Hi Keith and Ghery:
There are a number of effects of harmonic current
emission from non-linear loads.
1) When a large number of loads rich in triplen
harmonics are supplied from a 3-phase source,
the neutral current can be as high as root 3
of the phase current. (This effect does not
exist on a single-phase distribution system,
or on a 3-phase system where each phase has
its own neutral.)
Where the neutral wire is sized for a balanced
load, some authorities allow the neutral wire
to be one size smaller than the phase wire.
Such a wire is likely to be overheated by the
triplen currents. Indeed, it is possible to
overheat the neutral wire when it is sized the
same as the phase wire. In the USA, authorities
now require (for such loads) the wire to be
larger than the phase wire, or two, parallel
neutral wires.
2) Consider that the non-linear load generates
current at harmonics of the mains frequency and
injects it into the mains distribution system.
This current must circulate in the distribution
system and return to the source (load). Often,
this current circulates in the delta primary of
the first upstream delta-wye distribution
transformer, and causes the transformer delta
winding to overheat. (This effect is likewise
mostly due to triplen harmonics.)
In the USA, distribution transformers are
specially designed to dissipate this power
without overheating. Such transformers include
a K-factor rating, which is a measure of the
transformer to accommodate the current.
3) Depending on the source impedance, a large
number of non-linear loads can cause voltage
waveform distortion. Voltage distortion is
caused by all of the harmonics, not just the
triplen harmonics.
Voltage waveform distortion can cause motors to
overheat.
Each of these effects is a separate and independent
issue. They should not be lumped as a single issue.
For each effect, there can be one or more remedies.
The remedy can be either in the load or in the
source.
EN 61000-3-2 arises from the voltage distortion
effect. Mr. Van den Bergh's comments (as quoted by
Keith) appear to address voltage distortion, not the
other effects.
Because of the difference in the design of power
distribution systems, voltage distortion is more of
a problem in the EU than in the USA.
> I suspect the real reason for the computer industry's denial of harmonics
> problems, or else blaming them on a poor distribution system, is that US
> computer manufacturers simply want to make one model they can sell
> world-wide
> so they want whatever is permitted in their main market (the US) to be
> permitted everywhere else.
> Would you agree with this?
I believe this is an oversimplification of the
manufacturer's dilemma of addressing this problem.
The USA computer industry has been quite forward in
addressing effects 1 and 2. The computer industry
was the force behind a series of academia-based
seminars on the causes and solutions to effects 1
and 2 that resulted in changes to the USA National
Electrical Code and to distribution transformer
testing and ratings. (I presented in some of those
seminars.)
This is NOT denial.
> But whichever method is adopted, the customer pays the bill eventually and I
> have more confidence in the highly competitive world of electronic products
> to come up with a cost-effective solution in a timely manner.
One of the USA's major objections to EN61003-2 is
that remedying the load repeats with each new product
that is introduced, while remedying the source is a
one-time remedy. EN61000-3-2 requires continous
cost to the consumer with each product. (The cost
is NOT trivial -- nearly double the cost of the power
supply.) Indeed, this has forced manufacturers to
develop one supply for the EU, and one supply for the
remainder of the world. And, forced two products for
the world instead of one.
(One of the benefits of EN61000-3-2 has been a real
effort at power reduction so that more and more
products are below the 50-watt exemption limit.)
> I have some knowledge of power-factor correction techniques in switch-mode
> supplies, and some of them can cost very little indeed. So I really don't
> know why the US computer industry is making such a fuss about controlling
> harmonic emissions.
I certainly have not seen low-cost PF correction
techniques. My experience is that the cost is
nearly twice the cost of a non-PF corrected power
supply.
Best regards,
Rich
ps: EU power suppliers are taking an interesting
approach to their customers. If the customer
has a linear power factor problem, we will
correct it. If the customer has a non-linear
power factor problem, we will not correct for
it, and we will not sell power to you if the
effect is too great. Clearly a monopolistic
view.
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