Three-Level Voltage Source Inverter with Hysteresis-Band Current

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Three-Level Voltage Source Inverter with Hysteresis-Band Current Powered By Docstoc
					                         Three-Level Voltage Source Inverter with
                            Hysteresis-Band Current Control


      M. Lafoz and I.J. Iglesias                            C. Veganzones
      Centro de Estudios y Experimentación                  Universidad Politécnica de
      de Obras Públicas (CEDEX).                            Madrid (UPM) SPAIN.
      C/ Alfonso XII, 3 28014 Madrid. SPAIN.


Keywords.
Three-level inverters, current control, current hysteresis band, DC middle point voltage control,
permanent magnet synchronous generator, wind generation plant.

Abstract.
This paper presents the simulation results of a Double Three Level Voltage Source Inverter controlled
by means of a double current hysteresis band technique. The presented control strategy will be applied
to a 1.2 MW three-level VSI to drive a multipole Permanent Magnet Synchronous Generator in a
Wind-Generation Plant. A current strategy is chosen for this application in order to control the
electromagnetic torque in the generator and the power factor at the front-end inverter. The neutral
point voltage control, which is one of the critical aspects in this kind of topology, can be easily
achieved by means of progressive variation of the hysteresis bands limits.

                                         I. INTRODUCTION
The advantages of the Three-Level Voltage Source Inverter topology (Figure 1) are well known and
have been applied in medium and high power applications in the last years [Mertens 1999, Buschman
1997, Steinke. 1989]. The reduction of switching frequency and the increment of the voltage
supported by each device are very attractive features. The switching frequency can be reduced four
times with the same output current distortion when compared with a conventional two level voltage
inverter.




                            Figure 1. Three level VSI inverter topology.

This paper presents the simulation results of a high power double VSI driving a multipole Permanent
Magnet Synchronous Generator (PMSG) connected to the grid. The rating power and DC voltage
value (3 kV.) imply that the best option will be a Three-Level VSI topology based on the use of 2.5 kV
IGBT modules.

The most extended control system for three-level inverters is the application of the Space Vector
Modulation (SVM) technique [Bacigalupo 1997, Marchesoni. 1992 and Walczyna 1993]. This method
gives a very good line current with a low switching frequency and also permits the possibility of
controlling the neutral point voltage, which is a very important issue in this topology. However very
complex calculations are needed for this strategy, especially for the neutral point voltage control, and a


                                                                                                             1
powerful microprocessor or DSP is required. An important modification to this technique can be
achieved by means of combining natural sampling methods with the SVM principles [Mertens 1999,
Heras 2001]. In this case an easy way to balance neutral point voltage can also be achieved.

All the previous mentioned techniques are based on voltage control strategies. However, it is
interesting for the mentioned application, to control the electromagnetic torque in the generator and
also being capable of regulating both the active and reactive power supplied to the network. Therefore,
a current strategy appears as a good choice to control both inverters.

Saber® simulator has been used to simulate all the complete system. The current control technique
presented in this paper permits good performance of both converters, with very easy and fast control
of the neutral point voltage and a small line current THD for an average switching frequency of
around 1 kHz.

                   II. DOUBLE HYSTERESIS-BAND CURRENT CONTROL

The basic principle of the double hysteresis-band current control is based on the classical hysteresis
control applied to conventional two-level inverters. The difference in a three-level inverter is that three
voltage levels can be applied to each phase: +Vdc/2 (switching on the two upper switches, T1 y T2), -
Vdc/2 (switching on the two lower switches T3 y T4) and 0 volts (switching on the two central
switches T2 y T3). Therefore, to increase or decrease the line current, we have more possibilities than
with a two-level inverter. For this reason, as it is shown in Figure 2, we define two hysteresis bands
(Upper and Lower Conmutation Bands) around the current reference value. The hysteresis bands are
actually superimposed but to differentiate them, they will be named as Upper and Lower band. The
band change could be obtained by means of a simple logic circuit when using two slightly shifted
bands. However, to avoid the influence of noise, this option was rejected.
                                                                         T1


                      Iref (Reference Current)
                                                                         T2

                       Upper Commutation Band
                                                       -VDC/2
                      Iref (Reference Current)                  To DC
                                                                Middle
                                                       0
                     Lower Commutation Band                     Point
                                                           0             T3
                     Iref (Reference Current )
                                                      + VDC/2            T4




                       Figure 2. Double-Hysteresis Band Current Control Strategy.

When the upper band is operating, the control system commutates the line voltage between -Vdc/2 and
0 to maintain the line current inside this band. In this case, applying 0 volts cause a current increase. In
the opposite case, when the lower band is operating, 0 volts application causes a current decrease.

The exact time when the band must be changed is when the zero voltage passes from increasing the
current to decreasing it or viceversa. This moment occurs when the slope of the current coincides with
the slope of the reference current and can be determinated when the slope defined variable is equal to
zero as Figure 3 shows. When slope is less than zero, the upper-band is used (0, -Vdc). When slope is
greater than zero the lower-band is used (+Vdc, 0).

The commutation band change decision depends on phase voltage V, the current reference iref and
line inductance L. In the network side inverter the voltage can be measured and filtered, iref is
generated by the control strategy, and L is well known because it is the inverter to grid connection
inductance. In the generator side inverter, V is a square shape voltage which will be estimated and not
measured, by a simple synchronous machine model, iref is the current reference, and the parameter L



                                                                                                          2
is the machine inductance. It is important to adjust the inductance value in order to get a band change
at the appropriate instant in both inverters.
                                                                                                                                               Commutation Band Change. Current evolution whith zero voltage application in the different bands.

                                                                    M


                                         banda_hab_a

                                                  swt_1a

                                                  swt_2a




                                                                        400.0




                                                                        200.0


                                                                                                                                                                                                                                                                                                    Lower Band.
                                                                                                                                                                                                                                                                                                    Zero voltage application
                                                                                                                                                                                                                                                                                                    decrease the current




                                                           (A)
                                                                              0.0

                                                                                                                                              Upper Band.
                                                                                                                                              Zero voltage application
                                                                                                                                              increase the current

                                                                       -200.0




                                                                       -400.0




                                                                                                          0.106                       0.107                       0.108                    0.109                       0.11                            0.111                          0.112                     0.113                  0.114
                                                                                                                                                                                                                       t(s)
                                    (A) : t(s)            i(l.la)




       Figure 3. Band change condition. Detail of the current when operation band is changing.

A strategy for operating at a constant switching frequency has been also developed. Based on previous
analysis [Bose, 1990], a variable bandwidth expression is proposed in order to get the current
harmonics concentrated around a fixed switching frequency. This is shown in Figure 4. Figure 5
presents the variable bandwidth expression and Figure 6 shows a detail of the bands limits when this
strategy is applied.
                                                           Contenido en armonicos de la corriente con banda fija.                                                                                                                     Contenido en armonicos de la corriente con banda variable.

                      MAG(A)                                                                                                                                                                   MAG(A)
                          22                                                                                                                                                                       22


                          20                                                                                                                                                                       20


                                                                                                                                                                                                   18
                          18


                                                                                                                                                                                                   16
                          16

                                                                                                                                                                                                   14
                          14

                                                                                                                                                                                                   12
                          12


                          10
                                                                                                                                                                                                   10
                                                                                                                                                                                                                                                                                                   THD=4,5%
                                                                                                               THD=5,6%                                                                                8

                            8
                                                                                                                                                                                                       6

                            6
                                                                                                                                                                                                       4

                            4
                                                                                                                                                                                                       2

                            2
                                                                                                                                                                                                       0


                            0                                                                                                                                                                      -2


                          -2                                                                                                                                                                       -4
                                0      200          400          600            800         1k    1.2k       1.4k           1.6k   1.8k       2k           2.2k   2.4k    2.6k     f(Hz)                   0    200           400             600       800          1k        1.2k        1.4k      1.6k      1.8k     2k      2.2k      2.4k   2.6k     f(Hz)
                      MAG(A) : f(Hz)                             (6)i(l.la)                                                                                                                    MAG(A) : f(Hz)            (5)i(l.la)




Figure 4. Fourier analysis of the line current with fixed (left side) and variable (right-side) hysteresis-
                                                  band
                                                                                                             CONSTANT SWITCHING FREQUENCY

                                                    Vload
                                                                                                                                                   1 Vload  2L Vload 
                                          Vdc                                                                         HW=                                 + m1−      + m
                                                                                                                                                   fc  L      Vdc  L                                                                                                                                      Hystheresis
                                          diref
                                       m=                                                                                                                                                                                                                                                                       Band Width
                                           dt                                                                                                       Parameters: L, fc

   Figure 5. Variable hysteresis-bandwidth calculation for constant switching frequency operation
                                                                                                                                      Detail of the variable width band for constant switching frequency in the network side inverter current control.

                                                                        M

                                                     swt_1a

                                                     swt_2a

                                                     swt_3a

                                                     swt_4a




                                                 500.0



                                                 400.0                   400.0



                                                 300.0                   300.0


                                                 200.0                   200.0


                                                 100.0                   100.0
                                       (V)




                                                             (-)




                                                    0.0                        0.0


                                                 -100.0                 -100.0



                                                 -200.0                 -200.0



                                                 -300.0                 -300.0



                                                 -400.0                 -400.0



                                                 -500.0                 -500.0

                                                                                    0.112        0.113              0.114          0.115           0.116          0.117          0.118         0.119            0.12                  0.121            0.122              0.123             0.124           0.125       0.126          0.127      0.128
                                                                                                                                                                                                                t(s)
                                    (-) : t(s)            (ban_sup_a(control_3n_red.sym53)+(banda_var_af(control_3n_red.sym53)/2.0))                                                         (ban_sup_a(control_3n_red.sym53)-(banda_var_af(control_3n_red.sym53)/2.0))



                                    (V) : t(s)            imed_a




Figure 6. Detail of the current commutating between the two limits of the variable hysteresis band for
                                constant switching frequency operation.


                                                                                                                                                                                                                                                                                                                                                                  3
It is necessary to connect the DC middle point to the neutral of the network or the machine to get
constant switching frequency. If it is not connected, no constant switching frequency is obtained as
shown in Figure 7., and also the DC middle point voltage is not very well controlled. Therefore in the
machine inverter side, when the neutral machine is not accessible, it is not possible to operate at a
constant switching frequency. In this case the only advantage is that the switching frequency is lower
than when the neutral point is connected.

              Corriente suministrada por el inversor del lado de la red. Bandas y disparos con neutro conectado.                                                                                                                          Graph 2
  t(s) = 56.6m                                                                                                                                                     t(s) = 26.4m

                            600                                                                                                                                                               600

                            400                                                                                                                                                               400



                            200                                                                                                                                                               200



                  (-)             0                                                                                                                                                    (-)          0



                          -200                                                                                                                                                               -200


                                                                                                                                                                                             -400
                          -400

                                                                                                                                                                                             -600
                          -600
                                                                                                                                                                  (9)swt_1a                    0
 (8)swt_1a                   0
 (8)swt_2a                   0                                                                                                                                    (9)swt_2a                    1
 (8)swt_1b                   0                                                                                                                                    (9)swt_1b                    1
 (8)swt_2b                   1                                                                                                                                    (9)swt_2b                    1
 (8)swt_1c                   0                                                                                                                                    (9)swt_1c                    1
 (8)swt_2c                   1                                                                                                                                    (9)swt_2c                    1




                                                                                                                                                                                                    20m   25m            30m        35m       40m              45m            50m   55m   60m   65m   70m
                                                                                                                                                                   t(s)
                                 54m        56m          58m           60m           62m            64m          66m   68m   70m   72m     74m    76m      78m      (-)       : t(s)                      (7)...+(banda_var_af/2)                   (7)...-(banda_var_af/2)
  t(s)
   (-)   : t(s)                              (9)...+(banda_var_af/2)                        (9)...-(banda_var_af/2)                                                 (-)       : t(s)                      (9)ia
   (-)   : t(s)                              (8)ia




 Figure 7. Line current commutation and switching pulses with neutral connected (constant switching
                     frequency) and not connected (variable switching frequency)


                                                                                              V. NEUTRAL POINT VOLTAGE CONTROL

As mentioned before, one of the inherent problems of the three-level topology is to maintain the DC
middle point voltage in a narrow band around Vdc/2. If this condition is not satisfied, some individual
switches have to support a higher voltage and the current harmonic distortion increases rapidly. With
the control strategy presented in this paper, the neutral point voltage can be easily controlled by means
of moving the hysteresis-bands position with respect to the initial situation. When zero voltage is
applied to any phase, the sum of the currents flowing through the clamping diodes, called icharge,
produces the unbalanced DC capacitors charging process. By this way, a positive current icharge from
the inverter produces a charge in the capacitor 1 (C1), and a negative current produce a discharge in
the same capacitor. A DC Middle Point unbalance appears when iC1 ≠ iC2 or icharge≠0, as it is
shown in Figure 8.

                                                                                                                                          C1
                                                                                                                                                                    i c h arg e = (C 1 + C 2 )
                                                                                                                                                                                                                                            dVC1
                                                                                           i a ,b,c
                                                                                                                                   i c h a rg e         iC 1                                                                                 dt

                                                                                                                                                                 i ch arg e = − (C 1 + C 2 )
                                                                                                                                                        iC 2                                                                                 dVC 2
                                                                                                                                          C2
                                                                                                                                                                                                                                              dt



                                                Figure 8. DC Middle Point unbalance appears when iC1 ≠ iC2 or icharge≠0.

It is possible to control the DC middle point voltage by means of modifying the duration of the
charges or discharges in one of the DC capacitors. For example, when the capacitor 1 (C1) voltage is
less than Vdc/2, working in the upper band and with positive icharge , a progressive positive
displacement of the upper band generate an increasing of this voltage. If C1 voltage is greater than
Vdc/2 a progressive negative displacement of the upper-band generates a decrease in this voltage. This
is shown in Figure 9.




                                                                                                                                                                                                                                                                                                        4
                                                                                                                               0 V
                                                                                                                                                                                                                               0 V


                            Figure 9. Increasing and decreasing the charge of capacitors by means of displacement of the
                                                           conmutation hysteresis-band.

Moreover, the control action depends on the current sign and on the conmutation band used. Table I
shows the change of the control action sign. This is implemented in the control_change block.

                                                                                                    Conmutation Band                                                               Current sign
                                                                                                                       0       (lower)                                        1      (positive)                ⇒                              Control action = - control_ ref
                                                                                                                       1       (upper)                                        1      (positive)                ⇒                              Control action = control_ ref
                                                                                                                       0       (lower)                                        0      (negative)                ⇒                              Control action = control_ ref
                                                                                                                       1       (upper)                                        0      (negative) ⇒                                             Control action = - control_ ref


                                                                                                                                             Table I. Control-Change block operation.

Figure 11 shows the DC middle point voltage control strategy, which consists of shifting the current
reference from its initial value by means of a control_ref signal obtained by means of a P.I controller
and depending on the operation band and the current sign. A first order filter is applied to the final
control action in order to reduce the radical variations of the control signal when operation band or
current sign changes occur.
                                                                                                    V d c /2_ r e f
                                                                                                                                                                     P.I
                                                                                                                                                                                               control_ ref

                                                                                                                              V C1                                                                                                                     control_ r e f _ a
                                                                                                                                                                     Hab_band_ a
                                                                                                                                                                                                                                                                          1er Order
                                                                                                                                                                                                              Control Change                                                Filter
                                                                                                                                                                                                                                                                                                                                              Ia_ref_mod
                                                                                                                                               Ia_ref
                                                                                                                                                                                   Ia sign
                                                                                                                                                                                                                                                                                     Ia_ref
                                                                                                                                                                                                                                                       control_ r e f _ b
                                                                                                                                                                     Hab_band_ a
                                                                                                                                                                                                                                                                          1er Order
                                                                                                                                                                                                              Control Change                                                Filter
                                                                                                                                                                                                                                                                                                                                              Ib_ref_mod
                                                                                                                                               Ib_ref
                                                                                                                                                                                   Ib_ sign
                                                                                                                                                                                                                                                                                     Ib_ref
                                                                                                                                                                                                                                                       control_ r e f _ c
                                                                                                                                                                     Hab_band_ a
                                                                                                                                                                                                                                                                          1er Order
                                                                                                                                                                                                              Control Change                                                Filter
                                                                                                                                                                                                                                                                                                                                              Ic_ref_mod
                                                                                                                                               Ic_ref
                                                                                                                                                                                   Ic sign
                                                                                                                                                                                                                                                                                     Ic_ref


                                                                                                                   Figure 11. DC middle point voltage control system.

Finally, Figures 12 and 13 show the simulation result of the neutral point voltage control strategy.
Figure 12 shows the control signal, maintaining the DC middle point voltage within a narrow band of
±40V around a given voltage reference of 1500 V.
                                                                   DC middle point voltage control signal. DC middle point voltage reference and measured voltage.                                                                                                                              DC middle point voltage when control is operating and not operating.


             40.0

                                                                                                                                                                                                                                  1.0



             20.0
     (-)




                                                                                                                                                                                                                         (-)




              0.0




             -20.0



                                                                                                                                                                                                                                  0.0
             -40.0

           1560.0                                                                                                                                                                                                              1600.0



           1540.0



                                                                                                                                                                                                                               1500.0
           1520.0
   (-)




                                                                                                                                                                                                                   (-)




           1500.0

                                                                                                                                                                                                                               1400.0

           1480.0




           1460.0
                                                                                                                                                                                                                               1300.0

                     0.0                0.02         0.04   0.06          0.08                0.1               0.12                0.14               0.16           0.18   0.2        0.22        0.24
                                                                                                                                                                                                                                        0.0             0.02       0.04       0.06       0.08                0.1                0.12               0.14                0.16   0.18   0.2   0.22   0.24
                                                                                                                       t(s)
                                                                                                                                                                                                                                                                                                                                       t(s)
(-) : t(s)            control_ref_maq
                                                                                                                                                                                                                (-) : t(s)               activo_control_maq


(-) : t(s)            ref_tens_vc1_maq         tens_c1
                                                                                                                                                                                                                (-) : t(s)               tens_c1               tens_c1




                                    Figure 12. DC middle point voltage                                                                                                                                                                                 Figure 13. Effect of the operation or not of
                                  control signal, voltage reference and C1                                                                                                                                                                                the DC middle point voltage control.
                                             measured voltage.
Figure 13 present the simulation results of the DC middle point voltage evolution when the control
action is not operating. At 150ms the control action operates once again.


                                                                                                                                                                                                                                                                                                                                                                                                         5
It is not necessary to control the DC middle point voltage in both inverters. There is an only DC
middle point and while the machine side inverter controls the DC middle point by means of a
displacement of the machine current references, the front-end inverter is able to work without a DC
middle point voltage control and therefore without current distortion due to this effect.

                                    IV. CONTROL SYSTEM DESCRIPTION
Figure 14 presents the control block diagram for the Wind-Generator connected to the grid.
                                                                                        DC Link
                                                                                                                         Inductance
                                              P.M.S.M.
                                                                     Generator
                                                                        Side                        Front-End
                                                                      Inverter                       Inverter
                                                                                                                               LC Filter
                                        wr

                                             Machine Line Currents
                                                                                        V C1                           Network Line Currents
                                                                      Currents
                                                                                                     Currents
                                 V C1          DC Middle              Hysteresis
                                                                                                     Hysteresis
                                                 Point               Comparator
                                                                                                    Comparator
                                                Voltage
                                                Control

                        Ia_ref
                                                                  Operation Band                   Operation Band
                        Ib_ref                                   Selection and Band               Selection and Band
                                                                 Width Calculation                Width Calculation
                        Ic_ref

                     Rotational Speed                                              Estimated
                                                  P.M. Synchronous                 Generator        Network Voltages
                      Line Currents               Generator Model                   Voltages




                                                           Figure 14. Control Diagram Block.
Machine Side Inverter
The generator side inverter receives a torque or power reference from a block which simulates the
wind turbine behaviour. This turbine model is based on a model which optimises the operation point in
order to get the maximum wind power. In an appropriate d-q axes system, when the d-axes coincides
with the machine e.m.f., the electromagnetic torque is proportional to the iq current component. By
means of this strategy the machine works with the minimum current for a given turbine torque.
For a given DC voltage, the machine output voltage has to be limited under the inverter voltage
operation limit. For this reason when the machine output voltage exceeds this limit, a PI controller
generates a negative d-axes current reference, reducing the inner machine flux and maintaining the
output voltage. Some simulations will be presented in this paper to show this phenomena.
From these iq and id components, three senoidal current references are generated based on Park
transformations. A double hysteresis current control with a variable bandwidth strategy is used for the
inverter control. Some simulations will be also presented later.
Front-End Inverter
The front-end inverter is connected to the grid through a 900µH inductance and a 2.4 kHz LC filter.
This is the frequency of the main output current harmonic, which is double the average switching
frequency. The current THD is reduced from 10 % to 4 % with this filter.
The front-end inverter controls the DC voltage and varies the active power supplied to the grid
according to the DC voltage deviation. By this way, all the active power is transmitted from the wind
generator to the network. It is necessary to synchronise the inverter current with the network voltage,
thereby generating the current references according to the active and reactive power requirements. A
network voltage phase detector has been developed for this goal, based on Park transformations.

                                                                V. SIMULATIONS
The previously described control system has been tested by simulation with Saber® simulator. Some
results of the developed simulations are presented in the next figures.

A constant torque is applied to the generator shaft by the wind turbine and a power ramp is proposed
by the control system to be injected into the network. As it is shown in Figures 15 and 16, to maintain


                                                                                                                                               6
the DC voltage, the front-end inverter varies its iq reference in a similar way to the input power
variation.
                                                                                                                                                          Variation of a power reference. Machine current, DC voltage evolution, power control signal and grid current.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          (-) : t(s)
                                  600.0                         800000.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          ref_potencia

                                  400.0
                                                                600000.0                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  (V)        : t(s)
                                  200.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          ia_med_maq
                   (V)




                                                   (-)



                                     0.0                        400000.0


                                -200.0
                                                                200000.0
                                -400.0


                                -600.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          (-) : t(s)
                                                                  3100.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          tens_dc



                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          ref_tens_dc
                                                                  3000.0
                                                          (-)




                                                                  2900.0




                                                                  2800.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          (A)        : t(s)
                                1000.0                             600.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          i(l.la_red)

                                                                   400.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          (-) : t(s)
                                                                   200.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          iqref
                                                          (A)
                   (-)




                                  500.0                              0.0


                                                                  -200.0


                                                                  -400.0


                                     0.0                          -600.0

                                                                           0.0                          0.02                            0.04                              0.06                  0.08                  0.1                 0.12                            0.14                                0.16                         0.18                                                                0.2                                               0.22                                                      0.24
                                                                                                                                                                                                                                                 t(s)




         Figure 15. Power reference, machine current, DC voltage evolution, control signal and network line
                                                      current.
                                                                                                                                                                                   Three phases current in the machine side inverter and the network side inverter.


                                  600.0



                                  400.0



                                  200.0
                   (V)




                                     0.0



                                -200.0



                                -400.0



                                -600.0

                                  600.0



                                  400.0



                                  200.0
                   (A)




                                     0.0



                                -200.0



                                -400.0



                                -600.0

                                                                     0.02                              0.04                                    0.06                               0.08                   0.1                       0.12                          0.14                                        0.16                                       0.18                                                                  0.2                                                            0.22                                                     0.24
                                                                                                                                                                                                                                          t(s)
             (V)      :    t(s)                 ia_med_maq                   ib_med_maq                                ic_med_maq



             (A)      :    t(s)                 i(l.la_red)                  i(l.lb_red)                               i(l.lc_red)




                                                         Figure 16. Three phases Line currents in the machine side and front-end inverters.

The machine side inverter line voltage and current are shown in Figure 17 together with the switching
pulses. The switching frequency is around 200 Hz and the Fourier analysis of this current is presented
in Figure 18. The machine neutral is not connected to the DC middle point. The switching frequency is
not constant and the current harmonic distortion (THD) is around 7%. Fourier analysis of this current
it is shown in Figure 18.
                                                                                        Line Current, line voltage and switches operation in the machine side inverter.                                                                                                                                                                                                                               Fourier Analysis of the machine line current.

                                M
                                                                                                                                                                                                                                                                 500.0
                          1a

                          4a
                                                                                                                                                                                                                                                                 450.0

                                                                                                                                                                                                                                                                                                                                                                                                                                             Fourier Analysis of the machine line current (Detail).


             600.0               3000.0
                                                                                                                                                                                                                                                                 400.0


                                                                                                                                                                                                                                                                                                                                               25.0


             400.0               2000.0                                                                                                                                                                                                                          350.0


                                                                                                                                                                                                                                                                                                                                               20.0



                                                                                                                                                                                                                                                                 300.0
             200.0               1000.0
                                                                                                                                                                                                                                                                                                                                               15.0




                                                                                                                                                                                                                                                                 250.0
                          (V)
   (-)




                0.0                 0.0                                                                                                                                                                                                                                                                                                        10.0




                                                                                                                                                                                                                                                                 200.0

                                                                                                                                                                                                                                                                                                                                                5.0

             -200.0             -1000.0

                                                                                                                                                                                                                                                                 150.0
                                                                                                                                                                                                                                                                                                                                                0.0

                                                                                                                                                                                                                                                                                                                                                      0.0    100.0       0.2k        0.3k      0.4k     0.5k    0.6k   0.7k    0.8k   0.9k      1.0k      1.1k      1.2k           1.3k   1.4k        1.5k   1.6k   1.7k     1.8k   1.9k    2.0k   2.1k   2.2k     2.3k   2.4k   2.5k
                                                                                                                                                                                                                                                                                                                                                                                                                                                                           f(Hz)
             -400.0             -2000.0
                                                                                                                                                                                                                                                                 100.0                                                                Mag(A)     : f(Hz)      i(r.r10)     i(r.r7)          i(r.r8)




                                                                                                                                                                                                                                                                  50.0
             -600.0

                                          0.0            0.02       0.04         0.06              0.08               0.1               0.12              0.14             0.16          0.18      0.2         0.22         0.24
                                                                                                                                               t(s)                                                                                                                0.0
(V) : t(s)            (va_maq-vb_maq)
                                                                                                                                                                                                                                                                         0.0    0.2k      0.4k        0.6k       0.8k   1.0k   1.2k   1.4k                  1.6k           1.8k                2.0k            2.2k      2.4k         2.6k             2.8k              3.0k              3.2k              3.4k          3.6k      3.8k          4.0k          4.2k       4.4k        4.6k   4.8k   5.0k   5.2k
                                                                                                                                                                                                                                                                                                                                                                                                                                      f(Hz)
(-) : t(s)            _n5374                                                                                                                                                                                                                            Mag(A)    : f(Hz)      i(r.r10)     i(r.r7)          i(r.r8)




                      Figure 17. Machine line voltage and current.                                                                                                                                                                                                             Figure 18. Line current Fourier analysis.

Figure 19 shows the current, the three level output line voltage and the switching pulses in the front-
end inverter. A constant switching frequency strategy is applied and a LC filter is able to reduce the
current THD from 10% to 4% when the switching frequency is 2.4 kHz. The output inductance is
0.9mH and the inverter is supplying an output current of 400 A. Figures 20 and 21 present the current


                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    7
shape and the harmonic distortion reduction that can be obtained with the output LC filter. A higher
THD reduction can be achieved by means of increasing the switching frequency. For example
operating at a frequency of 5 kHz (instead 2.4) a THD of 2% is achieved.
                                                                                                          Line Current, line voltage and switches operation in the network side inverter.                                                                                                                                                                         Fourier analysis of the line current before and after the LC filter in the network side inverter.

                                    M
                                                                                                                                                                                                                                                                                        600.0                                                                                                                                                                           (Detail) Fourier analysis of the line current before and after the LC filter in the network side inverter.




                 swt_1a

                 swt_2a                                                                                                                                                                                                                                                                 500.0
                                                                                                                                                                                                                                                                                                                                                                                                            20.0




                                                                                                                                                                                                                                                                                        400.0                                                                                                               15.0


         750.0

                                                                                                                                                                                                                                                                                        300.0                                                                                                               10.0




         500.0                      2000.0                                                                                                                                                                                                                                              200.0                                                                                                                5.0




                                                                                                                                                                                                                                                                                                                                                                                                             0.0
                                                                                                                                                                                                                                                                                        100.0                                                                                                                      0.0         0.5k      1.0k    1.5k     2.0k   2.5k     3.0k         3.5k         4.0k         4.5k          5.0k         5.5k         6.0k         6.5k           7.0k     7.5k   8.0k    8.5k   9.0k    9.5k   10.0k
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               f(Hz)
                                                                                                                                                                                                                                                                                                                                                                                                   Mag(A)     :    f(Hz)   i(l.la)


         250.0                      1000.0
                                                                                                                                                                                                                                                                                          0.0

                                                                                                                                                                                                                                                                                        600.0                                                                                                                                                                           (Detail) Fourier analysis of the line current before and after the LC filter in the network side inverter.


                                                                                                                                                                                                                                                                                                                                                                                                            25.0
   (A)




                          (V)




               0.0                      0.0
                                                                                                                                                                                                                                                                                        500.0
                                                                                                                                                                                                                                                                                                                                                                                                            20.0




                                                                                                                                                                                                                                                                                        400.0
                                                                                                                                                                                                                                                                                                                                                                                                            15.0


         -250.0                     -1000.0

                                                                                                                                                                                                                                                                                        300.0                                                                                                               10.0




                                                                                                                                                                                                                                                                                        200.0
         -500.0                     -2000.0                                                                                                                                                                                                                                                                                                                                                                  5.0




                                                                                                                                                                                                                                                                                        100.0                                                                                                                0.0

                                                                                                                                                                                                                                                                                                                                                                                                                   0.0         0.5k      1.0k    1.5k     2.0k   2.5k     3.0k         3.5k         4.0k         4.5k          5.0k         5.5k         6.0k         6.5k           7.0k     7.5k   8.0k    8.5k   9.0k    9.5k   10.0k
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               f(Hz)
                                                                                                                                                                                                                                                                                                                                                                                                   Mag(A)     :    f(Hz)   i(l.la_red)


         -750.0                     -3000.0
                                                                                                                                                                                                                                                                                          0.0
                                              0.12                    0.125          0.13            0.135                 0.14            0.145              0.15                  0.155          0.16           0.165          0.17            0.175
                                                                                                                                                                                                                                                                                                0.0         0.5k            1.0k   1.5k           2.0k   2.5k     3.0k        3.5k        4.0k              4.5k                         5.0k             5.5k             6.0k                        6.5k                           7.0k                      7.5k                        8.0k            8.5k           9.0k            9.5k       10.0k
                                                                                                                                                               t(s)
                                                                                                                                                                                                                                                                                                                                                                                                                                         f(Hz)
(V) : t(s)            (fase_a-fase_b)
                                                                                                                                                                                                                                                                               Mag(A)    : f(Hz)      i(l.la)



(A) : t(s)            i(l.la_red)
                                                                                                                                                                                                                                                                               Mag(A)    : f(Hz)      i(l.la_red)




                                         Figure 19. Line voltage, line current and                                                                                                                                                                                                        Figure 21. Line current Fourier analysis. DC
                                        switching pulses in the front-end inverter.                                                                                                                                                                                                          middle point connected to the neutral.
                                                                                                                                                                                              Line Current before and after the LC filter and switches operation in the network side inverter.

                                               M


               swt_1a

               swt_2a




                                                     400.0


                                                     200.0
                                  (A)




                                                            0.0


                                                 -200.0


                                                 -400.0


                                                 -600.0

                                                     600.0


                                                     400.0


                                                     200.0
                                  (A)




                                                            0.0


                                                 -200.0


                                                 -400.0


                                                 -600.0

                                                                     0.0                           0.02                                  0.04                                0.06                         0.08                           0.1                           0.12                             0.14                                  0.16                          0.18                                                         0.2                                                          0.22                                                                                  0.24
                                                                                                                                                                                                                                                                              t(s)
         (A)          :    t(s)                            i(l.la)




         (A)          :    t(s)                            i(l.la_red)




                                         Figure 20. Front-end inverter line current before and after the LC filter. Switching pulses.

When some reactive power is required by the network control system, the front-end inverter has to be
able to supply it. This can be achieved by means of varying the d-axes current component. Figure 22
shows the current evolution, before and after the LC filter, when the network requires some reactive
power.
In this figure it can be appreciated that the band change point is modified depending on the relative
position between the current and the voltage. Therefore as the reactive power varies, the current shape
is modified, and the THD is slightly increased (6%).
                                                                                                            Current and line voltage when a reactive power is required from the network.                                                                                                                                                                            Current before and after the LC filter and line voltage when a reactive power is required.
                                                                                                                                                                                                                                                         (A) : t(s)                                                                                                                                                                                                                                                                                                                                                                               (A) : t(s)
             1000.0                  400.0                                                                                                                                                                                                                                               1000.0
                                                                                                                                                                                                                                                         i(l.la)                                                                                                                                                                                                                                                                                                                                                                                  i(l.la)



                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  (V) : t(s)
              800.0                                                                                                                                                                                                                                      i(l.la_red)
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  fasea15kv
                                     300.0                                                                                                                                                                                                                                                 500.0                200.0
                                                             300.0

                                                                                                                                                                                                                                                         (V) : t(s)
              600.0
                                                                                                                                                                                                                                                         fasea15kv
                                                                                                                                                                                                                                                                                 (V)




                                                                                                                                                                                                                                                                                                      (A)




                                     200.0                                                                                                                                                                                                                                                      0.0                 0.0

                                                             250.0
              400.0                                                                                                                                                                                                                                      (-) : t(s)

                                                                                                                                                                                                                                                         idref

                                                                                                                                                                                                                                                                                          -500.0                -200.0
                                     100.0
              200.0
                                                             200.0



                                                                                                                                                                                                                                                                                         -1000.0                -400.0
                            (A)
   (V)




                                                     (-)




                0.0                     0.0                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       (A) : t(s)
                                                                                                                                                                                                                                                                                         1000.0                 400.0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  i(l.la_red)
                                                             150.0

             -200.0                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               (V) : t(s)
                                     -100.0
                                                                                                                                                                                                                                                                                           500.0                200.0                                                                                                                                                                                                                                                                                                                             fasea15kv


             -400.0                                          100.0


                                     -200.0
                                                                                                                                                                                                                                                                                 (V)




                                                                                                                                                                                                                                                                                                      (A)




                                                                                                                                                                                                                                                                                                0.0                 0.0
             -600.0

                                                              50.0

                                     -300.0
                                                                                                                                                                                                                                                                                          -500.0                -200.0
             -800.0



                                                               0.0
         -1000.0                     -400.0
                                                                                                                                                                                                                                                                                         -1000.0                -400.0

                                                                     0.0      0.01          0.02     0.03           0.04          0.05          0.06       0.07              0.08           0.09   0.1     0.11           0.12    0.13         0.14
                                                                                                                                                                                                                                                                                                                          0.1             0.105            0.11              0.115                    0.12                                       0.125                              0.13                                               0.135                                            0.14                        0.145
                                                                                                                                                                      t(s)
                                                                                                                                                                                                                                                                                                                                                                                                                                                   t(s)




 Figure 22. Front-end inverter current and network                                                                                                                                                                                                                                               Figure 23. Detail of the voltage and current
    voltage when a reactive power is required.                                                                                                                                                                                                                                                  before and after the LC filter when a reactive




                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  8
Due to the delay produced by the output LC filter, the desired current phase is not exactly obtained.
This delay has to be compensated by the reactive power control system. This is observed in Figure 23.

The machine side inverter control has to be able to maintain the generator voltage below the maximum
value for the converter operation. Figure 24 shows how the id current component (pink line in the
middle graph) has to be increased in order to reduce the output machine voltage (pink line in the upper
graph). The output machine currents are shown in the lower graph, without voltage control (blue
curve) and with this control actuating (pink curve).
                                                                         Current and line voltage when a reactive power is required from the network.
                                                                                                                                                                                                                       (-) : t(s)
                                1100.0
                                                                                                                                                                                                                       u



                                1000.0
                                                                                                                                                                                                                       usmax
                          (-)




                                 900.0

                                                                                                                                                                                                                       u

                                 800.0




                                                                                                                                                                                                                       (-) : t(s)

                                                                                                                                                                                                                       ref_isd_maq


                                 300.0

                                                                                                                                                                                                                       -ref_isq_maq
                                 200.0
                          (-)




                                 100.0
                                                                                                                                                                                                                       ref_isd_maq


                                   0.0


                                -100.0
                                                                                                                                                                                                                       (V) : t(s)
           1000.0                400.0
                                                                                                                                                                                                                       ia_med_maq



                                 200.0
                                                                                                                                                                                                                       ia_med_maq
                          (V)
   (-)




               0.0                 0.0

                                                                                                                                                                                                                       (-) : t(s)

                                -200.0                                                                                                                                                                                 ea_estim



          -1000.0               -400.0

                                         0.0   0.02   0.04      0.06            0.08             0.1                 0.12          0.14           0.16          0.18         0.2          0.22          0.24
                                                                                                                            t(s)




                                                                Figure 24. Machine output voltage control.
                                                                                               Machine Line Voltage Control.
                                                                                                                                                                                                               (-) : t(s)
          3080.0
                                                                                                                                                                                                               tens_dc


          3060.0
                                                                                                                                                                                                               tens_dc


          3040.0
    (-)




          3020.0


          3000.0


          2980.0
                                                                                                                                                                                                               (V)   : t(s)

                                                                                                                                                                                                               (va_maq-vb_maq)
          3000.0

          2000.0

          1000.0
  (V)




              0.0

          -1000.0

          -2000.0

          -3000.0

                                                                                                                                                                                                               (V)   : t(s)

                                                                                                                                                                                                               (va_maq-vb_maq)
          3000.0

          2000.0

          1000.0
  (V)




              0.0

          -1000.0

          -2000.0

          -3000.0



                    0.0              0.02      0.04    0.06       0.08             0.1                 0.12                 0.14          0.16           0.18          0.2         0.22          0.24
                                                                                                              t(s)




                     Figure 25. DC voltage and machine line voltage when machine voltage control is applied and
                                                       not applied.

Figure 25 shows how the inverter operation can be perturbed if the voltage limit control system does
not operate. In the upper graph the DC voltage with (blue) and without (pink) is shown. The middle
graph shows the inverter line voltage (blue) when the DC voltage is maintained below its limit.
Finally, the lower graphic shows (pink) the inverter line voltage which is seriously distorted when the
DC voltage surpasses its upper limit.

                                                              VI. EXPERIMENTAL PROTOTYPE
A Three-Level Inverter prototype is being developed and constructed now at CEDEX Labs. An
optimal IGBTs modules distribution will be achieved in order to reduce the stray inductances. This is a
critical aspect in the operation of this inverter topology because there is not a commercial module for a
three-level inverter.




                                                                                                                                                                                                                                      9
A control model is being developed with MATLAB and directly compiled and downloaded to a
DS1103 dSPACE board which will generate the firing IGBT pulses for the Double Three-Level
Inverter prototype. A 7,5kW, 750rpm permanent magnet synchronous generator will be driven by a
three level inverter controlled by means of the double hysteresis band current strategy.

                                         VII. CONCLUSIONS
Simulation results with Saber® of a three level inverter with a double hysteresis-band current strategy
are presented in this paper. The strategy has been validated in the following respects:
• A reduced harmonic distortion and a small switching frequency have been achieved, both in the
  network and in the machine side converters. For the same harmonic distortion, the switching
  frequency can be divided by four, compared to use of a two-level converter.
• An easy way to control the DC middle point voltage, based on the double hysteresis-band strategy,
  has been proposed and tested.
• Developed simulations have shown that when DC middle point is connected to the neutral, a better
  currents control and for the DC middle point voltage is achieved.
• This control strategy can be easily implemented in a DSP with a brief program.
• A voltage phase estimator has been developed . Based on its estimation the inverter regulates the
  reactive power supplied to the grid.
• A machine voltage control is applied to the generator in order to reduce the output voltage. This
  feature permits an inverter power rating reduction.

                                             REFERENCES
[1] C. Bacigalupo et al. “Modulation Techniques Applied to High Power 3 Level GTO’s Converter for
    Asynchronous Motor Drives”. Proceedings of the 7th European Conference on Power Applications EPE’97.
    Trondheim. Norway. 1997. Pp 1.158-1.164.
[2] B.K. Bose. “An Adaptive Hysteresis-Band Current Control Technique of a Voltage-Fed PWM Inverter for
    Machine Drive System”.IEEE Trans. On Industrial Electronics, vol 37, nº 5. Pp 402-408.1990
[3] M.K. Buschmann and J.K. Steinke. “Robust and Reliable Medium Voltage PWM Inverter with Motor
    Friendly Output” Proceedings of the 7th European Conference on Power Applications EPE’97. Trondheim.
    Norway. 1997. pp3.502-3.507.
[4] Carrara, G, Gardella,S, Marchesoni,M, Salutari,R and Sciutto,G. “A New Multilevel PWM Method: A
    Theoretical Analysis”. IEEE Trans. on Power Electronics, Vol 7, No 3, July 1992. pp 497-505.
[5] A. Heras. “Development and simulation of a PWM-SVM hybrid strategy for Three-level power inverter”.
    Student proyect. 2001.
[6] M. Lafoz, I.J. Iglesias, “ A Novel Double Hysteresis-Band Current Control for a Three-Level Voltage Source
    Inverter”, Proceding of the 31st Annual Power Electronics Specialist Conference, Vol. 1, pp 21.
[7] Ph. Lataire. “White paper on the new ABB medium voltage drive system, using IGCT power semiconductors
    and Direct Torque Control”. EPE Journal. Vol 7. No3/4. December 1998. Pp 40-45
[8] M. Marchesoni. “High-Performance Current Control Techniques for Applications to Multilevel High-Power
    Voltage Source Inverters”. IEEE Trans. on Power Electronics. Vol 7, No 1, January 1992.
[9] A. Mertens, M. Bruckmann, R. Sommer. “ Medium Voltage Inverter using High-Voltage IGBTs”
    Proceedings of the 8th European Conference on Power Applications EPE’99. Lausanne, Switzerland. 1999
[10] J.K. Steinke. “Switching Frequency Optimal PWM Control of a Three-Level Inverter”. Proceedings of the
    3rd European Conference on Power Applications EPE’89. Aachen, Germany.1989. pp. 1267-1272.
[11] A.M. Walczyna and R.J. Hill. “Space Vector PWM Strategy for 3-Level Inverters with Direct Self-
    Control”. Proceedings of the 5th European Conference on Power Applications EPE’93. Brighton. U.K. 1993.
    pp 152-157.




                                                                                                           10