Ampacity unbalance

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QPEgeorge

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I have been contacted by a facilities manager for a large chiller plant that supplies cold water for a portion of a town. The problem with this is a pronounced ampacity imbalance. How is it occuring? Here are the particulars:A phase 814A, B phase 832A. C phase 731A. ACV 484, ABV 490, BCV 489. The service grounds are excellent reading less than one ohm. There are two chillers in the building. Only one was running while I was there, but I was assured that the second one had the same issue. Two power company pad mount transformers feed two sets of switchgear in the building. Both sets of gear were scanned with FLIR cameras with no hot spots. All connections appear clean, bright and tight. The power company switched their primaries, A to B, B to C, C to A, to see if the problem would follow. It did. The low ampacity originally was on B phase and now is on C phase. The power company says it is only responsible to keep voltage in a 1 percent range, and they are very close. I have not researched all possibilities yet, but was reaching out for ideas. Any body got some? Thanks, George
 
Lots more info is needed.
Where are you measuring the current and what are you using to measure it?
Are there VFDs on the chillers?
Are there VFDs anywhere on that distribution system?
Can you isolate the chillers to see if the problem exists if only one of them is running?
When the utility swapped the primary leads, did they do it on both xfmrs at the same time or somewhere upstream so that it fed both xfmrs?

All that aside though and assuming that the primary lead swap affected both xfmrs, if the problem follows the primary lead swap, the cause would not exist in the load circuit. It is either a problem or lack of capacity in the xfmr primary(s), or in the utility supply itself. The advice given by buckofdurham would be the telltale sign of that.
 
Thank you both for the responses. One, I cannot check voltage with nothing on at this time. Demand is too great. Will try to do so in the future. Second, the voltage is a tad low, on the same phase, on both services. The chiller that was running has its own 1600 amp switchboard, ergo, its own transformer. The second switchboard, 2600 amps, feeds the other chiller (not in use at this time), the building power, and the tower and circulation pumps. All the pumps have VFD's. The chiller controller did not appear to be a VFD. It had what seemed to be a two winding setup with a soft start contactor. I saw very little electronics. It is possible the VFD was in another enclosure mounted on the chiller which I had no access to. Yes, both transformer primary leads were swapped. The power company even fed the transformers from two directions by isolating their feeds on the overheads. The paperwork I have showing the graphs they did show increases in current unbalance corrolating to the voltage unbalance. They admit that by isolating the feed direction, the problem was not fully corrected. I have their time log. Will try to make sense of all of it. The current was measured on the line side of the switchgear with a true RMS Fluke amp meter. The current and voltage at the chiller were measured by the control panel instruments. I fully agree that the power company is responsible but getting them to admit it is the issue.
 
The current was read on the line side of the switch gear. What else is fed

off of this gear ? What are the current readings at the chiller ?
 
The utility is giving you around 1% voltage imbalance, and you are thinking they are at fault? In my opinion, this is more of a VFD parameter setting issue or a chiller issue than a supply issue. What is the current imbalance trip set point on the VFD's? What is the maximum current imbalance recommended by the chiller manufacturer?
 
From what is described here, the chillers in question are not on a VFD, but there are other VFDs in the plant.

Is it possible that the supply phase angles are not a nice even 120 degree difference?

-Jon
 
Thanks to everyone for your replies. I want to quote from an article written by Joe Evans, Ph.D., in Pumps & Systems, July 2008. Dr. Evans states, "Voltage unbalance occurs when individual phase voltages in a three phase circuit are not equal. The major effect of this unbalance is an increase in stator and rotor I2R losseswhich results in an even larger phase current unbalance. A voltage unbalance of just 1 percent can result in a current unbalance of 6 to 10 percent (even higher when VFD's are involved)." Dr. Evans continues, "Unbalanced voltage can originate with the utility or within your own distribution system. Another source of current unbalance is voltage distortion. This condition, due to harmonics produced by non-linear loads, is characteristic of solid state devices. These harmonics may not lead to measurable voltage unbalance but can cause significant current unbalance. Current unbalance can be due to problems on the motor side of the circuitthat usually do not result in voltage unbalance. Since current unbalance can exist without an accompanying voltage unbalance. an accurate systems analysis will require measurement of both voltage and current in the circuit". This gives me a lot to chew on. All of you who responded touched on parts of this article. It seems I just scratched the surface of what I need to do. Time to breakout the recording meter and look for the harmonics. It is also possible another large commercial user could be generating harmonics and dumping them on the utiity. Also, demand and time of day may come into play. If you have any more suggestions or comments, please reply. We all learn from each other, and that is what makes us better electricians and people. Thanks again, George
 
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