Novel Soft-switching Isolated Three-phase Bidirectional AC_DC

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					              Novel Soft-switching Isolated Three-phase
                  Bidirectional AC/DC Converter
                                                               Staffan Norrga
                                                             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
                                                                                                                               utr                              Ud
Furthermore, it is shown that resonant commutation of the                                                                                                            side
voltage source converter is possible by utilizing the cycloconverter
                                                                                                                Ntr: 1
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 [1]-[4]. Both circuits with single            iac,1       iac,2      iac,3                                            or
phase terminal and three phase terminal on the ac side have
                                                                                      AC side
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 [5] and [6] 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 [7], [8]. In the present article the prospects for
direct converter can either operate by source commutation [3]
                                                                         adapting the ideas from [5] 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
[4]. In order to reduce the switching losses of the VSC it is
                                                                                                              II. TOPOLOGY
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 [1].                              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
 side                                                            side
                                                                         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)
                                                                                      itr                                 itr
becomes valid.
  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
                                                                                     3.                                 4.
 1.                             2.                            3.
                           Lλ                           Lλ                    Lλ
                                                                                          itr                             itr
      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
                                   N tr
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
             U 
           =  d  + ( itr ,1 + ienh )
                                       2                                                                                                          t
itr , pk                                                                    (7)
              Zr 
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.



uac,1                                                                     uac,3*

uac,2                                                                      uac,1


            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

  C. Modulation
                                                                                          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)
                           800.0                                                                                                         400.0

                           500.0                                                                                                         250.0
voltage (V), current (A)

                           250.0                                                                                                         125.0

                                                                                                                           voltage (V)
                               0                                                                                                             0

                           -250.0                                                                                                        -125.0

                           -500.0                                                                                                        -250.0

                           -800.0                                                                                                        -400.0
                                    4.200m        4.300m              4.400m           4.500m         4.600m    4.700m                            4.200m     4.300m        4.400m        4.500m        4.600m   4.700m

                                                                           time(s)                                                                                             time(s)
                           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

                           100.0                                                                                                            30
                                                                                                                         current (A)

                            50.0                                                                                                            10
  current (A)

                                                                                                                                         700 m
                                                                                                                                         500 m
                                                                                                                                         300 m
                                                                                                                                         200 m

                                                                                                                                          80 m
                           -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)

                           250.0                       2itr
                                                                                                                                       VI. CONCLUSIONS
                               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
                                                    during VSC
                                                                                                            auxiliary semiconductors. Simulations verify the operation of
                                                                                                            the concept.
                                    9.600m   9.650m           9.700m          9.750m   9.800m     9.850m                                  REFERENCES
                                                                                                            [1]   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                                              [2]   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 [9]. The converter stations in such a system                                                    1993.
are equipped with three-phase voltage source converters, in                                                 [3]   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.                                                   [4]   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                                             [5]   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                                          [6]   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.
                                                                                                            [7]   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                                                  [8]   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.
                                                                                                            [9]   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
                                                                                                                                                  phase leg

                                                          Figure 14a,b. HVDC application. Comparison between a conventional system and the studied solution

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