Power Distribution Measured Amp Load is incorrect.

[Trublmkr]

I have a power panel that is fed by 208 3 phase coming from a UPS and a second or alternate power feed from the city utility line, they are synced and within a fraction of a volt of each other. Does anyone know why if the load is measured with a clamp on meter, why when the feeds are sharing the load they read 60% to 75% higher. If 1 feeder is connected and the other not we see approximately 40 amps per leg, but when they are both on and should be sharing the load, 1 feeder shows 50 to 60 amps per leg and the other feeder shows 70 to 90 amps per leg. But when we check Power Logic it reports the load at 40 amps each. Does anyone have an explanation of what could be happening here? We have this same thing happening in at least 2 different buildings. It is not causing any problems, we just can't figure what would cause this. Thanks for your time.

 

[jghrist]

Sounds like the UPS is feeding loads on the utility line. Can you get power measurements to indicate if you are getting reverse power from the utility?

 

[PHovnanian]

I concur with jghrist's hypothesis. And I'm wondering why you've got a UPS (or several) that are running in parallel with utility power.

Are these actually some sort of grid tie inverter systems (like part of a solar power system)? If so, depending on their settings, you might actually expect to see power feeding back toward the utility at certain times. That's not what I'd expect to see for a simple backup power system where the object is to keep the batteries topped off from utility power. But if they (the batteries) are charged from some other source (PV panels for example) then the excess power can go back to the power company.

 

[waross]

Are the UPS batteries charging?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

To clarify- are both feeds to the panel connected at the same time so there are operating in parallel? If so, the UPS is pushing power to the utility or there are circulating currents flowing through the interconnections.

Is there a single common neutral point for both the UPS and the utility feed? Or is the transformer supplying the utility source grounded at a different location than the UPS output transformer?
If the 208/120V volt transformer has a neutral grounded at its location and the UPS 208/120V output is grounded at the UPS even slight differences in the output voltages or phaseing will circulate current through the transformers.

The UPS inverter output may not be able to sense the circulating currents, so its display only shows the actual power it is delivering and not the circulating currents flowing in its output winding.

Back to a fundamental question? Why are you reducing the reliability of your system by paralleling the utility supply with the UPS? Any fault on the 208 V utility system will trip the UPS feed as well as the utility feed. Your load is not protected from utility failures by the UPS. Or maybe I'm missing something.

 

[Trublmkr]

Thanks for all the reply's. I need to clarify that 1 feed is coming from the UPS in bypass mode which is at the time of this anomaly being fed by an emergency generator and the alternate feed is paralleled and being fed by the same Emergency Generator Dist. Board.

No it was not charging the batteries the UPS was being pm'ed.

The power feeds are paralleled to facilitate UPS maint. so there is no power interruption.

Thanks again.