Download A DSP based reliable fuel cell power system with input

International Journal on Advanced Computer Theory and Engineering (IJACTE)
“CONTROL FOR RELIABLE FUEL CELL POWER SYSTEM
WITH INPUT RIPPLE CURRENT COMPENSATION”OVERVIEW
1
Sachin N.Phad, 2 A.M.Jain
1
Student of Master of Engineering K.K.Wagh College of Engineering & Research Centre Nasik.
2
K.K.Wagh College of Engineering & Research Center, Nashik
Email: - [email protected]
Abstract: This paper work is carried out to reduce ripple
current which is flow to the fuel cell/Battery through
power electronics devices during dc-ac operation. FCs
have advantages such as high efficiency, zero or low
emission (of pollutant gases), and flexible modular
structure With clean operating environment and high
energy conversion efficiency, fuel cell is getting more and
more attention, especially for the stationary power
application. Such an application, either delivering
electricity with utility intertie or directly supplying to
residential area as a standalone power source, can be
used for future distributed generation systems
Figure 1 :- Block diagram of a stationary fuel cell
power system
Some existing commercial-off-the-shelf proton
exchange membrane (PEM) fuel cells also have their
nominal voltage set at 48 V and below for either
telecommunication or residential applications. In order
for low voltage dc fuel cell to generate 50/60 Hz,
120/240 V ac voltage for residential applications, a dc–
dc converter is needed to boost the fuel cell voltage to
a level that can be converted to the desired ac output.
Fig. 1 shows such a system structure that contains a
low-voltage high-current dc–dc converter and a dc–ac
inverter. The output of the dc– dc converter, or the
input of the dc–ac inverter for 120 V ac is typically 200
V; and for 240 V ac is about 400 V. For a nominal 20
V, 5 kW fuel cell under full load condition, the voltage
is 20 V, and the average current is 250 A. The ripple is
added on top of the average current with a peak current
that tends to overload the fuel cell periodically. The
fuel cell can experience nuisance tripping with such a
ripple related overload situation.
This paper initially describes problems associated with
the fuel cell due to the ripple current such a ripple
current may shorten fuel cell life span and worsen the
fuel efficiency due to the hysteresis effect. The most
obvious impact is it tends to reduce the fuel cell output
capacity because the fuel cell controller trips under
instantaneous over-current condition. I section two
different methods are discuss , I section three appropriate
method is discuss & i section four conclusion is given.
This paper consists of different methods through which
the ripple current analyses and control.
Keywords: Fuel cell, Ripple current, dcac conversion, active
filter, power electronics
I. INTRODUCTION
Fuel cells (FCs) are static electric power sources
that convert the chemical energy of fuel directly into
electrical energy. FCs have advantages such as high
efficiency, zero or low emission (of pollutant gases),
and flexible modular st FCs are good energy sources to
provide reliable power at steady state; however, due to
their slow internal electrochemical and thermodynamic
characteristics, they cannot respond to electrical load
transients as quickly as desired.[1] .They are connected
to the power grid through power electronic interfacing
devices, and it is possible to control their performance
by controlling the interfacing devices. Modelling of
FCs can therefore be helpful in evaluating their
performance and for designing controllers [1,2].
Figure 2 :- Block diagram of ripple current generation
in fuel cell power system
II. METHODOLOGY
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International Journal on Advanced Computer Theory and Engineering (IJACTE)
Some approaches have been proposed to prevent this
120 Hz ripple from being imposed on the fuel cell. One
of the methods proposed [3] uses a LC series response
circuit tuned to twice the output frequency connected
in parallel to the DC bus capacitor in a composite
PWM voltage source inverter. This approach requires
bulky components. A multiloop control for a high
frequency link DC-AC inverter system is proposed in
[4]. Another topology using a high frequency link and
an active filter is proposed in [5]. The high frequency
link necessities the use of a high frequency transformer
and other switching devices. Method using an active
filter for current ripple reduction was proposed in [6,
7]. However these methods are based on using a
separate energy storing device like battery or ultra
capacitor.
capacitor or a passive L-C filter circuit is generally
connected to the DC line in order to reduce low
frequency ripple voltage. Furthermore, when batteries
are connected to the DC output., most of the DC ripple
current generated by the PWM converter flows into the
battery because the impedance of the battery is
comparatively much lower than that of the prior-art
circuit Battery ripple current particularly at low
frequencies, results in battery heating and a
corresponding rise in temperature. It is well known that
battery life time decreases as temperature increases.
Method -1 The present paper introduces a new
topology for a single phase PWM voltage source
converter which produces not only sinusoidal input
current but also zero ripple output current This new
topology is accomplished by adding only a pair of
switching devices and a reactor to a conventional
single-phase PWM converter circuit. Using a simple
control technique, the ripple energy in the DC line is
converted by the action of the additional switches into
energy which is stored in the additional inductor. Fig.3
shows the configuration of the main circuit of the
proposed PWM converter.
Adding energy storage capacitor either on the
high-side dc bus or on the low side fuel cell dc bus may
help reduce the ripple, but the cost and size of added
energy storage component are objectionable when the
ripple is reduced to an acceptable range. Laboratory
test indicated a peak-to-peak Ripple of 34% is obtained
with a typical 1.2- kW design. This Ripple current
component implies that fuel cell requires a power
handling capability 17% higher than its nominal output
rating. Current ripple not only affects the fuel cell
capacity, but also the fuel consumption and life span
[2], [3]. The results indicate that the fuel cell not only
needs higher power handling capability, but also
consumes 10% more fuels [2]. Furthermore, as pointed
out by [3], the 100-Hz harmonic current exhibits a
hysteresis Behavior with PEM fuel cells. Injecting
ripple current around this frequency to fuel cell may
result in thermal problem among stacks and impair the
stack lifetime.
Figure 3 :-Low frequency ripple current generation y
single phase full-ridge inverter
The fuel cell current Ripple reduction is thus a
major issue for the fuel cell converter Design.
Reference [4] suggested that the ripple current be
limited to less than 10%. Passive energy storage
compensation Method was suggested and tested
extensively in [5]. Active Compensation with external
bidirectional dc–dc converter method was suggested in
[6], [7]. These methods require externally Added
components or circuits and are not preferred
a]
The rectifier portion of the single phase PWM
converter consists of U-phase switches (SI, Sz), Vphase switches (S9,S4), a DC capacitor (CO),a and an
input AC filter circuit (LAC, CAC). The ripple
reducing portion consists of additional switches (S5,
s6) which we call W- phase switches, V phase switches
which are shared with the rectifier portion, and an
additional inductor (Lw) that is connected between the
junction of the V-phase switches and the junction of
the W-phase switches.
DC Ripple Current Reduction Method on A
Single Phase PWM Voltage Source Converter
The generation of harmonics and their
subsequent propagation into utility lines is a topic of
increasing concern for power supply authorities. To
reduce harmonics in power lines, unity power factor
PWM converters are used. However, single phase
PWM converters have serious defects including low
frequency ripple current which appears on the DC
output increases in proportion to the input current into
the PWM converter, resulting in low frequency ripple
voltage in the DC output Therefore, a very large
Figure 4 :-Modulation principle.
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International Journal on Advanced Computer Theory and Engineering (IJACTE)
Figure 8 :- Equivalent circuit of the ripple reducing
portion
Operation principle A Rectifier Portion
For PWM modulation in the rectifier portion
and ripple reducing portion, a triangular signal with a
frequency of 20kHz is used in common. The switching
frequency of switches S1 - s6 is then comparatively
higher than the AC input frequency. Fig.4&5 shows the
waveforms of the & signals U t ~ i and control signals
Vu and Vv. Fig 5 shows the detailed waveforms of
Vtri, Vu, and Vv during one switching period. SI and
S2 are controlled based on a comparison of Vu and
Ubi, and .S3 and S4 are controlled based on a
comparison of Vv and Vtri. Hence, the "ON duty
factor" Du of S1 and the "ON duty factor" Dv of S3 are
defined as shown in Fig.3. Depending on the
combination of Du and Dv, switching stages, stage 6a stage 6a, of the main circuit of the rectifier portion are
defined and the switching sequence during one
switching period is determined. Fig 6 (a) - (c) show the
equivalent circuits that correspond to each stage. AC
input current icr flows into the DC output in stage2a,
but circulates in the switching devices in stages la and
IC
Figure 5 :- Switching stages of the rectifier portion.
B. Ripple Reducing Portion
In order to reduce the DC ripple current, both
V- phase and W- Phase switches are used. Similar to
the rectifier portion, the "ON duty factor" Dw of S5 is
defined and switching stages, stage lb - stage4b, of the
ripple reducing circuit
Figure 6 :- Equivalent circuit of the rectifier portion
III. DSP BASED CONTROL
Method 2 In this a mathematical analysis of a single
phase inverter has been presented to verify the
appearance of the second harmonics current ripple at
the inverter stage. An active filter method to cancel the
second harmonics current ripple drawn from the fuel
cell source in a single phase fuel cell system is
proposed. Texas instrument DSP TMS 320 F 2808 is
used for the overall control and monitoring of the
system.
Figure 7:- Switching stages of the ripple redwing
portion
Block diagram of the proposed system and the
second harmonic current flow
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International Journal on Advanced Computer Theory and Engineering (IJACTE)
Figure 9 :- Block diagram of the proposed system and
the second harmonic current flow
component at 120 Hz. The total ripple factor is around
55%. For the proposed system with DC bus voltage
25V the amplitude of the DC component of the fuel
cell current around 1.9A.
Figure 9 shows the proposed system block
diagram along with the second harmonics current path .
The proposed system uses an active filter connected
across the DC-DC converter. The active filter provides
an alternate path for the second harmonics current and
prevents the ripple to appear on the fuel cell side. The
active filter block consists of two controllable switches
and an inductor. When using DSP based control, these
two switches can be controlled easily using the same
DSP chip used for the control of the inverter and/ or
boost switches. The second harmonics current
component now flows from the inverter through the
boost circuit into the active filter and back to the
inverter.
IV. CONCLUSION
Table 1 : -Comparison between convectional system &
proposed system
Quantity
Conventional Proposed
System
System
Vdc (Bus Voltage)
25V
25V
1 fuel cell 0Hz (DC 2A
1.9A
Current drawn)
1fuel-cell @120 Hz (120 1.2A
0.6A
Hz
ripple
current
imposed on the fuel cell)
Ripple factor due to 120 50%
29.5%
Hz current
Total Ripple factor
>55.7%
<36%
A DSP based reliable fuel cell power system
with input ripple current compensation using an active
filter is presented. The proposed system provide an
alternate path for the ripple current through the active
filter which prevent the ripple current from circulating
through the fuel cell source. The cancellation current is
derived from the DC bus capacitor itself and hence no
external energy storing device is required.
IV. REFERENCES
Figure 10 :- Current Ripple Under Dynamic Condition
without Adding Capacitors
Figure 11 :- proposed filter operation Fuel cell voltage,
fuel cell current
Figure 12 :- Simulation results for the proposed
system; (a)Fuel cell current and output current
For a convectional system with DC us voltage
of 25V the amplitude of the DC component of the fuel
cell current is around 2A. There is a significant ripple
[1]
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[3]
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[5]
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International Journal on Advanced Computer Theory and Engineering (IJACTE)
[6]
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[7]
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[8]
DSP based control for reliable fuel cell power
system with input ripple current compensation
IEEE 2008
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