A new European standard, EN61000-3-2, which sets severe limits on
allowable harmonic current emissions from equipment with an input current
up to and including 16A/phase, became effective at the beginning of this
year. Malcolm Staff explains how this will affect users of variable speed
drives
Measurement results from several European countries have shown that the
levels of harmonic voltage distortion in the electricity supply have been
increasing over the last decade. As a consequence, the IEC has been
working on standards that define how to limit the harmonic currents from
installed equipment, and ensure compatibility with the distortion levels
stated in the existing IEC61000-2-2 standard.
The aim of these standards is to limit the potential hazards caused by
harmonic voltage distortion and its adverse effects such as overheating
of transformers, cabling and capacitor banks, and the subsequent
malfunction of other products connected to the same supply. These
standards impose specific requirements on the use of variable frequency
drives that draw their power from the public electricity supply network.
Voltage harmonic distortion affects the ‘quality’ of the mains voltage
delivered to other users of the electricity supply. This differs from
some other types of electrical disturbance (RFI for example), which is
normally confined to the user’s own premises. Not surprisingly, the
utility companies have been active in setting acceptable limits for
harmonic distortion on their networks. As a result, a new European
standard, EN61000-3-2, was applied at the beginning of this year imposing
very demanding limits on allowable harmonic currents. Moreover, the
standard is valid for all equipment installed on public low-voltage
distribution systems.
Origins of power line distortion
Most variable frequency drives operate by using a bridge rectifier to
convert the incoming ac voltage into a dc voltage. The inverter of the
drive then converts the dc voltage into a controlled voltage and
frequency for motor speed control. The most common types of drives in use
today, use a diode bridge rectifier to convert the ac power line into a
fixed-voltage dc bus. A dc bus capacitor bank is then used to filter out
the ac ripple. Some three-phase drives (including all product series
manufactured by the author’s company) incorporate dc coils, which
considerably limit harmonic currents, reduce the ripple on the dc link
and hence increase the lifetime of the dc bus capacitors.
Indeed, most modern items of electronic equipment, including computers,
fax machines, copy machines and electronic lighting ballasts, use this
type of bridge rectifier power supply. As the current is not proportional
to the supply voltage the load is described as being ‘non-linear‘, and
such equipment generates current harmonics, resulting in harmonic voltage
distortion.
Public low-voltage distribution systems can be defined as those systems
in which one or more clients are connected to the same low-voltage public
utility network. Therefore, if a user has a dedicated transformer and
does not share his low voltage supply with any other users, this standard
does not have to be applied. The limits in EN61000-3-2 are absolute and
specified up to the 40th harmonic of the base frequency, typically 50Hz.
The standard is valid for 220/230/240V single-phase and 380/400/415V
three-phase operating at 50 or 60Hz.
The limits in EN61000-3-2 apply to all equipment sold directly to the
general public with a maximum phase current of 16A. The standard applies
to ‘professional’ equipment (ie that which is not sold directly to the
general public) with a total input power less than or equal to 1kW.
Limits for professional equipment higher than 1kW are under
consideration. Professional equipment includes frequency converters and
EN 61000-3-2 must be followed for products rated up to 1kW of input power.
Self-