In message <00ab01c80445$9d6a67f0$2301a8c0@JohnOptiplex>, dated Mon, 1
Oct 2007, John McAuley <[email protected]> writes:
>The latest version of EN 61000-4-7 includes interharmonics.
Well, sort of. What has happened is that in the latest edition the
measurement bandwidth has been raised from 5 Hz (which included only a
few interharmonic products spectrally close to each harmonic) to 50 Hz
(which includes all interharmonic products around each harmonic).
BUT there is a concession to use the 5 Hz bandwidth (or the older
measuring instrument) until certain issues resulting from the change
have been resolved.
>
>Electromagnetic compatibility (EMC) Part 4-7: Testing and measurement
>techniques General guide on harmonics and interharmonics measurements
>and instrumentation, for power supply systems and equipment connected
>thereto
>
>At present European standards such as EN 61000-3-2 do not require the
>measurement of interharmonics.
Not correct. By changing the bandwidth in the Basic standard, the
inclusion of (more) interharmonics is automatic, not requiring any
change at all to IEC 61000-3-2 or -12.
>
>A manufacturer of harmonics measuring equipment has told me that
>interharmonics is required for any testing that is being carried out
>for the Japanese market, particularly Japan Pharmaceutical Affairs Law,
>and will be required in the future in Europe.
>
>Can anyone confirm this?
I don't know about Japan, but the statement about Europe is just an
assumption.
>
>As I understand it, EN 61000-4-7 will not be listed in the Official
>Journal. Therefore, interharmonics would only be required if specified
>in standards such as EN 61000-3-2 or EN 61000-3-12. And as far as I am
>aware, there is no proposal to include interharmonics in either of
>these standards.
Not true, as explained above.
>
The UK national committee has proposed one way (there may be others) of
resolving the 'certain issues' mentioned above. For almost all products
that produce interharmonics (either by direct generation or by fast
amplitude modulation of the load current), the inclusion ('grouping') of
the interharmonics around an odd harmonic doesn't result in a 'pass'
becoming a 'fail'. But the limits for even harmonics are much more
stringent, and the grouping can tip the balance to 'fail'.
The UK proposal is to compare each individual spectral component with
the limit for the nearest harmonic. For example, it doesn't seem likely
that if an emission of 2 A at 400 Hz can be tolerated, an emission of 2
A at 409 Hz cannot, in general, be tolerated.
This may not work for absolutely all interharmonic frequencies; the
problem is 'ripple control', which is used in some countries, notably
France, for tariff switching and other purposes. The equipment is often
elderly and doesn't have a very narrow bandwidth, so it can suffer
interference from interharmonic emissions. But it is possible to set
special limits on those critical frequencies, if they have to be lower
than the limit for the harmonic nearest in frequency.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of 2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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