plants with a high percentage of rectifier load, maintain harmonic cur-
rents at a low level to avoid overload of the transformer and high tem-
perature in the cables.
NB!
Some of the harmonic currents might disturb communication equipment connected to the same transformer or cause resonance in
connection with power-factor correction batteries.
NB!
To ensure low harmonic currents, the frequency converter is equipped with intermediate circuit coils as standard. This normally reduces
the input current I
RMS
by 40%.
The voltage distortion on the mains supply voltage depends on the size of the harmonic currents multiplied by the mains impedance for the frequency
in question. The total voltage distortion THD is calculated on the basis of the individual voltage harmonics using this formula:
THD % = U 25 + U
2
7 + ... + U
2
N
(U
N
% of U)
2.9.5 Harmonics Emission Requirements
Equipment connected to the public supply network:
Options:
Definition:
1
IEC/EN 61000-3-2 Class A for 3-phase balanced equip-
ment (for professional equipment only up to 1 kW total
power).
2
IEC/EN 61000-3-12 Equipment 16A-75A and professio-
nal equipment as from 1 kW up to 16A phase current.
2.9.6 Harmonics Test Results (Emission)
Power sizes up to PK75 in T2 and T4 complies with IEC/EN 61000-3-2 Class A. Power sizes from P1K1 and up to P18K in T2 and up to P90K in T4 complies
with IEC/EN 61000-3-12. Power sizes P110 - P450 in T4 also complies with IEC/EN 61000-3-12 even though not required because currents are above 75
A.
Table 4, R
sce
>= 120, THD <= 48% and PWHD >=46% provided that the short-circuit power of the supply S
sc
is greater than or equal to:
SSC = 3 × RSCE × Umains × Iequ = 3 × 120 × 400 × Iequ
at the interface point between the user’s supply and the public system.
It is the responsibility of the installer or user of the equipment to ensure, by consultation with the distribution network operator if necessary, that the
equipment is connected only to a supply with a short-circuit power S
sc
greater than or equal to specified above.
Other power sizes can be connected to the public supply network by consultation with the distribution network operator.
Automation VT Drive FC322 Design Guide
2 Introduction to VLT Automation VT Drive
MG.20.X1.22 - VLT
®
is a registered Danfoss trademark
37
2
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