Effect Of Harmonics In A Power Distribution System - Danfoss VLT Refrigeration Drive FC 103 Operating Instructions Manual

Introduction
1 1
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 Illustration 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 and feeds a number of loads. The point of common
xfr
coupling where all loads are connected is PCC2. Each load
is connected through cables that have an impedance Z
Z , Z .
2 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 1.10 Small Distribution System
16
VLT
U
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Refrigeration Drive FC 103 Low Harmonic Drive
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 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
1
the PCC (S
sc
(S ).
equ
S
sc
R =
sce S
equ
where S =
sc
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
) and the rated apparent power of the load
2
U
and S = U × I
Z equ equ
supply
MG16N102