Novel Soft-switching Isolated Three-phase
Bidirectional AC/DC Converter
Royal Inst. of Technology
Electr. Machines and Power Electronics
SE-100 44 Stockholm, Sweden
Abstract-- A novel ac/dc converter topology comprises a Phase Phase Phase Phase Phase
leg 1 leg 2 leg 3 leg 1 leg 2
voltage source converter with capacitive snubbers and a 3-by-2
cycloconverter, connected by an MF transformer. By alternately
commutating the two converters it is possible to achieve soft itr Cs Cs
switching conditions for all of the semiconductor elements. DC
Furthermore, it is shown that resonant commutation of the side
voltage source converter is possible by utilizing the cycloconverter
for short-circuiting the transformer terminals. Cs Cs
I. INTRODUCTION uac,2
Isolated ac/dc converters incorporating a voltage source uac,3
converter (VSC) and a direct converter connected by an MF
transformer, as shown in Figure 1, have been treated
extensively in the literature -. Both circuits with single iac,1 iac,2 iac,3 or
phase terminal and three phase terminal on the ac side have
been studied. A wide range of applications including UPS
systems and active filters has been envisaged. The basic Figure 2. Topology of the proposed converter.
function of the converter concept is that the VSC produces a
of the transformer and thereby reduce the benefits of using an
medium frequency ac voltage that magnetises the transformer
MF transformer. In  and  a concept is described which
and that this voltage is converted to a desired PWM voltage by
utilises snubber capacitors in the VSC and a specialised
the direct converter. The PWM voltage is applied to an
commutation algorithm in order to achieve soft commutation
inductive filter on the output of the direct converter whereby
for all semiconductor elements in all points of operation.
the power flow may be controlled. Several options exist
Similar concepts have also been proposed for multilevel
concerning the commutation mode of the two converters. The
converters , . In the present article the prospects for
direct converter can either operate by source commutation 
adapting the ideas from  to a system with a three-phase
in which case the VSC acts as source, or by forced
terminal on the ac side will be analysed. Furthermore, a brief
commutation. In case forced commutation is employed the
discussion of the prospects for applying the concept in HVDC
switching losses can be reduced by having the VSC output
converter stations will be made.
zero voltage during the commutations of the direct converter
. In order to reduce the switching losses of the VSC it is
possible to have the ac side current freewheeling through the
direct converter during commutations of the VSC whereby The topology of the proposed converter system is displayed
zero-current switching can be achieved . in Figure 2. A voltage source converter with two phase legs is
The above mentioned methods of reducing the switching connected to one of the windings of a single phase
losses can generally not be applied in such a way that the loss transformer. This converter is equipped with snubber
reduction is obtained in both the VSC and the direct converter capacitors Cs connected in parallel to each of the
simultaneously. The semiconductor switching losses in either semiconductor valves. The capacitors should be sufficiently
converter may limit the maximal possible operation frequency large to allow for zero-voltage turn-off of the switches of the
converter. The other winding of the transformer is connected
Direct MF to a 3-by-2 cycloconverter, which in turn is connected to a
converter transformer passive line filter. The valves of the cycloconverter do not
need any turn-off capability, they may well be realised by
LF AC DC thyristors connected in anti-parallel. However, for practical
reasons it may be more beneficial to use a configuration with
diodes and IGBT transistors as indicated in the figure.
Figure 1. Isolated ac/dc converter topology with In order to simplify the analysis of the operation of the
MF transformer and direct converter.
system coupling functions that relate the relevant voltages and This expression is positive which implies that utr and itr are
currents are defined. For each phase leg of the cycloconverter of the same sign, i. e. the instantaneous power flow is directed
a coupling function kac,i may be defined such that kac,i=+½ from the dc side to the ac side. Thereby the conditions are set
when the phase leg i. connects the corresponding terminal of for a snubbered commutation of the VSC. The stages of such a
the filter to the upper transformer terminal and kac,i=-½ for the commutation are outlined in Figure 4 for the case where itr is
lower terminal. Thus follows positive. The process is initiated by turning off the switches
uac ,i = kac ,i utr N tr (1) that conduct. Thereby the current is diverted to the snubber
capacitors which thus are recharged. When the potential of
and both phase terminals have fully swung to the opposite dc rail
itr = N tr ∑ kac ,i iac ,i . (2) the diodes in the incoming valves take over the current.
i Finally, the switches that are anti-parallel to these diodes are
turned on at zero-voltage and zero-current conditions. Thereby
III. PRINCIPLE OF OPERATION the VSC is prepared for a subsequent current direction
A. Commutation The reversal of the transformer voltage utr by the VSC
By alternately commutating the two converters it is possible commutation causes the condition in expression. (3) to become
to allow the direct converter to solely operate by source fulfilled for all three cycloconverter phase legs. Thus the cycle
commutation (natural commutation) whereas snubbered or may be repeated by again commutating the cycloconverter etc.
zero-voltage commutation is always enabled for the VSC. In The principal voltage and current waveforms during a
this section such a commutation scheme will be analysed. sequence of commutations as devised above can be found in
In order for a natural commutation of one of the phase legs Figure 7. It should be pointed out that the durations of the
of the cycloconverter to be possible the condition commutation processes have been greatly exaggerated in this
figure, for clarity. In practice they occupy only a very minor
utr kac,i iac ,i < 0 (3) fraction of the commutation cycle.
has to be fulfilled. Figure 3 schematically shows an example of B. Resonant VSC commutation at low load
such a commutation. For simplicity the assumption is made
At low currents the commutation of the VSC may become
that the transformer can be represented by its leakage
unduly lengthy as the recharging of the snubber capacitors
inductance whereas the voltage source converter can be
becomes slower. In the extreme case of zero load it is not
represented by a constant voltage throughout the process. The
possible at all to commutate the VSC in the fashion described
commutation is started by turning on the non-conducting valve
above. However, a quasi-resonant mode of commutation is
in the direction of the current through the phase terminal. The
proposed that solves this problem. By short-circuiting the
voltage supplied by the VSC appears across the leakage
primary winding of the transformer using the cycloconverter
inductance and the incoming valve gradually takes over the
during the commutation of the VSC it is possible to initiate a
current. Finally the initially conducting valve turns off as the
resonant process, governed by the snubber capacitors and the
current through it goes to zero and the switches in this valve
leakage inductance of the transformer. This resonant process
can be gated off. The current derivatives during both turn-on
can be utilised for recharging the snubber capacitors. In
and turn-off are determined by the transformer leakage
Figure 5 the stages of such a resonant commutation are shown.
inductance and are thus relatively low. As a result of the
commutation the coupling function changes sign, i. e. 1. 2.
utr kac,i iac,i > 0 (4)
When all of the cycloconverter phase legs have been
commutated it follows from eq. (2) and (4) that
utr itr = N tr ∑ utr kac ,i iac ,i = N tr ∑ utr iac ,i . (5)
i i 2
1. 2. 3.
Lλ Lλ Lλ
Lfilt Lfilt Lfilt
-NUd -NUd -NUd
Figure 3. Commutation of a cycloconverter phase leg. Figure 4. VSC Commutation.
1. Power flow from AC side to DC side 2. Enhancement stage 3. Resonant stage
4. Ramp-down stage 5. Cycloconverter natural turn off 6. Power flow from AC side to DC side
Figure 5. Stages of a resonant commutation of the VSC.
The corresponding voltage and current waveforms are side current. At this level the cycloconverter valves that were
displayed in Figure 6. Initially, stage 1, the power flow is turned on in stage 2 turn off as the current through them goes
directed from the dc side to the ac side just as in the case of the to zero, stage 5. Finally the switches in these valves are gated
conventional VSC commutation. The process is started, off at zero-current conditions, stage 6.
stage 2, by turning on valves in the cycloconverter so as to The enhancement current allows for a complete
provide a path in the direction of the voltage applied by the commutation despite losses in the resonant circuit and
VSC. This causes the transformer current to start increasing variations the ac side currents during the resonant process.
linearly as the voltage from the VSC appears across the Although only one of the phase legs of the cycloconverter is
transformer leakage inductance. This is allowed to continue shown in the figure it is obvious that all of the phase legs may
until the current has increased by a certain predefined amount, be switched in the described fashion. This provides additional
hereafter named enhancement current, ienh. Thereafter the two paths for the resonant current and thereby reduces the stress on
switches in the VSC that carry current are turned off whereby the semiconductors.
the resonance process takes place, stage 3. The duration can be In summary, the described method offers an alternative way
shown to be, of commutating the VSC that is not dependent on the ac side
currents. This is of vital importance for the feasibility of the
i + i
tr = Lλ Cs π − 2 arctan Z r tr ,1 enh (6) system as otherwise hard switching would have to be used to
U d commutate the current from switch to diode with sufficient
speed at low load. Hard switching in this case amounts to rapid
discharge of the snubber capacitors into the IGBT that turns
Z r = Lλ Cs and itr ,1 =
ac ,i on. This would lead to very high switching losses and possibly
neglecting losses, where Lλ is the leakage inductance in tr
secondary side quantities and Ntr is the turns ratio of the utr
transformer. From the expression it is obvious that both the ac
side currents as well as the enhancement current act to shorten
the resonant interval as they contribute to the recharging of the uAC,i
capacitors. The peak transformer current during the resonant
process amounts to itr ienh
= d + ( itr ,1 + ienh )
itr , pk (7)
When the transformer voltage has been fully reversed the
diodes in the VSC valves that initially blocked take over the
current and the switches anti-parallel to these diodes can be
turned on at zero-voltage and zero-current conditions.
1. 2. 3. 4. 5. 6.
Subsequently, stage 4, the current through the transformer is
forced down linearly to the value that corresponds to the ac Figure 6. Current and voltage curve shapes
during a resonant VSC commutation.
D A1 A3 A2 D A1 A3 A2 D
D: VSC commutation
Ai: Commutation of cycloconverter phase leg i
Figure 7. Principal curve shapes during commutation
sequence enabling constant soft commutation.
Sawtooth carrier for Sawtooth carrier for
also destruction of the IGBT. iac,i<0 iac,i>0
Figure 8. Carrier-based modulation method
The control system for the proposed converter should fulfil enabling constant soft commutation.
two main demands apart from maintaining soft commutation as
outlined in the previous two sections. Firstly, proper operation direction, are used as shown in Figure 8. For simplicity, the
of the transformer should be ensured by avoiding low impact on the curveforms caused by the commutation
frequency or dc components in the voltage applied to the processes is not shown in the figure. In practice it may be
transformer utr. This is achieved by always making the VSC necessary to adjust the timing of the switchings for the voltage-
commutations at constant intervals, whereby essentially a time area lost or gained during the commutations.
square voltage will be applied to the transformer. Secondly it
should be possible to obtain desired PWM patterns in the ac- IV. SIMULATION STUDY
side phase potentials uac,i. By making the commutations of the In order to verify the control algorithm a simulation model
cycloconverter phase legs at appropriate instants in the interval of a 40 kVA converter has been created in the simulation
between two VSC commutations the width of the pulses can be software package Simplorer. The parameters of the
chosen freely. This may be effected in a multitude of ways. To simulated system can be found in TABLE 1. The parameters
show the potential of the concept a rather simple and have been chosen such that it will be possible to build a
straightforward modulation derived from carrier-based prototype using IGBT modules with a blocking voltage of
methods will be described. 1200 V. The control logic needed to achieve the desired
Following each VSC commutation the sign of each phase
potential uac,i is opposite to the sign of the current through the
corresponding phase outlet iac,i which follows from expressions TABLE 1. - Parameters of the simulated converter.
(1) and (3) (expression (3) is fulfilled in this case). This Rated power 40 kVA
dependency implies that the instants for commutating the Dc link voltage 700 V
cycloconverter phase legs have to be determined in different Transformer operating frequency 4 kHz
ways depending on the direction of iac,i. This may be Transformer turns ratio 1.0
Transformer leakage inductance 5 µH
interpreted as a carrier based method where sawtooth carriers
Snubber capacitance, per valve 0.3 µF
with either positive or negative slope, depending on the current Filter inductor, (per phase) 1.5 mH (0.35 pu)
voltage (V), current (A)
4.200m 4.300m 4.400m 4.500m 4.600m 4.700m 4.200m 4.300m 4.400m 4.500m 4.600m 4.700m
Figure 9. Simulated curve shapes of transformer voltage and current. Figure 10. Simulated curve shape of phase voltage uac,1.
Operation at rated load. Operation at rated load.
commutation sequence was implemented using a number of the phase voltages is shown in Figure 10 for the same time
coupled state machines. All semiconductor elements were interval. The curveshape follows a two-level PWM pattern
modeled as essentially ideal. Small RC-snubbers were except during commutations of the cycloconverter when the
connected in parallel to the cycloconverter valves as such voltage momentarily goes to zero. The three phase currents
snubbers most likely would be used in a real system to handle can be seen in Figure 11, showing the curve shape during an
diode reverse recovery. In the simulations the converter system entire fundamental cycle. The curves are essentially sinusoidal
is connected to a 230 V/50 Hz three-phase ac voltage source with minor notches occurring at zero crossings. These result
and it is controlled to operate at rated power in such a fashion from the previously mentioned changes between positive and
that either cosφ = 1 or cosφ = -1 at the connection point. This negative carrier slopes in the modulation algorithm. Figure 12,
implies that the PWM voltages, uac,i, are slightly out of phase finally, displays a frequency spectrum of one of the ac side
with the ac side currents, iac,i, in order to compensate for currents, iac,1. The dominant harmonics are grouped around
reactive power in the filter inductors. even multiples of the transformer operating frequency.
Figure 9 through Figure 12 display results from a simulation In Figure 13 the VSC output voltage and current are
in which the system operates at rated load with power flowing displayed during a couple of commutation cycles when the
from the dc side to the ac side. Figure 9, firstly, shows VSC system operates at low load and resonant commutation has to
output voltage and current during a few commutation cycles be used for the VSC. The curveshapes are principally the same
(cf. Figure 7). The voltage has essentially a square waveform as during full load (cf. Figure 9) but during the resonant
and the frequency equals the switching frequency. Stepwise commutations the current momentarily increases.
changes in the current occurs at commutations of the
cycloconverter. The transients that appear in the current are V. APPLICATION IN HVDC CONVERTER STATIONS
likely to be caused by the cycloconverter RC snubbers. One of High voltage direct current (HVDC) systems based on
iac,3 iac,2 iac,1 200
-200.0 50 m
2.00m 5.00m 10.00m 15.00m 20.00m 25.00m 30.00m -1.00k
0.0k 10.00k 20.00k 30.00k 40.00k 50.00k
Figure 11. Simulated curve shapes of ac side currents. Operation at rated load. Figure 12. Frequency spectrum of ac side current iac,1. Operation at rated load.
Also for interfaces between HVDC transmission systems
and renewable energy sources, such as wind generators, the
500.0 utr studied converter topology may represent a promising
voltage (V), current (A)
0 A three-phase isolated bi-directional ac/dc converter
topology is described together with a control algorithm. It is
-250.0 shown that soft commutation is possible for all semiconductor
resonant current elements in all points of operation without the need for any
auxiliary semiconductors. Simulations verify the operation of
9.600m 9.650m 9.700m 9.750m 9.800m 9.850m REFERENCES
 T. Kawabata, K. Honjo, N. Sashida, K. Sanada, M. Koyama, "High
Figure 13. Simulated curve shapes of transformer voltage and current. frequency link DC/AC converter with PWM cycloconverter",
Operation at low load. Resonant commutation employed for the VSC. Conference Record of the 1990 Industry Applications Society Annual
Meeting, pp. 1119 -1124, vol.2, 1990.
voltage source converters have lately emerged as a competitive  I. Yamato and N. Tokunaga, “Power loss reduction techniques for three
alternative to classic HVDC technology based on current phase high frequency link DC-AC converter”, 24th Annual IEEE Power
Electronics Specialists Conference, PESC '93 Record., pp. 663 -668,
source converters . The converter stations in such a system 1993.
are equipped with three-phase voltage source converters, in  M. Matsui, M. Nagai, M. Mochizuki and A. Nabae, “High-frequency
which each valve is realised by several series-connected IGBT link DC/AC converter with suppressed voltage clamp circuits-naturally
modules, see Figure 14a. On the ac side generally transformers commutated phase angle control with self turn-off devices”, IEEE
Transactions on Industry Applications, vol. 32, no. 2, pp. 293 -300,
are attached in order to adapt the output voltage to March-April 1996.
transmission networks operating at medium or low voltage.  M. A. Rodrigues, E.R. da Silva, C.B. Jacobina and A.M.N. Lima,
With the studied converter topology the dc/ac conversion can “PWM strategy for switching loss reduction in a high frequency link DC
to AC converter”, 30th Annual IEEE Power Electronics Specialists
be integrated with transformation by a medium frequency Conference, PESC ‘99 Record , vol. 2 , pp. 789 -794, 1999.
transformer thus enabling lower transformer losses. In addition  S. Norrga, “A Soft-switched Bi-directional Isolated AC/DC Converter
soft commutation for all semiconductor valves is achieved. An for AC-fed Railway Propulsion Applications”, IEE PEMD 2002
conference proceedings pp. 433-438, Bath UK, 16-18 April 2002.
interesting option is furthermore to utilise a modification of the  S. Norrga, “A Novel Soft-switched Bi-directional Isolated AC/DC
studied topology employing only one phase leg in the VSC, Converter”, PCIM 2002 conference proceedings, Nuremberg Germany,
see Figure 14b. In this case one of the transformer terminals is 2002.
 F. Iturriz, P. Ladoux, “Phase-Controlled Multilevel Converters Based on
connected to a midpoint in the dc link created by bus-splitting
Dual Structure Associations”, IEEE Trans Power Electronics, vol. 15,
capacitors. This solution implies a very significant reduction in no. 1,pp 92-102, Jan 2000.
the number of series-connected IGBT sub-valves on the high  C. Chabert, A. Rufer, “Multilevel Converter with Two-stage
voltage side compared to the conventional case. As these sub- Conversion”, EPE 2001 Conf. Proceedings, Graz, Austria, 2001.
 L. Stendius, K. Svensson, “HVDC Light - an excellent tool for city
valves require complex gate drives and voltage-sharing center infeed”, PowerGen Conference Proc., Singapore, 1999.
circuitry they tend to be expensive and a reduction in their
number is therefore highly desirable.
a.) Conventional VSC-based solution b.) Studied topology with MF transformer
Three HV main
phase legs One HV main
Figure 14a,b. HVDC application. Comparison between a conventional system and the studied solution