Document 6597881

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Document 6597881

SSRG International Journal of Electrical and Electronics Engineering (SSRG-IJEEE) – volume 1 Issue 9 –November 2014
Simulation of Three-Phase Transformer-less Neutral Point
Clamped Inverter in Fuel Cell Systems
J.Sanjeeva Rao*1, Indira Rani.G*2
M-Tech Student Department of EEE, VBIT, Aushapur, Ghatkesar, R.R (Dt), Telangana, India.
Assistant Professor, Department of EEE, VBIT, Aushapur, Ghatkesar, R.R (Dt), Telangana, India.
ABSTRACT
In this paper we presents the
traditional grid-connected Fuel cell inverter includes
either a line frequency or a high frequency
transformer between the inverter and grid. The
transformer provides galvanic isolation between the
grid and the Fuel cell. In order to increase the
efficiency to reduce the size and cost of the effective
solution is to remove the isolation transformer. The
common mode current reduces the efficiency of
power conversion stage affects the qualities of grid
current deteriorate the electric magnetic compatibility
and give rise to the safety threats. In order to
eliminate the common mode leakage current in
transformer-less Fuel cell system the concept of
virtual DC bus is proposed in this paper. By
connecting the grid neutral line directly to the
negative pole of the DC bus the stray capacitance
between the Fuel cell and the ground is bypassed.
The CM ground leakage current can be suppressed
completely. Virtual DC bus is created to provide the
negative voltage level for the negative AC grid
current generation. The virtual DC bus is realized
with the switched capacitor technology that uses less
number of elements. Therefore, the power electronic
cost can be reduced. This topology can be modulated
with the unipolar SPWM to reduce the output current
ripple. A smaller filter inductor can be used to reduce
the size and magnetic losses. The simulation result of
the proposed topology using MATLAB/SIMULINK
is presented.
increasing and fulfilling of these higher energy
demands is the main problem for the power suppliers
or worldwide.
FUEL CELLS are electrochemical devices
that process H2 and oxygen to generate electric
power, having water vapor as their only by-product.
The voltage resulting from the reaction of the fuel
and oxygen varies with the load, and ranges from 0.8
V at no-load to about 0.4 V for full-load. Due to their
low output voltage, it becomes necessary to stack
many cells in series to realize a practical system. For
low-power applications, the number of cells that
needs to be connected in series is small, but as power
increases, the number of cells that are required in the
stack increases rapidly [1], [2]. An example 100 V
fuel cell stack consists of 250 cells in series and to
produce 300 V at full-load requires 750 cells stacked
in series. A conventional fuel cell system consists of
a stack of cells and a dc–dc converter to step-up its
terminal voltage and compensate for its no-load to
full-load variation [3]–[5]. Since this fuel cell
structure is equivalent to connecting several voltage
sources in series, each with its own internal
impedance [6], [7], the output power of the stack is
limited by the state of the weakest cell. The state of a
cell can be inferred from the voltage across its
terminals, which is affected by parameters such as
fuel and air pressure, and membrane humidity.
Furthermore, if a stack contains malfunctioning or
defective cells, the whole system has to be taken out
of service until major repairs are done.
Key
words:
Transformer-less Inverter,
Common Mode Leakage Current, PWM.
I.
INTRODUCTION
Form the long time, Fossil fuels, such as coal,
natural gas, and petroleum are main supplier of
energy to the world. But these are available with
limited stock and non renewable sources of energy.
And their use gives increased carbon dioxide
emissions, environmental pollution, global warming
and climate change. But with all above mentioned
problems the energy demand of world is still
ISSN: 2348 – 8379
Fig1. Utility scale fuel cell stack and dc–dc/dc–ac
converter.
www.internationaljournalssrg.org
Page 66
II.
RELATED WORK
It is known that the 19th Century was the
century of the steam engine and the 20 th Century was
the century of the internal combustion engine. On the
other hand, the 21 st Century will be likely to be the
century of the fuel cell, and as a result fuel cells will
revolutionize the way to currently generate electric
power offering the prospect of supplying the world
with clean, efficient, sustainable electrical energy
because they use hydrogen as a fuel.
A fuel cell is defined as an electrical cell, which
unlike other storage devices can be continuously fed
with a fuel in order that the electrical power can be
maintained. The fuel cells convert hydrogen or
hydrogen-containing fuels, directly into electrical
energy, heat, and water through the electrochemical
reaction of hydrogen and oxygen.
Since hydrogen and oxygen gases are
electrochemically converted into water and energy as
shown in the above overall reaction, fuel cells have
many advantages over heat engines: high efficiency
and actually quiet operation and, if hydrogen is the
fuel, no pollutants are released into the atmosphere.
As a result, fuel cells can continuously generate
electric power as long as hydrogen and oxygen are
available.
Among several types of the fuel cells
categorized by the electrolyte used, four types are
promising for distributed generation systems:
Phosphoric Acid fuel cell (PAFC), Solid Oxide fuel
cell (SOFC), Molten Carbonate fuel cell (MCFC),
Proton-Exchange- Membrane fuel cell (PEMFC).
All types of the fuel cells produce electricity
by electrochemical reaction of hydrogen and oxygen,
and the oxygen can be easily obtained from
compressing air. On the contrary, hydrogen gas
required to produce DC power is indirectly gained
from the reformer using fuels such as natural gas,
propane, methanol, gasoline or from the electrolysis
of water.
A typical configuration of an autonomous
fuel cell system is described in Figure 1.2. As shown
in this figure, the fuel cell plant consists of three main
parts: a reformer, stack, and a power conditioning
unit (PCU). First, the reformer produces hydrogen
gas from fuels and then provides it for the stack.
Second, the stack has many unit cells in series to
generate a higher voltage needed for their
applications because a single cell that consists of
electrolyte, separators, and plates, produces
approximately 0.7 V DC. Last, the PCU including
power converters converts a low voltage DC from the
fuel cell to a high voltage DC and/or a sinusoidal AC.
Figure.2 Basic fuel cell operation
Figure shows a block diagram of basic fuel cell
operation. As illustrated in this figure, the fuel such
as natural gas, coal, methanol, etc. is fed to the fuel
electrode (anode) and oxidant (oxygen) is supplied to
the air electrode (cathode). The oxygen fed to the
cathode allows electrons from the external electrical
circuit to produce oxygen ions. The ionized oxygen
goes to the anode through the solid electrolyte and
combines with hydrogen to form water. Even though
chemical reactions at anode and cathode may be a
little different according to the types of fuel cells, the
overall reaction can be described as follows:
Fig:3 Configuration of the fuel cell system
Overall reaction: 2 H2 (gas) + O2 (gas) → 2 H2O +
energy (electricity, heat)
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Types of fuel cells:
Proton exchange fuel cells:
In the archetypal hydrogen–oxygen proton exchange
membrane fuel cell (PEMFC) design, a protonconducting polymer membrane, (the electrolyte),
separates the anode and cathode sides. This was
called a "solid polymer electrolyte fuel cell" (SPEFC)
in the early 1970s, before the proton exchange
mechanism was well-understood. (Notice that
"polymer electrolyte membrane" and "proton
exchange mechanism" result in the same acronym.)
On the anode side, hydrogen diffuses to the anode
catalyst where it later dissociates into protons and
electrons. These protons often react with oxidants
causing them to become what is commonly referred
to as multi-facilitated proton membranes. The protons
are conducted through the membrane to the cathode,
but the electrons are forced to travel in an external
circuit (supplying power) because the membrane is
electrically insulating. On the cathode catalyst,
oxygen molecules react with the electrons (which
have traveled through the external circuit) and
protons to form water. The materials used in fuel
cells differ by type. In a typical membrane electrode
assembly (MEA), the electrode–bipolar plates are
usually made of metal, nickel or carbon nanotubes,
and are coated with a catalyst (like platinum, nano
iron powders or palladium) for higher efficiency.
Carbon paper separates them from the electrolyte.
The electrolyte could be ceramic or a membrane.
III.
PROPOSED WORK
To take advantage of the modular fuel cell stack, an
appropriate dc–dc converter and control scheme are
required. The converter should have as many
independently controllable inputs as there are
sections in the stack. In addition, since the positive
terminal of one section in the stack also serves as the
negative terminal for the next section, the converter
should provide isolation between its input and output
to avoid circulating currents. A converter meeting
these specifications can be constructed by using an
arrangement of isolated dc–dc converter modules,
where the inputs of each module are connected across
each of the sections of the stack and their outputs are
connected in series in order to add the output voltages
of the different modules, thus obtaining a higher
output voltage. Such a modular dc–dc converter is
shown in Fig. 9, where the converter is composed of
three push–pull modules.
As discussed earlier, another advantage of
constructing a fuel cell stack with several sections is
that faulty portions of the stack can be bypassed,
while the rest of the stack can continue operation. To
implement this function, each of the modules used to
construct the dc–dc converter should be able to stop
extracting power from the section they are connected
to and set its output impedance to zero. This function
can be accomplished by removing the gating signals
to the transistors. In addition, it is necessary to add a
switch (Sx ) at the output of each module to shortcircuit the output capacitor of the module and bypass
it.
In order to optimize the power extraction from
each of the sections in the fuel cell, an appropriate
control scheme needs to be devised. From Figs. 7 and
8, it can be observed that the voltage cross the
terminals of each section in the stack is a good
indication of how much power it can generate; thus,
this information can be used to better distribute the
power extracted from each section.
A section producing a higher voltage can
generate more power than a section that produces a
lower voltage. Therefore, by controlling the load
current on each section in the stack as a function of
the voltage, they produce results in healthier sections
supplying more power than underperforming
sections. This, in turn, reduces internal losses and
improves the overall efficiency of the system. Since
the outputs of the modules are connected in series,
their output currents io are identical.
Now, if the modules are constructed by push–
pull converters, the input current of every module is
given by (6), where Dn is the duty cycle of the nth
module and N1 and N2 are the transformer primary
and secondary turns
(6)
ISSN: 2348 – 8379
Page 68
controllers. The calculation of the reference signals
for each of the modules is done digitally by means of
a DSP. The reference signals are then feed to analog
controllers located on each of the dc–dc modules.
IV.
Fig.4.Proposed control scheme.
Thus, the input current of each module can be
controlled by setting an appropriate duty cycle. The
duty cycle for each module is calculated as shown in
Fig. 10. In this block diagram, the output voltage of
the converter is maintained constant to the value set
by Vo, ref . The output of the main voltage loop
compensator is then used to calculate the required
duty cycle for each dc–dc converter by multiplying it
with the corresponding reference signal for each
module. These reference signals are calculated by
taking into account the voltage produced by each of
the sections in the stack and the number of modules
that compose the dc–dc converter. Each of the
reference signals is calculated by the weighting
function shown as
SIMULATION RESULTS
To verify the operation of the proposed fuel cell stack
and converter, a laboratory prototype was built. The
test system is composed of a 12-V/150-WH2 –air
PEM modular fuel cell stack consisting of three
sections of eight cells, each with an active area of 50
cm2 , and a modular dc–dc converter composed of
three push–pull modules. The dc–dc converter is
designed to supply a 22-V load; thus, if all the
sections in the fuel cell produce the same voltage
across their terminals, each module needs to provide
one-third of the total output voltage Vo and output
power. However, since the dc–dc converter has to be
designed for continue operation under the condition
of having.
(7)
Where VSn is the voltage produced by the “nth”
section in the fuel cell stack, VSi is the voltage
produced by the “ith” section in the stack, and NAC
is the total number of active sections in the stack.
Thus, the reference signal for the “nth” module is
given by the ratio between the voltage produced by
the “nth” section in stack and the total voltage
produced by the stack. The number of active sections
is defined by all the sections that produce a voltage
above a minimum value. Now, if one of the sections
produces a voltage below this threshold level, then
that section can be considered faulty. Thus, it cannot
produce power and needs to be discarded. In this
case, the controller reduces NAC by 1 and sets the
reference signal to the respective module to zero.
Additionally, this has the effect of increasing the
reference signals of the remaining modules to
compensate for the loss of one stack section.
The implementation of this control scheme can
be carried out by combining digital and analog
ISSN: 2348 – 8379
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Fig. 6. Simulated output voltage (vAO) and load
current (iA) in phase a with load 2 for (a) SVPWM,
(b) 3MV and (c) 2MV1Z.
Fig. 5. Simulated leakage current (iL) of the threelevel PV inverter with Load 1: (a) SVPWM, (b) 3MV
and (c) 2MV1Z.
Fig. 7. Simulated output voltage (vAO) and load
current (iA) in phase a for SVPWM using Ts/2.
V.
CONCLUSION
The proposed idea a novel inverter topology is
proposed with virtual DC bus concept by adopting
the switched topology. This topology is suitable for
small three phase power applications where as the
output current is relatively small so that the current
stress caused by switched capacitor does not cause
the serious reliable problems for power device
capacitors. The proposed virtual DC bus concept
provides a promising solution for the transformer less
grid connected Fuel Cell inverters.
REFERENCES
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Aldabas, "A New High-Efficiency Single-Phase Transformerless
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J. Malvar, R. Teodorescu, J.Doval-Gandoy, "Eliminating Ground
Current in a Transformerless Photovoltaic Application," Energy
Conversion, IEEE Transactions on , vol.25, no.1, pp.140-147,
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ISSN: 2348 – 8379
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[3] E. Gubía, P. Sanchis, A. Ursúa, J. Lopez, and L. Marroyo,
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