Harmonics on load side of a UPS

[mshields]

I've got a UPS feeding an ATS which in turn feeds an 800A a series of panels with electronic loads.

The UPS is very lightly loaded resulting in THD current of 39 to 43 percent.

The ATS was switching over due to the frequency sensor. Russelectric took the frequency relay out of the circuit and that problem went away. My questions are these.

I realize that this level of harmonics is far above anything indicated in IEEE 519 as being acceptable. But I also realize that UPS's are often times lightly loaded and such levels are by no means unprecidented.

Should I be concerned about this level of THD on my computer loads being fed via this system?

thanks,

Mike

 

[ghostbuster]

What is your voltage T.H.D. and waveform look like ?

 

[dbuckley]

Should I be concerned about this level of THD on my computer loads being fed via this system?

No.

Contrary to popular belief, computers don't need 'clean' power, they will run on any old sh*t you throw at them.

 

[ghostbuster]

No.

Contrary to popular belief, computers don't need 'clean' power, they will run on any old sh*t you throw at them.

Not necessarily:

We have had situations where the U.P.S. voltage waveform is flat topped and the power supplies are drawing 25-40% more current to compensate.

Result:

Constant power supply failures.

 

[barclayd]

Current harmonics in the range of 40% are not that uncommon.
Lightly loaded UPS generally run at very low efficiency, especially in the size to which you refer.
I don't think I have ever seen an ATS on the output of a UPS. Is this some sort of home-made maintenance bypass? What is the other feed?

 

[robbietan]

Not necessarily:

We have had situations where the U.P.S. voltage waveform is flat topped and the power supplies are drawing 25-40% more current to compensate.

Result:

Constant power supply failures.

As far as I know, flat topped voltages will result in zero current, as there will be no firing of the SCRs or IGBTs. these need a peak voltage value before they can operate.

But I have seen power supplies draw more current during a voltage sag or when you first turn the power supplies 'on'.

 

[robbietan]

I've got a UPS feeding an ATS which in turn feeds an 800A a series of panels with electronic loads.

The UPS is very lightly loaded resulting in THD current of 39 to 43 percent.

The ATS was switching over due to the frequency sensor. Russelectric took the frequency relay out of the circuit and that problem went away. My questions are these.

I realize that this level of harmonics is far above anything indicated in IEEE 519 as being acceptable. But I also realize that UPS's are often times lightly loaded and such levels are by no means unprecidented.

Should I be concerned about this level of THD on my computer loads being fed via this system?

thanks,

Mike

the IEEE is more concerned with current TDD (total demand distortion) rather than current THD (total harmonic distortion). you can have a high current THD (100%) but when the rms value of this current is just 10 amps and you have a total load of 1000 Amps, this doesnt bear much.

only when you measure high current THD at capacitors, should you worry.