Hey Forum Gurus,

I am having an issue with the power system in a building. There is an usually high electronic failure rate. I ran some diagnostics and found something peculator and wanted to get some expert opinion. My conserns was with "THD A". Total Harmonic Distortion Amperage.

Here are the results of the diagnostic:

L1-
Voltage: 464.131
Current: 174.15
frequency: 60.03
THD V: 0.6%
THD A: 5.2%

L2-
Voltage: 467.893
Current: 192.682
frequency: 60
THD V: 0.9%
THD A: 6.9%

L3-
Voltage: 459.715
Current: 172.691
frequency: 60.03
THD V: 0.8%
THD A: 7.8%

There are similar results for the 3 phase 208 THD A.

Thank you in advance

 

Is the voltage being measured L1 to L2, L2 to L3 and L3 to L1 in a Delta configuration or is the voltage being measured L1 ro N, L2 to N and L3 to N in a WYE configuration? What would you guess the source of the distortion to be? Meaning on the line coming in or could it be internally generated and fed back into the line?

As to the acceptable limits for THD on voltage:

“Computers and allied equipment, such as programmable controllers, frequently require ac sources that have no more than 5% harmonic voltage distortion factor [THD], with the largest single harmonic being no more than 3% of the fundamental voltage. Higher levels of harmonics result in erratic, sometimes subtle, malfunctions of the equipment that can, in some cases, have serious consequences.”

This is a good read on the subject. You may want to get a copy of IEEE Std 519, RECOMMENDED PRACTICES AND REQUIREMENTS FOR HARMONIC CONTROL IN ELECTRICAL POWER SYSTEMS which should get things in perspective with realistic acceptable numbers. Lastly, you do not mention a location? I am not sure if acceptable limits vary from location to location.

Ron

 

Hi Ron,

Thanks for the reply. Yes the measurements are phase to phase and the transformer is of delta configuration. My guess the source of the distortions are related to the florescent lighting through out the building, variable speed controls for the roof top condensers, the copious amounts of power backup units, and computers.

This building is a 10 story apartment building located in Costa Rica.

Thanks for the reference and will be giving that a good read. As far as "THD A" is concerned my searches on google haven't turned up much info. I was looking at my data and found THD A as high as 11% I am sure thats not good.

Thanks
Robert

 

Here is a screen capture of some of my data.
Screen Shot

 

My thinking as to source, based on what you mention, would run with yours. The facility I retired from was 480 V 60 Hz 3 Phase Delta and there were plenty of VFD drives for large motors running pumps. While we always did have some THD it really never was much of a problem. Never looked at THD on current but years ago we were looking at the voltage THD and did notice as the load increased so went the THD on our voltage. Since we never saw any ill results I was never overly concerned. Eventually we brought a dedicated facilities engineer on board and I avoided facilities like a plague.

Ron

 

http://electrical-engineering-portal.com/line-harmonic-currents-adjustable-frequency-control-created

http://electrical-engineering-portal.com/how-to-get-rid-off-the-harmonics-in-electrical-system

Basics, but it may point you in the right direction to get started

 

Thank you for your replies and links to very helpful information. I think my understanding of harmonic distortions are more complete now but I am still having trouble digging up info on the current distortions. There has to be something wrong there with all the dead electronics and elevator control computers and I believe that something has to do with THD A. That is the only thing I see that raises questions unless some else sees something I don't.

The following are more screen shots of my results.

These screen shots are for the 480v 3ph Delta
Link1: Harmonics
Link2: Frequency / Unbalance
Link3: Voltage and Current
Link4: Spreadsheet
These screen shots are for the 208v 3ph Delta
Link1: Harmonics
Link2: Frequency / Unbalance
Link3: Voltage and Current
Link4: Spreadsheet

Thanks Again
Robert

 

Having a hard time imagining how harmonic currents could be destroying electronics?

I can imagine really dirty power causing malfunctions, but damage points strongly to transient over voltage?

 

There is an existing network of surge suppressors and lighting dissipation. I know that suppressors are not infallible and that transient voltages can still get through but I don't think the amount of dead electronics are the result of spikes. But I guess could be if there's a grounding issue which i think there is not.

Thanks