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Performance Comparison of Input Current Ripple
Reduction Methods in UPS Applications with Hybrid
PEM Fuel Cell/Supercapacitor Power Sources
Yuedong Zhan a, *, Youguang Guo b, Jianguo Zhu b and Li Li a
a
Department of Automation, Kunming University of Science and Technology
Kunming, 650500, China
b
School of Electrical, Mechanical and Mechatronic Systems, University of
Technology, Sydney
PO Box 123, Broadway, NSW 2007, Australia
(Emails: [email protected], [email protected], [email protected])
*Corresponding author: Tel.: +86 871 5623806; Fax: +86 871 5916643, Email address:
[email protected] (Yuedong Zhan)
ABSTRACT
An uninterruptible power supply (UPS) system with different input current ripple
reduction methods is proposed, and a comparison research has been conducted about
these methods. The proposed UPS system consists of a 63-cell 300W proton exchange
membrane (PEM) fuel cell stack, two 16-cell supercapacitors (SCs) in series, a
high-efficiency push–pull DC/DC converter and a half-bridge DC/AC inverter. Besides
that the traditional push-pull DC/DC converter has inherent advantages of low
input-current stress and high voltage conversion ratio, the SCs, LC filter, and an active
clamp circuit are employed to reduce the input current ripples in the UPS system. First,
the input current ripple generation and performance without an external component are
analyzed and modeled in the PEM fuel cell. Then the input current ripple reduction
methods mentioned above are proposed and operated in the designed UPS system.
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Finally, the experimental results show that the input current ripple can be further
reduced by using different current ripple reduction approaches, and the active
compensation method has better performance than the passive compensation method.
The input current ripple is less than 5% of the rated input current.
Keywords: Uninterruptible power supply (UPS) system; Proton exchange membrane
(PEM) fuel cell; Input current ripple reduction; Push-pull DC/DC converter; Active
clamp circuit
1. Introduction
Uninterruptible power supply (UPS) systems are composed of a DC/DC converter and a
DC/AC inverter as the power conditioners of the backup and emergency power supply
for important applications. In the near future, the proton exchange membrane fuel cell
(PEMFC) and liquid-fed direct methanol fuel cell (DMFC) based on hydrogen are the
most promising fuel cells (FCs) due to their long continuous run-times, quietness, low
to zero emissions and excellent dynamic characteristics. The present lifetime
capabilities of PEMFC are suitable for backup UPS applications. However, there are
severe voltage and current ripples with twice the output AC frequency in the DC bus, as
shown in Fig. 1. In this case, the current ripples will react with the output terminal of
the FCs through the DC/DC converter so that the power sources will sustain extra loads.
Although the circuit capability of the UPS system could be raised to cope with extra
loads, this current ripple may shorten the lifetime of the FCs [1]. The low-frequency
current ripple and transient load will cause the decrease of FC efficiency [2]. Therefore,
the FC current ripple reduction is a major issue for the DC/DC converter design, and the
low frequency current ripple issue plays an important role in the development of
high-efficiency UPS system [3].
Fig. 1. Low frequency current ripples in UPS system.
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According to the suggestion of reference [4], the current ripple should be limited to less
than 10%. According to [5], it is recommended that the 100/120 Hz ripple component
be limited to within 15% of the total output and the 50/60 Hz ripple component be
limited to within 10% from 10% to 100% loads for an overall improved efficiency and
FC lifetime. Therefore, recently, many research works have been conducted with some
approaches to reduce the undesired current ripple as follows. (1) Passive compensation
approaches: design the supercapacitors (SCs) or filter capacitors in both low and high
voltage sides of a DC/DC converter [6], or use input and output inductor–capacitor
series-resonance circuit [7]. Using these methods, although the current ripples have
been reduced, the size increase of the passive energy storage device results in
considerable installation space and cost. (2) Active compensation approaches: according
to the references [2] and [8], the active compensation approaches are classified into two
main categories. One is the power conditioner self-controlling method [3], and the other
is the current-ripple-injecting method [9]. In the active compensation methods, the
control strategies have been carried out using the proportional-integral (PI) control,
neural network control (NNC) and sliding-mode control (SMC). The SMC is one of the
effective nonlinear robust control approaches since it provides the dynamic
performances with an invariance property to uncertainties once the system dynamics are
controlled in sliding mode [2]. (3) Passive and active comprehensive compensation
methods. For instance, Ostrožnik et al. [10] presented the results of a comparative study
of two possible hybrid filter topologies, comprised of a passive stage and an active stage,
which can be implemented in any general DC supply distribution system.
Besides the methods mentioned above, recently, Zhu et al. [11] presented a waveform
control method that can mitigate such a low frequency ripple current being drawn from
the FC while the FC system delivers AC power to the load using a differential inverter.
Liu et al. [12] proposed an FC power conditioning system based on the current-fed
dual-half-bridge DC/DC converter that can achieve low-frequency ripple free input
current using a control-oriented power pulsation decoupling strategy when an inverter
load is connected to the FC system. Li et al. [13] presented a high-efficiency FC
power-generation system with an active-clamp flyback–forward converter to boost a 12
V DC voltage into a 220 V 50 Hz AC voltage, including a high-efficiency high-step-up
interleaved soft-switching flyback–forward converter and a full-bridge inverter. Ahmad
et al. [14] designed a PI controller for the front-end boost converter in two-stage power
converters based on series and parallel feedback theory. Jang et al. [15] designed a
single-phase buck–boost inverter controlled by voltage-mode and a backup unit
controlled by current-mode, which supplied the low-frequency current ripple and
minimized the detrimental effects of such ripple being drawn directly from the FC itself.
Wai et al. [2] focused on the design of a dual active low-frequency ripple control for a
clean-energy power-conditioning mechanism with an aim to achieve both the alleviation
of the low-frequency current ripple of clean-energy sources, such as the solar
photovoltaic and fuel cell, and the improvement of the AC power quality of a power
conditioner. Kwon et al. [16] proposed a high-efficiency FC power conditioning system
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with input current ripple reduction, which consists of a high-efficiency high-step-up
current-fed resonant push–pull converter and a full-bridge inverter. The converter
conserves the inherent advantages of a conventional current-fed push–pull converter
such as low input-current stress and high voltage conversion ratio. Kim et al. [17]
proposed a high-efficient current-fed push–pull converter for high output voltage
applications supplied by low-voltage and high-current sources such as FCs and solar
cells. The ripple current of a boost inductor was reduced, compared with the
conventional one. Moreover, as the duty ratio approaches 0.5, the ripple of the inductor
current is close to zero. Wang et al. [18] proposed a novel parallel-series full bridge
DC/DC converter to improve its phase shifting scheme, and also proposed a novel
controller for low frequency ripple current suppressing applied on the converter.
Ramos-Paja et al. [19] proposed the cascade connection of a low-ripple voltage doubler
pre-regulator and a classical boost converter regulator to improve the efficiency of the
full system due to the reduction in average currents of all boost converters and the
smaller duty cycle required for the conventional boost regulator.
This paper conducts the comparison study on current ripple reduction methods through
a single-phase high-frequency UPS system with the hybrid PEMFC/SC power sources
developed for backup and emergency power applications. Fig. 2 shows the schematic
diagram of the UPS system, including a 300 W PEMFC stack, two 16-cell SCs in series,
a single phase high frequency UPS, and the intelligent control and communication units.
First of all, because of the slow dynamic performance of the stack, two 16-cell SCs of
7.5 F in series are employed to improve the responding time to sudden change of the
load and reduce the current ripples produced by power conditioner of DC/DC converter
and DC/AC inverter in the UPS system. Then the designed DC/DC converter utilizes
LC filter to realize the current ripple reduction. Finally, because the passive device
increases the overall size and cost, an active compensation method is proposed based on
the designed push-pull DC/DC converter by adding the active-clamp circuit to further
reduce the current ripples in the UPS system.
Fig. 2. An intelligent UPS system with backup PEMFC and SC power sources.
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2. Generation and analysis of low frequency current ripples
In order to examine the generation and propagation process of low frequency current
ripple under the conditions of the external loads, a single-phase high-frequency UPS
system was considered. As shown in Fig. 3, the system consists of a push-pull DC/DC
converter and a single-phase half-bridge DC/AC inverter, where VFC , I FC , VBus , I Bus , vo
and io are the voltages and currents of the PEMFC, the DC link/Bus (inverter input)
and inverter output, respectively.
Fig. 3. Normal circuits of DC/DC converter and DC/AC inverter designed in UPS
system.
2.1 Generation mechanism of low frequency current ripple
According to the operating principle of half-bridge DC/AC inverter, the final output
voltage becomes a 50 Hz or 60 Hz (angular frequency) sinusoidal waveform after
filtering. The filtered output voltage and current of the inverter are expressed as follows
[20].
vo (t )  2Vo cos(t )
(1)
io (t )  2 Io cos(t   )
(2)
where Vo and I o are the root-mean-square (RMS) values of output voltage and output
current for inverter, respectively,  is the angular frequency, and  the phase angle
between the voltage and current.
By multiplying (1) and (2), the output power of inverter is calculated as
Po (t )  vo (t )io (t )  Vo I o cos(2t   )  cos  
(3)
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Based on the principle of the pulse width modulation (PWM) of DC/DC converter,
sinusoidal PWM (SPWM) of DC/AC inverter and the energy conservation law when
neglecting the power losses of the converter and inverter in this UPS system, the powers
at the PEMFC, DC link/Bus and AC output should be almost the same, which is

VFC I FC (t )  VBus I Bus
(t )  Po (t )
(4)

where VBus is the filtered DC link/Bus voltage, I Bus
the filtered DC link/Bus current,
which consists of a pure DC value and a sinusoidal value at twice the fundamental

frequency. The I Bus term is the inverter input current, which consists of I Bus
and the
high frequency values I Bus due to inverter switching on and off.
According to (4), there is

I Bus
(t ) 
Po (t ) Vo I o

cos   cos(2t   )
VBus VBus
(5)
Although the DC Bus output has the capacitors C Bus , its impedance is not infinite, only
low frequency current ripple is charged and discharged through the DC Bus capacitors,
which contains the AC value of double angular-frequency 2 (100 Hz or 120 Hz).
Therefore, the DC Bus capacitor voltage can be derived as follows.
vBus (t )  VBus 


V0 I 0
1

I Bus
(t )dt  VBus 1 
sin(2t   ) 
2

CBus
 2CBusVBus

(6)
When the PEMFC output voltage can be regarded as current controlled voltage source,
the FC current contains 100 Hz or 120 Hz ripple value, that is

I FC (t )  vBus (t ) I Bus
(t ) / VFC
(7)
Consequently, the low frequency current ripple is reflected to the FC stack and produces
harmful effects on the FC voltage and current.
2.2 Modeling of PEMFC stack
Because the PEMFC is a type of electrochemical energy conversion device, if the
parameters for each single cell are lumped to represent the stack, the output voltage of
the stack can be obtained as [21]
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Vstack  Ereversible  VactLOSS  VOhmicLOSS  VconcLOSS  VleakLOSS
 RT  i  in 
 
RT  1  
iL
 Ereversible  N 
ln 

  ROhmic  i  in  
1   ln 
nF     iL  i  in   
  nF  i 0 
(8)
where VactLOSS is the activation voltage loss (V), VOhmicLOSS Ohmic voltage loss (V),
VconcLOSS concentration voltage loss (V), VleakLOSS leakage voltage loss (due to internal
current) (V), Ereversible the reversible voltage (V), N the number of cells in a PEMFC
stack, α the transfer coefficient, n the number of electrons per molecule of H2 (2
electrons per molecule), R the universal gas constant (J/mol·K), T the stack
temperature (K), F the Faraday’s constant (C/mol), ROhmic the area-normalized
resistance, also known as area specific resistance (ARS) of the PEMFC measured
(Ωcm2), i0 the exchange current density (A/cm2), iL the limiting current density at
which the cell voltage will fall rapidly (A/cm2), in the internal current or parasitic
current density that is wasted (A/cm2), and i the PEMFC stack current density (A/cm2).
The reversible voltage at varying temperatures and pressures can be expressed as
1




 0 RT  PH 2 ( PO2 ) 2  s298.15 K

Ereversible  N  E 
ln 

(
T

298.15)


2F
P
nF


 H 2O 


(9)
where Pi is the partial pressure of species i (i is H2, O2/air, or liquid water at cathode
side) (kPa), respectively; E 0 the cell open-circuit voltage (OCV) at the Standard
Temperature and Pressure (STP); and s298.15K the change in the molar entropy at STP
(J/mol·K).
According to the above voltage output equation, an equivalent circuit for
electrochemical circuit model of PEMFC is obtained, as shown in Fig. 4, where C is the
equivalent capacitor due to the double-layer charging effect, Vc is the voltage across the
capacitor, Ract is the activation resistance, and Rconc is the concentration resistance.
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Fig. 4. Equivalent circuit of a PEMFC.
2.3 Low frequency current ripple impacts on PEMFC stack
According to the equivalent circuit of PEMFC stack, because of the equivalent capacitor
C of the double-layer charging effect, the activation resistance Ract and the concentration
resistance Rconc , the performance influenced on the FC stack depends on the frequency
and magnitude of current ripples and the time delay of the mechanical balance of plants
(MBOP), e.g., an oxygen supply module also affects the FC stack [8].
Based on the simplified equivalent circuit model without PEMFC degradation, as
shown in Fig. 4, the total impedance can be calculated by [22]
1
jC
Z ( j )  ROhmic 
1
Ract  Rconc 
jC
R R
 ( Ract  Rconc )
 ROhmic  act 2 conc
j
2
1  
1   2 2
( Ract  Rconc ) 
(10)
where   C ( Ract  Rconc ) is the time constant.
When there are the low frequency current ripples from the power conditioning system in
UPS system, the total impedance can be changed into
1
j (C  C )
Z ( j )  ( ROhmic  ROhmic ) 
1
Ract  Ract  Rconc 
j (C  C )
R  Ract  Rconc
 ( Ract  Ract  Rconc )
 ( ROhmic  ROhmic )  act
j
2 2
1   
1   2 2
( Ract  Ract  Rconc ) 
(11)
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where    (C  C )( Ract  Ract  Rconc ) .
In the normal manner, the low frequency current ripples will lead to an increase in the
charge transfer resistance of membrane, and a decrease in the capacitance of the double
charge layer. The equivalent circuit of a PEMFC is shown in Fig. 5.
Fig. 5. Equivalent circuit of a PEMFC with a low frequency current ripple.
The hysteresis appears considering that the linearized model parameters can be
identified on two different branches. As shown in Fig. 6, it is important to consider that
the slope of the major branch is different from that of the reversal curve. This is clearly
shown in Fig. 6, from which it can be argued how the role of hysteresis is important for
a correct identification of the parameters of the linearized model in a PEMFC [23].
Fig. 6. Voltage-current hysteresis curve of a PEMFC stack obtained for the ascending
and descending branches for the inversion point I= 8.2 A.
3. Experimental Setup
The experimental setup consists of a UPS hybrid system and its intelligent controller, SC,
PEMFC generating and testing system, hydrogen and air mass flow transport and control
devices and the data-acquisition system including multifunction I/O unit NI6036E,
analogue voltage output unit NI6713, parallel digital I/O interface PCI-6503 and
analogue multiplexer with temperature sensor AMUX-64T (National Instruments). The
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UPS hybrid system with backup PEMFC and SC provides the AC power source for both
the linear loads (e.g. lamp box) and nonlinear loads (e.g. PC). In the PEMFC generating
and testing system, both hydrogen and air are regulated by two mass flow controllers
(type: F-201C-GAS-22V and F-112AC-GAS-22V, Bronkhorst). The temperature and
humidity of air and hydrogen can be measured at the inlet by the hydrotransmitter (type:
HD2008TV1, Delta OHM) as well as the pressure transmitter (type: AUS EX 1354X,
Burkert) between the inlets of cathode and anode of FC stack. The output of the UPS is
connected to a lamp load that is used in a constant voltage mode. All physical parameters
such as currents and voltages of the UPS hybrid system, the PEMFC stack and SC, the gas
mass flow of the hydrogen, the pressure, relative humidity and temperatures of air and
hydrogen are recorded with the data-acquisition system. Fig. 7 shows a photo of the
experimental setup.
Fig. 7. Photo of the experimental setup.
4. Comparison and discussions of low frequency current ripple reduction methods
4.1. Normal performances of low frequency current ripple
As mentioned in Section 2, if no circuit is designed to prevent the rise of low frequency
current ripple from a power conditioners system in PEMFC power generation system,
the current ripple will then reflect on the FC stack, which has a negative impact.
The input voltage values of the DC/DC converter used in the UPS system of Fig. 3 are
from VFC  30 V to VFC  54 V , which are provided by the PEMFC stack under
current I=0-9A: temperature at 24-55°C, fuel pressure of hydrogen at 5.8-6.5 PSI, and
stoichiometry of hydrogen at 1.5.
Fig. 8 shows the current and voltage waveforms of the PEMFC without any input
current ripple reduction control circuits of the DC/DC converter and DC/AC inverter in
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UPS system. A large current ripple of the double grid frequency (100 Hz) exists. The
average input current value ( I FCa ve ) is 8.22 A, the peak-to-peak current ripple value
( I FC pp ) is 2.0 A, and the current ripple percentage (CR%) can be determined by
Current ripple (CR%) 
I FC pp
I FCave
100% 
2.0 A
100%  24.3% .
8.22 A
(12)
And according to the theory analysis mentioned above and the experimental results, the
input current ripple increases along with the increase of the output power of UPS loads.
Fig. 8. Input current and voltage waveforms without input current ripple reduction
circuits in the UPS system.
4.2. Performances of low frequency current ripple using SC
An SC is a type of electrochemical energy storage device, which is an electrochemical
capacitor that has an unusually high energy density when compared to common
capacitors, typically thousands times greater than that of a high capacity electrolytic
capacitor. They also have efficiency of almost 90%, which can improve the performance
of a power conditioning systems that normally uses FCs or batteries.
In Fig. 2, when the switches K1 and K2 are turn on, the PEMFC and SC can be
combined into a hybrid power system and supply the power for the UPS system. When
the utility grid power source is interrupted, the hydrogen will be supplied to PEMFC
stack. However, during the 10 s start-up time of the stack, or a sudden change of
external load, the hydrogen cannot be fed fast enough, and the FC stack may take a few
seconds to reach the required output voltage. To overcome this issue, the SCs are
employed in parallel with PEMFC, which is composed of a redundant system, as shown
in Fig. 9.
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Fig. 9. Adding SC in UPS system.
Fig. 10 shows the current and voltage waveforms of PEMFC when using 16-cell SCs of
7.5 F in series in the UPS system, with the capacity of Ccell  120 F (±25%), control
voltage of 2.7 V, and internal resistance Rcell  20 m (±25%). The average input
current is 8.17 A, the peak-to-peak current ripple is 1.50 A, and the current ripple is
18.4%. If using the two 16-cell SCs in parallel, the average input current is about 8.17 A,
the peak-to-peak current ripple can be reduced to about 1.19 A, and the current ripple is
14.6%.
By increasing the capacitance value, for instance, using three 16-cell SCs in parallel, the
current ripple can be reduced to 11.7 %. However, because being larger capacitor value
and its voltage cannot be mutated, when the PEMFC starts up, it will produce larger
over-limiting current and shuts down immediately, unless a group of 36 V batteries are
employed in parallel with SCs in the UPS system. Therefore, this current ripple control
strategy will result in a siginificant increase in the system size and manufacturing cost.
Fig. 10. Input current and voltage waveforms with SCs in UPS system.
4.3. Performances of low frequency current ripple with LC filter
Based on Fig. 3, an input current low-pass LC filter circuit (Lin and Cin) has been
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designed and used, as shown in Fig. 11, to protect the PEMFC against the low
frequency discontinuous input current ripple of the DC/DC converter. The LC filter
parameters are selected as Rin  50 m , Lin  0.03 mH and Cin  3300  F according to
the power of UPS system designed.
Fig. 11. LC filter circuit designed in UPS system.
Fig. 12 shows the PEMFC current and voltage waveforms when using LC filter in the
UPS system. The average input current is 8.17 A, the peak-to-peak current ripple is 1.38
A, and the current ripple is 16.9%. If the inductance value is changed from 0.01 mH to
0.1 mH, the average input current and the peak-to-peak current ripple are the same.
With the increase of the input inductance, the current ripple will be decreased. The
current ripple can be reduced sharply to 4.1 %, when the inductance value increases to
over 100 mH. Simultaneously, the inductor will produce a large voltage difference,
making the voltage of PEMFC stack decrease rapidly below 30 V, then the PEMFC will
be shut down immediately. Therefore, the UPS system cannot run normally, if the
current ripple is reduced to less than 10 % by increasing the inductor value.
Fig. 12. Input current and voltage waveforms with LC filter circuit in UPS system.
4.4. Performances of low frequency current ripple with SC and LC filter circuit
As mentioned above, if using the SCs or LC filter individually, the input current ripple
cannot be reduced to less than 10%, unless the SCs and LC filter can be applied
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simultaneously as shown in Fig. 13. In fact, the capacitor Cin is replaced by SC, i.e., the
increase of the capacitor value. The SCs and LC filter parameters include Rin  50 m ,
Lin  0.1 mH and SC  15F .
Fig. 13. SCs and LC filter circuits designed in UPS system.
Fig. 14 shows the PEMFC current and voltage waveforms when using SCs and LC filter
together in the UPS system. The average input current is 8.21 A, the peak-to-peak
current ripple is 0.81 A, and the current ripple is 9.9%. Therefore, based on a certain
amount of SC, by increasing the inductor value by a small value, the current ripple can
be reduced to less than 10 %. This current ripple control strategy for passive
compensation approaches is feasible.
Fig. 14. Input current and voltage waveforms with SCs and LC filter circuits in UPS
system.
4.5. Performances of low frequency current ripple with active clamp circuits
Besides the passive compensation approaches, active compensation approaches can
also make the current ripple reduce to less than 10 %.
The conventional PWM push–pull DC/DC converter is shown in Fig. 3. The switches
S1 and S2 operate with shift in phase by T/2 and the same duty ratio Dc. The duty ratio
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must be smaller than 0.5. When switch S1 is on, diode D1 and D4 conduct and diode D2
and D3 are off. Diode states are reversed when switch S2 is on. When both controllable
switches are off, the diodes are on and share equally the filter inductor current. The DC
voltage transfer function of the push–pull DC/DC converter under the condition of the
continuous current is
VBUS
2n

VFC 1  Dc
(13)
where n  N 2 / N1 , and Dc is the control output duty ratio to regulate the DC link/Bus
voltage based on the feedback Bus voltage without the active clamp circuits.
In this paper, to reduce the input current ripple without increasing the size and cost of
the UPS system, the push-pull DC/DC converter with active clamp circuits is designed
as shown in Fig. 15. Two 180◦ out-of-phase pulses of equal and adjustable widths are
supplied to drive the main switches S1 and S2 of the step-up current-fed push-pull
DC/DC converter. The main components of the converter with active clamp technique
in Fig. 15 are: two main switches S1 and S2, two anti-parallel diodes D1 and D2, two
auxiliary clamping switches S3 and S4, two clamping diodes D3 and D4, and two
clamping capacitor C1 and C2 ( C1  C2  10 F ). Furthermore, the current feeding iL is
provided
by the
constant
voltage
source
VFC
in
series
with
the
input
inductor L  20 H and in parallel with the input capacitor C  10  F . The Push-Pull
transformer is represented by the primary magnetizing inductance L1 and the secondary
magnetizing inductance L2. As usual, the leakage inductances are referred to the primary
side by Lk, which is the main concern of the active clamping concept. This half-bridge
inverter is controlled in order to force the current io to be injected into the load to follow
a sinusoidal reference waveform which is synchronized and in phase with the load
voltage vo.
Fig. 15. DC/DC converter with active clamp circuits designed in UPS system.
Fig. 16 shows the pulse width modulator controller of the proposed DC/DC converter
with active clamp circuit and this controller is mainly implemented with a UC3525
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PWM controller and an electrically programmable logic device (EPLD). The EPLD is
designed to convert one gate pulse generated by the UC3525 to the four gate pulses
required by the proposed DC/DC converter. Also, to protect and restart the DC/DC
converter, the protection and soft start-up circuits are used and operated in the
over-current and over-voltage modes.
Fig. 16. Structure diagram of input current ripple reduction control with active clamp
circuits in UPS system.
Based on the designed push-pull DC/DC converter with active clamp circuit, a PWM
voltage controller and a current ripple reduction controller are employed to regulate the
output voltage VBus and the input current ripple I FC by adjusting the total duty ratio Dout ,
i.e. [16]
vBus (t )
2n
2n


VFC
1  Dout 1  ( Dc  Dr )
(14)
where Dr is a ripple cancellation duty ratio for current-ripple reduction.
(14) can be rewritten as
Dr  (1  Dc )  2n 
VFC
vBus (t )
(15)
Substiting (6) and (13) into (15), we have
Dr  2n 
Arranging (16), we obtain
VFC
 2n 
VBus
V
VFC
.
V0 I 0

sin(
2

t


)
Bus
2
2C BusVBus
(16)
17
Dr  kVFC I 0 sin( 2t   )
where k 
(17)
nV0
.
2
C BusVBus
Using the ripple cancellation duty ratio for current-ripple reduction, the current ripple of
the PEMFC can be reduced to about 3.1 %, as depicted in Fig. 17.
Fig. 17. Input current and voltage waveforms with active clamp circuits in UPS system.
A perturbed duty ratio D is employed to further reduce the ripple current of PEMFC,
which is given by [16]
D  k p ( I FC  iFC )
(18)
where k p is the proportional control gain.
By adding the D current ripple reduction in UPS system, the experimental results show
that the average input current is 8.17 A, the peak-to-peak current ripple is 0.18 A, and
the current ripple is 2.2%.
In conclusion, the major harmonic component in the input current is 100/120 Hz in the
UPS systems, and in a conventional circuit, the input current ripple increases according
to the addition of the output power of loads. To reduce the input current ripple, when
using the passive compensation approaches, such as the SC and LC filter methods,
although the input current ripples have been reduced, the size increase of the passive
energy storage device, such as increasing the number of SCs and the values of
inductances, results in considerable installation space and cost. In contrast, when
employing the active compensation approaches, such as the proposed active clamp
circuits, the input current ripple decreases, which cannot be affected by the increase of
18
the output power of loads. Therefore, the active clamp circuits are suitable for
renewable energy system applications. Table 1 shows the input current ripple
differences between the approaches.
Table 1 Comparison of input current ripple reduction methods.
Input current ripple reduction methods
Current ripple rates (%)
Normal
24.3
SCs (C=15 F)
14.6
LC filter (L=0.01-0.1 mH) (C=3300 µF)
16.9
SCs + LC filter
9.9
Active clamp circuits
3.1
5. Conclusion
The performance comparison of different input current ripple reduction control methods
for a UPS system with backup PEMFC and SCs power sources is presented in this paper.
A UPS hybrid system was developed, including a PEMFC generating and testing
system and its data acquisition devices, DC/DC converter with active clamp circuit,
DC/AC inverter, and their controllers. To reduce the input current ripple of the UPS
hybrid system, the SCs, LC filter, and active clamp circuit and technology are employed
and implemented. Based on the designed UPS hybrid system, three stages of
experimental test and analysis are conducted. Firstly, the generation principle of input
current ripple is analysed and obtained experimentally. Then the different current ripple
reduction approaches and strategies designed are implemented and examined. Finally,
the current ripple performances of the UPS hybrid system are tested. The experimental
results indicate that the active compensation method is better than the passive
compensation methods for reducing the input current ripple, which is of less cost and
smaller size.
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Fig. 1. An intelligent UPS system with backup PEMFC and battery power sources
22
Fig. 2. Schematic diagram of the PEMFC generating and testing system
23
Fig. 3. Photo of the PEMFC stack
24
Fig. 4. Circuit schematic model of DC/AC inverter
25
Fig. 5. Single-phase active PFC AC/DC rectifier
26
Fig. 6. Schematic diagram of DC/DC converter
27
Fig. 7. Schematic diagram of AC/DC charger
28
Fig. 8. Schematic diagram of connection between PEMFC and battery
29
Fig. 9. Photo of the experimental setup
30
Fig. 10. Network UPS hybrid system interface
31
Fig. 11. Voltage-current and power-current characteristics of PEMFC
Fig. 12. Start-up performance of PEMFC
32
Fig. 13. Switching of the UPS power source from battery to PEMFC
33
Fig. 14. Transitional waveform when the utility grid power is interrupted
34
Fig. 15. Transitional waveform when the utility grid power recovers
35
Fig. 16. System efficiency of UPS hybrid system