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Accepted Manuscript Power source protection method for hybrid polymer electrolyte membrane fuel cell/ lithium-ion battery system Ya-Xiong Wang, Kai Ou...

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Accepted Manuscript Power source protection method for hybrid polymer electrolyte membrane fuel cell/ lithium-ion battery system Ya-Xiong Wang, Kai Ou, Young-Bae Kim PII:

S0960-1481(17)30279-3

DOI:

10.1016/j.renene.2017.03.088

Reference:

RENE 8681

To appear in:

Renewable Energy

Received Date: 21 June 2016 Revised Date:

22 March 2017

Accepted Date: 28 March 2017

Please cite this article as: Wang Y-X, Ou K, Kim Y-B, Power source protection method for hybrid polymer electrolyte membrane fuel cell/lithium-ion battery system, Renewable Energy (2017), doi: 10.1016/j.renene.2017.03.088. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Power Source Protection Method for Hybrid Polymer

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Electrolyte Membrane Fuel Cell/Lithium-ion Battery

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System

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Ya-Xiong Wang a, Kai Ou b, and Young-Bae Kim b,*

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a

School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou 350116, China

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b

Department of Mechanical Engineering, Chonnam National University, Gwangju, Republic of Korea

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* Corresponding author. Tel.: +82 62 5301677.

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E-mail address: [email protected] (Young-Bae Kim).

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Abstract

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Polymer electrolyte membrane fuel cell hybridized with lithium-ion battery possesses significant

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advantages, including the combination of large energy carrier feature with high power density to provide

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a power source for large fluctuated areas such as a vehicle or a construction equipment. A hybrid system

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obviously requires a suitable power management means to distribute each power source optimally and

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ensure safe and efficient power system operation. This study investigates hybrid system power

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distribution and the protection of power sources, namely, PEMFC and/or LIB, to extend their lifetimes

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under the condition of external load variations. Power distribution with the purpose of power source

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protection is developed to balance the power and stabilize the DC-link voltage with the developed hybrid

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model. In particular, two new power splitting methods are proposed: coordinated current–voltage control

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and dual-voltage control. Moreover, these two control schemes are selected depending on the threshold

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load current. The threshold load current is decided by fuzzy logic rules to prevent power shortage in

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PEMFC by current control for higher load and to regulate LIB’s state-of-charge for lower load. To

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validate the proposed power management approach, experimental tests are conducted on a hybrid

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PEMFC/LIB power system prototype.

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Keywords: power management; fuel cell protection; coordinated current–voltage control; dual-voltage

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control; equivalent electrical circuit model

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1. Introduction Polymer electrolyte membrane fuel cell (PEMFC) applied to the dynamic power systems such as

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automotive industry exhibits many advantages because of its high energy density, low operation

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temperature, zero emission, and high efficiency. To address PEMFC’s relatively low power density

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obstacle to transient load demands, batteries and/or supercapacitors are generally utilized in highly

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electric power systems to overcome PEMFC’s slow power response while retaining its advantage of high

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energy density [1–5]. In a hybrid power system, PEMFC functions as the main power source that shares

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the high steady-state load demand, and a battery and/or a supercapacitor act as the energy storage

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subsystem. They function as a transient power supply, overload carrier, and external power storage device

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[6]. DC/DC power converters of unidirectional and bidirectional types are generally employed to regulate

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power delivery in hybrid power systems [4, 7]. Even though dual converter system shows some

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complexities, it has been known that the dual converter structure displays advantages in flexibility,

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multifunctionality, and power source management [8] because two converters can be directly utilized to

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control the voltage and/or current of power supplies separately. The hybrid PEMFC/lithium-ion (LIB)

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power system structure with dual converters as shown in Fig. 1 has been applied to hydrogen fuel cell

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vehicles.

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The power or energy management strategies involved in operating hybrid PEMFC-based power

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systems safely and efficiently have captured the attention of many researchers. One of the power

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management objectives is to minimize the fuel consumption or reduce the operation cost. Optimal control

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of hydrogen consumption is considered one of the most effective means to improve the fuel economy of

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hybrid PEMFC-based power systems to satisfy this objective [9–13]. Zheng at al. [9] proposed fuel

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consumption minimization control based on the minimum principle with and without a battery state-of-

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charge (SOC) constraint. They compared the optimization effects on battery performance. Meanwhile, the

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optimal control method based on Pontryagin’s minimal principle (PMP) was studied in comparison with

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dynamic programming (DP) and charge depleting charge sustaining (CDCS) in fuel cell hybrid vehicle

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energy management [10]. The results demonstrated that PMP is the best solution among the three

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algorithms that minimized the operation cost for both simulation and real-time implementation. Currently,

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convex programming [11] and Markov chain prediction model based on PMP [12] are employed to

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regulate hybrid PEMFC vehicles and obtain improved performance that entails reduced computation time

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and a good power demand response.

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Another objective of power management is to prolong the lifetime of PEMFC. Some researchers

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studied the power management using optimal energy scheme based on the trade-off between PEMFC

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lifetime and fuel consumption, as explored in Ref. [13]. Optimal control-based energy management in this

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direction is important to extend the lifetime of expensive fuel cell vehicles and decrease the driving cost;

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however, this method is rarely implemented in actual fuel cell system application because the calculation

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procedure to obtain the optimization solution is highly complicated and the power splitting strategy with

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voltage and/or current regulation is deficient because DC-link voltage is in unstable and results in low

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efficiency for the electrical motor, which is connected through inverters. Moreover, PEMFC is generally

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unregulated, which may cause fuel starvation and even system performance degradation or fatal fuel cell

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breakdown.

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To address the essence of hybrid system power distribution and DC-link power conditioning, namely,

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each power source voltage and current regulation, a basic level of power management has been

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investigated. Thounthong et al. [14] studied a PEMFC protection power management method by using

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PEMFC current, battery current, and battery SOC cascade control to balance load power demand, regulate

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DC-link voltage, and protect PEMFC from fuel starvation (e.g., limiting current slop within 4 A·s-1 for a

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500 W PEMFC). To achieve improved DC bus stabilization with fast response characteristics to feed the

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subsequent motor system, nonlinear flatness-based controllers [15] have been proposed to regulate DC-

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link voltage and distribute power to each energy carrier. Motapon et al. [16] investigated a wavelet

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transform-based stress analysis method for energy sources to evaluate their life cycle. The histogram

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results showed that the frequency decoupling and fuzzy logic (FL) scheme leads to constant output power

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of PEMFC, indicating minimal stress. In the same manner, a power management strategy of realizing

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PEMFC power regulation, in other words, PEMFC current control, and DC-link voltage stabilization has

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been recently proposed to split the hybrid PEMFC-based power system. For example, polynomial control

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technique [17] was employed to conduct DC bus voltage and current control to split the power and protect

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PEMFC from transient currents of the load, which may result in accelerated aging. A PEMFC protection

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control-oriented management strategy that achieves both PEMFC output power and DC-link voltage

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stabilization was discussed in Ref. [18]. Simoes et al. [19] proposed a FL-based decision-making strategy

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to split the hybrid system power and force PEMFC and the battery to operate in the range of best

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performance; however, DC-link voltage regulation was ignored. Voltage control-oriented and current

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control-oriented techniques for fuel cell hybrid power source regulation were reported in detail by Bizon

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[20, 21]. The developed nonlinear controllers stabilized the hybrid power system’s output voltage at a low

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voltage ripple under voltage control mode and regulated the stack current of PEMFC governed by current

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control mode separately. Model predictive control (MPC) [22, 23] introduced a possible means to

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determine the reference current for each power source under various load profiles to ensure balance in the

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distributed power. The demonstrated methods were mostly validated by experimental results, and the

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obtained performance was relatively limited in terms of power conditioning and power source protection.

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In summary, most researchers in this direction mainly focused on power management by considering

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three objectives, namely, DC-link voltage stabilization, PEMFC current regulation, and LIB SOC

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regulation, to implement load demands for balance and power source protection. The present study aims

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to meet all the requirements of efficient power management by introducing new power splitting

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approaches. The proposed power splitting method using control of voltage and/or current coordinately is

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different from normal cascade current–voltage control [16–18], which may require the tuning of 5

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complicated control parameters. The proposed power splitting method can be divided into two types

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based on LIB operation mode (charge and discharge): coordinated current–voltage control (PEMFC

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current and LIB-fed converter output voltage) and dual-voltage control (PEMFC-fed converter output

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voltage and LIB voltage). Given that the proposed approach has two modes, the mode switching rule is

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very important and the fuzzy logic (FL) rule is implemented for the switching control between two modes.

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The contributions of this study are summarized as follows: power balancing with DC-link voltage

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stabilization and power source protection and power splitting by coordinated current–voltage control and

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dual-voltage control according to FL rules to satisfy the three power management objectives.

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The rest of the paper is organized as follows. The design and modeling of the hybrid PEMFC/LIB

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power system are described in Section 2. Section 3 presents the development of a control approach to

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protect the power sources of the hybrid system. Section 4 provides the simulation results and

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experimental validations. The conclusion is presented in Section 5.

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2. Hybrid PEMFC/LIB Power System Modeling

The DC hybrid power supply system is comprised of PEMFC, LIB, a DC/DC boost converter, and a

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bidirectional DC/DC converter. Many studies have been performed on PEMFC modeling with various

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purposes; however, an accurate and simple control-oriented dynamic model is required for power

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management study. PEMFC dynamics with auxiliary subsystem modeling techniques are usually

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employed to control the airflow rate and pressure [24, 25]. However, the presented models have numerous

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state variables that pose a real-time control challenge. A thermal model of PEMFC [26] described the

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energy conservation of released electrochemical energy, produced electrical power, and thermal energy

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generation and its loss; however, thermal dynamics is relatively slower than electrical dynamics, such as

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voltage or current response, which is unsuitable for hybrid power system control application. Similarly, a

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number of LIB thermal models [27, 28] have been presented to represent temperature-related

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characteristics, such as SOC variations, but they are insufficient for control purposes. Therefore, simple

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but accurate models of two power sources are necessary. In this study, both dynamical models of PEMFC

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and LIB are presented by an equivalent electrical circuit (EEC) model to represent the voltage transients.

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DC/DC converters are moderately modeled by average-value models [16] that disregard switching

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harmonics but retain all converter dynamics. A DC hybrid PEMFC/LIB power supply system model with

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the abovementioned sub-models is constructed with MATLAB/Simulink.

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2.1 Power sources modeling

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The static value of PEMFC output voltage VFC is presented by the following electrochemical relation

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[24].

VFC = n(ENernst − Vact − Vohm − Vcon )

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(1)

The voltage variables are defined in Table 1 in detail. According to the PEMFC EEC model using a

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variable double-layer capacitor Cact [7, 29], the voltage transient response of PEMFC vFC can be obtained

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as follows:

vFC = n(E Nernst − VC − Vohm − Vcon ) ,

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and

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where VC denotes the capacitor terminal voltage to simulate the dynamical loss and is determined by [7,29]

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(2)

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dVC 1 = dt C act

Cact =

 V  I FC − C R act 

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ξ5 ⋅ Ract

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,

  , 

(3)

(4)

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where Ract is the equivalent resistance of activation polarization calculated by [29]

Ract =

Vact . I FC

(5)

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The updated PEMFC voltage model describes the V-I static relation (polarization curves) and portrays

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the dynamics response with input current variations. The modeling parameters and experimental

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validation of the presented model can be found in our previous publications [4, 7].

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With respect to LIB modeling, a block function of MATLAB/Simulink is directly utilized. The LIB

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operation involves two separate discharge and charge modes. The discharge characteristic of LIB is

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presented by [30]

VLIB = E0 −

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K ⋅ Qm * K ⋅ Qm ⋅i − ⋅ Q + α ⋅ exp(− β ⋅ Q ) , Qm − Q Qm − Q

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Meanwhile, the charge mode of the battery is calculated by the following [30]:

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VLIB = E0 −

K ⋅ Qm K ⋅ Qm ⋅ i* − ⋅ Q + α ⋅ exp(− β ⋅ Q ) , Q + 0.1Qm Qm − Q

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where E0 is the constant voltage of LIB, K denotes the polarization coefficient, Qm is the maximum

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battery capacity, the extracted capacity is represented as Q (Q = ∫idt), the filtered battery current is

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provided by i*, and α, β represent the exponential voltage and capacity, respectively [30]. SOC is one of

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the crucial parameters of LIB determining its performance and life cycle. SOC is estimated by the

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Coulomb counting method as follows:

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SOC = SOC0 −

where SOC0 is the initial value of SOC. 8

t 1 ⋅ ∫ idτ . 0 Qm

(8)

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2.2 DC/DC power converter modeling The model of the DC hybrid power supply system is schematically shown in Fig. 2. This figure also

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shows the EEC models of PEMFC and LIB. The DC/DC converters are composed of a series of inductor

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L, capacitor C, diode D, and MOSFET switch S. The boost converter enhances the PEMFC voltage to

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provide high regulated output voltage and high torque for subsequent motor systems. The bidirectional

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converter is utilized to improve LIB voltage and decrease the charging voltage of LIB. The converters

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assume that operations are performed with continuous conduction mode. The equivalent series resistances

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of the inductors and capacitors are disregarded in the converter model. Then, the boost converter can be

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modeled as follows:

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V v I&FC = −(1 − u1 ) DC + FC L1 L1 I v V&DC = (1 − u1 ) FC − FC C1 rFCS C1

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Moreover, the bidirectional DC/DC converter is considered a combination of boost and buck functions,

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which can be described by the following:

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(for the boost mode)

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(9)

V V I&L = −(1 − u 2 ) DC + LIB L2 L2 , and u3 = 0 , I VLIB L V&DC = (1 − u 2 ) − C 2 rLIB ,disch C 2

(10a)

V V I&L = − LIB + u3 DC L2 L2 , and u 2 = 0 . I VLIB L V&LIB = − C3 rLIB ,ch C3

(10b)

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(for the buck mode)

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where u1, u2, and u3 are the manipulated variables of MOSFET switches by obtaining the value from the

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set of {0:1}, VDC denotes the voltage of the DC link, rFCS represents the resistance shared by the PEMFC-

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fed converter, rLIB,ch is the charging resistance of LIB, and rLIB,disch is the resistance shared by the boost

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functional converter of LIB. The parameters applied in the hybrid PEMFC/LIB power system modeling

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are listed in Table 2.

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3. Power Source Protection Control Approach

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Power balancing between PEMFC and LIB with DC-link voltage regulation is primarily required to

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implement the hybrid PEMFC/LIB power system; however, external load varies unpredictably and may

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cause a sudden change in PEMFC and/or LIB power, which can in turn result in fuel starvation of

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PEMFC for large external load or high-power filling into LIB for small external load. The frequent

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sudden power peak shortens the lifetime of PEMFC and LIB. To protect the power sources of the hybrid

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PEMFC/LIB system from these sudden power peaks and unregulated charging voltage, a new power

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management approach with the purpose of power source protection control is designed, as shown in the

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flowchart in Fig. 3. The new approach is mainly developed to accomplish three objectives, i.e., power

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balancing with DC-link voltage stabilization, PEMFC current regulation under large external load, and

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LIB charging regulation at small external load. Therefore, the proposed method applies two power

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splitting structures, namely, coordinated current–voltage control and dual-voltage control, to address

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PEMFC current reference tracking and LIB charging regulation, respectively. Another contribution of the

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present power splitting method is the use of fuzzy logic (FL) rules to select the power splitting guidance

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between coordinated current–voltage control and dual-voltage control in accordance with PEMFC power,

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PEMFC current slope, and LIB SOC to improve hybrid system performance.

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3.1 Power splitting structures

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Two power splitting structures, namely, coordinated current–voltage control loop and dual-voltage

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control loop, are proposed, as shown in Fig. 4. The main concept of the coordinated current–voltage

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control (Fig. 4(a)) is to regulate the combination of PEMFC current control and LIB-fed bidirectional

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converter boost mode output voltage control. In other words, the controlled variables are IFC and VDC. The

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control loop is utilized for large load cases to guarantee PEMFC current limitation to protect the fuel cell

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and DC-link voltage stabilization simultaneously. Meanwhile, Fig. 4(b) describes dual-voltage control,

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which is implemented by controlling PEMFC-fed boost converter output voltage to stabilize DC-link

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voltage and regulating LIB charging voltage when the load is small. VDC and VLIB are the controlled

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variables in the dual-voltage control loop. To address the low-level controller design to regulate the

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controlled variables, the converters’ voltage and/or current proportional–integral (PI) control is applied.

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Parameter tuning for PI controllers is implemented based on the frequency responses with heuristics to

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complement the experimental results because the hybrid system mode is frequently switched according to

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LIB charge and discharge modes; this condition may render the system unstable if theoretical control

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tuning parameters are utilized.

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3.2 Fuzzy logic (FL) control of current threshold decision

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Given that the power splitting method has two structures, control mode switching is important to

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determine the performance of the hybrid system. The threshold current of the load, Ith, is defined as the

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decision variable of switching the two power splitting structures. If the threshold current is high, PEMFC

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may supply peak load and LIB charging load simultaneously, which is not good for PEMFC long-time

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operation because of the high burden on the fuel cell. Meanwhile, a small threshold value may cause LIB

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over-discharge. Moreover, considering that the PEMFC current slope denotes the current stress of

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PEMFC [16], its high value reveals that PEMFC is suffering from transient demand that may result in a

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rapid change in fuel cell power, thereby causing instant fuel or air starvation. Consequently, an advanced

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FL control method is proposed to decide threshold current, Ith, by considering the power level of PEMFC,

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the slope of PEMFC current, and LIB SOC. The power of PEMFC, PFC, is selected within the range of [0

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to 120 W] and [−10 A/s to 10 A/s] for the slope of PEMFC current, I&ˆFC , and within [30% to 80%] for

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SOC input. The fuzzification of PFC and Iˆ&FC is based on the trapezoidal-shaped membership functions,

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and SOC applies the symmetric Gaussian function. The three degrees of the input membership functions

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are L (low), M (middle), and H (high), and their boundaries are decided with heuristics. Output Ith is

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divided into three groups: L, M, and H. Fig. 5 shows the FL distribution rules in determining the

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threshold Ith membership degree. All boundaries of the membership functions of inputs and output are

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adjusted through several experiments.

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The manipulated variables of the hybrid PEMFC/LIB power system based on the two power splitting

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structures integrated with FL rules can be obtained as follows:

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(for coordinated current–voltage control)

u1 = K P (I FC , ref − I FC ) + K I

u 2 = K P (V DC , ref − V DC

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2

u3 = 0

(for dual-voltage control)

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I2

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u3 = K P (VLIB ,ref − VLIB ) + K I

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FC , ref

u1 = K P (VDC ,ref − VDC ) + K I u2 = 0

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∫ (V

− I FC )dt

DC , ref

− V DC )dt , when I Load > I th ,

DC , ref

− VDC )dt

∫ (V

LIB , ref

− VLIB )dt

, when I Load ≤ I th .

(11a)

(11b)

4. Simulation and Experimental Results

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The numerical simulations and experimental validations are presented in this section. The test

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conditions involving step load and ramp load profiles are listed in Table 3 to verify the efficacy of the

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proposed power protection control method. Power downsized ECE-15 driving cycling is applied to

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demonstrate the possible application of the proposed power management rule in PEMFC vehicles.

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4.1 Simulation results

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First, the proposed power management approach is validated in MATLAB/Simulink with the hybrid

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PEMFC/LIB control-oriented model. Figs. 6(a) and 6(b) show the voltage step responses by using

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coordinated current–voltage control and dual-voltage control, respectively. In the coordinate current–

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voltage control loop, VDC,ref and IFC,ref are tracked well, leading to DC-link voltage and PEMFC voltage

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being stabilized simultaneously. Meanwhile, VDC and VLIB are well regulated in dual-voltage control, as

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shown in Fig. 6(b). The present power splitting method is tested under Condition 2, in which step and

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ramp loads are mixed. Fig. 7(a) shows the voltages of the hybrid PEMFC/LIB system based on the

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coordinated current–voltage control. Although the external load exceeds the threshold value, DC-link

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voltage tracks the reference value well, with only 1.628 V undershoot when abrupt disturbance is

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encountered at 9.5 s. The varying external load is covered mainly by LIB power because PEMFC

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maintains constant power to protect the fuel cell. The load current profile and current responses of

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PEMFC and LIB are shown in Fig. 7(b), which demonstrates that PEMFC current is fixed at 6 A except

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for a small overshoot of 0.362 A at 9.5 s when the current disturbance suddenly engaged. When the load

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is smaller than the threshold value, dual-voltage control is implemented, as shown in Fig. 8. LIB is

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charged by PEMFC with a constant charging voltage, and the load change is delivered by PEMFC power

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with a stabilized DC-link voltage. Subsequently, the integrated power splitting approach combining

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coordinated current–voltage control and dual-voltage control is tested under Condition 3. The voltage and

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current results are shown in Fig. 9, wherein the DC-link voltage is stabilized at 24 V, LIB is charged at a

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constant voltage of 16.8 V for a low-level load, and PEMFC current is regulated when high external load

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is applied. The tests of Conditions 2 and 3 involve abrupt disturbance changes, and the maximum over-

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/under-shoot of DC-link voltage caused by these disturbance variations (Fig. 9(a) at 7 s) is 6.636 V as the

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power splitting structure changes. Overall, the simulation results indicate that the proposed power

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splitting methods can distribute power with voltage and/or current regulation as expected.

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4.2 Experimental implementations

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Experimental tests are conducted to validate the real performance of the proposed power management

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approach and power source protection method. The hybrid PEMFC/LIB power system prototype consists

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of 100 W PEMFC, 2400 mAh LIB, a unidirectional DC/DC boost converter, and a bidirectional DC/DC

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converter. Although the experimental system has small power, our experimental system has the

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same fuel cell hybrid vehicle topology; therefore, the algorithm we developed can be directly used

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for the real vehicular system. The signal inputs and outputs of the hybrid power system are implemented

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by NI DAQ devices, including PCI-6229 board, SCXI-1000 chassis, SCXI-1125, and SCXI-1313A signal

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conditioner. The DC load is generated by an electronic load bank. The experimental test platform for the

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hybrid power system prototype based on NI DAQ devices is shown in Fig. 10.

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Condition 1 is the first to be tested in the experimental platform. Please refer to Table 3 for the detailed

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experimental conditions. The voltage transient responses obtained by employing coordinated current–

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voltage control and dual-voltage control are shown in Figs. 11(a) and 11(b), respectively. The step

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response of the dual-voltage control is faster than that of the coordinated current–voltage control.

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However, the experimental results exhibit a slower response than that in the simulations for the two

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power splitting methods. Step and ramp load variations are applied to test the hybrid power system

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prototype. Fig. 12(a) shows the coordinated current–voltage control results under Condition 2; the results

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confirm that DC-link voltage and PEMFC voltage are regulated well, and LIB shares the varied load

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power. Conversely, LIB voltage and DC-link voltage are stabilized under dual-voltage control, as shown

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in Fig. 12(b), where the changed load is provided by PEMFC. The result of DC-link voltage regulation

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shows that the two power splitting methods display small (approximate 1 V) DC link voltage deviations

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under the ramp load condition. The deviations are considered by the slow response characteristics of the

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PI controllers. Fig. 13 presents the voltage response of the two integrated power splitting structures. The

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result shows that the power is distributed by two power sources with DC-link voltage tracking set-point.

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Before mode switching (see Stage I), LIB voltage is fixed to be charged, and after the mode change as

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shown on Stage II, PEMFC voltage is regulated to be protected. The experimental results for Conditions 1

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to 3 confirm that the proposed power source protection power management approach is suitable for real

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hybrid PEMFC/LIB power system prototype applications. The power sources are protected in both low-

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and high-load conditions, and the DC-link voltage is well regulated, which proves that this power

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management scheme works well in fuel cell vehicle operation.

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4.3 Power downsized driving cycling simulation

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To further examine the possible application of the proposed scheme for PEMFC vehicle control, power

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downsized ECE-15 driving cycling is applied to verify the developed approach. Fig. 14 reveals the

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response performance in detail. As our objective is to see the protection of the fuel cell and to keep

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battery SOC constant, the reduced ECE-15 driving cycle with 200 s period is used in our study without

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changing the slew-rate of power. DC-link voltage tracks the reference voltage with an acceptable error

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range. The over-/under-shoots are mainly caused by threshold change and mode switching. Given that

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ECE-15 cycling is a low-speed cycle, LIB is charged with regulated charging voltage most of the time.

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When a large load is applied, the power splitting method may switch to the coordinated current–voltage

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control based on FL control of the threshold, which leads to PEMFC power stabilization, LIB discharge,

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and sharing of the varied load power. The LIB charging and PEMFC protection areas are indicated in Figs.

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14(a) and 14(b). The proposed power management approach is applied to limit LIB SOC within a desired

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range. Different initial SOCs of the hybrid power system are controlled and evaluated based on FL rules

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and a constant current threshold. Fig. 14(c) indicates that FL-based SOC is relatively improved at low

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initial SOC, and FL-based SOC decreases at high initial SOC, which can benefit LIB lifetime in long-

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term application. The real power distribution is described in Fig. 14(d). The load power is well fitted with

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the desired power profile, and it is suitably shared by PEMFC and LIB via the proposed power source

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protection control approach.

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5. Conclusion

A power source protection control approach was developed based on proposed power splitting

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structures with FL rules to balance the load demand, regulate DC-link voltage, and regulate PEMFC

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current or LIB charging voltage. A control-oriented hybrid PEMFC/LIB power system model was first

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constructed and verified with EECs. The EEC modeling techniques reduced the modeling process while

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retaining the important dynamical characteristics of the hybrid system. The power source protection

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control power management strategy was proposed to fulfill power distribution with voltage and/or current

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regulation. The developed method is composed of two power splitting control structures: coordinated

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current–voltage control and dual-voltage control. Mode switching control was employed according to FL

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rules. Then, the power management strategy was designed and simulated based on the control-oriented

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hybrid power system model. The simulation results indicated that the coordinated current–voltage control

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stabilized DC-link voltage and regulated PEMFC current at a large load level. Dual-voltage control

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tracked the DC-link voltage reference and LIB charging voltage setpoint when the load was small. The

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mixed load condition was also tested with the power splitting structures. To validate the efficacy of the

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proposed power management strategy, experimental implementation was conducted with large, small, and

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mixed load profiles. The experimental results were in good agreement with the simulation results.

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Furthermore, power downsized ECE-15 driving cycling was employed to examine the feasibility of the

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proposed method for real application. The results showed that the power demand and DC-link voltage

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were balanced and stabilized well, PEMFC was protected at a large load demand, and LIB charging was

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regulated at a small load demand by using load level determination with FL rules.

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The new power splitting method via coordinated current–voltage control and dual-voltage control can

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benefit low-level controller design because control parameter tuning is much easier than that in normal

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cascade control. Moreover, FL rules are applied to manage mode switching that improves LIB SOC

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management by locating it within a better operation range. The proposed power source protection control

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provides a new possible means to distribute the power and regulate the voltage and current of the hybrid

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PEMFC/LIB power system in vehicular applications.

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Acknowledgement

This work was supported by the National Research Foundation of Korea (15H1C1A1035825 and

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15R1A4A1041746), Korea Electric Power Company (KEPRI-16-07), and Fujian Provincial Collaborative

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Innovation Center for High-end Equipment Manufacturing.

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List of figures Fig. 1. Hybrid PEMFC/LIB power system for vehicular application.

Fig. 3. Flow chart of power management for power sources protection.

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Fig. 2. Equivalent electrical circuit of hybrid PEMFC/LIB power system dynamic model.

Fig. 4. Proposed two power splitting structures: (a) coordinated current-voltage control; (b) dual-voltage control. Fig. 5. FL rules distribution for threshold decision.

Fig. 6. Voltage transient response simulations: (a) with coordinated current-voltage control; (b) with dual-voltage control.

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Fig. 7. Voltage and current response governed by coordinated current-voltage control in the simulation: (a) voltage result; (b) current result.

Fig. 8. Voltage and current response governed by dual-voltage control in the simulation: (a) voltage result; (b)

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current result.

Fig. 9. Voltage and current response governed by the integrated two control structures in the simulation: (a) voltage result; (b) current result.

Fig. 10. NI DAQ devices-based hybrid power system prototype experimental test platform. Fig. 11. Voltage transient response experimental results: (a) with coordinated current-voltage control for DC-link voltage regulation and PEMFC current control; (b) with dual-voltage control for DC-link voltage regulation and LIB charging voltage control. Notes on the oscilloscope: Channel 1 is the DC-link voltage; Channel 2 is the voltage of

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LIB; Channel 4 is the voltage of PEMFC.

Fig. 12. Voltage response under step and ramp load variation experimental results: (a) with coordinated currentvoltage control; (b) with dual-voltage control. Notes on the oscilloscope: Channel 1 is the DC-link voltage; Channel 2 is the voltage of LIB; Channel 4 is the voltage of PEMFC.

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Fig. 13. Voltage response governed by the integrated two control structures experimental results. Notes on the oscilloscope: Channel 1 is the DC-link voltage; Channel 2 is the voltage of LIB; Channel 4 is the voltage of PEMFC. Fig. 14. Stress analysis for experimental results: (a) LIB voltage response with dual-voltage control on Stage I; (b)

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LIB voltage response with coordinated current-voltage control on Stage II; (c) PEMFC voltage response with dualvoltage control on Stage I; (b) PEMFC voltage response with coordinated current-voltage control on Stage II. Fig. 15. Simulation results of power sources protection control power management for downsized ECE-15cycling (SOC0 = 30%): (a) voltage response result; (b) current response result; (c) LIB SOC profiles compared with different SOC0 and constant current threshold; (d) power distribution result.

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Fig. 1. Hybrid PEMFC/LIB power system for vehicular application.

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Fig. 2. Equivalent electrical circuit of hybrid PEMFC/LIB power system dynamic model.

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Fig. 3. Flow chart of power management for power sources protection.

Fig. 4. Proposed two power splitting structures: (a) coordinated current-voltage control; (b) dual-voltage control.

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Fig. 5. FL rules distribution for threshold decision.

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control.

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Fig. 6. Voltage transient response simulations: (a) with coordinated current-voltage control; (b) with dual-voltage

Fig. 7. Voltage and current response governed by coordinated current-voltage control in the simulation: (a) voltage result; (b) current result.

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Fig. 8. Voltage and current response governed by dual-voltage control in the simulation: (a) voltage result; (b)

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current result.

Fig. 9. Voltage and current response governed by the integrated two control structures in the simulation: (a) voltage

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result; (b) current result.

Fig. 10. NI DAQ devices-based hybrid power system prototype experimental test platform.

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Fig. 11. Voltage transient response experimental results: (a) with coordinated current-voltage control for DC-link

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voltage regulation and PEMFC current control; (b) with dual-voltage control for DC-link voltage regulation and LIB charging voltage control. Notes on the oscilloscope: Channel 1 is the DC-link voltage; Channel 2 is the voltage of

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LIB; Channel 4 is the voltage of PEMFC.

Fig. 12. Voltage response under step and ramp load variation experimental results: (a) with coordinated currentvoltage control; (b) with dual-voltage control. Notes on the oscilloscope: Channel 1 is the DC-link voltage; Channel 2 is the voltage of LIB; Channel 4 is the voltage of PEMFC.

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Fig. 13. Voltage response governed by the integrated two control structures experimental results. Notes on the

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oscilloscope: Channel 1 is the DC-link voltage; Channel 2 is the voltage of LIB; Channel 4 is the voltage of PEMFC.

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Fig. 14. Simulation results of power sources protection control power management for downsized ECE-15cycling (SOC0 = 30%): (a) voltage response result; (b) current response result; (c) LIB SOC profiles compared with different

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SOC0 and constant current threshold; (d) power distribution result.

List of Tables

Table 1. PEMFC output voltage component expression. Table 2. Hybrid PEMFC/LIB power system parameters. Table 3. Test conditions for simulations and experimental implementations.

Table 1. PEMFC output voltage component expression.

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Voltage variables

Mathematical expression

  PH  1  PO   + ln    E Nernst = 1.229 − 8.5 × 10 − 4 (Tst − 298.15) + 4.3085 × 10 −5 Tst ln    1.01325  2  1.01325   Vact = − ξ1 + ξ 2 ⋅ Tst + ξ 3 ⋅ Tst ln (C O ) + ξ 4 ⋅ Tst ln (I FC ) 2

[

Activation loss, Vact Ohmic loss, Vohm

Vohm = I FC (RM + RC )

Concentration loss, Vcon

Vcon = −

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RTst  j   ln1 − 2F  jmax 

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Nernst voltage, ENernst

Table 2. Hybrid PEMFC/LIB power system parameters.

Parameter description Cell number of PEMFC Rated power of PEMFC Stack temperature of PEMFC Hydrogen partial pressure Oxygen partial pressure Nominal voltage of LIB Maximum capacity of LIB Capacitor of unidirectional boost converter Inductor of unidirectional boost converter Capacitor of bidirectional converter boost mode Capacitor of bidirectional converter buck mode Inductor of bidirectional converter DC-link voltage setpoint

Parameter

20 100 W 60 ˚C 2 bar 0.21 bar 14.8 V 2400 mAh 200 µF 500 µH 200 µF 500 µF 500 µH 24 V

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n PFC,rated Tst PH2 PO2 VLIB,nom Qm C1 L1 C2 C3 L2 VDC,ref

Value

Table 3. Test conditions for simulations and experimental implementations. Test conditions

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Condition 1: step response

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Condition 2: step and ramp response

Condition 3: step and ramp response by the two power splitting structures

Load profile 6 A for coordinated current-voltage control 0.2 A for dual-voltage control 6 A @ [0, 5] s→ [6, 4] A @ [5, 7] s → 4 A @ [7, 9.5] s → 5 A @ [9.5, 12] s for coordinated current-voltage control 0.2 A @ [0, 5] s→ [0.2, 2.5] A @ [5, 7] s → 2.5 A @ [7, 9.5] s → 0.8 A @ [9.5, 12] s for dual-voltage control 1 A @ [0, 2] s→ [1, 2.5] A @ [2, 5] s → 2.5 A @ [5, 7] s → 6 A @ [7, 9.5] s → 4 A @ [9.5, 12] s

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Fig. 15. Simulation results of power sources protection control power management for downsized ECE-

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15cycling (SOC0 = 30%): (a) voltage response result; (b) current response result; (c) LIB SOC profiles

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compared with different SOC0 and constant current threshold; (d) power distribution result.

ACCEPTED MANUSCRIPT Hybrid models including PEMFC, LIB, and dc converters are developed.



Power source protection is designed and proved through simulation and experiment.



Fuel cell protection from sudden power requirement is achieved using currentvoltage control scheme.



Battery protection from overcharge is prohibited using dual voltage control scheme.



Current-voltage control method and dual voltage control method is selected through fuzzy logic rules.

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