Lust CDA3000 - Page 272

Engineering Guide CDA3000
6-5
6 Selection of supplementary components
DE
EN
1
2
3
4
5
6
7
A
The influence of the lowest system resonance can be estimated with an
acceptable margin of error as follows:
Example: Calculation of system resonance
S
Tr
= 1600 kVA
U
k
= 6 %
Q
C
= 120 kVA
S
k
= S
Tr
/ U
k
= 1600 kVA / 6 % = 266 kVA
f = n
res
x 50 Hz = 1.3 x 50 Hz = 65 Hz
Result:
The lowest resonance point is at the harmonic number 1.3 - correspond-
ing to 65 Hz.
This calculation is used to estimate the lowest resonance point under
ideal system conditions.
If the calculated resonance point is not in the vicinity of the harmonic
numbers generated by the inverter of 5, 7, 9, 11, 13 etc., it can be
assumed that no resonance problems will occur.
It should be mentioned that additional motor loads shift the resonance
points toward higher values and the ohmic resistance component in the
system brings about an attenuation of the resonant circuit. In complex
system layouts in particular, pre-calculation of any possible resonance
points is very difficult, so in such cases too it is advisable to perform
measurements only after installation of a harmonics generator. Appro-
priate remedial measured should be initiated accordingly.
S
k
= short-circuit power
Q
C
= power of capacitor battery
n
res
= harmonic number at which resonance occurs
n
res
S
k
Q
C
⁄=
n
res
S
k
Q
C
⁄ 266kVA()120kVA()⁄ 13,== =