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EMC and Harmonics
Current
Harmonic current
I
RMS
Input current
1.0
Table 9.8 Harmonic Currents Compared to the RMS Input
Current
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 (THDi) is calculated based on the
individual voltage harmonics using this formula:
U25 + U27 + ... + U2n
THDi =
U
9.8 Effect of Harmonics in a Power
Distribution System
In Illustration 9.13, a transformer is connected on the
primary side to a point of common coupling PCC1, on the
medium voltage supply. The transformer has an impedance
Z
and feeds several loads. The point of common coupling
xfr
where all loads are connected is PCC2. Each load is
connected through cables that have an impedance Z
Z
.
3
PCC
Point of common coupling
MV
Medium voltage
LV
Low voltage
Z
Transformer impedance
xfr
Z
Modeling resistance and inductance in the wiring
#
Illustration 9.13 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.
MG37N102
Design Guide
I
I
I
I
1
5
7
11-49
0.9
0.5
0.2
<0.1
, Z
,
1
2
Danfoss A/S © 6/2016 All rights reserved.
Current distortion relates to apparatus performance and it
relates to the individual load. Voltage distortion relates to
system performance. It is not possible to determine the
voltage distortion in the PCC knowing only the harmonic
performance of the load. To predict 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
between the short circuit apparent power of the supply at
the PCC (S
) and the rated apparent power of the load
sc
S
sc
(S
).R
=
equ
sce
S
equ
2
U
where S
=
and S
= U × I
sc
equ
Z
supply
Negative effects of harmonics
•
Harmonic currents contribute to system losses (in
cabling and transformer).
•
Harmonic voltage distortion causes disturbance
to other loads and increases losses in other loads.
. R
is defined as the ratio
sce
sce
equ
101
9
9
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