The Energy Management Control Strategy for Electric Vehicle ...

International Conference and Utility Exhibition 2014 on Green Energy for Sustainable Development (ICUE 2014) Jomtien Palm Beach Hotel and Resort, Pattaya City, Thailand, 19-21 March 2014

The Energy Management Control Strategy for Electric Vehicle Applications A. Wangsupphaphol1, Student Member, IEEE, N.R.N. Idris1, Senior Member, IEEE, A.Jusoh1,N.D.Muhamad1 and Low Wen Yao1 Abstract--This paper presents the control system of energy management for electric vehicle (EV) applications, based on the actual speed of the vehicle and the terminal voltage of supercapacitors (SCs). The cascade control of voltage and current were implemented on SCs for tracking the energy during acceleration and braking. The performances of SCs concerning the dynamic power were examined. A reliable power system composed of Lithium-ion Batteries (LBs) and SCswas selected. AC drive was simplified by a DC drive system, simulated using fully driven acceleration cycle and also during braking condition. The actual vehicle dynamic and tractive loads were modeled for the motor driving load. The energy consumption of pure Battery Electric Vehicle (BEV) and the proposed Batteries-SCs Hybrid Electric Vehicle (BHEV) were then compared for the driving cycle. The driving power portions, bus voltage regulations, andSCs actual voltage and current were also investigated. Numerical simulations using MATLAB had proven the effectiveness of the proposed system over the BEV. Index Terms-- Energy management, Electric vehicles, Control design, DC-DC power converters, Supercapacitors, Batteries.

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I. INTRODUCTION

HE energy management system is aimed to establishthe systems and consequently improve the energy performances, including energy efficiency, usage and consumption [1]. To effectively implement the concept of the energy management system into an EV system, the overall subsystem participation must be designed, considered and carefully selected in order to obtain an optimized solution. LBs are one of the most suitable and effective battery technology widely implemented for all electric loads, mainly to supply the propulsion unit and also for auxiliary loads. This battery has about two times higher specific energy and power density than nickel metal hydride battery while lower in weight [2]. The battery specific energy and power density are huge enough for a compromising acceleration and driving range, comparable to the internal combustion engine (ICE) vehicle at the present. However, when operating under 0°C,

This research is financially supported by the PhD. Merit Scholarship of Islamic Development Bank (IDB) for conducting the research at UTMProton future drive laboratory. 1 In collaboration with the UTM-PROTON Future Drives Laboratory, Faculty of Electrical Engineering, Universiti Teknologi Malaysia 81310 Skudai, Johor, Malaysia(e-mail: [email protected]).

the problem of recharging often occurs. The battery has the capability of supplying high transient accelerating powers and capturing moderate transient regenerative braking powers. Transient powers occur when the motor is started and then is put to braking, whereas the rate of change in motor speed or acceleration results in high power variations. These two powers are inherently composed in a driving cycle namely dynamicpower; while tractive power is developed by the tractive load resistance which is not varies to the acceleration. Dynamic and average tractive power are combined as a driving power as shown in Fig. 1.

Fig. 1. Driving power of propulsion load [2]

The BEV dynamic power is generally not satisfactory, particularly in long term because it generates high temperature and energy loss, which means the battery life, will be shortened. To ensure a long life of battery, decreasing the battery charging and dischargingrate caused by the use of dynamic power has to be avoided.In the present development of energy storage devices, SCs have proven their suitability of pulse discharging and charging huge amount of power in a very short period of time, with lower equivalent resistance compared to LBs. SCs is definitely one of the alternativeenergy storages which can be potentially utilized for onboard auxiliary power supply. With regard to this, safety concern can be achieved and economic perspective can be reviewed from its internal rate of return [3]. Thus, it is expected that the combination of two sources between LBs and SCs, called BHEV, is one of the best solutions to overcome the problems existing in BEV. II. SYSTEM CONFIGURATION A. Battery Electric Vehicle BEV is essentially based on Proton SAGA EV. It has a curb weight 𝑴𝑴𝑽𝑽 = 1000 kg. The car was a front 2-wheel drive and had a single gear ratio 1.4288, wheel diameter, 𝒓𝒓𝒘𝒘𝒘𝒘 = 0.26 m, and frontal area 𝑨𝑨𝒇𝒇 = 2.098𝒎𝒎𝟐𝟐 . The vehicle maximum speed in this design was limited to 𝒗𝒗𝑽𝑽,𝒎𝒎𝒎𝒎𝒎𝒎 = 𝟑𝟑𝟑𝟑. 𝟑𝟑m/s, having acceleration rate from 0-100 km/h on the zero degree slope within20 seconds, 𝐚𝐚 = 𝟏𝟏. 𝟑𝟑𝟑𝟑𝟑𝟑/𝐬𝐬𝟐𝟐, with maximum angular speed of wheel 𝛚𝛚𝐰𝐰𝐰𝐰,𝐦𝐦𝐦𝐦𝐦𝐦 = 128.08 rps. The traction unit was composed of a three-phase induction motor 52 HP,


Fig. 2. Power train configuration of BHEV

connected to a three-phase inverter. The drive system had been simplified by utilizing a DC machine block of SimPowerSystem in MATLAB Simulink, which had the specification as followed: 50 HP, 240 V, maximum angular speed, 𝝎𝝎𝒎𝒎,𝒎𝒎𝒎𝒎𝒎𝒎 = 183.2 rps, total motor inertia, 𝑱𝑱𝒎𝒎 = 0.2 𝒌𝒌𝒌𝒌𝒎𝒎𝟐𝟐 and viscous friction coefficient, 𝑩𝑩𝒎𝒎 =0.007032 Nm-s. The proposed power system is composed of the battery 7.5 V/unit, 32 cells connected in series; provided 60 kWwhich is a bank of LBs possessing 240V, 250A maximum current with equivalentbattery resistance,𝒓𝒓𝒃𝒃𝒃𝒃𝒃𝒃 =52 mΩ.

B. Power supply system in BHEV A hybrid power supply system is a combination of two or more power sources in order to achieve the optimum performance for a specific purpose as in Fig. 2. In present BHEV, one of the best solutions is hybridization of LBs and SCs [4]. There are a number of possible hybridizations in BHEV. However, EV applications, the selection of converter configuration should be the most reliable, less complex, having low weight, low loss and cost. The configuration in Fig. 3was proposed to be studied in this work based on a good reliable system, particularly on its dynamic cascade control design.

Fig. 3.Converter configuration of proposed hybrid power system

C. Converter topology for BHEV The purpose of the converter in BHEV is to control the dynamic power forth and back between SCs and propulsion load; in conventional case, it works as a boost converter while supplying power and buck converter when absorbing power.

The topology mostly used in an EV is a half bridge nonisolated bi-directional DC-DC power converter, as shown in Fig. 2. Amongst converter topologies, the half bridge converter has more advantages than Cuk and combined SEPIC/Luo converters, which offers high efficiency, being most compact, having lowest cost and less weight and simple control. The converter needs only half size of inductor and other components compared to the Cuk and combined SEPIC/Luo converters. Switching and conduction losses are lower because of the lesser number of switching components and inductor. However, half bridge converter needs larger size of output capacitor than other topologies in order to maintain continuous output current [5]. D. Axiliary power supply The auxiliary power supply is a bank of SCs with a DCDC converter. SCs can be a connection of an elementary cell in series and parallel or connection of a single automotive module. As known, the dynamic power requested during the acceleration has been designed to be supplied by SCs and the tractive power is supplied by LBs. The power from SCs is idled during coasting but supplied alone by LBs. During braking, SCs are charged to their setting maximum voltage by the load and LBspower. As noted, the size of SCs is based on the acceleration period for providing the acceleration,translating mass and the maximum speed of the EV are taken into account. In this study, to evaluate the actual energy availability, the mean efficiencies of the SCs 𝜂𝜂𝑆𝑆𝑆𝑆𝑆𝑆 , converter efficiency 𝜂𝜂𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 , and propulsion chopper efficiency 𝜂𝜂𝑐𝑐ℎ𝑜𝑜𝑜𝑜 , DC motor efficiency 𝜂𝜂𝐷𝐷𝐷𝐷𝐷𝐷 , and mechanical efficiency 𝜂𝜂𝑚𝑚𝑚𝑚𝑚𝑚 ℎ had been taken into consideration. Based on the efficiency specifications provided by manufacturers, the SCs efficiency was selected according to design guide of use in high power pulsing to 𝜂𝜂𝑆𝑆𝑆𝑆𝑆𝑆 = 0.95. The efficiency of auxiliary converter,propulsion chopper and DC motor, were simulated for full load cycle, which produced 𝜂𝜂𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 0.97, 𝜂𝜂𝑐𝑐ℎ𝑜𝑜𝑜𝑜 = 0.97, 𝜂𝜂𝐷𝐷𝐷𝐷𝐷𝐷 = 0.91 and 𝜂𝜂𝑚𝑚𝑚𝑚𝑚𝑚 ℎ = 0.98 respectively. The kinetic energy and energy stored in the SCs are balanced


with a certain difference by losses according to the following equation: 2 (𝑀𝑀𝑉𝑉 + 𝑀𝑀𝑆𝑆𝑆𝑆𝑆𝑆 )𝑣𝑣𝑉𝑉,𝑚𝑚𝑚𝑚𝑚𝑚 = 𝜂𝜂𝑡𝑡𝑡𝑡𝑡𝑡 𝐶𝐶𝑆𝑆𝑆𝑆𝑆𝑆 �𝑉𝑉2𝑆𝑆𝑆𝑆𝑆𝑆,𝑚𝑚𝑚𝑚𝑚𝑚 − �

1 2

𝑉𝑉𝑆𝑆𝑆𝑆𝑆𝑆,𝑚𝑚𝑚𝑚𝑚𝑚 �

2

�(1)

where 𝜂𝜂𝑡𝑡𝑡𝑡𝑡𝑡 = 𝜂𝜂𝑆𝑆𝑆𝑆𝑆𝑆 𝜂𝜂𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝜂𝜂𝑐𝑐ℎ𝑜𝑜𝑜𝑜 𝜂𝜂𝐷𝐷𝐷𝐷𝐷𝐷 𝜂𝜂𝑚𝑚𝑚𝑚𝑚𝑚ℎ . SCs terminal voltage is designated to vary between maximum terminal voltage, 𝑉𝑉𝑆𝑆𝑆𝑆𝑆𝑆,𝑚𝑚𝑚𝑚𝑚𝑚 and half of the maximum voltage in order to utilize 75% of energy content so that weight and size are compromised. In order to evaluate the SCs capacitance, 𝐶𝐶𝑆𝑆𝑆𝑆𝑆𝑆 , Equation (1) can be transformed to:

𝐶𝐶𝑆𝑆𝑆𝑆𝑆𝑆 =

2 (𝑀𝑀𝑉𝑉 +𝑀𝑀𝑆𝑆𝑆𝑆𝑆𝑆 )𝑣𝑣𝑉𝑉,𝑚𝑚𝑚𝑚𝑚𝑚

(2)

3 4

2 𝜂𝜂𝑡𝑡𝑡𝑡𝑡𝑡 � 𝑉𝑉𝑆𝑆𝑆𝑆𝑆𝑆 ,𝑚𝑚𝑚𝑚𝑚𝑚 �

The mass of SCs is preliminary approximated to be zero but subjected to change after finalized. The maximum voltage of SCs is chosen to obtain the highest efficiency of the boost converter or the maximum voltage gain which should be lower or equal to 3 [3]. To find the capacitance of single cell in formercase, the overall connection units can be derived by using the following equation:

𝐶𝐶𝑆𝑆𝑆𝑆,𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 =

𝐶𝐶𝑆𝑆𝑆𝑆𝑆𝑆 ∙𝑁𝑁𝑠𝑠 𝑁𝑁𝑝𝑝

(3)

where𝑁𝑁𝑠𝑠 is number of series connection and 𝑁𝑁𝑝𝑝 is number of parallel connection. The𝑁𝑁𝑠𝑠 can be achieved by dividing the SCs terminal voltage with the voltage rating of 𝐶𝐶𝑆𝑆𝑆𝑆,𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 provided by manufacturer. 𝑁𝑁𝑝𝑝 is a selection of an integer number to achieve 𝐶𝐶𝑆𝑆𝑆𝑆,𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 according to the supercapacitor manufacturer.In this study, the evaluation of 𝐶𝐶𝑆𝑆𝑆𝑆𝑆𝑆 produced 49 Farad, where SCs terminal 200 V was chosen. This was made by a parallel connection of series 𝐶𝐶𝑆𝑆𝑆𝑆,𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 2000 Farad, 74 cells, 2.7 V/cell. The connection produced 𝐶𝐶𝑆𝑆𝑆𝑆𝑆𝑆 = 54 Farad and 53 kg of total weight. However, the final calculation still had to be ensured after preliminary evaluation, which increased the capacitance by 10% and weight by 5% of curb weight, satisfying the vehicle performance as prior. To find the maximum power rating, maximum acceleration rate has to be designed. According to the EV performance, 𝑎𝑎 = 1.37 𝑚𝑚/𝑠𝑠 2 was used in order to design the converter capacity as the following equation: 𝑃𝑃𝑆𝑆𝑆𝑆𝑆𝑆 ,𝑚𝑚𝑚𝑚𝑚𝑚 =

𝑀𝑀𝑡𝑡𝑡𝑡𝑡𝑡 𝑣𝑣𝑉𝑉,𝑚𝑚𝑚𝑚𝑚𝑚 𝑎𝑎

𝜂𝜂 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝜂𝜂 𝑐𝑐ℎ 𝑜𝑜𝑜𝑜 𝜂𝜂 𝐷𝐷𝐷𝐷𝐷𝐷 𝜂𝜂𝑚𝑚𝑚𝑚𝑚𝑚ℎ

= 57.1 kW

(4)

where𝑃𝑃𝑆𝑆𝑆𝑆𝑆𝑆 ,𝑚𝑚𝑚𝑚𝑚𝑚 is maximum power of SCs and 𝑀𝑀𝑡𝑡𝑡𝑡𝑡𝑡 = 𝑀𝑀𝑉𝑉 + 𝑀𝑀𝑆𝑆𝑆𝑆𝑆𝑆 is total vehicle mass. Then, the current was calculated by using the following equation: 𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆 ,𝑚𝑚𝑚𝑚𝑚𝑚 =

𝑃𝑃𝑆𝑆𝑆𝑆𝑆𝑆,𝑚𝑚𝑚𝑚𝑚𝑚

𝑉𝑉 𝑆𝑆𝑆𝑆𝑆𝑆 ,𝑚𝑚𝑚𝑚𝑚𝑚 /2

= 571 A

(5)

where𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆 ,𝑚𝑚𝑚𝑚𝑚𝑚 is maximum current of SCs. From the calculation, the current of each string would be shared by 286 Awhich is validated by the maximum peak current of 1600 A. III. MATHEMATICAL MODEL OF THE PHYSICAL SYSTEM A. Model of Propulsion Load Power requested by the propulsion chopper 𝑃𝑃𝑐𝑐ℎ𝑜𝑜𝑜𝑜 can be modeled mathematically as the following equations:

𝑃𝑃𝑐𝑐ℎ𝑜𝑜𝑜𝑜 �

𝑃𝑃𝑑𝑑 �𝜂𝜂𝑐𝑐ℎ𝑜𝑜𝑜𝑜 𝜂𝜂𝐷𝐷𝐷𝐷𝐷𝐷 ; 𝑃𝑃𝑑𝑑 𝜂𝜂𝑐𝑐ℎ𝑜𝑜𝑜𝑜 𝜂𝜂𝐷𝐷𝐷𝐷𝐷𝐷 ;

𝑃𝑃𝑑𝑑 > 0

𝑃𝑃𝑑𝑑 < 0

𝑃𝑃𝑑𝑑 = 𝑃𝑃𝑡𝑡𝑡𝑡 + 𝑃𝑃𝑑𝑑𝑑𝑑

(6) (7)

where𝑃𝑃𝑑𝑑 is the driving power, 𝑃𝑃𝑡𝑡𝑡𝑡 is the tractive load power and 𝑃𝑃𝑑𝑑𝑑𝑑 is the dynamic load power at motor shaft. To derive the tractive load power, vehicle dynamic resistance need to be derived by using the following equations: 𝐹𝐹𝑡𝑡𝑡𝑡 = 𝐹𝐹𝑟𝑟𝑟𝑟 + 𝐹𝐹𝑎𝑎𝑎𝑎 + 𝐹𝐹𝑔𝑔𝑔𝑔 𝐹𝐹𝑟𝑟𝑟𝑟 = 𝜇𝜇𝑟𝑟𝑟𝑟 𝑀𝑀𝑡𝑡𝑡𝑡𝑡𝑡 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 2 𝐹𝐹𝑎𝑎𝑎𝑎 = 0.5𝜌𝜌𝐴𝐴𝑓𝑓 𝐶𝐶𝑑𝑑 𝑣𝑣𝑉𝑉,𝑚𝑚𝑚𝑚𝑚𝑚 𝐹𝐹𝑔𝑔𝑔𝑔 = 𝑀𝑀𝑡𝑡𝑡𝑡𝑡𝑡 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝛼𝛼 ⎨𝑇𝑇 = 𝐹𝐹 𝑟𝑟 ; 𝑇𝑇 = 𝑇𝑇 𝜂𝜂 𝑤𝑤ℎ 𝑡𝑡𝑡𝑡 𝑤𝑤ℎ 𝑒𝑒𝑒𝑒 𝑤𝑤ℎ 𝑚𝑚𝑚𝑚𝑚𝑚ℎ ⁄𝐺𝐺 ⎪ 𝐺𝐺 = 𝜔𝜔𝑚𝑚 ,𝑚𝑚𝑚𝑚𝑚𝑚 ⁄𝜔𝜔𝑤𝑤ℎ ,𝑚𝑚𝑚𝑚𝑚𝑚 ⎪ 𝑃𝑃 = ⎩ 𝑡𝑡𝑡𝑡 𝑇𝑇𝑒𝑒𝑒𝑒 𝜔𝜔𝑚𝑚 ,𝑚𝑚𝑚𝑚𝑚𝑚 = 18.38 kW ⎧ ⎪ ⎪

(8)

where 𝐹𝐹𝑡𝑡𝑡𝑡 is the tractive resistance, 𝐹𝐹𝑟𝑟𝑟𝑟 is the rolling resistance generated between tires and road surfaces, 𝐹𝐹𝑎𝑎𝑎𝑎 is aerodynamic resistance dragging the vehicle motion to move into the air , 𝐹𝐹𝑔𝑔𝑔𝑔 is the grading resistance that downgrades the force of vehicle from going up, 𝑇𝑇𝑤𝑤ℎ is the load torque at wheel of the vehicle, 𝑇𝑇𝑒𝑒𝑒𝑒 is the equivalent load torque transferred through overall gear ratio 𝐺𝐺. Any coefficient values for calculation were as follows: rolling resistance 𝜇𝜇𝑟𝑟𝑟𝑟 = 0.0048, Air density 𝜌𝜌 = 1.25 𝑘𝑘𝑘𝑘/𝑚𝑚3 , aerodynamic drag𝐶𝐶𝑑𝑑 = 0.353, gravity acceleration rate g = 9.8 𝑚𝑚/𝑠𝑠 2 , and road grading angle 𝛼𝛼 = 0 degree. Because the LBs were not being controlled. The maximum power was derived by taking the efficiency of propulsion chopper and DC motor efficiency into account as: 𝑃𝑃𝑏𝑏𝑏𝑏𝑏𝑏 ,𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑃𝑃𝑡𝑡𝑡𝑡 ⁄ 𝜂𝜂𝑐𝑐ℎ𝑜𝑜𝑜𝑜 𝜂𝜂𝐷𝐷𝐷𝐷𝐷𝐷 = 20.82 kW

(9)

We observed that LBs in BHEV had been resized by 65 % as used in BEV. Nonetheless, the control strategy had much effect on battery size, which will be described in the next sections. The rest of power 𝑃𝑃𝑑𝑑𝑑𝑑 was responded by SCs, which varied to the angular acceleration rate of the propulsion load, according to the following equations: ⎧

𝑃𝑃𝑑𝑑𝑑𝑑 = 𝑇𝑇𝑑𝑑𝑑𝑑 𝜔𝜔𝑚𝑚 ,𝑚𝑚𝑚𝑚𝑚𝑚

𝑇𝑇𝑑𝑑𝑑𝑑 = 𝐽𝐽𝑒𝑒𝑒𝑒

𝑑𝑑 𝜔𝜔 𝑚𝑚 ,𝑚𝑚𝑚𝑚𝑚𝑚 𝑑𝑑 𝑡𝑡

⎨ 𝐽𝐽 𝑤𝑤 ℎ ⎩𝐽𝐽𝑒𝑒𝑒𝑒 = 𝐽𝐽𝑚𝑚 + ( 𝐺𝐺 2 )𝜂𝜂𝑚𝑚𝑚𝑚𝑚𝑚ℎ

(10)

where𝑇𝑇𝑑𝑑𝑑𝑑 = dynamic load torque, 𝐽𝐽𝑒𝑒𝑒𝑒 = equivalent moment of inertia referred to motor shaft, 𝐽𝐽𝑚𝑚 = total motor inertia and 𝐽𝐽𝑤𝑤ℎ = moment of inertia of vehicle referred to wheel. By solving the equation, 𝑃𝑃𝑑𝑑𝑑𝑑 = 48.31 kW, the maximum SCs power can be resized as: 𝑃𝑃𝑆𝑆𝑆𝑆𝑆𝑆,𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑃𝑃𝑑𝑑𝑑𝑑 ⁄𝜂𝜂𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝜂𝜂𝑐𝑐ℎ𝑜𝑜𝑜𝑜 𝜂𝜂𝐷𝐷𝐷𝐷𝐷𝐷 = 56.5 kW

(11)

B. Model of Auxiliary Power supply and battery system. The equivalent of SCs can be modeled based on the equivalent RC circuit in series as shown in Fig. 2. The equivalent electric circuit model is derived from the following equations:


𝑖𝑖𝑆𝑆𝑆𝑆𝑆𝑆 = −𝐶𝐶𝑆𝑆𝑆𝑆𝑆𝑆

𝑑𝑑𝑢𝑢 𝑆𝑆𝑆𝑆𝑆𝑆 𝑑𝑑 𝑡𝑡

�𝑣𝑣𝑆𝑆𝑆𝑆𝑆𝑆 = 𝑢𝑢𝑆𝑆𝑆𝑆𝑆𝑆 − 𝑟𝑟𝑆𝑆𝑆𝑆𝑆𝑆 𝑖𝑖𝑆𝑆𝑆𝑆𝑆𝑆 𝑢𝑢𝑆𝑆𝑆𝑆𝑆𝑆 (0) = 𝑉𝑉𝑆𝑆𝑆𝑆𝑆𝑆,𝑚𝑚𝑚𝑚𝑚𝑚

(12)

where𝑢𝑢𝑆𝑆𝑆𝑆𝑆𝑆 is SCsinternal voltage, 𝑟𝑟𝑆𝑆𝑆𝑆𝑆𝑆 is SCsequivalent resistance, 𝑣𝑣𝑆𝑆𝑆𝑆𝑆𝑆 is SCs terminal voltage and 𝑖𝑖𝑆𝑆𝑆𝑆𝑆𝑆 is SCs current.

LBs are the main power supply for the EV, which has the bus voltage𝑣𝑣𝑏𝑏𝑏𝑏𝑏𝑏 , as described by Equation (13). The internal battery voltage 𝑣𝑣𝑏𝑏𝑏𝑏𝑏𝑏 varies to its state of charge (SOC) as similar in SCs but having much slower rate of change. In real application, 𝑣𝑣𝑏𝑏𝑏𝑏𝑏𝑏 is changed by SOC and the voltage drop, the production of battery current 𝑖𝑖𝑏𝑏𝑏𝑏𝑏𝑏 and its equivalent battery resistance𝑟𝑟𝑏𝑏𝑏𝑏𝑏𝑏 , as in the following equation: 𝑣𝑣𝑏𝑏𝑏𝑏𝑏𝑏 ∝ 𝑆𝑆𝑆𝑆𝑆𝑆 � 𝑣𝑣𝑏𝑏𝑏𝑏𝑏𝑏 = 𝑣𝑣𝑏𝑏𝑏𝑏𝑏𝑏 − 𝑟𝑟𝑏𝑏𝑏𝑏𝑏𝑏 𝑖𝑖𝑏𝑏𝑏𝑏𝑏𝑏

(13)

IV. CONTROL STRATEGY AND DESIGN A. Control strategy The main objective of control strategy to reduce stress on the battery discharge phase and save more the braking energy that cannot be captured by the battery alone. As shown in Equation (1), the reference voltage of SCs can be used for catch up the kinetic energy as the following equation: 𝑢𝑢𝑆𝑆𝑆𝑆𝑆𝑆,𝑟𝑟𝑟𝑟𝑟𝑟 ≅ �𝑉𝑉2𝑆𝑆𝑆𝑆𝑆𝑆,𝑚𝑚𝑚𝑚𝑚𝑚 −

𝑀𝑀𝑡𝑡𝑡𝑡𝑡𝑡 𝑣𝑣2 𝑉𝑉 𝜂𝜂𝑡𝑡𝑡𝑡𝑡𝑡 𝐶𝐶𝑆𝑆𝑆𝑆𝑆𝑆

(14)

To vary the terminal voltage of SCs, the power has to be supplied or absorbed by controlling the amount and direction of current, 𝑖𝑖𝑆𝑆𝑆𝑆𝑆𝑆 . In acceleration, the maximum positive SCs current is a gain of chopper current, 𝑖𝑖𝑐𝑐ℎ𝑜𝑜𝑜𝑜 , taken the converter efficiency. In deceleration, the current can be set to maximum limit of the battery discharge current in order to support the regeneration power if not enough for charging the SCs. The current strategy can be derived as the following equation: �

𝑖𝑖𝑏𝑏𝑏𝑏𝑏𝑏 = 𝑖𝑖𝑐𝑐ℎ𝑜𝑜𝑜𝑜 − 𝑖𝑖𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑖𝑖𝑐𝑐ℎ 𝑜𝑜𝑜𝑜 𝑣𝑣𝑏𝑏𝑏𝑏𝑏𝑏

; 𝑖𝑖𝑆𝑆𝑆𝑆𝑆𝑆 = � 𝜂𝜂𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑣𝑣𝑆𝑆𝑆𝑆𝑆𝑆 −𝐼𝐼𝑏𝑏𝑏𝑏𝑏𝑏 ,𝑚𝑚𝑚𝑚𝑚𝑚 ;

𝑃𝑃𝑡𝑡 > 0

(15)

⎧ ⎪ 𝐴𝐴1 = � ⎪ ⎪

−(𝑟𝑟𝐿𝐿 +𝑟𝑟𝑆𝑆𝑆𝑆𝑆𝑆 )

𝐿𝐿

0

0

1

� , 𝐵𝐵1 = � 𝐿𝐿 � , 𝐶𝐶1 = [1 − 0 𝐶𝐶(𝑅𝑅+𝑟𝑟𝐶𝐶 ) 1

0]

; system matrices of on stage

(16) −R ⎨ 1 𝐿𝐿(𝑅𝑅+𝑟𝑟𝐶𝐶 ) 𝐶𝐶 ) ⎪ 𝐴𝐴2 = � 𝐿𝐿(𝑅𝑅+𝑟𝑟 � , 𝐵𝐵2 = � 𝐿𝐿 � , 𝐶𝐶2 = [1 0] R −1 ⎪ 0 ( ) ( ) 𝐶𝐶 𝐶𝐶 𝑅𝑅+𝑟𝑟 𝑅𝑅+𝑟𝑟 ⎪ 𝐶𝐶 𝐶𝐶 ; system matrices of off stage ⎩ where𝑅𝑅is equivalent resistance of maximum powersupply by SCs, 𝐿𝐿 is converter inductance, 𝑟𝑟𝐿𝐿 is equivalent inductive resistance, 𝐶𝐶 is converter capacitance and 𝑟𝑟𝐶𝐶 is equivalent capacitive resistance. Equation (16) presents the system matrices of on and off interval, which are used for linearizing the system at the quiescent operating point. To obtain the transfer function of duty ratio, 𝐷𝐷, to the SCs current, the state equation can be arranged by the following equation: −�𝑅𝑅𝑟𝑟𝐶𝐶 +𝑅𝑅(𝑟𝑟𝐿𝐿 +𝑟𝑟𝑆𝑆𝑆𝑆𝑆𝑆 )�

𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆 (𝑠𝑠) 𝐷𝐷(𝑠𝑠)

= 𝐶𝐶𝑠𝑠 (𝑠𝑠𝑠𝑠 − 𝐴𝐴𝑠𝑠 )−1 [(𝐴𝐴1 − 𝐴𝐴2 )𝑋𝑋 + (𝐵𝐵1 − 𝐵𝐵2 ) 𝑈𝑈] + (𝐶𝐶1 − 𝐶𝐶2 )𝑋𝑋

(17)

where𝑋𝑋 is equilibrium state vector and 𝑈𝑈 is equilibrium input vector. Equation (17) can be transformed into standard equation of MATLAB transient response analysis for calculating the transfer function as in the following equation: 𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆 (𝑠𝑠) 𝐷𝐷(𝑠𝑠)

= 𝐶𝐶𝑠𝑠 (𝑠𝑠𝑠𝑠 − 𝐴𝐴𝑠𝑠 )−1 𝐵𝐵𝑠𝑠 + 𝐸𝐸𝑠𝑠

(18)

where𝐴𝐴𝑠𝑠 , 𝐵𝐵𝑠𝑠 , 𝐶𝐶𝑠𝑠 and 𝐸𝐸𝑠𝑠 are the combinations of matrices as shown in the following equations: 𝐴𝐴𝑠𝑠 = 𝐴𝐴1 𝐷𝐷 + 𝐴𝐴2 (1 − 𝐷𝐷) 𝐵𝐵𝑠𝑠 = (𝐴𝐴1 − 𝐴𝐴2 )𝑋𝑋 + (𝐵𝐵1 − 𝐵𝐵2 )𝑈𝑈 𝐶𝐶𝑠𝑠 = 𝐶𝐶1 𝐷𝐷 + 𝐶𝐶2 (1 − 𝐷𝐷) ⎨ 𝐸𝐸𝑠𝑠 = (𝐶𝐶1 − 𝐶𝐶2 )𝑋𝑋 ⎩ ⎧

(19)

By doing so, the duty ratio to inductor current can be obtained and the outer loop, SCs current to SCs voltage, is its pure integrator of capacitance. After that, PI controller can be designed by using PID controller block in MATLAB/Simulink. Figure 4 shows the overall transfer functions and their controllers. The block diagram of the BHEV power train system is shown in Fig. 5.

𝑃𝑃𝑡𝑡 < 0

B. Converter design Cascade control, which has a good stability and disturbance rejection performance, is utilized to control the driving power by controlling the duty cycles current of the inner loop receiving the reference current from the outer voltage control loop. In order to control the voltage and current, the converter’s state equations have to be linearized. State space averaging technique is used to analyze timevarying nonlinear systems in designing the controllers. The system matrices, 𝐴𝐴1,2 , 𝐵𝐵1,2 and 𝐶𝐶1,2 , can be achieved by using KVL in the on and off stage of the boost converter [6] as shown in the following equations:

Fig. 4. Cascade control for the converter

Fig. 5.MATLAB/Simulink block diagram for the BHEV power train system

International Conference and Utility Exhibition 2014 on Green Energy for Sustainable Development (ICUE 2014) Jomtien Palm Beach Hotel and Resort, Pattaya City, Thailand, 19-21 March 2014 220

(a)

120

Voltage [V]

80 60 40 20

x 10

10

20

Time [s]

30

40

4

Voltage [V]

Power [W]

Time [s]

30

50

40

0

10

Time [s]

30

40

50

250

235 0

-300 0

300

255

Vbus BHEV Vbus BEV

240

20

0 -100

250

245

LBs power SCs power Chopper power Motor power

100

(e)

260

2

-6 0

20

265

(d)

200

-200 10

50

4

-4

140

270

6

-2

160

100 0

-20 0

8

180

120

Reference speed Vehicle speed

0

(c)

300

Energy [Wh]

Speed [km/h]

100

400

(b)

Usc reference Vsc actual

200

SCs Current [A]

140

10

20

Time [s]

30

40

200

10

20

Time [s]

30

40

BEV energy BHEV energy SCs energy Kinetic energy

50

(f)

150 100 50 0

50

-50 0

10

20

Time [s]

30

40

50

Fig. 6. Simulation results of a wide open throttle driving cycle: (a) Speed reference and actual profile of vehicle simulation(b) SCs voltage ref. and act.(c) SCs current, (d)LBs, SCs, propulsion chopper and motor power(e)Bus voltage of BEV and BHEV, and (f) Energy comparison between BEV and BHEV

V. SIMULATION RESULTS The simulation had been carried out by using the mathematical model as discussed. In order to receive the same performance, BEV and BHEV weight were set the same. The torque-speed curve of the permanent magnet DC motor was constantat 300 Nm as its characteristic until reaching the maximum speed 183.2 rps. The driving cycle test is the step reference full speed, maximum speed 120 km/hstarting at 1 s. After acceleration, test was allowed for cruising for a few seconds then braking from t=30-50 s, with the same torque as shown in Fig. 6(a). At the acceleration startingfrom t = 1 s in Fig. 6(d), SCs almost supplied poweralone until t = 5 s, the battery had taken part of power until theend of acceleration period at t = 25 s. Power supplied from SCs was decline and could not reach maximum value designedbecause of the non-linear production of actual voltage, affected by the propulsion load as shown in Fig. 6(b), and current as shown in Fig. 6(c). However, it had been compensated by the battery power. The acceleration 0-100 km/h within 20 s was proved.In cruising phase t = 25-30 s, SCs further supplied the power as it had not reached the reference voltage. Thereafter, the LBs began to feed power for the tractive load per design. At t = 30-50 s, the motor decelerated, causing the motor to become generator, supplying power back to the power supply about 55 kW. At this moment, SCs absorbed deeper power than LBs, which is around 30 kW while remaining power was absorbed by the battery. The SCs was recharged by the load as long as the motor power existed and also by the battery,since t = 34 s. The current was steadily supplied to SCs according to the control strategy and caused of their voltage linearly increased to maximum value as shown in Fig. 6(c) and Fig. 6(b), respectively. Figure6(e) shows the bus voltage profile comparison between BEV and BHEV that highest bus voltage regulation of BEV, 9.8 %, occurred in acceleration phase while BHEV,7.5 %, occurred in deceleration phase. The reduction of bus voltage regulation by 2.3 % was derived by BHEV.

Figure 6(f) shows the energetic consumed by each component of BEV and BHEV, in which the kinetic and SCs energy conformed to the design objective that always turned to zero at the end of cycle. The energy consumption comparison between BEV and BHEV had been improvedby 7 %, consumed by 199 and 185 wh, respectively. VI. CONCLUSION This paper presents a control strategy and a reliable DCDC converter of SCs auxiliary power supply, in order to improve the energy economy and battery life for the BHEV. The proposed strategy is by using the cascade voltage and current control of SCs, for tracking the kinetic energy, whereas the maximum power is derived from acceleration and observed through the simulation. This paper also presents the comparison on energy saving of the BHEV over BEV, which can save up to 7% by the proposed system and alsodecrease the bus voltage regulation by 2.3 %. VII. REFERENCES [1] [2] [3]

[4] [5]

[6]

ISO, “Win the energy challenge with ISO 50001,” no. ISBN 978– 92–67–10552–9, 2011. M. Ehsani, Y. Gao, and A. Emadi, Modern electric, hybrid electric, and fuel cell vehicles: fundamentals, theory, and design. CRC press, 2009. D. Iannuzzi and P. Tricoli, “Speed-based state-of-charge tracking control for metro trains with onboard super capacitors,” Power Electronics, IEEE Transactions on, vol. 27, no. 4, pp. 2129–2140, 2012. S. Pay and Y. Baghzouz, “Effectiveness of battery-supercapacitor combination in electric vehicles,” in Power Tech Conference Proceedings, 2003 IEEE Bologna, vol. 3, 2003, p. 6–pp. R. M. Schupbach and J. C. Balda, “Comparing DC-DC converters for power management in hybrid electric vehicles,” in Electric Machines and Drives Conference, 2003. IEMDC’03. IEEE International, vol. 3, 2003, pp. 1369–1374. J. Wong, N. Idris, M. Anwari, and T. Taufik, “A parallel energysharing control for fuel cell-battery-ultra capacitor hybrid vehicle,” in Energy Conversion Congress and Exposition (ECCE), 2011 IEEE, 2011, pp. 2923–2929.