Harmonic Current Standards - The End of the Line?

By Malcolm Burchall C.Eng FIEE, EMC and Power Electronics Consultant

The Need for Limitations of Harmonic Current Emissions

The Finite Impedance of the Mains Network

The mains voltage that we are used to using from our wall sockets does not have a negligible impedance. As shown later it has a finite value and varies from location to location and from country to country. This variability is one of the foundation stones on which the use of conditional connection is based (as detailed in Technical Report IEC61000-3-51 and the upcoming Standard IEC61000-3-112) since it isn't the actual harmonic currents that are so important (other than in their heating effect on neutral wires, transformers, etc.) but the voltages they impress on the network by interaction with the source impedance.

The Effect of the Harmonic Voltages Generated on Components Connected to the Mains Network

The principle effect of distorted mains voltage is on power components such as phase-correction capacitors and electric motors which experience an increase in temperature and hence a reduction in lifetime3. There is very little effect on most domestic electrical and electronic equipment.

The Proliferation of Harmonic Generating Loads

There is some evidence that the rise in the installed base of harmonic generating loads has plateaued4 in which case the harmonic voltage problem is not going to get any worse. The statements made by USA experts on their lack of harmonic voltage problems may reflect the fact that the installed base of TV's in the USA is static and has been for many years. In spite of an increasing base of PC's the actual total load is being reduced by the increasing use of "Green" PC's which switch to a low power state if unused for a while. In fact, the USA "Green" programme has probably done more to reduce the harmonic problem in the EU than EN61000-3-2 will ever do.

Impedance of the LV Network

This is defined in IEC725:19815 as

Z=(O.4+jO.25)ohms = O.4ohms + O.8mH

This represents the entire impedance of the generating system including the LV, MV & HV networks as seen by the consumer. For major countries, using the 230V supply system, at the extremes the UK is the best at less than (0.25+j0.23)ohms and Ireland is the worst at (1.03+j0.55)ohms both at 90% of locations. In the UK only 2% of consumers have supply impedances above the standard value whereas in Ireland 40% have. These are 1981 figures but there is no reason to think that they have changed substantially over the last 19 years.

Compatibility Levels

Maximum voltage distortion allowed = 8%.

This is based on the ability of AC capacitors to survive the extra currents generated by the harmonic voltages.

Historical Situation

  1. Pre 1980 TV sets were mainly half-wave and generated DC and even harmonic distortion.
  2. BS54066 produced (= IEC555-2) which required equipment to have full-wave rectification to meet the limits.
  3. Post 1980 TV sets started to use switched mode power supplies with their "peaky" waveform.
  4. Introduction of personal computers with switched mode power supplies.
  5. Introduction of high frequency fluorescent ballasts also with a "peaky" waveform.
  6. 1990. IEC1000-3-2 (BSEN61000-3-2) written7.
  7. 1995. IEC1000-3-2 (BSEN61000-3-2) published with effective application date 1997.
  8. 1996. EMC Directive became mandatory.
  9. Generic Standard BSEN55081-18 called for EN60555-2 (=IEC555-2) to be met by applicable equipment, i.e. domestic.
  10. Application date of BSEN61000-3-2 delayed until 1.1.2001.
  11. 1999. BSEN61000-3-2 declared unsatisfactory due to ambiguities and uncertainties.
  12. 2000. Amendments being made to BSEN61000-3-2 to make it acceptable for general use.

    Harmonic Limits of BS5406 (EN60555-2/IEC555-2)

    Harmonic Limits of BSEN61000-3-2/IEC61000-3-2

    Present Classification

    CLASS A Balanced 3-phase equipment and all equipment not in another class.
    CLASS B Portable tools.
    CLASS C Lighting equipment including dimmers.
    CLASS D Equipment up to 600W having the special "peaky" waveshape.

    Problems with the Existing Standard

    1. Difficulty in establishing which Class to use.
    2. Some equipment moves between Class A and Class D during operation.
    3. Different manufacturer's test equipment gives different results particularly with fluctuating loads9.
    4. EMCTLA declaration that the standard is unusable in its present form.
    5. Standard covers a multiplicity of equipment not manufactured in sufficient quantity to be a problem.
    6. Uncertainty over the application date of the reduction of the lower limit of Class D from 75W to 50W.

    Proposed Amendment to the Standard

    This amendment is now available from the BSI as a Draft for Public Comment as document 00/241586DC10 with a closing date of the end of June. If you want to comment this is almost your last chance. The amendment doesn't actually change any of the limits but it does alter the classification of equipment, places the onus on the equipment manufacturer to specify the testing conditions and simplifies the measurement of fluctuating loads.

    CLASS A Balanced 3-phase equipment and all equipment not in another Class, household appliances excluding those in Class D, non-portable tools, dimmers for incandescent lamps, audio equipment.
    CLASS B Portable tools.
    CLASS C Lighting equipment.
    CLASS D Personal computers and their monitors and TV receivers up to a limit of 600W.

    The most important two aspects of the above changes are that all professional equipment is now in Class A and not, therefore, subject to proportional harmonic current limits and that the definition of Class D by the waveform shape has been removed and the definition is now by equipment classification.

    This doesn't remove the need for a harmonic current reducing inductor, particularly for Class D where it is still the same size as is needed by the present Standard, but for many lower powered equipments which have moved into Class A the inductance required is comparable with that already in use for series-mode RF attenuation. For example, one TV company already uses a special multi-cored series inductor that consists of an iron or iron-powder core for the LF attenuation (line frequency harmonic currents) and a ferrite core for the HF attenuation enclosed by one winding.

    Conclusions

    The amendments being proposed are very close to those that I proposed many 1000's of Committee hours and millions of pounds of development money ago in 1992 and I quote from that paper11:

    "In attempting to resolve a real problem for the majority of industry, a small but not insignificant part of the industry is being unduly penalised. The regulatory bureaucrats are using a sledgehammer to crack the last little nuts hiding in a corner!"

    We haven't quite got rid of the sledgehammer but we have come as close to it as we are ever likely to.

    The final moral of this story is that, if you don't want these problems to surface again in another guise or area, keep abreast of what your Trade Association is doing and make sure your views are heard this time before the event rather than after. There are lots more of these standards in the pipeline where this one came from.

    References

    1. IEC61000-3-5:1994, "Electromagnetic Compatibility (EMC) Part 3: Limits. Section 5. Limitation of voltage fluctuations and flicker in low-voltage power supply systems for equipment with a rated current greater than 16A". (Technical Report).
    2. Draft IEC61000-3-11. "Electromagnetic Compatibility (EMC) Part 3: Limits. Section 11. Limitation of voltage fluctuations and flicker in public low-voltage power supply systems, for equipment with a rated current 75A and subject to conditional connection".
    3. "The background to the problem - the power supply view", Geoff Finlay, The Electricity Association, EMCTLA meeting 19th May 2000.
    4. "Recent moves in CENELEC to produce a standard", Maarten Vrolijk, Phillips, EMCTLA meeting 19th May 2000.
    5. IEC725:1981, "Considerations on reference impedances for use in determining the disturbance characteristics of household appliances and similar electrical equipment".
    6. BS5406:1988/EN60555-2:1987, "Disturbances in supply systems caused by household appliances and similar electrical equipment. Part 2. Specification of harmonics".
    7. IEC61000-3-2:1995/BSEN61000-3-2:1995, "Electromagnetic Compatibility (EMC) Part 3: Limits. Section 2. Limits for harmonic current emissions (equipment input current < 16A per phase)".
    8. BSEN55081-1:1992, "Electromagnetic compatibility - Generic emission standard. Part 1. Residential, commercial and light industry".
    9. "The background to the harmonics problem - the test laboratory view", Stephen Kirk, RFI.
    10. Final draft EN61000-3-2, amendment prA14, BSI document 00/241586DC.
    11. Burchall, M A, "Harmonics - The cost of conformity", EMC'92, "Designing EMC into your product - Conference Proceedings", London, 12-13th February 1992, ERA Report 92-0011, Leatherhead,U.K.

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