[RE-wrenches] THD

[drake . chamberlin]

Hello Wrenches,

How much total harmonic distortion is acceptable for a generator feeding 
an inverter, house loads, etc. I'm looking at one that advertises less 
than 6%.


From what I've been able to find on the internet, inverter generators 
are 3 to 5%. Does this jive with your experience?


Does less than 6% seem like a good THD number?

Thank you,

Drake

Drake Chamberlin
Athens Electric LLC
athens-electric.com

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[RE-wrenches] THD problem solved

[William Miller]

Friends:



I was whining on this forum recently about an off-grid system

that was tripping GFI and AFCI breakers and devices.



The problem seemed to be related to high THD on the voltage waveform
experienced when the generator was charging the batteries.  We first
discovered this THD problem when inexpensive AC voltmeters disagreed with
True-RMS AC voltmeters.  Subsequent testing with Power Quality Monitors
indicated 28% THD of the voltage waveform.



Well it turns out that when these three-stack Radian inverters were set for
Generator AC input mode, the charger circuits generated a lot of THD (28%f
in our case).  However, when we selected Backup, Support or UPS mode the
THD went down to 3.5%f.



For those of you that have had various appliances that perform poorly with
a given inverter, you might check to see if the failures coincide with
generator run.  It could be that it is the charging circuit that is the
culprit and not the inverting circuit.  Knowing this could be helpful.

I hope this information may prevent one or more of you from tearing out
hair.  For a full report of our travails, see this link

.



William



PS:  It has only been a few days since we adjusted the settings, but so far
no more tripping.



WM



Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com

CA Lic. 773985
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Re: [RE-wrenches] THD

[Ray]

My understanding is anything over 5% THD is getting to be a problem.  
I've measured that high on grid here before.  Do you have any idea what 
harmonics this distortion is at?  Have you looked at it on an O scope?  
What generator is this?  I've seen portable generators with terrible 
wave forms under load.  They have a front end spike on the waveform, and 
then they flatten out under load.  Anything that isn't a pure sine wave 
is distortion.


Have you checked the peak of the wave form?  I've seen generators under 
load, flatten to 135 v peak, which is really not even a sine wave 
anymore.  Very poor performance, most AC electronics do not like a DCish 
wave form.


Also, I have a Seaward meter that does THD, its a little clunky to 
navigate, but it gives real time readings, is battery powered, and can 
break out the separate harmonics too.  The harmonics just give you an 
idea at what frequency the distortion is at, which "might" let you 
figure out what is causing problems.  I don't know enough about AC GFI 
electronics to know what effect THD would have, but 19% THD is total 
trash, and all bets are off; it could damage all sorts of other 
equipment too.


I think you've already done all that needs to be done: They need a 
better generator.


Ray Walters
Remote Solar
303 505-8760

On 1/23/20 1:16 PM, William Miller wrote:


Colleagues:

In a recent post about some AC GFI tripping problems, I mentioned I am 
measuring some high THD readings in certain scenarios—the highest 
being when the generator is charging at high percentages:


Inverter only: 2.5% THD

Gen Max: 19% THD

Gen 5 Amps: 8% THD

I measured the THD using a Fluke VR1710 .  This is a 
handy little gizmo and pretty expensive considering its size.  The 
device has limitations:


1.It is cord connected.

2.Results cannot be viewed in real-time.

3.It measures voltage parameters only.

Here are my questions:

1.Has anyone here had to consider problems with high THD in any 
off-grid scenarios?


2.What readings are normal?

3.What is the significance of voltage harmonics versus current harmonics?

4.What instruments are you using to measure THD?

5.Has anyone seen high THD causing AC GFI tripping?

6.What other symptoms might occur with high THD?

Tip: The reason we first started suspecting high THD is that cheap 
voltmeters registered abnormally high (including the onboard Radian 
metering) during periods of high THD, while true RMS meters (Fluke 87) 
continued to measure normal AC voltages.  If you experience this 
situation, high THD is suspect.


Answers are always gratefully appreciated.

William Miller

PS: If you are receiving a BCC of this email, it is because you are 
part of the team working on this problem.  The message is directed to 
an industry forum in hopes of gleaning additional knowledge.  I will 
share any results I receive.


Wm

Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com 

CA Lic. 773985


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[RE-wrenches] THD

[William Miller]

Colleagues:



In a recent post about some AC GFI tripping problems, I mentioned I am
measuring some high THD readings in certain scenarios—the highest being
when the generator is charging at high percentages:



Inverter only: 2.5% THD

Gen Max: 19% THD

Gen 5 Amps: 8% THD



I measured the THD using a Fluke VR1710 .  This is a handy
little gizmo and pretty expensive considering its size.  The device has
limitations:

1.   It is cord connected.

2.   Results cannot be viewed in real-time.

3.   It measures voltage parameters only.



Here are my questions:



1.   Has anyone here had to consider problems with high THD in any off-grid
scenarios?

2.   What readings are normal?

3.   What is the significance of voltage harmonics versus current harmonics?

4.   What instruments are you using to measure THD?

5.   Has anyone seen high THD causing AC GFI tripping?

6.   What other symptoms might occur with high THD?



Tip: The reason we first started suspecting high THD is that cheap
voltmeters registered abnormally high (including the onboard Radian
metering) during periods of high THD, while true RMS meters (Fluke 87)
continued to measure normal AC voltages.  If you experience this situation,
high THD is suspect.



Answers are always gratefully appreciated.



William Miller



PS:  If you are receiving a BCC of this email, it is because you are part
of the team working on this problem.  The message is directed to an
industry forum in hopes of gleaning additional knowledge.  I will share any
results I receive.



Wm



Miller Solar

17395 Oak Road, Atascadero, CA 93422

805-438-5600

www.millersolar.com

CA Lic. 773985
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