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Vol.06,Issue.09,
October-2014,
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Soft-Switching Techniques for DC-to-DC Converters in Electrolyzer Application VISWAMOHAN K
1, JAYAKRISHNA G
2
1PG Scholar, Dept of EEE, Narayana Engineering College, Gudur, AP, India. 2Professor, Dept of EEE, Narayana Engineering College, Gudur, AP, India.
Abstract: An Electrolyzer is part of a renewable energy
system and generates hydrogen from water electrolysis that
is used in fuel cells. A dc-to-dc converter is required to
couple the electrolyzer to the system dc bus. This paper
presents the design of three soft-switched high-frequency
transformer isolated dc-to-dc converters for this application
based on the given specifications. It is shown that LCL-type
series resonant converter (SRC) with capacitive output filter
is suitable for this application. Detailed theoretical and
simulation results are presented. Due to the wide variation
in input voltage and load current, no converter can maintain
zero-voltage switching (ZVS) for the complete operating
range. Therefore, a two-stage converter (ZVT boost
converter followed by LCL SRC with capacitive output
filter) is found suitable for this application. Experimental
results are presented for the two-stage approach which
shows ZVS for the entire line and load range.
Keywords: DC-to-DC Converters, Electrolyzer, Renewable
Energy System (RES), Resonant Converters.
I. INTROCUTION
A Renewable energy system (RES) converts the energy
found in sunlight, wind, falling water, waves, geothermal
heat, or biomass into a useable form, such as heat or
electricity. A typical RES that is under development at the
Institute for Integrated Energy Systems (IESVic) laboratory,
University of Victoria, is shown in Fig. 1 [1]. Renewable
energy storage in the form of hydrogen may overcome the
inherent weakness of battery-based energy storage systems
like physical size, limited life span, initial capital cost of the
battery bank coupled with transportation, maintenance, and
battery disposal issues. During periods when the renewable
resources exceed the load demand, hydrogen would be
generated and stored through water electrolysis. For this
purpose, electrolyzer that breaks water into hydrogen and
oxygen [1]-[3] is used as an integral part of RES in Fig. 1.
During periods when the load demand exceeds the
renewable resource input, a fuel cell operating on the stored
hydrogen would provide the balance of power. To ensure
proper flow of power between the system elements, the
available energy from different sources is coupled to a low
voltage dc bus. A direct connection of dc bus to the
electrolyzer is not suitable because it lacks the ability to
control the power flow between the renewable input source
and the electrolyzer. Therefore, a power conditioning
system, usually a dc–dc converter, is required to couple the
electrolyzer to the system bus.
Fig1. Block diagram of a typical RES.
The Electrolyzer used for hydrogen generation in Fig. 1 is
a Stuart SRA 6-kW alkaline electrolyzer (Stuart Energy
Systems, Mississauga, ON, Canada). The cell stack used in
the [1] electrolyzer is a low-pressure alkaline design with a
maximum 900-L/h H2 production capacity. The control
system allows hydrogen production provided that the input
voltage is between 42 and 56 V DC. According to the
manufacturer, hydrogen Production is roughly linear with
respect to input power. Standard equipment includes inlet
water purification, hydrogen gas purification, compression
to 17 bars, and an integrated safety monitoring system.
Section II, III, IV&V discusses fixed frequency filters,
design of fixed frequency operation of LCL SRC
Capacitive, inductive and phase-shifted ZVS PWM full-
bridge converter, two stage approaches and the conclusion.
II FIXED FREQUENCY FILTERS
High-frequency (HF) transformer isolated, HF switching
dc to-dc converters are suitable for this application due to
their small size, light weight, and reduced cost. To increase
their efficiency and to further increase the switching
frequency while reducing the size, cost, and electromagnetic
interference problems, soft-switching techniques [4]-[10]
VISWAMOHAN K, JAYAKRISHNA G
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.09, October-2014, Pages: 1021-1026
will be used in this paper. Due to the high power
requirement, an interleaved multi cell configuration that
uses three cells (each rated at 2.4 kW) in parallel (both at the
input and output) with each cell being phase shifted by 1200
( = 3600/3) is adopted. Each cell shares equal power and the
thermal losses are distributed uniformly among the cells.
Also, the input/output ripple frequency of three-cell
configuration becomes three times the input/output ripple
frequency of each cell. There are three major types of HF
transformer isolated soft switching converter configurations
possible: 1) voltage fed resonant converters 2) current fed
resonant converters and 3) fixed-frequency resonant
transition zero-voltage switching (ZVS) pulse width
modulation (PWM) [8]-[10] bridge converters. Our studies
show that current fed resonant converters require HF
switches rated at 5–6 times the input voltage (reducing the
efficiency) in the present application, and therefore, they are
not considered further. Voltage fed resonant converters can
be operated either in variable-frequency mode or fixed-
frequency mode. But the operation in variable frequency
mode suffers from several disadvantages: wide variation in
switching frequency making the design of filters and control
circuit difficult.
Therefore, fixed-frequency operation is adopted in this
paper. From the previous discussions, we are left with
mainly the following six soft-switching converter
configurations for the electrolyzer application:
1. Fixed-frequency series resonant converter (SRC);
2. Fixed-frequency parallel resonant converter (PRC);
3. Fixed-frequency series-parallel or LCC-type
resonant converter (SPRC);
4. Fixed-frequency LCL SRC with a capacitive
output filter;[6].
5. Fixed-frequency LCL SRC with an inductive
output filter;[7].
6. Fixed-frequency phase-shifted ZVS PWM full-
bridge converter.[8]-[10]
Among the aforementioned six converter configurations,
the SRC and SPRC can operate with the ZVS, only for very
narrow variations in supply and load variations in the
present application. In the case of PRC, the inverter peak
current does not decrease much with reduction in the load
current and there is no coupling capacitor in series with the
HF transformer. Therefore, the first three configurations are
not considered for further study and we will study only the
latter three configurations.
III. DESIGN AND DETAILED VIEW OF FIXED
FREQUENCY CONVERTERS
The selected converters are designed for the worst
operating conditions of minimum input voltage Vin = 40V,
maximum output voltage Vo = 60V, and maximum output
power (2.4 kW for each cell); switching frequency fs =100
kHz; inverter output pulse width δ = π. The detailed view of
LCL SRC with capacitive filter, LCL SRC with inductive
output filter and phase-shifted ZVS PWM full-bridge
converter as below.
A. Fixed-Frequency LCL SRC with Capacitive Output
Filter
The basic circuit diagram of the modified series (LCL-
type) resonant converter with capacitive output filter is
shown in Fig 2 and its typical operating waveforms for
fixed-frequency operation using phase-shifted gating signals
are shown in Fig3. This converter has been analyzed using
the Fourier series and approximate (complex ac circuit)
analysis approaches. It has been shown that the converter
operates in lagging power factor (PF) mode for a very wide
change in load and the supply voltage variations, thus
ensuring ZVS for all the primary switches. The peak current
through the switches decreases with load current. The
converter is designed based on the analysis and design
procedure given in. The following values are found to be a
near optimum for the design specifications: resonant
inductance ratio Lr /L, t = 0.1; normalized load current J =
0.427; converter gain M = 0.965; normalized switching
frequency F = 1.1, where Lp = L’t = (nt 2 )Lt , J = (Id /nt
)/IB , IB = Vin /Z, Z = (Lr /Cs )1/2 , M = (nt Vo )/Vin , F =
ωs /ωr , ωs = 2πfs , ωr = 1/(LrCs )1/2 , and fs = switching
frequency.
Fig.2.LCL series resonant dc-to-dc converter with a
capacitive output filter.
Fig.3. Typical operating waveforms for fixed-frequency
operation of the converter.
Soft-Switching Techniques for DC-to-DC Converters in Electrolyzer Application
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.09, October-2014, Pages: 1021-1026
Table I: Design Values for 2.4-Kwcell
B. Fixed-Frequency LCL SRC with Inductive Output
Filter:
The basic circuit diagram of the modified series (LCL-
type) resonant converter with inductive output filter is
shown in Fig4, and its typical operating waveforms for
fixed-frequency operation using phase-shifted gating signals
are shown in Fig5. This converter has been analyzed using
the approximate (complex ac circuit) analysis approach. It
has been shown that this converter also operates in lagging
PF mode for a very wide change in load and the supply
voltage variations, thus facilitates ZVS for all the primary
switches. The peak current through the switches decreases
with load current and is approximately clamped to the load
current. This converter is designed based on the analysis and
design procedure given in. The following values are found
to be a near optimum for the design specifications:
Fig.4. LCL series resonant dc-to-dc converter with an
inductive output filter.
Fig5. Typical operating waveforms for fixed-frequency
operation of the converter.
The ratings of various components obtained theoretically
for the three converter configurations are summarized in
Table II. Since the voltage and current ratings are different
for Vin,min = 40V and Vin,max = 60V, note that the
maximum voltage or current stresses are to be selected.
TABLE II: Component Ratings for 2.4-Kwcell at Vin,
Min = 40v, Vo = 60v, Values Shown In Brackets Are for
Vin,min = 60 V
C. Fixed-Frequency Phase-Shift Controlled ZVS Full-
Bridge PWM Converter:
The phase-shifted ZVS PWM dc-to-dc full-bridge
converter shown in Fig6 and its typical waveforms is shown
in fig7. It reduces peak current stresses compared to a
resonant converter. The ZVS for the switches is realized by
using the leakage inductance of the transformer (together
Fig.6. Fixed-frequency phase-shifted ZVS PWM dc-to-
dc full-bridge converter.
with an external inductor) and the output capacitance of the
switch. Although various improvements have been
suggested for this converter, all of them use the increased
number of components and suffer from one or another
VISWAMOHAN K, JAYAKRISHNA G
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.09, October-2014, Pages: 1021-1026
disadvantage (limited ZVS range or high voltage ringing on
the secondary-side rectifier diodes or loss of duty cycle).
Therefore, for comparison purpose, we have used the basic
configuration. In the case of last two configurations, a
snubber circuit is needed across the output rectifier to clamp
the voltage ringing due to diode junction capacitance with
the leakage inductance of the transformer. The ratings of
various components obtained theoretically for the three
converter configurations are summarized in Table. Since the
voltage and current ratings are different for Vin,min = 40V
and Vin,max = 60V, note that the maximum voltage or
current stresses are to be selected.
Fig.7. Typical operating waveforms for fixed-frequency
phase-shifted ZVS PWM dc-to-dc full-bridge converter
TABLE III: Theoretical Efficiency Comparison for 2.4-
Kw Cell At Full-Load with Vin,Min = 40v And Vo = 60v
The major problems associated with these converters are
listed in Table. From Tables I–IV, it is concluded that LCL
SRC with capacitive output filter is suitable for the present
application. The advantages of the LCL-type SRC with a
capacitive output filter as compared to the other
configurations are summarized as follows:
1. This (scheme) has the highest efficiency among the
three converters.
2. There is no duty cycle loss. Duty cycle loss occurs with
inductive output filter converters due to the overlap
time during which all the output rectifier diodes
conduct. This causes a decrease in the output voltage
and increases the primary peak current for the same
power output.
3. The transformer turns ratio (= 1/nt = Ns /Np ) is less
compared to other two configurations and this is
possible since there is no duty cycle loss as mentioned
previously.
4. This configuration does not have the ringing problem of
the rectifier; therefore, this scheme does not need lossy
snubber at the output. The current through the rectifier
diodes are sinusoidal, and therefore, the rectifier
switches smoothly, and they do not suffer from di/dt
and reverse recovery problems. The rectifier diode
voltages are clamped to the output voltage. Therefore,
100-V Schottky diodes can be used with low forward
voltage drops.
5. This scheme has a wide ZVS range for the MOSFETs.
Also the current in the tank circuit reduces with load
current; therefore, this scheme has very good part load
efficiency.
6. The variation in duty cycle required is narrow for a
wide range in power control.
Based on the advantages discussed previously, the LCL-
type SRC with capacitive output filter is selected for the
electrolyzer application.
Fig.8. Calculated and simulated results for LCL SRC
with capacitive output filter for Vin = 40V. (a) Vin,min =
40V, Vo = 60V, and Id = 40 A. (b) Vin,min = 40V, Vo =
60V, and Id = 20 A. (c) Vin,min = 40V, Vo = 60V, and Id
= 4A.
Soft-Switching Techniques for DC-to-DC Converters in Electrolyzer Application
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.09, October-2014, Pages: 1021-1026
TABLEIV. Major Problems Associated with the
Converters
Fig.9. Simulated results for LCL SRC with capacitive
output filter for different operating conditions. (a)
Vin,max = 60V and Vo = 60V at full-load and half-load
conditions. (b) Vin = 40V and 60V, Vo = 40V, and Id =
10 A.
IV. TWO-STAGE APPROACH
From the presented results, it can be observed that
although LCL SRC with capacitive output filter has better
performance compared to other configurations, this
converter cannot also maintain ZVS for wide change in
input voltage and requires small Lr, which is very difficult to
realize in practice.
Fig.10. Two-stage boost-LCL SRC with capacitive
output filter.
Fig.11. Typical operating waveforms for Two-stage
boost-LCL SRC with capacitive output filter.
Therefore, the solution is to boost the input voltage and
then use the LCL SRC with capacitive output filter as a
second stage. When this converter is operated with almost
fixed input voltage, duty cycle variation required is the least
among all the three converters. Thus, in this two-stage
approach, a ZVT boost converter generates approximately
100V as the input (Vbus) to the resonant converter for the
specified input voltage (40–60 V) while delivering the
output voltage of Vo = 60V. Fig. 15 shows the circuit
diagram of a two-stage boost- LCL SRC with capacitive
output filter. This approach not only achieves ZVS for all
the switches but also simplifies the design of Lr and Cs
resonant components.
Base values:
VB = Vin, min, ZB = (Lr /Cs)1/2 , and IB =VB /ZB .
Converter gain: M = Vo ’/VB , Vo ’ = nt Vo .
Normalized load current: J = (Id /nt )/IB .
Normalized switching frequency: F = ωs /ωr = fs /fr , ωr =
1/(LrCs )1/2 .
V. CONCLUSION
A comparison of HF transformer isolated, soft-switched,
dc- to- dc converters for electrolyzer application was
presented. An inter leaved approach with three cells (of
2.4kW each) is suitable for the implementation of a 7.2-kW
converter. Three major configurations designed and
compared are as follows: 1) LCL SRC with capacitive
VISWAMOHAN K, JAYAKRISHNA G
International Journal of Advanced Technology and Innovative Research
Volume. 06, IssueNo.09, October-2014, Pages: 1021-1026
output filter; 2) LCL SRC with inductive output filter; and
3) phase-shifted ZVS PWM full-bridge converter. It has
been shown that LCL SRC with capacitive output filter has
the desirable features for the present application. It has been
shown that none of the converters maintain ZVS for
maximum input voltage. However, it is shown that LCL-
type SRC with capacitive output filter is the only converter
that maintains soft-switching for complete load range at the
minimum input voltage while overcoming the drawbacks of
inductive output Filter. But the converter requires low value
of resonant inductor Lr for low input voltage design.
Therefore, it is better to boost the input voltage and then use
the LCL SRC with capacitive output filter as a second stage.
When this converter is operated with almost fixed input
voltage, duty cycle variation required is the least among all
the three converters while operating with ZVS for the
complete variations in input voltage and load. A ZVT boost
converter with the specified input voltage (40–60 V) will
generate approximately 100V as the input to the resonant
converter for Vo = 60V. Therefore, it is investigated that the
performance of a ZVT boost converter followed by the LCL
SRC with capacitive output filter.
VI. REFERENCES
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Author’s Profile:
K.VISWAMOHAN received B.Tech
degree in Electrical and Electronics
Engineering from Sri Raghavendra
Institute of Science and Technology.
Vinjamur, in 2012. He is currently
working towards M.Tech degree in
Power Electronics in Department of
Electrical and Electronics Engineering, Narayana
Engineering College, Gudur, A.P, India.
G.Jayakrishna received B.Tech,
M.Tech and Ph.D degrees in Electrical
Engineering from Jawaharlal Nehru
Technological University, Anantapur,
India in 1993, 2004 and 2013
respectively. Currently he is working
as professor & Head of Department of
Electrical and Electronics Engineering, Narayana
Engineering College, Gudur, A.P, India. His research
interests include Power Quality, Electrical drives and
Power Systems. He is life member of ISTE.