Harmonics Overview - Danfoss VLT AutomationDrive FC 302 Design Manual

Electrical Installation Con...

10.16 Harmonics Overview

Non-linear loads such as found with drives do not draw
current uniformly from the power line. This non-sinusoidal
current has components which are multiples of the basic
current frequency. These components are referred to as
harmonics. It is important to control the total harmonic
distortion on the mains supply. Although the harmonic
currents do not directly affect electrical energy
consumption, they generate heat in wiring and
transformers that can affect other devices on the same
power line.
10.16.1 Harmonic Analysis
Since harmonics increase heat losses, it is important to
design systems with harmonics in mind to prevent
overloading the transformer, inductors, and wiring. When
necessary, perform an analysis of the system harmonics to
determine equipment effects.
A non-sinusoidal current is transformed with a Fourier
series analysis into sine-wave currents at different
frequencies, that is, different harmonic currents I
50 Hz or 60 Hz as the basic frequency.
10 10
Abbreviation
f
1
I
1
U
1
I
n
U
n
n
Table 10.41 Harmonics-related Abbreviations
Current
Frequency
Table 10.42 Basic Currents and Harmonic Currents
Current
Input current
Table 10.43 Harmonic Currents vs. 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:
172
Description
Basic frequency (50 Hz or 60 Hz)
Current at the basic frequency
Voltage at the basic frequency
Current at the n th harmonic frequency
th
Voltage at the n harmonic frequency
Harmonic order
Basic Harmonic current (I
current (I )
1
I I I
1 5 7
50 Hz 250 Hz 350 Hz
Harmonic current
I I I
RMS 1 5
1.0 0.9 0.5
Danfoss A/S © 01/2018 All rights reserved.
VLT® AutomationDrive FC 302
THDi =
10.16.2 Effect of Harmonics in a Power
In Illustration 10.28, 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 connects
through cables that have an impedance Z
with
N
PCC
MV
LV
Z
xfr
Z
)
n #
I Illustration 10.28 Small Distribution System
11
550 Hz
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
I I
7 11-49 impedances result in higher levels of voltage distortion.
0.2 <0.1
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.
U25 + U27 + ... + U2n
U
Distribution System
Point of common coupling
Medium voltage
Low voltage
Transformer impedance
Modeling resistance and inductance in the wiring
, Z , Z .
1 2 3
MG38C202