Harmonics considerations for panel builders

Should mains harmonic distortion be a major concern for panel builders?

At present, the generic and product standards listed under the EMC Directive call up just one basic test standard on the emissions of harmonics into the AC mains supply.

This is EN61000-3-2 Limits for harmonic current emissions (equipment input current up to and including 16A per phase), which only applies to equipment connected to the public low-voltage mains distribution network.

There is also the IEC standard IEC61000-3-4, which will one day be replaced by IEC61000-3-12, for equipment connected to public low-voltage networks with an input current of between 16 and 75A per phase.

As yet this is not called up by any EMC Directive listed standards but, like all standards it can be used in technical specifications and contracts.

The definition of a public supply is one that is shared between more than one organisation or household.

A site that is owned and operated by one company, and has a dedicated MV or HV distribution transformer providing an LV mains supply that is not shared with any other company or residence, is a private mains supply and so although EN61000-3-2 still applies to equipment within its scope, its limits do not apply.

There are a number of other detailed exclusions and variations in EN61000-3-2, for instance, up until now professional equipment has been excluded, although it does apply to domestic equipment such as computers, air-conditioning units, fridges and washing-machines - but this is about to change.

For example, the new EMC standard for lifts and elevators which are used in environments that are difficult to define as being either industrial or domestic.

The new standard EN12105 is quite explicit regarding total harmonic distortion (THD) and partial weighted harmonic distortion (PWHD) and sets out specific values for permanent and short duration harmonic emissions for specific orders of harmonics (5th,7th,11th and 13th).

Then there is the Distribution Code G5/4 -1which is published by the Energy Networks Association which is used for managing harmonic distortion.

This applies to all consumers at their point of common connection to the public supply network (the point at which the consumer is connected to other consumers on the network) and effectively it forms part of the consumer's agreement to connect with the network operating company.

When an organisation wishes to install new equipment, if it conforms to the limits in EN61000-3-2 then it may be connected without any further assessment.

But if IEC61000-3-4 (or -12) has been applied there are three possibilities.

If the equipment meets the limits in EN61000-3-2 no notification or further assessment is required, and the equipment can be connected freely.

If the equipment meets a relaxed set of limits or rules given by IEC61000-3-4 (or -12) the network operator must be notified of the connection and relevant details.

They may subsequently choose to perform an assessment to G5/4 -1 but they cannot prohibit the initial connection.

But if the equipment does not meet even the relaxed limits or rules in IEC 61000-3-4 (or -12) the network operator must be notified and must give its permission before connection is undertaken.

For installation of equipment above 75A per phase, if the pre-existing total harmonic voltage waveform distortion is close to 5%, the network operator must be notified first, and could call for an assessment of the total harmonic currents generated by all equipment connected to a point of common coupling.

Usually this can be produced fairly easily by using manufacturer's data.

Another set of rules apply for equipment which draws its power from a dedicated transformer connected to the medium voltage distribution network (6.6 to 22kV) or the high-voltage network (33kV or greater).

In these cases more complex procedures are called for.

Harmonic reduction may also be required for self-protection of a site.

Harmonics can produce many problems such as excessive heating of cables (especially the three-phase neutral conductor in the case of single phase loads), abnormal heating and acoustic noise from transformers, busbars and switchgear; interference with equipment that is sensitive to voltage waveform and damage to power-factor correction equipment.

The excessive heating of cables and transformers can be so severe, causing actual damage, even smoke and fire hazards.

Fuses and thermal circuit-breakers detect the excessive heating effects of harmonics (but only in the conductors they protect) and will protect accordingly, which can lead to unreliable supplies - but an electrician measuring the current with a typical low-cost meter will not measure the true current and so may be tempted to fit a larger fuse or CB, thereby risking damage and fire.

These days, all site electricians should be equipped with true-RMS reading ammeters.

Harmonics can be produced by any semiconductor switching device connected to the supply - rectifiers in linear and switch-mode power supplies and motor drive inverters, thyristors and triacs being the most notorious offenders.

Also the contributions of the various harmonic emissions sources tend to be cumulative in a power network.

It would seem to be good practise, therefore, to fit harmonic filters onto all new high-power equipment such as frequency inverters or large power supplies, particularly if they are rated, say, above 30kW.

By adopting this practice there is at least some harmonic management, which could help to prevent future problems and the need for an expensive solution later on.

Alternatively, as a general rule because there are no statutory requirements just at the moment, it might be considered sensible to ensure that the total harmonic distortion of the supply waveform on a mains distribution network does not exceed 8% because most equipment appears to be immune to this level.

There are commercially-available standard solutions for harmonic filtering, and these fall into two categories - active and passive.

Active filters adapt to the harmonic signature of the load and produce compensating currents to reduce the voltage waveform distortion.

They are effective but generally quite expensive.

However, passive filters are usually inductors (so-called line reactors) or tuned circuits comprising inductors and capacitors, and are not so expensive but they are certainly not energy efficient.

It is expensive and time consuming to measure harmonics, especially over the long term, and difficult to predict overall levels harmonics at the planning stage, especially as the supply impedance can have a great influence on the level of harmonic distortion (the harmonic voltage waveform distortion caused by a given harmonic current is proportional to the network's impedance at that frequency).

So it is fairly common practice to connect a line reactor in front of equipment that is likely to generate harmonics.

Although it is an additional installation cost, there is good justification for doing this because the inductor also offers some protection against surge over voltages on the mains network, which are potentially harmful to equipment containing semiconductors such as drives and power supplies.

REO offer a series of practical guides on harmonics and other related power quality issues and these can be obtained by contacting REO.

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