Danfoss VLT AHF005, Руководство по конструкции

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limits based on these planning levels is described. G5/4 is a
system level standard.
For 400V the voltage THD planning level is 5% at the PCC.
Limits for odd and even harmonics in 400V systems are given
in Table 2 in the standard. An assessment procedure for the
connection of non-linear equipment is described. The
procedure follows three stages, aiming to balance the level
of detail required by the assessment process with the degree
of risk that the connection of particular equipment will result
in unacceptable voltage harmonic distortion.
Compliance of a system containing VLT
®
frequency
converters depends on the specific topology and population
of non-linear loads. AHF can be employed to meet the
requirements of G5/4.
3.3 Harmonic Mitigation
To mitigate the harmonics caused by the frequency
converter 6-pulse recitifier several solutions exist and they all
have their advantages and disadvantages. The choice of the
right solution depends on several factors:
•
The grid (background distortion, mains unbalance,
resonance and type of supply - transformer/
generator)
•
Application (load profile, number of loads and load
size)
•
Local/national requirements/regulations (IEEE519,
IEC, G5/4, etc.)
•
Total cost of ownership (initial cost, efficiency,
maintenance, etc.)
IEC standards are harmonized by various countries or supra-
national organizations. All above mentioned IEC standards
are harmonized in the European Union with the prefix “EN”.
For example the European EN 61000-3-2 is the same as IEC
61000-3-2. The situation is similar in Australia and New
Zealand, with the prefixes AS/NZS.
Harmonic solutions can be divided into two main categories:
passive and active. Where the passive solutions consist of
capacitors, inductors or a combination of the two in different
arrangements.
The simplest solution is to add inductors/reactors of typically
3% to 5% in front of the frequency converter. This added
inductance reduces the amount of harmonic currents
produced by the drive. More advanced passive solutions
combine capacitors and inductors in trap arrangement
specially tuned to eliminate harmonics starting from e.g. the
5
th
harmonic.
The active solutions determine the exact current that would
cancel the harmonics present in the circuit and synthesizes
and injects that current into the system. Thus the active
solution can mitigate the real-time harmonic disturbances,
which makes these solutions very effective at any load
profile. To read more about the Danfoss active solutions Low
Harmonic Drive (LHD) or Active Filters (AAF) please see MG.
34.OX.YY and MG.90.VX.YY.
Introduction to Harmonics a... AHF005/010 Design Guide
MG.80.C4.02 - VLT
®
is a registered Danfoss trademark 11
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