March 2001 saw the introduction of new regulations designed to combat the
effects of harmful harmonics. Geoff Brown spells out the implications for
variable speed drives installations
EA Recommendation G5/4, which affects every consumer connected to the
public electricity supply, proposes reduced levels of mains harmonic
current distortion (particularly the fifth harmonic). Users of variable
speed drives are especially affected as the fifth harmonic content of the
input current to an unfiltered inverter can be as high as 60%, which
could be well outside the G5/4 limits.
'Planning Levels' limit the overall voltage distortion at each level in a
network and are applied at the 'Point of Common Connection' (PCC)where
the supply is shared with other users. The consumer must ensure that the
appropriate procedures to agree connection of new loads are obtained,
otherwise they could find themselves having to implement costly remedial
measures in the event of a problem/complaint from other users. G5/4
defines three stages of assessment, which increase in complexity.
Stage 1. This concerns only 230V single phase and 400V three phase
supplies. In principle, any equipment, or combination of items of
equipment, under 16A input current, which meets Harmonised European
Standard EN 61000-3-2, may be connected without further assessment. This
standard basically covers domestic equipment - drives over 1kVA are
considered to be professional equipment and are accepted with no limits.
For multiple items of equipment, a table is presented to limit the
aggregate harmonic currents permitted. To calculate the resulting level,
the arithmetic sum of all the harmonics for a number of drives is taken.
If the connection is not acceptable at Stage 1 or is at medium voltage
(6.6 kV to 22 kV), it is possible to undertake a Stage 2 assessment. It
is not clear within G5/4 what procedure is applicable for a new 400V
connection when Stage 1 current limits have been exceeded.
Stage 2. If the total converter load is lower than 130kVA (6-pulse diode
rectifier) or 250kVA (12-pulse), for a medium voltage (MV) PCC, then
there is no need for further assessment. For converter loads greater than
above, Stage 2 connection is still permissible after an assessment to
determine whether the harmonic currents imposed on the MV supply are
within the limits set within Stage 2 of G5/4. Otherwise, the network
operator must measure the existing background network voltage distortion.
This measurement is preferably based on a minimum of seven days
recording, in order to capture the peak distortion.
The overall voltage distortion value is calculated as 95% of the
accumulated % time values (that is for a reading taken every ten
minutes), then %time would be equal to %xten minutes, or %minutes. Adding
all these %time values over the measurement period yields a grand total
of %minutes. The 95% value is thus 0.95x grand value divided by the total
measuring time.
If the harmonic currents due to the load exceed the limits defined in
G5/4 and/or the 95% value of existing distortion exceeds 75% of the
appropriate MV planning level, then the contribution to the overall level
of distortion, due to the connection of the new load, will have to be
determined and the resultant value assessed. Should the resultant overall
level and/or the level of 5th harmonic remain within the planning levels,
then connection may be agreed.
Stage 3. If the levels of harmonics exceed those for Stage 2, or if the
point of common coupling is at 33 kV or over, then a different and
substantially more complex procedure is called for. The method adopted
will need to be established between the supply utility, the consumer and
the manufacturer. Notching This is a new requirement within G5/4
consistent with the compatibility limits set out in the standards IEC
61000-2-2 and 61000-2-12. This requires that the