Effect Of Harmonics In A Power Distribution System - Danfoss VLT AQUA FC 202 Design Manual

VLT® AQUA Drive FC 202
Design Guide
Frequency [Hz]
Table 97: Harmonic Currents versus RMS Input Current
Current
Input current
The voltage distortion on the mains supply voltage depends on the size of the harmonic currents multiplied by the mains impe-
dance for the frequency in question. The total voltage distortion (THDi) is calculated based on the individual voltage harmonics
using this formula:
2 + I 7 2 + ..... I n 2
I 5
THDi =
I

10.17.2 Effect of Harmonics in a Power Distribution System

In Illustration 82, a transformer is connected on the primary side to a point of common coupling PCC1 on the medium voltage sup-
ply. The transformer has an impedance Z
together is PCC2. Each load is connected through cables that have an impedance Z
M V
L V
Z Z
1 2
P C C3 P C C3
R e c t i e r 1 R e c t i e r 2
Illustration 82: Small Distribution System
Harmonic currents drawn by non-linear loads cause distortion of the voltage because of the voltage drop on the impedances of the
distribution system. Higher impedances result in higher levels of voltage distortion.
Current distortion relates to apparatus performance and it relates to the individual load. Voltage distortion relates to system per-
formance. It is not possible to determine the voltage distortion in the PCC knowing only the load's harmonic performance. To pre-
dict the distortion in the PCC, the configuration of the distribution system and relevant impedances must be known.
A commonly used term for describing the impedance of a grid is the short circuit ratio R
short circuit apparent power of the supply at the PCC (S
S ce
R =
sce
S equ
where
U 2
S = and S = U ∗ I
sc equ
Z supply
154 | Danfoss A/S © 2020.09
Basic current (I )
1)
50
Harmonic current
I
RMS
1.0
and feeds several loads. The point of common coupling where all loads are connected
xfr
P C C1
Z
x f r
P C C2
Z
3
P C C3
O t h e r
L o a d s
R e c t i e r 3
) and the rated apparent power of the load (S
sc
equ
Harmonic current (I
250
I I
1 5
0.9 0.5
, Z , Z .
1 2 3
. R
sce sce
Electrical Installation
Considerations
)
n)
350 550
I I
7 11–49
0.2 <0.1
is defined as the ratio between the
).
equ
AJ300847815559en-000101 / 130R0337