[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 <TJ3fTpspG2DGCzC>.  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 <http://www.millersolar.com/>
>
> CA Lic. 773985
>
>
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