... What type system would this be?

150226-2358 EST

Looks like a concocked problem with voltages that are not quite correct.

... but precise voltage measurements are needed.

Check my math.

Gar is dead on. There is only one scenerio I see that works.
They got the generator voltages swapped with utility voltages. The voltages given are not very accurate. The generator is 12 lead and mis-connected

Ceb - you need to hang around a better class of dumpsters:roll:

ice

 

[ceb58]

Gar is dead on. There is only one scenerio I see that works.
They got the generator voltages swapped with utility voltages. The voltages given are not very accurate. The generator is 12 lead and mis-connected

Ceb - you need to hang around a better class of dumpsters:roll:

ice

They are saying that

utility voltage in the ATS is 125v A-N 125v B-N and 250v C-N. A-B 208v A-C 208v B-C 375v.

The generator voltage is fine. 120-N on all 3 legs 208 phase to phase.

From what Gar is saying, with out drinking 1/2 bottle of liquor, is that the poco may have something out of whack. ????

 

[kwired]

This was posted on another forum and I am curious about the voltage. This concerns a 75Kw 120/208 3 phase generator. The utility voltage in the ATS is 125v A-N 125v B-N and 250v C-N. A-B 208v A-C 208v B-C 375v. The generator voltage is as should be 120v Phase to Neutral on all 3 and 208 Phase to Phase. I know why they moved the high leg to the C phase because the ATS relies on voltage from A and B phase for the controls and solenoid and they are single phase. Its the 375v that has me confused? What type system would this be?

Confusion on utility supply here let alone generator voltage. Assuming you are stating the utility is the 125/125/250 to N, is the generator 208/208/375 phase to phase?

If I am right on that - the utility is probably a 4 wire delta with high leg - but high leg would be about 216 volts not 250 to N if the other two legs are 125. Any other reading would mean something is not right with it.

375 phase to phase on point plus the 250 to N also sounds like something not connected right or malfunctioning if that is what is coming from your generator. I'm leaning toward misconfigured generator output leads.


Possibly a misunderstanding here but seems as though we are backing up a 240 volt 4 wire delta system with a 208 volt 4 wire wye system. May or may not be problems with that, but it is not really a good match in some instances, others maybe just fine, but motors would draw more current when running on generator power.

 

[iceworm]

They are saying that

utility voltage in the ATS is 125v A-N 125v B-N and 250v C-N. A-B 208v A-C 208v B-C 375v.

The generator voltage is fine. 120-N on all 3 legs 208 phase to phase.

Yep - I got that - both items - first time

... From what Gar is saying, with out drinking 1/2 bottle of liquor, is that the poco may have something out of whack. ????

What I heard him say is the problem is concocked and the measurements are inaccurate. I absolutely agree.

For the POCO to be putting out this:

A and B are 120V, 120deg apart, and C is 180 out from B and doubled up to 240V - this matches gar's math.​

The utility would have to have somewhere around 27 miswires, generator problems, and at least one transformer that no one has ever had a reason to make. The mob attacking the utility offices would look like a bad Frankenstein movie.

The picture I attached is the only explanation I can see.

if you are planning on hanging around that site - a half a bottle seems like a lot. I wouldn't be able to find the key board. But at a minimum, 3+ stiff drinks would seem reasonable.

ice

 

[11bgrunt]

if I read this post correctly, it looks like a wye secondary to me coming from POCO. Is the POCO serving load with three overhead transformers? One is probably connected with polarity different than the other two. Crews reconfigure 120-240 winding to 120 only. AC BD. Usually the newest hand is given that task because you have to get in the transformer oil. With no supervision a green hand can screw it up.
Normally a utility would guarantee voltage is correct before passing on to the customer. What is their position?

 

[11bgrunt]

Posted Twice

 

[gar]

150227-2357 EST

I have now drawn the vectors, and made some assumptions.

I assume the 125 V and 250 V values are correct' I also assume the angle between the two 125 V values is 120 deg. Now calculated line-to-line voltage is 216.504 (actually measured length of vector) and not 208. With a line to line of 375 V, then the other line-to-line is 216.504 instead of 208. Now things fit into place nicely.

Obviously the third transformer is installed with the wrong turns ratio ( 1/2 the ratio it should have), and fed from the wrong primary lines.

The measured voltages were not suffficiently accurate, and it really looks like the 208 values were assumed and not measured.

Change the third transformer to the correct ratio and properly connect the primary wires, and you get a correct three phase set of vectors.

.

 

[ceb58]

The OP on the other forum just posted that the owner of the generator had called the poco to look at the utility voltage. They are now in the process of changing a transformer.

 

[iceworm]

...The picture I attached is the only explanation I can see. ...

The OP on the other forum just posted that the owner of the generator had called the poco to look at the utility voltage. They are now in the process of changing a transformer.

On the other hand - I could be all wet. And in this case it appears I am. Unlike the rest of the whizes on here - that can happen to me.

ice

 

[ceb58]

On the other hand - I could be all wet. And in this case it appears I am. Unlike the rest of the whizes on here - that can happen to me.

ice

Ceb - you need to hang around a better class of dumpsters:roll:

Don't worry Ice we wont kick you to the curb :happyno::happyno:

 

[robbietan]

maybe the service is an open delta? that would explain the voltages.

confirm if there are only 2 cans outside

 

[kwired]

maybe the service is an open delta? that would explain the voltages.

confirm if there are only 2 cans outside

If the intent is to supply 240/120 4 wire service, that can be done with open or closed delta, something has gone wrong or was misconfigured to come up with voltages described in OP.