Effect Of Harmonics In A Power Distribution System - Danfoss VLT HVAC Drive FC 102 Operating Instructions Manual

Introduction
1 1
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 =
1.6.3 Effect of Harmonics in a Power
Distribution System
In Figure 1.10, 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
feeds a number of loads. The point of common coupling
where all loads are connected is PCC2. Each load is
connected through cables that have an impedance Z
Z .
3
PCC
MV
LV
Z
xfr
Z
#
Figure 1.10 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.
16
VLT
U
Point of common coupling
Medium voltage
Low voltage
Transformer impedance
Modeling resistance and inductance in the
wiring
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®
HVAC Drive FC 102 Low Harmonic Drive
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
sc
(S ).
equ
S
sc
R =
sce
S
and
equ
xfr
where
S =
sc
, Z ,
1 2
Negative effects of harmonics
•
Harmonic currents contribute to system losses (in
cabling, transformer).
•
Harmonic voltage distortion causes disturbance
to other loads and increase losses in other loads.
. R is defined as the ratio
sce sce
) and the rated apparent power of the load
2
U
and
S = U × I
equ equ
Z
supply
MG16I322