4 Introduction to Advanced Harmonic Filters
4.1 Operation Principle
The Danfoss Advanced Harmonic Filters (AHF) consist of a
main inductor L
0
and a two-stage absorption circuit with the
inductors L
1
and L
2
and the capacitors C
1
and C
2
. The
absorption circuit is specially tuned to eliminate harmonics
starting with the 5
th
harmonic and is specific for the
designed supply frequency. Consequently the circuit for
50Hz has different parameters than the circuit for 60Hz.
L
0
L
1
L
2
C
2
C
1
M
AHF
Supply
Motor
130BB578.11
Frequency
converter
AHFs are available in two variants for two performance
levels: AHF005 with 5% THiD (total current harmonic
distortion) and AHF010 with 10% THiD. The strategy behind
the two levels is to offer a performance similar to 12 pulse
rectifiers with the AHF010 and a performance similar to 18
pulse rectifiers with AHF005.
The filter performance in terms of THiD varies as a function
of the load. At nominal load the performance of the filter
should be equal or better than 10% THiD for AHF010 and 5%
THiD for AHF005.
At partial load the THiD has higher values. However, the
absolute value of the harmonic current is lower at partial
loads, even if the THiD has a higher value. Consequently, the
negative effect of the harmonics at partial loads will be lower
than at full load.
Example:
An 18.5kW frequency converter is installed on a 400V/50Hz
grid with a 34A AHF010 (type code AHF-DA-34-400-50-20-A).
Following values are measured for different load currents,
using a harmonic analyzer:
I line RMS [A]
Fundamental
current at 50Hz
I
1
RMS [A]
THiD [%]
Total harmonic
current Ih RMS
[A]
1
9.6
9.59
5.45
0.52
15.24
15.09
13.78
2.07
20.24
20.08
12.46
2.5
25.17
25
11.56
2.89
30.27
30.1
10.5
3.15
34.2
34.03
9.95
3.39
1
)The total harmonic current has been calculated. The THiD
vs. load plot is shown in the following figure.
AHF-DA-34-400-50-20-A
0
2
4
6
8
10
12
14
16
10
15
20
25
30
35
Iline [A]
THiD [%]
0
0,5
1
1,5
2
2,5
3
3,5
4
Harmonic current Ih [A]
130BB579.10
THiD [%]
Harmonic current Ih [A]
It can be observed that at partial load, 15A, the THiD is
approximately 14%, compared to 10% at the nominal load of
34A. On the other hand, the total harmonic current is only
2.07A at 15A line current against 3.39A harmonic current at
34A line current. Thus, THiD is only a relative indicator of the
harmonic performance. The harmonic distortion of the
voltage will be less at partial load than at nominal load.
Factors such as background distortion and grid unbalance
can affect the performance of AHF filters. The specific figures
are different from filter to filter and the graphs below show
typical performance characteristics. For specific details a
harmonic design tool such as MCT 31 or Harmonic
Calculation Software (HCS) should be used.
Background distortion: The design of the filters aims to
achieve 10% respectively 5% THiD levels with a background
distortion of THvD = 2%. Practical measurements on typical
grid conditions in installations with frequency converters
show that often the performance of the filter is slightly
better with a 2% background distortion. However, the
complexity of the grid conditions and mix of specific
harmonics can not allow a general rule about the
performance on a distorted grid. Therefore we have chosen
to present worst-case performance deterioration character-
istics with the background distortion.
Introduction to Advanced Ha...
AHF005/010 Design Guide
12
MG.80.C4.02 - VLT
®
is a registered Danfoss trademark
4
4
Содержание VLT AHF005
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