Low-frequency conducted emissions

By J. M. Woodgate B.Sc.(Eng.) C.Eng.  MIET SMIEEE FAES Hon FInstSCE MIOA

Low-frequency conducted emissions

Did you think that IEC/EN 61000-3-2 and -12 (and -3 and -11) were just about finalized? Well, they are not; a lot of work is still going on.

IEC 61000-3-2: Even after the last major re-write, the committee sees further opportunities to make the standard easier to read. There is also an ambiguity about how Partial Odd-order Harmonic Current is determined. There are two possible ways to treat the raw data from the IEC 61000-4-7 measuring instrument, and if the harmonic content doesn't vary much during the observation period, they give nearly the same result. But now it has been found that they don't give the same result if the harmonics vary quite a bit. This is a tricky subject and we may have to wait for an answer.

Another issue was raised about the splitting up of a multi-box appliance into separate units that each meet the standard, but the whole appliance does not. This is probably a regulatory issue that cannot be addressed in a standard. However, the standard already includes prohibitions on the use of certain techniques, which could also be regarded as regulatory. The solution there is to replace the prohibition buy requirements (probably emission limits) that the unwanted techniques (such as half-wave rectification) cannot meet, But there has to be a decision in the committee to implement that.

A further question is whether the limit for the 3rd harmonic in Table 2 (which applies to only some Class C (lighting) products should still be dependent on the power factor ?. This dates from the days of linear fluorescenet lamps with magnetic ballasts, for which ? was about 0.5. European and Energy Star regulations now include relevant requirements, but they differ. They depend on the lamp power (active power, not VA), but the main requirement in Europe is that the displacement power factor (caused by phase-shift between applied voltage and fundamental input current) exceeds 0.7. Energy Star has the same 0.7, but it specifies 'power factor' and defines it so that it includes the effect of the current waveform possibly being distorted. In fact, the value of 0.7 doesn't justify the elimination of the factor ?, but the lamp manufacturers may present a case based on the actual power factor of the relevant lamps is much higher than 0.7

A yet further issue is how to apply the limits to a product having multiple functions. It's not a problem if each function can be operated separately, but that is not always the case. A simple solution would be to apply the limits for whichever Class in IEC 61000-3-2 draws the greater power (or perhaps VA?), but this needs to be carefully studied in case anomalies might be created.

In the somewhat longer term, the evolution of the 'network environment' needs to be considered. The provisions of IEC 61000-3-2 have actually not changed much for 40 years. In those days, we had direct-on-line TV bridge rectifiers, other products with mains transformers feeding one or another type of full-wave rectifier, commutator motors and other loads that hardly exist these days. Now we have switch-mode power supplies and adjustable-speed drives, mostly with automatic power-factor correction.

This causes low-order harmonic current levels to decrease, but some higher-order, such as the 15th, have increased and a whole chunk of spectrum from a few kHz (due to very large switching converters) to the low megahertz (due to very low-power products such as phone chargers) is now populated by conducted emissions from these switching circuits. This study may (or very possibly may not) result in changes to emission limits.

IEC 61000-3-12: This standard is undergoing a full maintenance overhaul. There are two main ussies. One is 'grouping' – the change of measuring bandwidth for harmonic from 5 Hz (only harmonics) to 50 Hz (harmonics and interharmonics). One reason for controlling interharmonics is that some networks still use very old system of network configuration control that depends on the injection of audio-frequency signals into the MV network. Grouping will also apply for IEC 61000-3-2, but it needs to be discussed only in one committee at a  time!

Changing the measurement bandwidth by a factor of 10 should, if the spectrum is 'noise-like', justify raising limits by the same factor. The spectrum isn't noise-like around odd-order harmonics, but it is , in some networks, around even-order harmonics. This makes for great complications, because the limits on even-order harmonics are very stringent and widening the bandwidth practically ensures that many current products would fail revised limits. The solution being studied is that if a product does not meet the present limits with grouping, but does meet them without grouping, it is then tested to a higher set of limits with grouping. This is 'sort-of' agreed in principle, but the higher limits are not yet agreed.

New standards

IEC 61000-1-8: General – Phase angles of harmonic current emissions and voltages in the public supply networks – Future expectations

This huge study of network conditions in Europe was prompted by the suspicion that the assumption of a 'prevailing phase angle' for the 5th harmonic  might no longer be valid. Some requirements in both emission standards depend on this assumption. The main finding of the study is that there is a prevailing phase angle at some sites but not at others. It is unlikely that the emissions standards will be changed, but discussions between Distribution System Operators (DSOs) and their large customers about products needing consent to connect may be more enlightened.

In addition to the above, there are two new standards and a Technical Report that may be published within the next two years.