soft-switching techniques for dc-to-dc converters in ... · couple the electrolyzer to the system...

www.ijatir.org

ISSN 2348–2370

Vol.06,Issue.09,

October-2014,

Pages:1021-1026

Copyright @ 2014 IJATIR. All rights reserved.

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



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




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



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




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

[1] A. P. Bergen, “Integration and dynamics of a renewable

regenerative hydrogen fuel cell system,” Ph.D. dissertation,

Dept. Mechanical Eng., Univ. Victoria, Victoria, BC,

Canada, 2008.

[2] D. Shapiro, J. Duffy, M. Kimble, and M. Pien, “Solar-

powered regenerative PEM electrolyzer/fuel cell system,”J.

Solar Energy, vol. 79, pp. 544–550, 2005.

[3] F. Barbir, “PEM electrolysis for production of hydrogen

from renewable energy sources,” J. Solar Energy, vol. 78,

pp. 661–669, 2005.

[4] A. K. S. Bhat, “Analysis and design of a fixed-frequency

LCL-type series resonant converter with capacitive output

filter,” IEE Proc.: Circuits, Devices Syst., vol. 144, no. 2,

pp. 97–103, Apr. 1997.

[5] A. K. S. Bhat, “Analysis and design of LCL-type

resonant converter,” IEEE Trans. Ind. Electron., vol. 41, no.

1, pp. 118–124, Feb. 1994.

[6] A. K. S. Bhat, “Analysis and design of a fixed frequency

LCL-type series resonant converter,” IEEE Trans. Aerosp.

Electron. Syst., vol. 31, no. 1, pp. 125–137, Jan. 1995.

[7] L. H. Mweene, C. A. Wright, and M. F. Schlecht, “A 1

kW, 500 kHz frontend converter for a distributed power

supply system,” in Proc. IEEE Appl. Power Electron. Conf.,

1989, pp. 423–432.

[8] J. A. Sabate, V. Vlatkovic, R. B. Ridley, F. C. Lee, and

B. H. Cho, “Design considerations for high voltage, high

power, full-bridge ZVS PWM converters,” in Proc. IEEE

Appl. Power Electron. Conf., 1990, pp. 275–284.

[9] S. V. Dhople, A. Davoudi, A. D. Dominguez-Garcia,

and P. L. Chapman, “A unified approach to reliability

assessment of multiphase DC–DC converters in

photovoltaic energy conversion systems,” IEEE Trans.

Power Electron., vol. 27, no. 2, pp. 739–751, Feb. 2013.

[10] Y. Jang and M. M. Jovanovic, “A new family of full-

bridge ZVS converters,” IEEE Trans. Power Electron., vol.

19, no. 3, pp. 701–708, May 2004.

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.

soft-switching techniques for dc-to-dc converters in ... · couple the electrolyzer to the system...

Documents