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Nr 62
Prace Naukowe Instytutu Maszyn, Napędów i Pomiarów Elektrycznych
Politechniki Wrocławskiej
Nr 62
Studia i Materiały
Nr 28
2008
harmonics,
total harmonic distortion
rectifier
Stanislav KOCMAN*, Vítězslav STÝSKALA*, Miroslav SAMEŠ*
TECHNICAL SOLUTIONS FOR HARMONIC REDUCTION
IN THE AREA OF ADJUSTABLE SPEED DRIVES
Semiconductor converters in structures of such devices as adjustable speed drives, uninterruptible
power systems or data processing and electronics devices represent sources of harmonic currents generated by them into the network to which these devices are supplied. The quality of supply voltage
can be significantly affected by their operation. Thus supply voltage is distorted, which can have negative influences on a function of devices connected to the same network. Besides voltage distortion
generated harmonic currents can cause other serious problems in the network and in devices connected to it. So the corresponding attention should be devoted to create convenient technical solutions resulting in the reduction of generated harmonic currents. There are various technical solutions of various levels of performance or efficiency in sense of their ability to reduce generated harmonics. In this
sense, effects of some chosen technical devices are presented in this paper.
1. INTRODUCTION
Harmonic currents generated by devices having in their structure semiconductor
converters have various negative effects on the supply network and devices connected
into it. The consequences of harmonics are linked to the increase in RMS and peak values of electrical quantities and to the frequency spectrum of voltages and currents,
which can cause failures in some protective devices operations, accuracy loss of some
measurement instruments, negative influences on operating of sensitive devices or even
their failure, current overload producing excessive overheating and leading to premature
ageing of equipment, destruction of capacitors by thermal overload, abnormal vibrations
and acoustic noise of such devices as motors or transformers, mechanical stress (pulse
__________
* Department of Electrical Engineering, VŠB-TU Ostrava, 17.listopadu 15, Ostrava-Poruba, 708 33,
[email protected], [email protected], [email protected]
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torques in asynchronous motors), overload and excessive overheating of the neutral
conductor, degradation of network voltage.
These negative effects always have economic impacts resulting from the additional
costs linked to energy losses, over sizing of equipment or to loss of productivity. There
are various professional texts dealing with harmonics, their origin, influences and possibilities of their reducing, for example [1, 2, 3]. One of the main sources of harmonics is
AC adjustable speed drive using in its structure a semiconductor converter. Nowadays
such type of this drive is widely produced and used in technical applications having
voltage-fed inverter and input diode six-pulse rectifier in its structure. The input rectifier
together with capacitor in DC circuit of the converter have the dominant influence on
harmonic spectrum of current drawn by the adjustable speed drive from the network.
This drawn current is very distorted as it can be seen in Figure 2 or 3 and as it is published in literature, e.g. [1, 3, 4, 5, 6]. There are various possibilities for its improving in
the sense of reducing harmonic currents appearing in its harmonic spectrum. From them,
such possibilities are presented and briefly discussed in this paper as AC input reactor,
multi-pulses rectifiers and broad-band harmonic filter.
2. SIMULATIONS RESULTS AND COMPARISON OF ANALYSED DEVICES
AC input reactors, multi-pulses rectifiers including special multi-windings supply
transformers and input passive broad-band harmonic filter used for harmonic reductions are located in the laboratory of the Department of Electrical Engineering at
VŠB-TU Ostrava. Series of experimental measurements and analyses have been performed on them. Wiring scheme of the measurement circuit is in Figure 1 for these
tested devices as follows:
• A: six-pulse diode rectifier with AC input reactor with effective impedance
Zef=3% (6p+3% AC reactor)
• B: twelve-pulse diode rectifier in series connection Dd0y1 (12p-series Dd0y1)
• C: eighteen-pulse rectifier in series connection Dy1z1+200z1-200 (18p-1D)
• D: six-pulse diode rectifier with input passive broad-band harmonic filter (broadband filter)
Since during individual measurements there were not identical conditions in the
supply network as regards the voltage quality, in which magnitude, level of distortion
and unbalance of supply voltage fluctuated even within each of realized measurement,
a simulation model of each of these circuits was created in the simulation program
ATP (Alternative Transients Program) to ensure the same required conditions of power supply for each of tested devices. After this an analysis of the harmonic currents
was performed for chosen characteristics of the supply voltage and for each device
separately. The voltage total harmonic distortion THDu was set to the value of 4% and
the voltage unbalance to 0%, because these levels are prevalent within work-
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B
A
+
Zef
2
-
d
D y
1
1
3
2
+
+
-
3
C
+
-
S2
1
S1
P
S3
D
2
1
+
-
L
Z
+
--
+
2
-
3
p
3
3 3PEN~50 Hz, 400 V
2
Inverter with
capacitor
3
1
C
load
R
3
Power
analyser
Fig. 1. Circuit for sequential measuring and analysis of tested devices
ing hours in the point of connection of the tested devices to the network. So the tested
devices were compared each other by utilization of analysis results of simulation
models. Because the dominant influence on harmonic spectrum of input current has
the input rectifier together with the capacitor in DC circuit of the drive converter, and
the influence of inverter together with the type of connected load is minor in this case,
so variant of resistive load was chosen for both measurements and simulations.
The simulation results can be seen in Figures 2, 3, and 4. In Figure 2, the waveforms of input currents are shown and illustrate the influence of tested devices on the
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24
[A]
18
current (A)
12
6
0
-6
-12
-18
-24
0,156
0,161
0,166
0,171
THDu_f iltr_síť.pl4: c:X0005A-X0086A
thdu_seriov é_dd0y 1.pl4: c:X0091A-X0025A
6p.pl4: c:X0069A-X0135A
18p_1d_thdu.pl4: c:X0002A-X0003A
0,176
0,181
0,186
0,191 [s] 0,196
time (s)
___ i6p ___ i12p ___ i18p ___ iBBF
Fig. 2. Waveform of input current of tested devices under their nominal loading
(simulation, THDu= 4%, voltage unbalance= 0%)
50
45
40
I h/I 1 (%)
35
6p+3% AC reactor
30
12p-series Dd0y1
25
18p-1D
20
broad-bad filter
15
10
5
0
3
5
7
9
11
13
15
17
19
21
23
25
27
29
harmonic order
Fig. 3. Harmonic spectrum of input current of tested devices from Figure 2
(simulation, THDu= 4%, voltage unbalance= 0%)
100
90
80
THD i (%)
70
6p+3% AC reactor
60
12p-series Dd0y1
50
18p-1D
40
broad-bad filter
30
20
10
0
20
30
40
50
60
70
80
90
100
load (%)
Fig. 4. THDi of input current of the tested devices versus their loading
(simulation, THDu= 4%, voltage unbalance= 0%)
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current waveform shape when especially current drawn by the eighteen-pulse rectifier
is already very close to the sinusoidal one (i6P: 6p+3% AC reactor, i12P: twelve-pulse
rectifier, i18P: eighteen-pulse rectifier, iBBF: 6p+broad-band filter). In Figure 3, the
harmonic spectrum of input current of all tested devices is presented showing the
ability of the tested devices to reduce the expected harmonics. In Figure 4, the input
current total harmonic distortion THDi versus the level of loading, from which is evidently seen the highly positive influence of both multi-pulses rectifiers and the broadband harmonic filter on the harmonic reductions with comparison to the standard sixpulse rectifier fitted with the AC input reactor in addition.
3. CONCLUSION
In this paper four devices used for harmonic reduction of AC adjustable speed drives
were tested and compared, such as AC input reactor, twelve-pulse and eighteen-pulse rectifiers and input passive broad-band harmonic filter. The analyses confirmed markedly better efficiency in harmonic reduction of multi-pulses rectifiers and broad-band harmonic
filter in comparison with the AC input reactor. While the total harmonic distortion THDi of
drawn current is in the case of using of the AC input reactor 48,9% at the nominal loading,
so at the twelve-pulse rectifier already only 9,2%, at the eighteen-pulse one even only 4,7%
and at the broad-band harmonic filter 11,5%. So it can be said on the basis of presented
analyses results and also by performed experimental measurements that the installed multipulses rectifiers and broad-band harmonic filter in the laboratory of Department of Electrical Engineering at VŠB-TU Ostrava represent very good devices for reducing of harmonic
currents. But it is necessary to bear in mind that the level of ability of the harmonic reduction is quite strong depended on some factors such as the level of device load, voltage distortion and unbalance in the mains.
REFERENCES
[1] CHAPMAN D., SANTARIUS P., GAVLAS J., KUŽELA M., Kvalita elektrické energie-průvodce.
Část 3.1. Harmonické. Příčiny a účinky. Elektrotechnika v praxi, 3/2003.
[2] COLLOMBET C., LUPIN J.-M., SCHONEK J., Harmonic Disturbances in Networks, and Their
Treatment. Cahier Technique Merlin Gerin, No. 152, 1999.
[3] FERRACI P., Power Quality. Cahier Technique Merlin Gerin, No. 199, 2000.
[4] KŮS V., Vliv polovodičových měničů na napájecí soustavu. BEN, Praha 2002.
[5] KŮS V., Harmonické polovodičových měničů. Automatizace 3/1999.
[6] LEXA V., Harmonické-lepší prevence než náprava. Elektro 4/2000.
[7] HINK K. M., Harmonic Mitigation of 12-Pulse Drives with Unbalanced Input Line Voltages. MTE
Corporation Technical Articles.
[8] HINK K. M., 18-pulse Drives and Voltage Unbalance. MTE Corporation Technical Articles.