Control for Power Conditioner Based PEM Fuel Cell and Back-Up Power System with Supercapacitor

Juan C. Trujillo Caballeroc,Oriol Gomis-Bellmunta,b,c, Daniel Montesinos-Miracle b,c, Andreas Sumpera,b,c , Antoni Sudrià-Andreua,b,c ,Adrià Junyent-Ferrec.

a IREC Catalonia Institute for Energy Research, C. Josep Pla, 2, edifici B2, Planta Baixa, 08019 Barcelona, Spain. b Centre d’Innovació Tecnològica en Convertidors Estàtics i Accionaments (CITCEA-UPC), Departament d’Enginyeria Elèctrica, Universitat Politècnica de Catalunya ETS d’Enginyeria Industrial de Barcelona, C. Comte d’Urgell, 187, Pl. 1, 08036 Barcelona, Spain. c Centre d’Innovació Tecnològica en Convertidors Estàtics i Accionaments (CITCEA-UPC), Departament d’Enginyeria Elèctrica, Universitat Politècnica de Catalunya EU d’Enginyeria Tècnica Industrial de Barcelona, Av. Diagonal, 647,Pl. 2, 08028 Barcelona, Spain.

Abstract—This paper presents a new control strategy to operate a Proton Exchange Membrane Fuel Cell (PEMFC) 1,2kW and Supercapacitor (SC) through a hybrid converter proposed. The Fuel Cell (FC) has a slow dynamic device by its nature; therefore it is recommended to have an additional auxiliary power such as the SC to improve system performance in all its applications. So has been made a study of the DC/DC converters as well as control strategies employed recently used to operate the FC and SC system. This paper shows an experimental validation of the system which has been simulated in Matlab-Simulink. For this two test have been proposed using different load demands of power and cut off FC power . Under disturbances from FC voltage and load voltage these tests are possible to verify that the system achieves an excellent output voltage (Vo) regulation and SC Voltage (VSC) control.

Keywords-component; DC/DC Converters; Bidirectional converter; hybrid system; Supercapacitor, Fuel Cells and PI controllers.

I. INTRODUCTION

Fuel cells (FCs) are set to become the power source of the future [1]. The interest in FCs has increased during the last years due to the fact that the use of fossil fuels for power has resulted in many negative consequences [2]. Some of these include severe pollution, climate changes, melting of ice caps, rising sea levels, acid rains, ozone layer depletion, oil spills, forest and agricultural land damage etc [3]. A new power source is needed that is energy efficient, has low pollutant emissions, and has an unlimited supply of fuel [4]. FCs are now closer to commercialization than ever, and they have the ability to fulfill all of the global power needs while meeting the efficiency and environmental expectations [5]. PEMFCs are the most popular type of FC, and traditionally use hydrogen as the fuel [6]. In this sense FCs are an attractive power supply because they are clean, highly efficient, and reliable. FCS have higher efficiency than heat engines particularly at part load- zero emissions, good load-following characteristics and quick start-up time.

The FCs still have some disadvantages like: The response is slow caused by the time constant of the hydrogen and gas supply systems that can be in the order of several seconds [7]. In the mean time the FC may be starved of fuel which is not good for the electrocatalyst shortening its life [8]. Therefore, it becomes necessary to study the control strategies to operate the FC so it can mitigate the disadvantages itself. A DC/DC converter, like boost, offers higher efficiency and less component count comparated to other DC/DC converter topologies like half bridge, push pull, flyback and full bridge etc [9], which can be used to interface the FC system to the load [10]. So considering that the FC has a slow response, a secondary energy source is often desired to help maintain bus voltage during transients and start-up [11]. The SC are suitable choices and can be interfaced out the FC or at the high-voltage DC link via a bidirectional DC/DC converter. But an energy storage e.g. SC requires an additional circuit to manage the power flow operations during charging and discharging conditions [12]. Several researches have studied the different topologies with their respective control proposals to operate FC and SC. Sánchez-Squella and Ortega et al. [13] studied energy management in electrical systems fed by multiple sources. They proposed a topology that is used in some electric cars. To ensure energy exchange, the interconnection of the storage and load devices is performed by using power converters. In this case the power converters are electronically switched circuits capable of adapting the port voltage or current magnitudes to the desired value. The current and voltage are tracked with control loops, usually proportional integral (PI). Thoungthon et al. [14] studied control strategies of FC/SC hybrid power source for electric vehicles. He presented a control principle for utilizing FC as main power source which operates with a boost converter and SC as auxiliary power source which also operate a bidirectional converter called two quadrants chopper. The concept is that the hybrid system

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control proposed a regulation of the DC bus voltage through the power, only delivered by the fast auxiliary source. Cervantes Isle and Perez-Pinal F. et al. [15] studied hybrid control techniques applied in FC/SC electric vehicle platform. In this paper the proposed hybrid controller guarantees stability in the selected operation region, meanwhile it achieves an excellent output voltage control under disturbances either from supply voltage, load voltage or other perturbations. Gebre T. and Undelant M. et al. [16] Studied the control of converters to operate FC and SC. In this case the control objective is carried out by using an external DC-link state of charge control loop which takes the bus voltage set point as reference and produces a FC current reference. This outer control can be a simple proportional controller with gain set to a value which would draw maximum FC current when the DC link reaches the maximum depth allowed. The control strategy for the SC is determined by the status of DC bus voltage. In this paper a new control strategy has been developed for hybrid power system with the objective to operate FC and SC with their maximum efficiency. The system itself consists of a primary FC converter and on the other hand, an auxiliary SC converter. In this sense this paper intends to study the voltage regulating characteristics and its dynamic limitations under varying load conditions. Another important point is the cost considerations. This paper is organized as follows: the Section II presents a proposed configuration, which includes the control system. The Section III describes simulation results in Matlab-Simulink and the Section IV discusses the conclusion of the control implemented in hybrid converter.

II. PROPOSED CONFIGURATION.

A. Analisys and control of hybrid converter proposed to FC and SC system.

Fig. 1 shows the proposed power conditioner and the control structure of the system. The hybrid converter has two power supplies: FCs and SCs. The primary converter which is in this case a boost converter operates with the FC. Therefore it is called FC converter in this paper. The second converter a bi-directional converter is used to operate with the SC. And it is called SC converter. It operates in two modes, the first is used while the SC charges. Then the SC converter acts as a boost converter delivering energy to the resistive load and when the SC is charged then the SC converter acts as a buck converter delivering energy to SC. These two converters are connected in parallel through a DC bus. The proposed control of the FC converter is shown in fig.1 whose function is controlling the SC voltage (Vin-SC) and FC current (Iin-FC) through Proportional-Integral (PI) controllers that are connected in cascade, in this case PI_VSC and PI_IFC. While the structure of control for the SC converter has the

function of controlling the (V0) and current of the SC (ISC) through the cascaded PI controllers PI_Vo and PI_Isc. For managing the control system 4 measurements, are required, two for the currents IFC and ISC and two more for V0 and VSC.

FCα

SCα

Fig. 1

To analyze the operation of the system it is necessary to describe it in the following order: The FC module, FC converter and SC converter. A.1 FC module. A simplified model of the FC module has been developed to operate the electrical characteristics and dynamics of FC Ballard Nexa 1.2 kW seen in Figure 2. The simplified model represents a particular FC stack. A diode is used to prevent the flow of negative current into the stack [17].

Fig 2. Equivalent FC

This model is based on the equivalent circuit of the FC stack. Where:

))ln(( onOC iAENE −= (1)

FZRT

aα

= (2)

R=8.3145 J/(mol K) and F=96485 A s/mol.

nE = Nerns voltage, which is the thermodynamics voltage of the cells and depends on the temperatures and partial pressure of the reactants and products inside the stack.

oi =Exchange current, which is the current resulting from the continual backward and forward flow of electrons from and to the electrolyte at no load. α =Charge transfer coefficient, which depends on the type of electrodes and catalysis's used. A.2 FC converter. The scheme for the FC converter is shown in Fig.1. It has been an analysis in continuous conduction mode to get the value of the inductor of FC. In this case the FC converter has two possible states of the switch.

1) During turn on. It happens when IGBT (Q1) turns on, and diode (D4) is polarized in reverse. During this sequence the inductor stores energy while the capacitor keeps the output voltage. 2) During turn off. When Q1 is turned off, the inductor forces the diode D4 to conduct and it discharges the output energy in the load. The value of FC inductor (L1) is obtained from the equations (3) for the turn on time, where FCα is the duty cycle in the FC converter.

( )

FC1

1

_

IN FC

FCL ON

VL

i fsα−=

Δ

(3)

It is necessary to analyze the FC converter to determine the transfer function in this case the input current-control and output-control. Using the

_ 2

22 2

12 2( )' 1

' '

LOAD

IN FCid FC

LOAD

LOAD

R Cs

VG s

L LCD R s sD R D

−

+=

+ +

(4)

1 2_ 0 2

20 0

1 1( )

1

z zvd FC d

s sw w

G s Gs s

w Q w

+ −=

+ +(5)

1 2( )( ) Kp s Ki

C ss−

+= (6)

By simplifying the FC converter we get the transfer function of the closed loop.

)()(1

)()()(2

22 sGsC

sGsCsT

vd

vd

+= (7)

-60

-40

-20

0

20

Mag

nitu

de (

dB)

100

101

102

103

104

90

135

180

225

270

315

Pha

se (

deg)

Bode Diagram

Frequency (Hz)

Fig.3

A.3 SC converter

Considering his FCs slow response, an auxiliary converter is needed to help maintain bus voltage during start-up and transient. In Fig. 1 is show the SC converter based on bidirectional converter which operated in mode buck for charge and boost for discharge. The main characteristics of this topology is the current IL2 because it can flow from the bus voltage to the SC and vice-versa, however, the Vo in bus voltage must be always greater than the voltage of SC (Vsc). There are two operating modes described below:

A.3.1 Charge Mode

When the SC is being charged, IL2 is positive, and Q2 and D3 alternately conduct current, thus the bidirectional converter operates in buck mode. The analysis for obtaining the inductor value of this converter in Buck mode is shown below.

02

2 ( _ )

( )SC SC

L O N SC s

V VL

i fα− ⋅=

Δ

A2.2 Charge Mode

When the SC is being discharged, IL2 is negative, and Q3 and D2 alternately conduct, thus the bidirectional converter operates in boost mode. The analysis for receiving the output capacitor value is obtained from this equation (17).

00

0 0

SC PC

V V fsα ⋅=

Δ ⋅ ⋅ Fig.1 shows the control scheme for the bidirectional converter, there are two control loops a current (IIN-SC) and an output voltage (Vo) loop. For the design of the controllers, two PIs has been used. Analyzing the SC converter in small signal it is possible to obtain the transfer functions for current and voltage. In this case it is done when the SC operates on boost mode. The equation (10) corresponds to the transfer functions for input current-control:

_ 22

2 2

12 2( )' 1

' '

LOAD

IN SCid SC

LOAD

LOAD

R Cs

VG s

L LCD R s sD R D

−

+=

+ +

1 2_ 0 2

20 0

1 1( )

1

z zvd SC d

s sw w

G s Gs s

w Q w

+ −=

+ +

3 4( )( ) Kp s Ki

C ss−

+= (12)

By simplifying the SC converter transfer function of the closed loop is received.

4 _

44 _

( ) ( )( )

1 ( ) ( )vd SC

vd SC

C s G sT s

C s G s=

+ (13)

For choosing the coefficient values of the controllers the same criteria as for the FC converter is used. In this case the PWM signal generation is applied to the transistors Q3 and Q4, respectively.

TABLE II CONTROLLERS TO SUPER-CAPACITOR CONVERTER

Fig.4

III. SIMULATION RESULTS In this section the proposed control to operate a hybrid converter has been simulated in Matlab-Simulink with objective to study their response in closed loop. As a result it has been proposed to operate the hybrid module in two scenarios; the first is connecting a variable load and the second consists in cutting off the FC energy operating with a constant load.

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The specifications of converters are shown in table III.

TABLE III SPECIFICATIONS TO OPERATE FC AND SC

CONVERTERS

FC converter

VVin 2633−= 0.8LI AΔ =

1 1L mH= kHzfs 20=

* 25SCV V=

WP 5000 =

SC converter

30inV V= 2 3.4L mH=

*

0 80V V=

AIL 2.0=Δ

WP 5000 =

kHzfs 20=

A.1 Test with variable load

For this test it has been considered that the SC was fully charged at 30V,the graphs obtained from voltage, current and power for this test are shown below.

A.1.1 Voltage waveforms

Fig. 5 shows the Vo regulation where the value is 80V. This value remains constant with changes in load power. Also it can be seen that the value VIN-SC remains constant. Some changes are observed in VIN-FC synchronized with changes in load power.

0 1 2 3 4 5 6 7 8 9 10-20

0

20

40

60

80

100

Time (Sec)

Vol

tage

(V

)

Vin FCVin SCVo LoadVo REFVSC REF

Fig.5 Voltage measurements.

A.1.2 Current waveforms

Fig. 6, shows the evolution of IIN-FC, IIN-SC and I0 load, it can be seen that IIN-SC helps IIN-FC. It makes sense because intrinsically the dynamic response of FC is slow and therefore it requires an auxiliary device that helps deliver the energy required in the load with a rapid response. Also the system is capable of recharging the SC and always trying to keep it charged fully for any eventuality or disturbance which might exist in the load.

0 1 2 3 4 5 6 7 8 9 10-10

-5

0

5

10

15

20

25

30

35

Time (Sec)

Cur

rent

(A

)

Iin FCIin SCIo Load

Fig.6

A.1.2 Power waveforms

Fig. 7 shows the evolution of the powers, starting by FC, SC and load, it can be seen that the back-up system is able to help the FC when there are load variations. Also the structure of the hybrid converter is able of recharging the SC.

0 1 2 3 4 5 6 7 8 9 10-200

0

200

400

600

800

1000

Time (Sec)

Po

we

r (W

)

Po LoadPo REFPin SCPin FC

Fig.7 Evolution power measurements.

A.2 Test with cut off FC energy.

In this section, the hybrid system had a constant load, which has made it a cut off power by the FC, in order to observe the response from the back-up system.

A.2.1 Voltage waveforms.

0.9 0.92 0.94 0.96 0.98 1 1.02 1.04 1.06 1.08 1.120

30

40

50

60

70

80

90

Time (Sec)

Volta

ge (V)

Vin FCVin SCVo Load

Fig.8 Voltage waveforms.

A.2.2 Current waveforms.

0.9 0.92 0.94 0.96 0.98 1 1.02 1.04 1.06 1.08 1.1-5

0

5

10

15

20

25

Time (Sec)

Cur

rent

(A

)

Iin FCIin SCIo Load

Fig.9 Response of currents when failure FC.

A.2.3 Power waveforms.

Fig. 10 shows the time evolution of PIN-FC, PIN-SC and P0 . A cut off of the FC is shown after 1 second of simulation. After this the back-up system is responsible for providing energy demanded by load, so this behavior describes a fast response by the control action which sends immediately the order to the supplies power backup system.

0.9 0.92 0.94 0.96 0.98 1 1.02 1.04 1.06 1.08 1.1-100

0

100

200

300

400

500

600

Time (Sec)

Pow

er (

W)

Pin FCPin SCPo Load

Fig.10

A.2.3 Economic analysis.

Total cost of ownership and operation of FCs will be a critical factor in their commercialization, along with the offered functionally and performance. These components include fuel cost, other operating cost such as maintenance cost, and the first cost of the equipment. This first cost has a significant impact on FCs competitiveness of this technology, the hybrid converter is more cheaper than FC module.

IV. CONCLUSION In this article the proposal for a new control strategy to operate a hybrid converter that includes a primary converter called FC converter, and an auxiliary converter called SC converter is shown. The control system, operates with 4 PIs controllers which were implemented in Matlab-Simulink together with the hybrid converter of the FC/SC. According to the results, the control system gets an excellent performance because it is able to compensate V0 and VIN-SC with changes of load and cut off energy. In this sense this proposed control provides a good platform to design a control system suitable for a power converter system for FC/SC applications.

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