Dear Rich Many thanks for your useful analyses. I was wrong to suggest that the 'computer industry' is in denial about mains harmonics - I realise that many people in that industry have made and are making valuable contributions in that field. But I am sure that the claims that there is 'no scientific evidence' for harmonics problems would not stand the light of day. In the UK harmonic problems due to fluorescent lighting have been discussed in public fora since the 1950's, and Arrilaga's important textbook on harmonics was published in 1985. The IEE (based in London) held an international conference on harmonics in power systems in 1981.
I haven't costed any PFC designs for a while, but there now appear to be solutions available that have much lower cost then the 'active PFC' front converters we used to use. For example: the 'charge-pump' method, which does not use additional switching devices - see Infineon Application Note: AN-TDA 1684X (version 1.2 dated June 2000) and a (probably) forthcoming article on improving this technique in a future issue of Compliance Engineering Magazine (www.ce-mag.com). Regards, Keith Armstrong In a message dated 23/01/02 20:11:32 GMT Standard Time, ri...@sdd.hp.com writes: > Subj:Harmonic current emissions > Date:23/01/02 20:11:32 GMT Standard Time > From: ri...@sdd.hp.com (Rich Nute) > Sender: owner-emc-p...@majordomo.ieee.org > Reply-to: <A HREF="mailto:ri...@sdd.hp.com";>ri...@sdd.hp.com</A> (Rich Nute) > To: cherryclo...@aol.com > CC: ghery.pet...@intel.com, emc-p...@majordomo.ieee.org > > 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. >
