First of all, thanks for the replies folks.

WRT THD, what I read might have been this:

"Two different definitions of THD are commonly used. One is expressed in

terms of percent RMS current or voltage. The other is based on percent

fundamental current or voltage...

Values below 20% (of either RMS or fundamental) are generally considered

tolerable; values exceeding 50% of RMS or 40% of fundamental are often

considered high"

from this article on compact fluorescents:

http://irc.nrc-cnrc.gc.ca/pubs/cp/lig3_e.html

There are two definitions of THD, but both use RMS values of

the various harmonics, so it is very confusing when the

author of the above article means when he states "One is

expressed in terms of percent RMS current or voltage. The

other is based on percent fundamental current or voltage. "

The first definition of THD, and the one used by the

lighting industry in the US (and perhaps other places), is

the ratio of the square root of the sum of the squares of

all the harmonics greater than 1, divided by the fundamental

frequency - the first harmonic. The dominator is often

expressed as the square root of the square of the

fundamental, which is just the fundamental. This definition

can be found in Section 3.7 of ANSI C82.77-2002, Harmonic

Emission Limits - Related Power Quality Requirements for

Lighting Equipment. A PDF copy of this document can be

obtained for no charge at

www.nema.org. Using this

definition the THD can range from 0 to infinity.

The second definition of THD is similar to the first except

that the dominator is the square root of the sum of the

squares of all the harmonics including the first. Obviously

using this definition the THD can vary from 0 to 100%

I see that article also suggests that a couple of "bad" CFLs likely won't be

a problem. Having heard that several times now I am feeling better about

the idea, so to speak.

Even relatively high CFL THD makes a only minor contribution

to the THD of a residential building. You can find a good

discussion of the relative contribution of CFL THD to total

residential building THD in NEMA LSD 8-1999, Power Quality

Implications of Compact Fluorescent Lamps in Residences. A

PDF copy of this document is also available for no charge at

www.nema.org.

--

Vic Roberts

http://www.RobertsResearchInc.com
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