BatterylUC Hybridization for Electric Vehicles via a ... - IEEE Xplore

2013 3rd International Conference on Electric Power and Energy Conversion Systems, Yildiz Technical University, Istanbul, Turkey, October 2-4, 2013

BatterylUC Hybridization for Electric Vehicles via a Novel Double Input DC/DC Power Converter

Furkan Akar Duzce University, Technology Faculty Electrical and Electronics Engineering Duzce, Turkey [email protected]

Bulent Vural Yildiz Technical University Electrical Engineering Istanbul, Turkey [email protected]

Abstract- In this work, for electric vehicles (EVs), a novel

double input DC-DC power converter, that enables utilization of a battery and ultra-capacitor (UC) in parallel while increasing the overall performance of electric vehicles and regenerative breaking energy, is introduced.

recovering

Since UCs have

higher power density when compared to batteries, by the use of a UC as a power input, DC bus voltage regulation at transients and peak loads is achieved easily, thus the life of the battery and efficiency of the system are increased. The average value model ® ® of the proposed converter is created in MATLAB , Simulink ® and SimPowerSystems environment, then its dynamic performance is tested under the load determined from the ECE-

15 drive cycle. Keywords-DC/DC converter, electric vehicles, hybrid electric energy systems, Ii-ion battery, ultra-capacitor

I.

INTRODUCTION

The fact that the global warming which has become a serious threat for the world, and the increasing prices of fossil fuels have accelerated the development of environmental friendly and high efficient vehicles that use alternative energy sources. Pure electric vehicles are the most popular technology among the others [1]. The batteries have higher energy densities however lower power densities when compared to the capacitor based energy storage systems [2]. Due to the fact that EVs require high power especially during the acceleration, in the case of using only a battery as a single power source in an EV, the performance and comfort of the vehicle are influenced adversely; moreover, a decrease in the battery life is inevitable. So, a hybrid system with a battery and another energy storage device needs to be formed. The batteries and UCs are foremost energy storage devices in hybrid EVs [3]. Unlike the batteries, UCs are known for their high power densities while suffering from their low energy storage capabilities [4]. By utilizing batteries and UCs simultaneously, a system that gathers advantages of these two devices can be developed. The performance of a hybrid system utilizing two different energy storage devices depends on the performance of the converter used in this system. Therefore, it is critical to have a proper DC-DC converter that can enable power transfer from the input energy sources to the output along with load sharing.

In [5], a hybrid system with a unidirectional converter and a bidirectional converter is developed. It can be seen that this topology allows load sharing despite of unsatisfying DC bus voltage regulation. In [6], a multi-input converter is introduced; this converter enables the input energy sources to power the output individually or simultaneously via a multi winding transformer. However, this kind of isolated DC-DC converter topology has several drawbacks, such as, complicated circuit structure, high cost, and the leakage inductance of transformer. In order to overcome these issues, a double input bidirectional DC-DC converter that utilizes a coupled inductor is given in [7]. This converter shows a really good performance on DC bus voltage regulation and load sharing while a fuel cell and a UC are used as input energy sources. However, it can be easily stated that this converter cannot recover the regenerative breaking energy due to the diode located at its output; in this work, it is aimed to come through this disadvantage, by adding a switch parallel to the aforementioned diode. Moreover, a reverse current diode is added and the diode that protects the fuel cell is removed. After these modifications, a novel double input bidirectional DC-DC converter having the capability of recovering regenerative breaking energy by both inputs is developed. As given in Fig.1, the battery and UC can be charged and discharged according to the state of switches, the instantaneous voltages and power of the inputs and output.

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Figure 1. The topology of the proposed DC/DC converter.

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IEEE


TABLE I OCELL 8QIP L1-ION BATTERY PARAMETERS

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Figure 2. The winding diagram of coupled inductor

II.

SYSTEM DESCRIPTION AND METHODOLOGY

A. Converter topology and its operating principle

The proposed converter topology is shown in Fig. 1. Here it can be seen that the input sources can feed the output simultaneously. In addition, in the case that one of these sources fails or disconnects, other can continue its operation. For the sake of control simplicity, Q3 switch is hold closed during the simulation, therefore it is assured that the regenerative breaking energy is recovered whenever possible. Furthermore, the duty cycle of Qo is set to 0.5 because of a possible over current problem. According to the duty cycles of QI and Q2 switches, the proposed converter can operate in buck-boost modes in both discontinues inductance current (DIC) mode and continues inductance current (CIC) mode. DIC mode decreases reverse-recovery loss and switching loss, as CIC mode decreases conduction loss and input current ripple; however, CIC mode increases the core losses. Thus, the inductors should be coupled as demonstrated in Fig. 2 [7]. The mutual inductance and leakage inductance values of a coupled inductance can be computed according to (1)-(3), (1)

LM = k.JL1Lz ' L1 = L1 - LM ' Lz = Lz - LM

(2) (3)

where, k is the coupling coefficient, LM is the mutual inductance, LI are L2 self inductances, LI' and L2' are leakage inductances. The proposed converter can be considered the combination of two boost converter and a one buck converter. Based on this observation, the output current equation can be written as (4). Here, ILM is the sum of currents flowing through the leakage inductances hI' and ILZ·. (4) L1 •

L2 .

Parameters

Values

Nominal Capacity

II0Ah

Nominal Voltage

100 V

Maximum Charge Voltage

116.8 V

Discharge Cut-off Voltage

80 V

Weight

133 kg

Approximate Resistance

60 mn TABLE II

BMOD165 UC PARAMETERS Parameters

165 F

Maximum ESR

6. 3 mn

Nominal Voltage

48 V

Maximum Peak Current

51 V

Leakage Current

1900 A

Maximum Charge Voltage

5.2 mA

Finally, the average value model of proposed converter can be constructed as shown in Fig. 3. As can been seen here, in this modeling technique, the switches are replaced by controlled current and voltage sources. Despite of decreasing the practicalness of the simulation model due to the ignored switching dynamics and losses, this method increases the speed of simulation, and allows long time analysis. It is also worthy to note that in this technique, the inductances are represented by resistors because sudden changes in the currents are ignored. Furthermore, from Fig.3, one can see that the currents flowing through the both leakage inductances are bidirectional. In other words, the both input sources can transfer their energy to the output or the other sources, while they can recover the regenerative breaking energy. B. Battery and UC modeling

According to the parameters given in Table I, the equivalent capacitance of the battery can be calculated. First, the battery energy in joule is computed as 39.6 MJ via (5). Then, the capacity can be found as 10kF via (6) for Vmax (116.8 V) and Vmin (80 V). In addition the serial resistance of battery is neglected in the simulation. Note that, a detailed battery model is not used here to save time. E =

Figure 3. The average value model of the proposed power converter

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(6) Vmax z - VminZ In (5), E denotes the energy in joule, Cnom is the nominal capacity in Ah while Vnom is the nomival voltage. In (6), C is the equivalent capacitance, Vmax and Vminare maximum and minimum battery voltages, respectively. UC is modeled based on the parameters in Table II as shown in Fig. 4. Here, ESR is the equivalent serial resistance while Rp denotes the parallel resistance that models the UC self-discharge; it can be easily computed as 9.23 kQ by dividing the nominal voltage to the leakage current. C

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SIMULATION AND RESULTS

In the designed system, an ultra-capacitor bank composed of three UCs whose parameters given in Table II is created for a successful DC bus voltage regulation. Since UCs are connected in serial, the UC bank equivalent capacitance, and voltage will be 55 F, and 120 V, respectively. Additionally, the initial voltage of battery is assumed to be 100 V. The batterylUC hybrid system used in the simulation is depicted in Fig. 5. As can be seen from this figure, it is aimed that the designed hybrid system operates to meet the required amount of energy of ECE-15 drive cycle. Here, DC bus voltage reference is determined 188 V [8]. In this work, in order to show the some of the operation modes of converter, a simple control algorithm is used. According to the this algorithm, UC bank current is controlled to regulate the DC bus voltage via one of the PI controllers that controls the duty cycle of switch Q/. In addition, the duty cycle of Q2 is controlled by the other PI controller to ensure that the demanded power below 9 kW is supplied by the battery. As mentioned earlier, the duty cycles ofQ3 andQo are kept constant at 1 and 0.5, respectively.

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Figure 5. The block diagram of the whole system

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Figure 6. a) Load current, b )De bus voltage

A. Simulation Results

In the Fig. 6, the variations in load current, and DC bus voltage versus the time are given when the demanded power is determined from the ECE-15 drive cycle. As can be seen from Fig. 6, the DC bus regulation is accomplished successfully after transients thanks to the high power density feature of the

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Fig. 7 shows the variations in UC bank current, voltage and duty cycle ofQ/. According to this figure, it can be easily seen that UC voltage usually tends to increase. In other words, it is typically charged because of the regenerative breaking enerry. Moreover, UC current reaches about 20 A around 1401 second since the demanded power is more than the maximum output power of battery in this period. At this stage, UC bank comes into play, and feeds the output for DC bus regulation.


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Fig. 9 presents the changes in the output power according to ECE-15 drive cycle, battery power and UC bank power. Here, one can see from Fig. 9-b that battery power is always positive, and less than 9 kW due to the control strategy, and the fact that the battery is never charged. Moreover, according to Fig. 9-c, UC bank power takes negative values when the regenerative breaking energy is available; this figure also shows that in the periods when the demanded power is higher than 9 kW, UC bank discharges. IV.

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REFERENCES [I]

[2] [3]

[4]

[5]

[6]

[7]

The battery current, voltage and duty cycle of Q2 switch are demonstrated in Fig. 8. As one can notice from this figure that the maximum current of battery is approximately 90 A due the fact that its maximum power is set to 9 kW. Furthermore, it seems that the battery is never charged since the duty cycle of Q2 is higher than the one of Q,. However, based on a different control strategy, it can be possible to charge the battery as well.

CONCLUSIONS

This paper presents a novel double input DC\DC power converter that forms a hybrid energy storage system which consists of a battery and an ultra-capacitor bank for EVs. Associated switches are controlled in a way that the output power less than 9 kW is supplied from the battery, and UC bank energy is transferred to the output when the battery is insufficient. In addition, UC bank recovers the regenerative breaking energy. Simulation results demonstrate that the proposed converter topology shows a good performance on both holding the DC bus voltage constant, and recovering the regenerative breaking energy. Besides, the peak power loads and transients do not affect the battery; by this way, it can be possible to increase its lifetime and efficiency. Due to the simplicity of the control, all operation modes of the converter cannot be shown in this work. For example, the regenerative breaking energy is only captured by the UC bank despite the fact that the proposed converter can enable a power transfer from the output to the battery as well; in the future work, it is targeted to improve the control strategy, and demonstrate the power transfer in all directions.

[8]

C. C. Chan, "The state of the art of electric and hybrid vehicles," in Proc. 18th Annu. IEEE Conf Computer Security Application, Washington, 2002, pp. 245-275. A. F. Burke, "Batteries and ultracapacitors for electric, hybrid, and fuel cell vehicles," Proceedings of the IEEE, 2007, pp. 806-820. G. Wang, P. Yang, J. Zhang, 'Fuzzy Optimal Control and Simulation of Battery-Ultracapacitor Dual-Energy Source Storage System for Pure Electric Vehicle", International Conference on Intelligent Control and Information Processing, August 13-15, 2010 Khaligh A, Li Z. "Battery, ultracapacitor, fuel cell, and hybrid energy storage systems for electric, hybrid electric, fuel cell, and plug-in hybrid electric vehicles." ,IEEE Trans Veh. Technol 2010;59(6):2806eI4. O. Erdinc, B. Vural, M. Uzunoglu, "A wavelet-fuzzy logic based energy management strategy for a fuel cell/battery/ultracapacitor hybrid vehicular power system", J Power Sources 2009;194(l):36ge80. B. Vural, O. Erdinc, M. Uzunoglu, "Parallel combination of FC and UC for vehicular power systems using a mUlti-input converter-based power interface", Energy Convers Manag 2010;51(12):2613e22. B. Vural, "FC/UC hybridization for dynamic loads with a novel double input DC-DC converter topology", International Journal of Hydrogen Energy 38 (2013) 1103-1110 B. Vural, O. Erdinc, M. Uzunoglu, "Parallel combination of FC and UC for vehicular power systems using a mUlti-input converter-based power interface", Energy Conversion and Management