Buck boost 208 to 460 odd voltages

[Horst]

Hi, have a new one on me. We have a 3 phase 100KVA 208/480 transformer that has been running an A/C for a few years. It started acting up yesterday with an odd problem that does not seem like it should effect the A/C, but it does. We have 205V on line side (each leg within a volt or two of each other)and 110ish each to earth and 470ish on each load side leg. But when you measure load side to earth A = 235 B = 241 C = 115 Since the power in and power out is only L-L load, I have no idea why this would affect anything, but it is surely not right. I am thinking a winding is shorted and will investigate tomorrow. Just seeing if anyone else has had an issue like this. Thanks!

 

[iwire]

It is not a buck boost, a buck boost is only used to raise or lower voltages a little bit.

It sounds like the 480 volt output is ungrounded which means voltage readings to earth are meaningless.

Can you provide more information about how it is wired and if there is an XO terminal or HO terminal being used?

 

[augie47]

Just FYI, under present Codes you would need to establish a ground on the 480v side or install a monitor {250.21(B)}.
When you state "it started acting up", exaclty how ?
As iwire states on an ungrounded system your voltage to ground are somewhat meaningless. You can monitor them as changes could indicate a leakage to ground.
(info requested by iwire might change this opinion :happyno

 

[Horst]

Hi, thanks for the replies. had buck/boost on the brain because of where this is being used, long story. The XO is grounded and HO floating. This is the way it was installed. I agree, there is no reference to ground, that is what has me perplexed. I simply stumbled upon the voltage reading. The A/C uses 3 phase 460V w/ no earth or N. The compressors starting cycling (short bursts of about 5-10 secs). So maintenance shut down and called us. The odd thing is both cycle at the same time. As an added note, we had a heck of a storm here last night, so... Maybe I should have them call the A/C guy. I did not think the odd L-G voltage was an issue, just curious since I have never seen it before.

 

[templdl]

Hi, thanks for the replies. had buck/boost on the brain because of where this is being used, long story. The XO is grounded and HO floating. This is the way it was installed. I agree, there is no reference to ground, that is what has me perplexed. I simply stumbled upon the voltage reading. The A/C uses 3 phase 460V w/ no earth or N. The compressors starting cycling (short bursts of about 5-10 secs). So maintenance shut down and called us. The odd thing is both cycle at the same time. As an added note, we had a heck of a storm here last night, so... Maybe I should have them call the A/C guy. I did not think the odd L-G voltage was an issue, just curious since I have never seen it before.

Is it possible for you to take a digital picture of the name plate of the transformer and we'll know for at least what your transformer is.
The name plate should include that ratings as well as the connection/wiring diagram. By providing that there will be no misinterpretation of a verbal description.

 

[augie47]

The XO should not be grounded !!!!!!

 

[kwired]

But when you measure load side to earth A = 235 B = 241 C = 115

Is it possible we have a corner grounded secondary and a 120 volt control or other accessory transformer in the unit and there is some issue with that 120 volt secondary causing these readings to be what they are? Not sure that is possible but something I would probably further investigate, or maybe a corner grounded C phase and we lost C phase somehow, the compressors are short cycling because of phase loss, and there is a 120 volt transformer somehow injecting voltage into the three phase system?

 

[hurk27]

Hi, thanks for the replies. had buck/boost on the brain because of where this is being used, long story. The XO is grounded and HO floating. This is the way it was installed. I agree, there is no reference to ground, that is what has me perplexed. I simply stumbled upon the voltage reading. The A/C uses 3 phase 460V w/ no earth or N. The compressors starting cycling (short bursts of about 5-10 secs). So maintenance shut down and called us. The odd thing is both cycle at the same time. As an added note, we had a heck of a storm here last night, so... Maybe I should have them call the A/C guy. I did not think the odd L-G voltage was an issue, just curious since I have never seen it before.

First the X0 is the low side (208) and should not be grounded, the H0 is the high side (480) and should be grounded, if wired as you said then that explains the voltage reading to ground you got, also if the X0 is bonded to the supply side EGC then you could have phase bucking since you should never bond the Y point on the primary side of a transformer with a common core.

What are the voltage readings when the compressors are trying to start? if you have a bad voltage drop it might not want to start the compressors and is why they are tripping out on their overloads, you could have a connection that went bad that caused it to show up now?

 

[Horst]

Hi, thanks for all the input. Keep in mind, we did not install this unit, so we are working from scratch. This is a Y Delta connection and there is no requirements for the XO ground and no bond. I opened each phase individually and checked input and output and C phase showed a drastic reduction in output voltage. Took an ohm meter and checked resistance on the windings and sure enough, C had a lower reading. after de-energizing and taking the back off the unit, we discovered a clearly visable hole about the size of a pencil lead on the winding. Apparently the winding shorted and we are only using half of it or so. Almost makes since. Still a bit confused on how we were getting some of the voltages we got, but, I will be happy to change this unit out. CRANE TIME!

 

[kwired]

Hi, thanks for all the input. Keep in mind, we did not install this unit, so we are working from scratch. This is a Y Delta connection and there is no requirements for the XO ground and no bond. I opened each phase individually and checked input and output and C phase showed a drastic reduction in output voltage. Took an ohm meter and checked resistance on the windings and sure enough, C had a lower reading. after de-energizing and taking the back off the unit, we discovered a clearly visable hole about the size of a pencil lead on the winding. Apparently the winding shorted and we are only using half of it or so. Almost makes since. Still a bit confused on how we were getting some of the voltages we got, but, I will be happy to change this unit out. CRANE TIME!

A transformer with a full delta secondary can lose a primary phase and still continue to operate as though nothing happened, as long as the thing is not loaded too heavily, but chances are in your application they didn't oversize any more than the next standard size higher than what was needed for the application. When this happens it is basically operating as an open delta system.