NEW 3 level Active Front End (AFE) technology from Schneider Electric allows trouble free operation, both for mains supply side and motor side. This technology allows exceptional low harmonics impact on the main side, which helps to protect other assets while saving energy. It also solves the major issue existing with 2 level AFE which can create premature wear on motors' bearings. By suppressing the common mode voltage, the 3 level AFE technology ensures the motor is running without potential issue, extending the motor’s lifetime and saving maintenance costs.

1. Highly efficient Active Front End
enables trouble-free operation
of low harmonic drives
by Dr. sc. ETH Michael Hartmann

2. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 2
Active Front Ends (AFEs) have emerged as the most powerful and effective way to
reduce input current distortions in variable speed drives (Fig. 1). Classic
implementations using a six-switch, two-level converter, as often used for the
inverter stage, show reduced efficiency and additional high-frequency common
mode (CM) voltage at the DC-link compared to a passive diode bridge. The novel
low-harmonic input stage, with or without regenerative capability, overcomes these
drawbacks. Using a three-level topology with an optimized input filter and lossless
damping of the filter stage by control makes it possible to implement a very
compact, low-harmonic input stage with high performance and efficiency. By
implementing an additional CM filter stage in the AFE, no additional high-frequency
CM voltage is generated at the DC-link, and the bearings’ lifetime is not reduced as
compared to a passive diode bridge rectifier. The three-level AFE topology with
additional CM filter stage now offers a real alternative to the passive three-phase
rectifier by making a low-harmonic input stage available.
Modern variable speed drives (VSDs) must fulfill many requirements. As resources
and energy are limited, the efficient use of energy becomes increasingly important.
VSDs clearly contribute to the efficient use of energy, but not only the application
must be efficient. The VSD itself must also be highly efficient. A passive diode
bridge rectifier with smoothing inductors on either the DC or AC side (cf., Fig. 2(A))
is the most robust and efficient way to implement a VSD.
Efficiency, however, is not the only requirement. Due to the basic operating principle
of three-phase passive rectifier circuits, their input currents are not sinusoidal but
rather show low-frequency harmonics, as seen in Fig. 2. On the one hand, low-
frequency harmonics distorts the grid, but on the other hand, they cause additional
losses in the grid. Power quality, therefore, becomes another important requirement
for VSDs so the demand for drives with low impact on the mains is also growing.
Abstract
Figure 1
Highly efficient
low-harmonic
input stage
Figure 2
(A) Passive three-phase
diode bridge rectifier with
AC-side connected
smoothing inductors.
(B) Typical input currents,
and (C) corresponding
harmonic spectrum of the
input currents where the
low-frequency harmonics
are visible (5
th
, 7
th
, 11
th
,
13
th
, etc.)
(A) (B)
Introduction
(C)

3. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 3
According to Fig. 2(C), a classic six-pulse passive diode bridge with AC chokes
shows mainly 5
th
, 7
th
, 11
th
, and 13
th
harmonic components, and the typical total
harmonic distortion of input currents (THDi) is slightly below 48%. These rectifier
systems do not fulfill the requirements outlined in standards such as IEEE 519,
where a total THDi of only 5% is demanded.
Several possibilities are available to comply with such stringent input current
distortions and the different approaches have already been discussed in another
white paper.
1
One possibility to mitigate the low-frequency harmonics is the
application of passive filters tuned to the low harmonics, mainly to the 5th/7th and
11
th
/13
th
harmonic components. Due to their low operating frequency, these filters
are very heavy and bulky, show a considerably large voltage drop across the filter
chokes at nominal load, and often result in increased DC-link voltage in no-load or
light load conditions. In addition, they show a low power factor at partial load. The
tuned filters could be excited by the mains and have to be damped, which reduces
their efficiency. Overall, these filters are not the best choice for implementing a low-
harmonic input stage.
Multipulse solutions, such as 12-pulse, 18-pulse, or 24-pulse rectifier systems, use a
transformer with two or more phase-shifted windings and diode bridges. Due to the
phase shift in the different windings, some low-frequency harmonics are
compensated on the primary side of the transformer and only harmonics with the
ordering number h=n·p±1 occur, where p is the pulse number of the rectifier system
(e.g., 17
th
/19
th
and 35
th
/37
th
harmonic for p = 18). These systems show a high
efficiency as well as the advantage of already including the transformer to the MV
side in the drive. However, the high complexity and cost of the transformer clearly
outweigh these advantages.
AFEs have become the most powerful and effective way to reduce input current
distortions. In Fig. 3, the most common structure of an AFE — the classic six-switch,
two-level active rectifier — is depicted. Many topologies have been invented for the
implementation of AFEs with unidirectional and bidirectional power flow, and a good
overview of these can be found in other articles.
2, 3, 4
Due to the active shaping of the
input currents, their THDi can be reduced to values significantly below 5%. In
addition, AFEs offer a controlled DC-link voltage, which allows stable operation even
in the event of weak mains.
Aside from the advantages of AFEs, there are also some limitations to note. The
efficiency of these topologies is slightly below the efficiency of passive rectifier
circuits. While the basic operating principle, which will be discussed in detail below,
generates additional high-frequency CM voltage at the DC-link, it also generates
some high-frequency emissions on the mains side, which must be filtered by a
properly designed input filter.
Figure 3
Schematic of the classic
six-switch, two-level AFE
with input filter stage
1
Schneider Electric, “Choose the best harmonic mitigation solution for your drive,” White paper WP2121101EN, 2012.
2
J. W. Kolar and T. Friedli, “The Essence of Three-Phase PFC Rectifier Systems—Part I,” IEEE Transactions on Power
Electronics, Vol. 28, No. 1, pp. 176-198, Jan. 2013.
3
T. Friedli, M. Hartmann and J. W. Kolar, “The Essence of Three-Phase PFC Rectifier Systems—Part II,” IEEE
Transactions on Power Electronics, Vol. 29, No. 2, pp. 543-560, Feb. 2014.
4
B. Singh et al., "A review of three-phase improved power quality AC-DC converters, "IEEE Transactions on Industrial
Electronics, Vol. 51, No. 3, pp. 641-660, June 2004.

4. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 4
These drawbacks are addressed by selecting the right topology, by properly
designing the input filter, and by applying a novel CM filter structure. The answer to
all these requirements is a three-level topology with an additional CM filter stage,
which shows very high efficiency and behaves like a passive rectifier circuit in terms
of CM voltage. The total structure of the AFE is given in Fig. 4. Starting from the
two-level topology, three additional (bidirectional) switches are connected between
each phase and the midpoint of the DC-link capacitors. The basic function, proper
design of the input filter, and the novel CM voltage mitigation strategy are discussed
later in this paper.
In order to generate sinusoidal input currents in phase with the mains voltage, the
current is shaped by the switching actions of the AFE semiconductors. The
converter can only generate discrete voltage levels because the particular switches
can only be opened or closed, and boost chokes LN are used as energy storage
elements. A simplified equivalent model is given in Fig. 5(A). The grid has a voltage
source with a purely sinusoidal mains voltage vN with mains frequency fN. The
converter system generates the voltage vr and the difference between the two
voltages applies across the boost choke. A positive voltage across this choke forces
the current to increase, while a negative voltage across this choke forces the current
to decrease. In order to achieve sinusoidal mains currents iN in phase with this
voltage, the converter system must generate a voltage vr that shows a small phase
shift to the mains voltage vN. The corresponding phasor diagram and voltage
waveforms are given in Fig. 5(B) and 5(C), respectively. This phase shift depends
on the size of the boost choke LN and is therefore dependent on the voltage and
power rating of the AFE. Whereas this phase shift is typically in the range of 0.1° –
0.2° for a power rating of 10 kW/400 V, phase shifts of 1° – 2° occur for 100
kW/400 V. These phase shifts are compensated so that the mains current is in
phase with the voltage at the input terminal of the drive.
As mentioned before, an AFE can only generate discrete voltage levels by either
closing or opening the particular switches. A classic two-level voltage converter
stage with six switches is only able to generate two different voltage levels per
phase, but the three-level topology shown in Fig. 4 is able to generate three voltage
levels per phase. Each phase can be connected to the positive DC-rail +VDC/2 by
closing switch T1 or due to the corresponding freewheeling diodes. To the DC-link
midpoint M by closing switches T3 or T4, or to the negative DC-rail –VDC/2 by closing
switch T2, or due to the corresponding freewheeling diodes. There are a number of
possible modulation techniques to generate the switching signals for the particular
semiconductors (e.g., pulse-width modulation (PWM), hysteresis control, space
vector modulation, etc.). An improved PWM strategy is often used to generate the
converter voltages vr. The average value of the converter voltage (averaged over
the switching frequency) must implement the required 50/60 Hz voltage component
Figure 4
Schematic of the
three-level AFE,
including LCL filter at
the input and
high-frequency CM
filter stage
Basic
operation
principle

5. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 5
for current shaping. The voltage differences between the discrete voltage levels and
the required average values cause additional current ripple in the boost chokes. It’s
evident that these voltage differences are smaller in a three-level converter system
where three voltage levels are available for current shaping. The AFE shown in
Fig. 4 is a boost-type converter, which means that the DC-link voltage is higher than
the mains voltage, e.g., 650 V for a 400 V mains. The DC-link voltage is, however,
directly related to losses as the switching losses are proportional to the DC-link
voltage. Also, ripple currents generated in the boost chokes LN are dependent on the
DC-link voltage. By applying the selected three-level topology, commutations with
only half of the DC-link voltage occur and switching losses are reduced
considerably. In addition, the DC-link voltage is not a fixed value but adapted
according to the mains situations with a dedicated controller,
5
and a novel thermal
model ensures the thermal protection of all semiconductors.
6
The switching
frequency can, therefore, be increased, which considerably reduces the size of the
boost choke and results in a very compact active input stage. A tradeoff between
efficiency and power density must be found for implementing an efficient and
compact drive with active input stage.
Figure 6
Qualitative voltage
emissions of an AFE. In
addition to the
fundamental voltage
component at 50/60 Hz,
the emissions at
switching frequency fs
and multiples of the
switching frequency are
also shown. These
emissions are well
attenuated by a
properly designed
lowpass filter to values
below 0.5%
Figure 5
(A) Simplified equivalent
model of the active front
end, (B) Phasor
diagram of the model,
and (C) corresponding
voltage and current
waveforms
(A) (B)
(C)
5
Schneider Electric, “Verfahren zur Regelung einer Gleichrichterschaltung“, AT 514684B1.
6
Schneider Electric, “Verfahren zur Ermittlung der thermischen Belastung von Halbleiterbauelementen“, AT patent
application AT 2015/51077.

6. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 6
Noise voltages
An AFE does not show low harmonic input current components like passive diode
bridges. The total harmonic distortion of input currents THDi is significantly below
5% and usually in the same range as the distortion of the feeding voltage. Due to
the switching actions of the active converter system to shape the input current, the
AFE generates noise voltages at switching frequency, which is in the range of
several kHz (cf., Fig. 6). These emissions are attenuated by an input filter to values
below 0.5% and will affect the mains only if the filter is not designed appropriately.
In Fig. 7, simulated voltage waveforms of the three-level AFE are shown together
with their averaged low-frequency components (averaged over one switching
period). There are three voltage levels available to form the phase voltage vr1. Next
to the sinusoidal 50 Hz component, a third harmonic component is also visible,
which is used to increase the modulation range of the converter system.
Due to the switching actions, not only differential mode (DM) voltages but also a CM
voltage is generated. A CM voltage is a voltage common to all three phases of the
DC-link and is defined by:
(1)
The CM voltage can be measured between the midpoint M of the two DC-link
capacitors and N, and is depicted in Fig. 7(B). In addition to the high-frequency CM
voltage, the third harmonic signal is also visible. In three-phase systems, the total
phase voltage vr consists of a DM voltage component and a CM voltage component
that are equal in all three phases:
(2)
3
)()()(
)( 321 tvtvtv
tvCM
++
=
)()()( ,, tvtvtv CMiDMir +=
Figure 7
Simulated voltage
waveforms of the three-
level AFE together with
the corresponding
average value
(averaged over one
switching period):
(A) Phase voltage
measured between
vr1 and N; (B) Common
mode voltage measured
between M and N;
(C) Differential phase
voltage measured
between vr1 and M;
and (D) Differential
mode voltage measured
between two phases
(L1 and L2)

7. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 7
The phase-related DM voltage (cf., Fig. 7(C)) can, therefore, be measured between
vr and M, and is the actual voltage component used to shape the mains currents.
The CM voltage component has no influence on the current shaping of the phase
currents, but its high frequency components may cause unwanted effects. It is
common to plot the differential voltage as the voltage measured between two
phases, as shown in Fig. 7(D). All voltages show high frequency components, and
their average values (averaged over one switching period) are either used to shape
the mains currents or show the third harmonic component used to increase the
voltage range.
As well as being the intended DM voltage component used to shape the mains
currents, the AFE also generates a high-frequency CM voltage. This CM voltage
adds to the CM voltage already generated by the inverter stage of the drive. This
results in uncomfortable high-frequency CM voltage variations of the DC-link and the
CM voltage. High-frequency CM voltage and its high dv/dt are the main sources of
bearing currents, which are known to reduce the lifetime of the bearings or even
destroy them in a very short period of time. A good overview on the drawbacks and
classification of bearing currents can be found in another article.
7
In addition, the
insulation of the motor is stressed by the high-frequency CM voltage when there is a
long cable.
A passive diode bridge rectifier shows only low-frequency harmonics, and only the
CM voltage generated by the inverter stage of the drive is present. An AFE, in
contrast, adds additional CM voltage and worsens the behavior of bearing currents.
The CM voltage is dependent on the topology and the implementation. The three-
level AFE topology shows considerably smaller CM voltage than a classic two-level
converter. In Table 1, the CM voltage of a two-level AFE is compared to the CM
voltage of a three-level AFE. Whereas the two-level AFE shows CM voltage levels of
±VDC/2 and ±VDC/6 and voltage steps of ±VDC/3, the three-level AFE shows only
voltage levels of ±VDC/3 and ±VDC/6, which result in voltage steps of only ±VDC/6, a
reduction of 55 Vrms. The reduction of the CM voltage levels, and consequently also
the CM voltage steps applied to the motor, reduces the stress of the insulation and
reduces the bearing currents. However, the dv/dt of 2 – 5 kV/µs still remains, which
is a main cause of the high-frequency currents in the bearings.
As bearing currents and the life span of bearings are among the main concerns of a
VSD, a CM voltage reduction method can be implemented in the three-level AFE.
This is done by connecting the DC-link voltage midpoint M to the artificial star-point
built by the filter capacitors CF as shown in Fig. 8(A). Due to this connection, the
high frequency CM voltage is nearly eliminated. In Fig. 8(B), an equivalent circuit of
the CM filter is shown. The total CM voltage VCM consists of a high- frequency
component VCM,HF and a low-frequency component VCM,~.
Figure 8
(A) Three-level AFE
with CM filter reduction.
The DC-link capacitor
midpoint M is
connected to the
artificial starpoint built
by the filter capacitors
CF. (B) Simplified
equivalent circuit of the
CM filter stage. The
boost chokes LN build,
together with the filter
capacitors, a second
order low-pass filter.
The total CM voltage
VCM consists of a high-
frequency component
VCM,HF and a low-
frequency component
VCM,~ (third harmonic
component)
Reduction of
CM voltage
7
A. Muetze and A. Binder. “Don't lose your bearings,” IEEE Magazine on Industry Applications. Vol. 12, No. 4, pp. 22-31,
July-Aug, 2006.

8. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 8
Topology
CM
voltage
Voltage
levels
± VDC/6, ± VDC/2 0, ± VDC/6, ± VDC/3
Low-frequency 3
rd
harmonic
Voltage
steps
± VDC/3 ± VDC/6 −
dv/dt dv/dt ≈ 2-5 kV/µs dv/dt ≈ 2-5 kV/µs dv/dt ≈ 0.050 V/µs
RMS
voltage
154 Vrms 99 Vrms 35 Vrms (3
rd
harmonic)
Highest CM voltage Reduced CM voltage No high-frequency
CM voltage
The boost chokes build, together with the filter capacitors, a second order low pass
filter that strongly attenuates the CM voltage components at the switching frequency
and at multiples of the switching frequency. Only the low-frequency CM voltage
remains across the filter capacitors and can therefore be measured at the DC-link
voltage midpoint M. A resonant tank for CM signals is built by the filter capacitors CF
and the boost chokes LN, which could be excited by the AFE. Hence, a particular
controller is implemented in the firmware to introduce a sufficient amount of damping
to prevent any unwanted oscillations of the CM filter stage. Due to the active
damping of the CM filter stage by a dedicated controller, damping resistors with high
dissipative losses are not required. This does not increase the losses of the AFE.
The CM voltage of the AFE with CM reduction is also given in Table 1. As a result,
no high-frequency CM voltage caused by the AFE is present at the DC-link, and only
the CM voltage generated by the inverter stage (not shown in Table 1) remains at
the same level of a drive with a passive rectifier bridge, something that has been
used in industrial applications for decades. This is a tremendous improvement as a
low-harmonic input stage using this concept doesn’t increase CM voltage and
doesn’t increase bearing currents. For the first time in terms of CM voltage and
bearing currents, it is possible to replace a passive diode bridge rectifier with a low-
harmonic input stage, with or without energy recovery.
Table 1
Comparison of the CM
voltage of a two-level
AFE, a three-level AFE
without CM reduction,
and a three-level AFE
with CM reduction for a
DC-link voltage of
600 V. The three-level
AFE with CM reduction
shows no high-
frequency CM voltage,
which is equal to the
situation of a passive
diode bridge rectifier.

9. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 9
Aside from the fundamental component, some voltage components at switching
frequency and multiples of the switching frequency are also generated (cf., Fig. 6)
inside an AFE. The high-frequency components are, however, filtered by a properly
designed input filter, as indicated in Fig. 4, and only very small values, significantly
below <0.5%, remain at the input terminal of the AFE. A proper design of the input
filter is essential for the reliable operation of the AFE. On the one hand, the filter
should implement sufficient attenuation so that the high-frequency voltage
components inside the AFE are not transferred to the mains terminals and don’t
disturb the grid. On the other hand, though, it must not be sensitive to disturbances
in the mains and should not reduce the efficiency of the converter. As the filter is
designed to attenuate the switching frequency components, which is usually in the
range of several kHz for drive systems, the size of the filter is much smaller than a
passive filter acting on the low-frequency harmonics (e.g., 5
th
and 7
th
harmonic). A
high attenuation increases the size of the filter, so a balance between filter
attenuation, filter losses, filter size, and damping of the filter must be found, as
shown in Fig. 9(B).
An LCL filter structure, which is a well-established and robust filter structure, is
chosen to meet these requirements (cf., Fig. 9(A)). The filter capacitors CF, together
with the boost chokes LN and the filter chokes LF, build a resonant tank. Without any
damping, this resonant tank could be excited by voltage variations on the grid and
must therefore be sufficiently damped. The grid usually shows inductive behavior
and the additional filter choke LF may not be required as it is already implemented
by the grid, itself. In this case, the resonance frequency of the LC tank given by
(2)
would strongly depend on the impedance of the mains LM, which is related to the
short circuit power of the feeding grid. A reliable damping of the resonant tank would
be very challenging and, in many cases, impossible. An insufficiently damped input
filter causes high currents in the filter capacitors, which would reduce its lifetime or
even result in the breakdown of the filter capacitors. The filter chokes LF decouple
the AFE and the filter capacitors from the grid and determine the resonance
frequency for the defined mains conditions, ensuring that the filter can be well
damped in all considered mains conditions. Oscillations of the filter do not occur,
and the filter’s performance and lifetime are not degraded.
There are several ways to damp the filter. The most common is to add a snubber
network, e.g., in parallel to the filter capacitors as shown in Fig. 10(A). Resistors Rs
in series to capacitors Cs are connected in parallel to the filter capacitors CF.
Although the snubber capacitors limit the current through the resistors Rs,
considerable losses are introduced by this snubber network and the efficiency is
reduced significantly. In addition, the size of this passive damping network does not
comply with the requirement of the small filter size. These arguments are valid for
( )( )MFNF
LCLres
LLLC
f
+
=
//2
1
,
π
Input
filter design
Figure 9
(A) LCL filter structure
of the AFE input filter,
and (B) main
constraints for the
design and optimization
of the filter
(A) (B)

10. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 10
almost all damping networks where passive elements are used to damp the filter
resonance.
A novel lossless control strategy can be applied to actively damp the input filter.
8
The control strategy is implemented in the firmware of the AFE controller and does
not require any additional passive components. The LC tank is, therefore, directly
damped by the converter stage itself. This means that no additional losses occur
due to passive damping elements and the efficiency of the AFE isn’t degraded. The
resonance frequency is mainly determined by the filter choke LF, which ensures that
the AFE operates well under all mains conditions specified in the datasheet.
A simulation is performed to demonstrate the implemented active damping strategy
of an AFE with a power rating of 160 kW. The AFE is operated in parallel with a
1 MVA thyristor controlled rectifier (cf., Fig. 11). Due to the commutation effects of
the thyristor controlled rectifier, the feeding mains voltage of the AFE is distorted.
The simulation results are shown in Fig. 11(B) and 11(C), where the characteristic
commutation notches are visible in the voltage. In Fig. 11(B), the active damping is
not activated and the LC tank is excited by the voltage variations (commutation
notches) of the grid. As a consequence of the insufficiently damped LCL filter
structure, considerably large oscillations occur in the capacitor voltage and the
phase current, which would considerably reduce the life span of the input filter.
8
Schneider Electric, “Verfahren zum Betrieb einer Umrichterschaltung mit LCL Filter,” AT 508390B1.
Figure 10
(A) AFE with input filter
in LCL structure,
including required
damping network in RC-
snubber configuration
(Rs, Cs) across the filter
capacitors CF. The
passive snubber
resistors cause
additional losses of the
input filter, which
reduce the efficiency
considerably.
(B) Damping of the LCL
filter using a lossless
active damping strategy
implemented in the
controller of the AFE
Figure 11
(A) Structure used to
demonstrate the
performance of the
active damping
strategy. The three-level
AFE is operated in
parallel to a thyristor
controller rectifier with a
power rating of 1 MW.
(B) Simulated input
current im and capacitor
voltage vCF when the
filter is not damped, and
(C) if the filter is
damped using the
lossless active damping
control strategy
implemented in the AFE
controller
(A) (B)
(A)
(B) (C)

11. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 11
After activating the active damping strategy in the controller, the filter is well damped
(cf., Fig. 11(C)), and almost no high-frequency oscillations occur, either in the
capacitor voltage or in the phase current. The phase currents follow the phase
voltage where the commutation notches are still present. This clearly demonstrates
the effectiveness of the applied damping strategy implemented in the AFE firmware
and shows that the AFE is able to operate in such a distorted environment.
Fig. 12 shows the measurements of the mains current of a 160 kW drive with AFE.
The drive is operated at a 400 V mains with 50 Hz mains frequency and at nominal
power (Pnom=160 kW). In Fig. 12(A), the drive is operated in motor mode where the
energy flows from the grid to the motor and, in Fig. 12(B), the drive is operated in
regenerative mode where the motor feeds energy into the grid. The currents are
nearly sinusoidal and in phase with the mains voltage (only mains voltage of
phase L1 is shown). In both cases, the measured THDi of the mains currents is
around 2%, however, the feeding voltage already shows a THDv of approximately
2%. The THDi of the mains current is primarily determined by the voltage distortion
of the feeding mains. Tests in the lab showed that the drive can easily generate
mains currents with a THDi ≈ 1% if the mains voltage is purely sinusoidal.
In addition to input current distortion, the efficiency of the AFE has also been
measured, resulting in an exceptionally high value of 98.5% at nominal load in motor
mode. This high value is mostly due to the reduced switching losses of the three-
level topology, the optimized DC-link voltage, and the lossless damping of the
optimized LCL input filter.
Performance
Figure 12
Measured mains current
of a 160 kW drive
operating at nominal
power (VLL = 400 V, fN =
50 Hz, Po = 160 kW).
(A) Operating in motor
mode (energy flow from
the grid to the motor),
and (B) operating in
regenerative mode
(energy flow from the
motor to the grid). The
feeding voltage already
shows a THDv of 2%
(A) Motor mode, THDi = 2.3% (B) Regenerative mode, THDi = 1.8%

12. Highly efficient Active Front End enables trouble-free operation of low harmonic drives 12
It is now possible for the first time to replace a passive diode bridge rectifier with a
low-harmonic input stage or a regenerative input stage. Due to the implemented CM
filter stage, no additional high-frequency CM voltage occurs at the DC-link. Only a
low-frequency CM voltage component similar to that of a passive diode bridge
rectifier is present. The bearing currents are, therefore, not increased compared to a
standard VSD with a passive diode bridge rectifier, and the life span of the bearings
is not reduced by applying the novel AFE. Due to the optimized LCL input filter,
high-frequency emissions caused by the switching actions of the AFE are well
attenuated to values below 0.5%. The possible resonance of the input filter is
damped by a novel lossless active damping control strategy implemented in the
controller of the AFE. The LC-tank built by the filter elements cannot be excited by
the grid, which is one less concern for the user. Due to the three-level topology, the
lossless damping of the input filter, and the optimized DC-link voltage, the novel
AFE shows very high efficiency. This high efficiency, combined with the high input
current quality, small size, and the elimination of the high-frequency CM voltage
caused by the AFE, make the three-level AFE with additional CM filter the ideal
choice for a drive system with low-harmonic input stage — with or without
regenerative capability.
About the author
Michael Hartmann received a B.S. degree (Hons.) and an M.Sc. degree (Hons.) in electrical
engineering from the University of Technology Vienna, in 2005 and 2006, respectively. He has a
Ph.D. from the Power Electronic Systems Laboratory, Swiss Federal Institute of Technology, Zurich,
Switzerland, where his research focused on active three-phase rectifier systems with ultrahigh
switching frequencies and three-phase rectifiers for aircraft applications. He is currently with
Schneider Electric Drive Technology and Systems, based in Vienna, where he is involved in
research on high-efficiency three-phase power converter systems for low- and medium-voltage
power drives. His research interests further include three-phase power conversion, electromagnetic
interference, and switched-mode power amplifiers using multicell topologies.
Summary
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