Download A method to increase the reliability in a 5

Power Circuit of the inverter: Fig. 1 represents the power circuit of
a 5-level inverter with two cells in each phase, used for medium
voltage drives. Each cell, shown in Fig. 2, has a three phase
secondary winding at the input side. The output part of the cell has
a single-phase inverter with Insulated Gate Bipolar Transistors
(IGBTs). The output voltage of the cell has three values: +VDC, 0
and –VDC. A more detailed description of this topology can be
found in [1] and [3]. A contactor is added at the output of each cell,
compared to the original solution.
N
Another strategy to correct the unbalance is to change the phase of
voltages VA, VB and VC, generated by the inverter, as observed in
Fig. 3b. This is achieved by giving to the reference voltages of
phases B and C the angles 135.52º and 224.47º, instead of 120º and
240º, respectively. This solution generates a higher load voltage.
VA
VA
VCA
12
0º
VCA
VAB
VAB
88.96º
120º
VC
º
52
5.
13
Introduction: Multilevel inverters have shown an important
development on the last years, due to their capability of increasing
the voltage and power delivered to the motor with semiconductors
which are available today [1]. Due to this high power, these
multilevel inverters have a big impact on production, generating
important losses when they fail. For this reason, the investigation
of methods to increase the reliability of these inverters is of high
interest [2]. This paper presents a method to increase the reliability
of a 5-level cascaded multicell inverter [3], introducing a reduced
amount of additional hardware and changing the reference voltages
of the modulator.
A simple method to generate balanced voltages is to bypass a cell
in each of the other two phases (B and C). However, this method
produces a reduction of 50% in the load voltage and does not use
all available cells.
13
5.5
2º
This paper presents a method to operate a cascaded 5level inverter with faulty cells. The addition of a
contactor and the displacement in the reference voltages
allows for a balanced operation, increasing the reliability
of the converter.
0º
J. Rodríguez , P. Hammond, J. Pontt and R. Musalem
Operation with one faulty cell: A damaged cell, for example A1 in
Fig. 1, is bypassed by changing the position of the bypass
contactor. The cell generates in this condition the output Vcell = 0.
The faulty phase (A) will generate only 50% of the voltage
generated by any of the other two phases. This will generate an
important voltage unbalance in the load, as shown in Fig. 3a, which
is unacceptable for a rotating electrical machine.
12
A method to increase the reliability in a 5-level
inverter
VB VC
VBC
VB
VBC
b)
a)
Fig. 3. Voltages generated with one faulty cell: a) unbalanced;
b) balanced.
Results: Fig. 4 represents the balanced operation of the inverter in
normal conditions, with two cells in each phase working properly.
2000
[v]
CELL
A1
Vcell
CELL
A2
CELL
B1
VB
CELL
B2
A
CELL
C1
a)
-2000
0
CELL
C2
B
VAB
a)
0
VC
2000
[v]
0
C
iB
0.005
0.01
0.015
0.02
0.025
0.03
0.035
b)
b)
-2000
0
iA
iC
2000
[v]
0
MEDIUM
VOLTAGE
INDUCTION
MOTOR
0.005
0.01
0.015
0.02
0.025
0.03
0.035
c)
c)
0
10
[A]
0.005
0.01
0.015
0.02
0.025
0.03
0.035
BYPASS
CONTACTOR
0.04
d)
d)
0
FROM DEDICATED
SECONDARY OF INPUT
TRANSFORMER
0.04
-2000
Fig. 1. Power circuit of the inverter
-10
COIL
0
Vcell
+
0.04
VDC
OUTPUT OF POWER
MODULE
Fig. 2. Power circuit of each cell.
Control of the inverter: Each cell operates with sinusoidal unipolar
pulse width modulation, comparing a sinusoidal control voltage
with a triangular carrier signal. The carrier has a frequency of 600
[Hz]. The carriers of the two cells in series connection are phase
shifted to reduce the distortion of the load voltage, producing an
additional equivalent switching frequency of 1200 [Hz].
0.005
0.01
0.015
0.02
Time [s]
0.025
0.03
0.035
0.04
Fig. 4. Balanced operation of a 5-level cascaded multicell inverter
(two cells per phase): a), b), c) line voltages in Volts [v]; d) load
currents (iA, iB, iC) in Amperes [A].
Fig. 5 shows the unbalanced operation with one cell bypassed in
phase A, maintaining the shifting of 120º between the reference
voltages. This situation generates a high unbalance in the load
voltages and currents.
Finally, Fig. 6 shows that the proposed method generates balanced
operation with one faulty cell. The reference voltages have the
phase-shifting of Fig. 3b. This solution produces a current 50%
higher than the simplest alternative of balanced operation with one
cell per phase.
1
a)
0
-1
2000
[v]
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
b)
-2000
2000
[v] 0
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
c)
-2000
2000
[v]
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
d)
-2000
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
10
[A]
e)
0
-10
0
0.005
0.01
0.015
0.02
Time [s]
0.025
0.03
0.035
0.04
Fig. 5. Unbalanced operation with one faulty cell: a) reference
voltages; b), c), d) line voltages in Volts [v]; e) load currents (iA,
iB, iC) in Amperes [A].
1
a)
0
-1
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
2000
[v]
0
0.04
b)
-2000
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
2000
[v]
0
0.04
c)
-2000
2000
[v]
0
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
d)
-2000
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
10
[A]
e)
0
-10
0
0.005
0.01
0.015
0.02
Time [s]
0.025
0.03
0.035
0.04
Fig. 6. Balanced operation with one faulty cell: a) reference
voltages; b), c), d) line voltages in Volts [v]; e) load currents (iA,
iB, iC) in Amperes [A].
Conclusions: The addition of a simple contactor at the output of
each cell will produce a significant increase in the reliability of the
inverter, permitting balanced operation with one faulty cell. The
solution is not expensive and it is easy to implement.
Acknowledgments: The authors acknowledge the support received
from the Chilean Research Fund CONICYT under grant 1030368
and from the University Federico Santa María.
References:
1 J. Rodriguez, J. S. Lai, F. Z. Peng, “Multilevel Inverters: A
Survey of Topologies, Controls and Applications”, IEEE
Transactions on Industrial Electronics, Vol. 49, No. 5,
October 2002, pp. 724-738.
2
C. Turpin, P. Baudesson, F. Richardieu, F. Forest, T.
Meynard, “Fault Management of Multicell Converters”, IEEE
Transactions on Industrial Electronics, Vol. 49, No. 5,
October 2002, pp. 988-997.
3
P. W. Hammond, “A new approach to enhance power quality
for medium voltage drives”, IEEE Transactions on Industry
Applications, Vol. 33, No. 1, January/February 1997, pp. 202208.