Comparison of Three Phase Current Source Inverters and Voltage Source
Inverters Linked with DC to DC Boost Converters for Fuel Cell Generation
Systems
Malte Mohr1, Friedrich W. Fuchs2
Christian-Albrechts-University of Kiel
Kaiserstr. 2, 24143 Kiel, Germany
Tel.: +49 – 431 8806104 / Fax: +49 – 431 8806103
1
mam@tf.uni-kiel.de 2fwf@tf.uni-kiel.de
Keywords
Fuel cells system, Renewable energy systems, Current Source Inverter (CSI), Voltage Source Inverters
(VSI), Distributed power, Design
Abstract
Three phase current source inverters (CSI) and voltage source inverters linked with a dc to dc boost
converter (VSI+BC) are appropriate solutions to convert electrical energy for distributed fuel cell
generation systems. The performance of CSI’s and VSI+BC’s in this application is shown and both
topologies are compared to each other concerning their power semiconductor design rating and their
semiconductor power losses.
I.
Introduction
Fuel cells can be an important component of future energy systems, enabling electricity and heat
generation from hydrogen or similar substances with zero or nearly zero emission. Their introduction is in
the stage of beginning as their development has not yet been finished.
Fuel cells provide a variable dc current at variable fuel cell voltage. They have to be connected to the ac
mains by means of inverters. Because of the comparatively low voltage of a fuel cell stack for low and
medium power, the fuel cell voltage is usually lower than the line voltage, the inverter has to increase the
voltage when feeding into the mains [1].
Apart from the requirements of dc current control capability, adaption of the voltage level and operation
with high efficiency, the inverter has to draw a well smoothed dc current from the fuel cell for a high
lifetime of the cell [2]. In addition, it has to deliver low current harmonics to the mains with respect to the
standards. Moreover, the inverter ideally provides a unity power factor in the mains, however, a variation
of the power factor cos(φ) from 0.95 to 1 for example is desirable.
The usual solution is a voltage source inverter in series with a dc/dc converter [3] [4]. The dc link voltage
of the voltage source inverter has to be greater than the maximum value of the phase to phase voltage of
the line. Up to a ratio between fuel cell voltage Vfc and the dc link voltage Vlink of Vlink/Vfc = 3 the boost
converter is an appropriate solution to achieve these requirements [4].
The current source inverter promises to be another interesting solution [5] [6]. The main advantage of the
current source inverter is that it increases the voltage towards the mains itself, so an additional dc/dc
converter may not be necessary. This results in less complexity of the system and its control. Apart from
[6], investigations of the current source inverter connected to fuel cells have not been found in the
literature.
In this paper, both converter systems, applied to a fuel cell and feeding into the mains, are compared by
analytical analysis concerning their semiconductor design rating and their semiconductor power losses. In
chapter II, the basic performance of the fuel cell and of both inverter systems is introduced. In chapter III,
the power losses of the power semiconductors are shown. Chapter IV shows the power semiconductor
design rating and its results. Chapter V shows the results of the power loss calculation. The two chapters
last mentioned include a comparison of the two systems. Chapter VI provides a short prospect of CSIs
with reverse blocking IGBT and chapter VII presents the conclusion.
II. Basic system performance
A Fuel cell characteristics
Figure 1 shows an example of the characteristic curve, fuel cell voltage Vfc vs. current density J, of a
single fuel cell [1]. Recommended operation of the cell is in the ohmic region (about a current density of
0.2 to 1 A/cm2 in the curve below). On the one hand operation in the concentration region (declining
section at about a current density of 1.1 to 1.2 A/cm2 in the characteristic curve) of the fuel cell yields to a
bad efficiency of the fuel cell and on the other hand it may damage the fuel cell and has to be avoided [2].
Vfc [V]
operating point
at no load
operating point at
nominal load
Fig. 1: Example of a characteristic curve of a single fuel cell
The fuel cell voltage typically specified in fig. 1 refers to one single fuel cell with typical current density J.
There are characteristic curves that show other or lower voltage at nominal load. The current, delivered by
the fuel cell, depends on its cell area, higher system voltages can be obtained by connecting several fuel
cells in series (fuel cell stacks).
B Voltage source inverter linked with boost converter
If a voltage source inverter is used for mains feed in, the dc link voltage has to be greater than the rectified
line to line voltage [7]. So the low level fuel cell dc voltage has to be increased towards the dc link
voltage. This can be achieved by using an additional boost converter, the whole system is shown in fig. 2.
The dc link capacitor decouples the voltage source inverter and the boost converter and keeps the dc link
voltage ripple to an adequate level. A feasible power flow control method could be to keep the dc link
voltage constant via the boost converter.
C Current source inverter
The current source inverter, whose topology is shown in fig. 3, increases the voltage towards the mains by
itself, so the voltage of the fuel cell must be lower than the lowest rectified line to line voltage [5] if the
fuel cell is directly connected to the CSI. At the CSI, similar to the VSI+BC system, the dc link inductor
Ld at the fuel cell side yields to an appropriate dc current ripple. The switches of the current source
inverter have to be reverse blocking. If IGBTs are used for the current source inverter, the reverse
blocking capability can at present only be achieved with diodes connected in series to the IGBTs. This
yields to relatively high semiconductor conduction losses. Another interesting semiconductor is the
reverse blocking IGBT (RBIGBT). The development of RBIGBT is in progress [8] so it can be supposed
that they will be available in the foreseeable future also for the CSI.
Fig. 2: Circuit diagram of a voltage source
inverter linked with a boost converter for a fuel
cell generation system.
Fig. 3: Circuit diagram of a current source inverter
for a fuel cell generation system.
III. Power Semiconductor power losses
A Semiconductor conduction and switching losses
Analytical derivation of semiconductor losses for the current source inverter (CSI) and for the voltage
source inverter has been carried out in [9]. In the following the forward characteristics of the
semiconductors have been linearised [9]. So forward or conduction losses PC of the semiconductors have
been calculated using (1). VCE0 and VF0 constitute the IGBT’s and diode’s threshold voltages, rCE and rF
constitute their differential ohmic resistance.
PCIGBT = VCE , 0 ⋅ iV + rCE ⋅ iV2 ; PCDiode = V F ,0 ⋅ iV + rF ⋅ iV2
(1)
The switching losses depend on switching voltage and current. The switching loss energies ES of the
IGBTs (EON, I, EOFF, I) and of the diodes (Erec) have been linearised according to (2). Here ESR is the rated
switching loss energy given from the datasheet for a reference commutation voltage Vref and a reference
current Iref while VV and IV are the actual commutation voltage and current of the valve [9].
V
I
E S = E SR ⋅ V ⋅ V
(2)
Vref I ref
The proposed modulation schemes used here are standard ones as Half Wave Symmetrical Modulation
(HSM) for the CSI [10] and so called Suboptimal Space Vector Pulse Width Modulation (SVPWM) for
the VSI [9]. Sinusoidal line current and line voltages are assumed as well as a power factor of cos(φ) = 1.
In the following Vline is the line to line voltage of the line and iL is the line current. Vfc characterises the
fuel cell or input dc voltage of the respective inverter, Id1 is the input dc current of the inverter and Vdc
denotes the dc link voltage of the voltage source inverter.
Equations (3) and (4) show the conducting losses for the current source inverter for one IGBT and one
diode, equations (5) and (6) show their switching losses [9].
2
2
PCCSI
⋅ I d 1 ⋅ (VCE ,0 ) + I d 1 ⋅ (rCE )
(3)
, IGBT =
6
2
2
PCCSI
⋅ I d 1 ⋅ (V F , 0 ) + I d 1 ⋅ (rF )
(4)
, Diode =
6
I
Vˆ
3
PSCSI
⋅ (EON , I + EOFF , I ) ⋅ d 1 ⋅ line
(5)
, IGBT = f S ⋅
6π
I ref Vref
[
[
]
]
I
Vˆ
3
⋅ E rec ⋅ d 1 ⋅ line
(6)
I ref Vref
6π
Equations (7) and (8) show the conducting losses, (9) and (10) the switching losses of the voltage source
inverter (VSI) for each semiconductor as derived in [9]. M is the modulation index of the voltage source
inverter; it describes the ratio between line to line voltage Vline and dc link voltage Vdc.
2
VCE , 0 ⋅ iˆL  M ⋅ π
2

 rCE ⋅ iˆL  π
PCVSI
=
⋅
1
+
⋅
cos(
ϕ
)
+
(7)
⋅  + M  ⋅ cos(ϕ )  


, IGBT
2π
4
2π


3

4
2
V F ,0 ⋅ iˆL 
M ⋅π
2

 rF ⋅ iˆL  π
VSI
⋅ 1 −
⋅ cos(ϕ )  +
⋅  − M  ⋅ cos(ϕ )  
(8)
PC , Diode =
2π
4
2π
3



4
V
iˆ
1
PSVSI
⋅ f S ⋅ (EON , I + EOFF , I ) ⋅ dc ⋅ L
(9)
, IGBT =
π
Vref iref
PSCSI
, Diode = f S ⋅
PSVSI
, Diode =
1
π
⋅ f S ⋅ (E rec ) ⋅
Vdc iˆL
⋅
Vref iref
(10)
Conducting losses for the boost converter (BC) can be easily derived as follows: depending on the
modulation index of the boost converter (neglecting voltage sags due to losses)
V fc
T
a = ON = 1 −
(11)
TS
Vdc
the IGBT V1 (see also fig. 2) is turned on for the time TON. It carries the input current Id1. This yields to its
conduction losses
PCBC
(12)
, IGBT = I d 1 (VCE 0 + rCE ⋅ I d 1 ) ⋅ a
The Diode V2 is conducting during TON-TS or (1-a). It carries also Id1 while it is turned on, so the diode’s
conducting losses are
PCBC
, Diode = I d 1 (V F 0 + rF ⋅ I d 1 ) ⋅ (1 − a ) ;
note: I d 1 =
Id 2
1− a
(13)
The commutaion voltage of the IGBT and the Diode is the dc link voltage Vdc. During their conduction
time, the semiconductors carry the current Id1 so the switching losses of IGBT and diode in the boost
converter are as described in (14) and (15).
Vdc I d 1
PSBC
⋅
(14)
, IGBT = f S ⋅ ( EON + EOFF )
Vref iref
PSBC
, Diode = E rec ⋅ f S ⋅
Vdc I d 1
⋅
Vref iref
(15)
Assuming that the inverter supplies constant power to the line at constant line voltage, it can be seen from
(7) - (10) that VSI’s semiconductor losses for constant dc link voltage, equal to a constant modulation
index, are constant. Losses for BC (12) - (15) and CSI (3) - (6) depend on their input dc voltage due to the
fact that dc current increases at constant power at decreasing dc voltage. Thus the power losses and
similarly the installed power of BC and CSI is increasing for decreasing voltages at a given constant
power.
IV. Power Semiconductor rating
A Definition of installed semiconductor power
Semiconductor costs for similar voltages and similar loss performance of the semiconductor are nearly
linear to their current carrying capability. To compare CSI and VSI+BC to each other concerning their
required semiconductor power, an installed semiconductor power, here also called rated semiconductor
power has been established. It is defined as
PSC , IGBT = VCES ⋅ I C
(16)
for the IGBTs with maximum dc collector current IC and
PSC , Diode = VCES ⋅ I F
(17)
for the diodes with maximum dc forward current IF and maximum blocking voltages VCES. Total rated
semiconductor power for one inverter is here defined as the sum of the separate rated powers for all
required semiconductors.
B Interpolation of semiconductor data
For an optimal dimensioning of the inverters, semiconductors should be diversified into fine steps.
Because there are only semiconductors of one type with coarse graded current carrying capability
available, the values needed for optimal dimensioning (EON, EOFF, VCE,0, rCE, Erec, VF,0, rF, Rthjc) have been
interpolated between available semiconductor data given from [11] which are shown in table I.
Table I: Semiconductor data for eupec BSM GB120 DLC low loss IGBT modules [11]
Nominal collector
current IC, nom
EON [mJ]
EOFF [mJ]
VCE, 0 [V]
rCE [mΩ]
Rthjc, IGBT [K/W]
Erec [mJ]
VF,0 [V]
rF [mΩ]
Rthjc, Diode [K/W]
35 A
50 A
75 A
100 A
200 A
4.5
4.3
1.25
33
0.4
2.8
1
20
0.7
6.4
6.2
1.25
23
0.27
4
1
14
0.6
7.5
9
1.25
15
0.18
6.2
1
9.6
0.5
10
12
1.25
11.5
0.16
9
1
7
0.3
22
23
1.25
6
0.08
14
1
3.5
0.18
C Approach
The dimensioning of the semiconductors was carried out in such a manner that the required semiconductor
power has been chosen as small as possible for the intended voltage and power range of the inverter. The
analysis has been carried out under the precondition of same power semiconductors in the voltage source
inverter with boost converter as well as in the current source inverter, including series diodes, to achieve a
good comparability.
Limiting factor for the semiconductors is the maximum permitted junction temperature. So the
semiconductor’s dimensioning was carried out by calculating the semiconductor’s junction temperature
[12] based on analytical calculation of semiconductor losses as shown in the next section using datasheet
values from Eupec®-modules, see table I. IGBTs and diodes for the different types of converters have been
dimensioned in that way that their junction temperature achieves 397 K (125°C) in the operation point in
which maximum power losses occur. Then so the total rated semiconductor power (respectively costs) is
as low as possible. As a result, also partial numbers of power semiconductors are possible. Each
semiconductor has been dimensioned separately, due to its own power losses: 6 IGBTs and 6 antiparallel
diodes for the VSI, 1 IGBT and 1 diode for the boost converter, 6 IGBTs and 6 serial diodes for the CSI as
shown in the circuits in fig. 2 and fig. 3.
D Operation conditions
In the following some requirements have been taken into account for the dimensioning process which will
be developed here. The defined operation conditions are shown in table II.
Table II: Defined operation conditions
Fuel cell
Vfc / J see fig. 1
Nominal inverter power
Pinv = 27 kW
Mains line to line voltage
Vline = 400 V +10% / - 15%
Mains power factor
cos (ϕ) = 1
Due to the line voltage of Vline = 400 V the blocking voltage of the power semiconductors has been chosen
to VRSM = VCES = 1200 V for all semiconductors. This is not an optimal rating for both systems of the
semiconductor’s blocking voltage. It has been done due to available semiconductors on the market.
(Semiconductor’s commutation voltage is different for both inverter topologies: at VSI and BC it is the dc
link voltage Vdc, CSI’s commutation voltage is the peak value of the line to line voltage Vline.) A constant
power module case temperature of 363 K (90°C) was supposed, so only the thermal resistance Rthjc
between junction and case has been taken into consideration.
Some further constraints concerning operation at the mains have to be taken into consideration: according
to standards, the fuel cell inverter system should operate at a mains voltage range of +10% / -15% from
nominal line voltage Vline. Moreover the aim is to keep the fuel cell at its operating point (constant current,
constant voltage) during mains voltage fluctuations while the inverter system is operating. The chosen
control method for the system VSI+BC is to keep the dc link voltage constant, so the VSI has to
compensate the mains voltage fluctuations while the BC adapts the fuel cell voltage fluctuations.
For the VSI, the power semiconductor losses at constant power have their maximum at lowest mains
voltage due to larger mains currents. Therefore, installed semiconductor power has to be estimated for
maximum power at the lowest mains voltage. The dc link voltage has to be greater than rectified
maximum line to line voltage Vˆline = 1.1 ⋅ 2 ⋅ 400 V [7]. Setting the dc link voltage to a maximum
allowable value of VdcVSI,max = 750 V in respect of maximum capacitor voltages and semiconductor blocking
voltages, there is on the one hand an sufficient overshoot capability for inverter control and on the other
hand the input voltage range in combination with a boost converter is as wide as possible. This maximum
voltage should not be exceeded to avoid damage of semiconductors or dc link capacitors due to
overvoltages for example.
The CSI doesn’t have a second decoupling energy storage like the dc link capacitor at the VSI+BC
system. The mains voltage fluctuations as well as the fuel cell fluctuations will therefore be compensated
by the CSI, adjusting the modulation index. For the CSI, the power semiconductor losses at constant
power have their maximum at lowest dc input or fuel cell voltage due to a larger input current. Therefore,
installed semiconductor power depends on input dc voltage and has to be estimated for maximum power
at the lowest impressed fuel cell voltage. For the current source inverter, there is a maximum allowed
input dc voltage it can work with. Assuming sinusoidal line currents, this voltage value is about
3 ~
VdcCSI
⋅ Vline
(18)
, max =
2
and depends among other things on the mains power factor and the CSI filter design [5]. Considering the
voltage fluctuation and taking into account that there should be a supposed overshoot capability of about
5% of the line to line voltage to control the dc current at lowest mains voltage, the maximum dc input
voltage for the CSI is
3
VdcCSI
⋅ 0.85 ⋅ 095 ⋅ 400 V = 396 V
(19)
, max =
2
For an appropriate choice of filter elements, as shown in [5], the effect of filter components can hereby be
neglected.
Regarding the characteristic curve of a fuel cell (fig. 1), it becomes obvious that the input dc voltage range
of an fuel cell inverter has to be from about 60 to 100 percent of the fuel cell no load voltage, assuming
operation over the entire power range. This results in an optimal fuel cell voltage from Vfc = 450...750 V
for the VSI+BC and from Vfc = 238...396 V for the CSI (fuel cell voltage at nominal load and fuel cell
voltage at no load).
E Installed / rated semiconductor power
The standardised rated semiconductor power PSC, total / PINV has been calculated according to the concept as
shown in the previous section. Figure 4 illustrates the results. It shows the total rated semiconductor power
of VSI, BC, CSI and VSI+BC which are rated for different minimum dc input voltages Vfc, min at constant
power PINV = 27 kW based on table III.
CSI (red dashed line)
VSI+BC (black solid line)
VSI (green chain dotted line)
BC (blue dotted line)
optimal rating for CSI
(entire fuel cell load range)
optimal rating for VSI
(entire fuel cell load range)
238
450
minimum inverter dc input voltage VV
dcfc,
min
Fig. 4: Rated semiconductor power PSC, total /PINV of BC+VSI and CSI. Operating conditions see table II.
The VSI+BC can be operated in the range below its highest dc voltage of Vdc, max = 750 V. Its rated
semiconductor power is low and rises moderately with lower minimum inverter dc input voltage. The
constant VSI semiconductor power is a result of its constant dc link voltage. The BC’s semiconductor
power rises with lower minimum dc input voltage according to the voltage boost function and higher
semiconductor current (see eq. (13)); Id1 gets higher while Id2 is constant due to constant power PINV and
constant dc link voltage Vdc.
The CSI can be operated at dc voltages Vfc lower than 396 V (at Vline = 0.85 · 400 V). With high “minimum dc inverter voltage“, it shows low rated semiconductor power. When reducing the minimum dc input
voltage of the CSI, the semiconductor power which has to be installed, rises significantly. This is due to
the fact that lower voltages cause higher currents at constant power and in addition the boost function of
the CSI, equal to input short circuit, is done with four power semiconductors in series (see fig. 3).
For fuel cell application the “minimum dc input voltage“ can be taken as the fuel cell voltage at its
operating point at nominal load as shown in fig. 1 (high current at low voltage). Assuming the voltage
range shown in chapter IV, section D, there are optimal ratings for the two inverters for operation over the
entire load range. The CSI then has to be rated for a fuel cell nominal load input voltage of 238 V (max.
voltage 396 V), the VSI+BC has its fuel cell nominal load voltage at 450 V (max. voltage 750 V). The
resulting optimal rating of both inverters for the entire voltage range of the fuel cell is shown in fig. 4
showing small to high advantages for the VSI+BC. For a given fuel cell stack the inverter’s input voltage
is given by the nominal voltage of the stack. For an nominal voltage of Vfc, min = 200 V for example, the
rated semiconductor power of the CSI is about 1.6 times larger than the VSI+BC’s rated semiconductor
power. Limiting the voltage range for the current source inverter yields to less rated semiconductor power
which is shown in [4]. The CSI needs lower total rated semiconductor power for a narrow input voltage
range at input dc voltages near to its maximum input voltage value of VDC, max = 396 V. Extending the
voltage range or applying fuel cells with lower voltage leads to an even higher dimensioning of the CSI
compared to the VSI+BC.
Table III gives the detailed analysis of the rated semiconductor power. It can be seen that for the CSI the
content of diode‘s rated semiconductor power is only a little greater than IGBT’s rated semiconductor
power. It has to be mentioned, that the semiconductor rating for the diodes especially in the CSI is
affected due to the comparative high thermal resistances Rthjc, Diode of the diodes. For the VSI main
contribution comes from the IGBT (9.4 compared to 4 for the diode).
Table III: Standardised rated semiconductor power for BC+VSI and CSI components (operation
conditions see tab. II)
Inverter
Type:
lowest
input
voltage
100 V
200 V
300 V
400 V
500 V
600 V
700 V
BC
IGBT diode
VSI
IGBT diode
IGBT
BC+VSI
diode
overall
BC
PSC
, Diode
VSI
PSC
, IGBT
VSI
PSC
, Diode
Pinv
Pinv
Pinv
Pinv
Pinv
14.5
7.8
5.1
3.1
2.2
1.6
1.3
8.9
5.6
4.5
4
3.6
3.3
3.3
9.4
9.4
9.4
9.4
9.4
9.4
9.4
4
4
4
4
4
4
4
23.9
17.2
14.5
12.5
11.6
11.0
10.7
CSI
overall
BC +VSI
PSC
,total
CSI
PSC
, IGBT
CSI
PSC
, Diode
Pinv
Pinv
Pinv
Pinv
CSI
PSC
, Diode
Pinv
12.9
9.6
8.5
8.0
7.6
7.3
7.3
36.8
26.8
23.0
20.5
19.2
18.3
18.0
40
18.7
13.4
9.4
-
53.4
24.0
16
10.7
-
93.4
42.7
29.4
20.1
-
BC +VSI
BC +VSI
PSC
PSC
, Diode
, IGBT
BC
PSC
, IGBT
CSI
IGBT
diode
From table III and fig. 4 a fact becomes obvious: the VSI+BC needs remarkably less total rated
semiconductor power in fuel cell application (about 1.8 times less than in the example shown at optimal
rating for the inverter) and is therefore the favourable system. For fuel cells with lower nominal voltage
than shown in fig. 1 this effect increases. The reason is obvious: the step up or boost function to increase
the low fuel cell voltage to the mains voltage level is done in the VSI+BC by one power semiconductor
conductiong at a time, IGBT or diode. For the CSI four power semiconductors, two IGBT and two diodes
carry the high fuel cell current and have to be dimensioned according to the high fuel cell current.
Here, in general semiconductor power of IGBTs and diodes have been weighted equally. When regarding
the results with lower weighting for the diodes because of their lower costs, the CSI would appear more
favourable, but still worse than the VSI+BC.
V.
Semiconductor losses
A Losses depending on input dc voltage and line current
In the following, semiconductor switching losses and conducting losses for the VSI and the CSI are
calculated as described above. As mentioned before, semiconductor losses of the CSI and VSI+BC for
lower input dc voltages are relatively high. For a given inverter system, power losses decrease at rising
input dc voltages.
B Losses including fuel cell characteristic curve
As a result of increasing efficiency at higher input dc voltages and lower dc input currents it is of interest
to take a closer look at semiconductor losses at partial fuel cell loads due to the fact that fuel cell voltages
increase at lower fuel cell currents.
To compare losses of CSI and VSI+BC for operation at fuel cells the fuel cell characteristic curve, given
in fig. 1, has been taken into consideration. In the following, CSI and VSI+BC of different
dimensioning/rating for different minimum voltages as described in the section before have been
compared. At the nominal respectively minimal fuel cell/converter input voltage level their rated power
has been chosen to Pmax = 27 kW. The fuel cell stack has been dimensioned and modelled in that way that
its current level has been adapted by scaling the cell area, stack voltage has been achieved by connecting
fuel cells in series, using data given from the characteristic curve (fig. 1).
Figure 5 shows power losses of different but adequate rated inverters of both topologies, connected to the
fuel cell. Higher fuel cell stack voltage yields to lower fuel cell current and therefore decreasing power as
can be seen from the characteristic curve of a fuel cell (fig. 1). It can be seen that in general the CSI causes
greater losses at its nominal load at minimum input voltage compared to an VSI+BC. This correspondes to
the necessarily higher installed and fully exploited semiconductor power (fig. 4) of the CSI. In the partial
load range at higher fuel cell voltages CSI’s semiconductor losses reach the VSI+BC losses. In a medium
voltage range at about 300 to 396 V CSI’s semiconductor losses fall minor below VSI+BC losses. But
note that the CSI is overdimensioned concerning power semiconductors (see chapter IV) and that the
voltage range of the CSI is limited due to its maximum permitted input dc voltage. At higher voltages the
VSI+BC shows lower total losses due to decreasing losses in the boost converter.
3000
CSI: thick red line
VSI+B: thin black line
power losses Ploss
power losses Ploss
2500
2000
1500
1000
500
0
100
200
300
400
500
600
700
800
fuel cell voltage V fc
fuel cell voltage Vfc
Fig. 5: Semiconductor power losses of VSI+BC and CSI connected to fuel cell stacks (operating
conditions see tab. II; nominal fuel cell voltage Vfc, nom = 100, 200, 300, 400, 500, 600, 700 V; fuel cell
curve from fig. I)
VI. Future prospects
A Reverse blocking IGBTs for the current source inverter
In the context of matrix converter research the development of reverse blocking IGBTs (RBIGBT) is in
progress [8]. These devices could also be used in a current source inverter. Due to the lack of separate
series diodes the conducting losses of the CSI could be reduced.
For matrix converter operation it is shown in [8] that a reduction of approximately 20 % of total
semiconductor losses can be achieved by using RBIGBT compared to conventional IGBTs and diodes in
series at a switching frequency of 15 kHz. Due to high switching losses of RBIGBT compared to
conventional IGBT with series diode [8], a total semiconductor loss reduction of about 40 % can be
estimated for a switching frequency of 3 kHz.
Relating to the the CSI this estimation shows that RBIGBT would significantly improve CSI’s efficiency
and rated power in the fuel cell application because of the abolition of conducting losses in the series
diodes.
VII. Conclusion
Two topologies of converters, the voltage source inverter linked with boost converter (VSI+BC) and the
current source inverter (CSI) can be applied for the power feed in from fuel cells to the three phase mains.
Both converters have been analyzed and compared to each other with regard to power semiconductor
design rating and power losses when feeding in power from fuel cells to the mains. The analysis has been
done by analytical calculation in steady state by average models. The comparison is based on a system
with a power of 27 kW at a line to line voltage of 400 V, with fuel cell stacks of different nominal
voltages. Nevertheless, the results should be valid in a great power range. The converter is equipped with
power semiconductors designed to the fuel cell power.
The analysis shows that for this application, the CSI always has to be equipped with a remarkably higher
installed semiconductor power, compared to the VSI+BC. This is due to the fact that a high voltage range
has to be covered and the CSI needs four power semiconductors to carry out the boost function at low fuel
cell nominal voltage with high dc current. The boost converter in the VSI+BC system needs only one
power semiconductor which carries the current during boost operation. This yields necessarily to a much
higher installed semiconductor power in the CSI.
The comparison of the losses for both systems shows moderate to high advantages for the VSI, mainly for
fuel cells with low nominal voltage and for operation in the full fuel cell voltage range. Evaluating this,
one has to keep in mind that the CSI‘s power semiconductors are necessarily higher dimensioned
compared to the VSI.
Finally it can be concluded that under the given circumstances and available IGBTs, regarding power
semiconductor design rating and losses, there is no need today to use CSIs to connect fuel cells to the
mains in spite of the CSI’s interesting inherent voltage boost function. For fuel cells or other power
sources with a more stiff dc link voltage the result would be more positive for the CSI in this application.
Future reverse blocking IGBTs could moreover modify the result of this evaluation if RBIGBT show
remarkable lower power losses than IGBTs with diodes in series.
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