Performance Evaluation of Ultra-Capacitor in Hybrid Energy Storage System for Electric Vehicles

*Shrikant Misal and Bangalore Divakar

Department of Electrical & Electronics Engineering, REVA Institute of Technology and Management, Bangalore, INDIA

*Corresponding Author: shrik88@gmail.com

Abstract- This paper deals with the simulation study of an ultra-capacitor based energy storage system for Electric Vehicle. A detailed performance analysis has been carried out on a configuration, where the ultra-capacitor is directly connected across the load and fed by the battery through a dc-dc converter. The simulation study mainly focuses on performance of this particular configuration pertaining to the charge/discharge cycles that capacitor is subjected to during forward torque and regenerative braking power. Various plots are obtained to aid capacitor selection so as to satisfy energy requirement. The study has also focussed on various strategies, by which the capacitor voltage is maintained within a band of voltage and their influences on energy delivered. Keywords - Battery, Ultra-capacitor (UC), Hybrid Energy Storage System (HESS), DC-DC Converter, Hysteresis Control, Electric Vehicle

I. INTRODUCTION

Energy Storage System (ESS) is the heart of an electric vehicle. A battery-based ESS has several challenges which need to be looked into such as: i) Low power density ii) Thermal iii) Cell balancing iv) Frequent charge- discharge cycles leading to deterioration v) Size and cost. Batteries in a typical EV are subjected to sudden discharge and charge cycle due to acceleration and braking actions respectively. As a result there will be a decline of battery life. Many research works have shown that the batteries perform well and last longer when they are subjected to constant load. Unfortunately the typical urban driving conditions do not offer such an ideal conditions for the batteries. Thus batteries cannot be made to operate under constant load conditions in practical conditions. Hence to overcome the above drawbacks of a conventional battery based energy system, a Hybrid Energy Storage System (HESS) in which an ultra-capacitor is interfaced with the battery is suggested in literatures [1]-[4]. Interfacing the battery and an ultra-capacitor (as an energy buffer) through a controlled dc/dc converter results in battery size reduction [1] and better overall performance [1]-[4]. Several configurations for HESS designs proposed in literatures differ from one another in the manner of interfacing of battery and energy buffer. Some of the confi-gurations are i) Ultra-capacitor/Battery configuration having UC at the input of dc/dc converter and Battery at its output ii)

Battery/Ultra-capacitor configuration having Battery at the input and Ultra-capacitor at the output of dc/dc converter iii) Multiple Input Converter configuration implementing two inputs to the dc/dc converter iv) Cascaded configuration implementing two dc/dc converters in cascade in a manner such that second converter is connected at the output of first and v) Multiple Converter configuration paralleling the output of two converters instead of cascading [1]. Among all the above configurations, the one which is most widely used is conventional HESS where the battery pack is directly connected to the dc link while a half-bridge converter is placed between the UC bank and the dc link. However in order to utilize the power density advantage of the UC, the half-bridge converter must match the power level of the UC. In most cases, the half-bridge converter is a significant portion of the cost. Although this design solves the problem of the peak power demands, the battery still suffers from frequent charge and discharge operations. Hence to take care of these issues related to the above configuration, a Battery/Ultra-capacitor configuration is implemented in [1]. The ultra-capacitor so connected is capable of meeting the sudden burst of power demand from the load. This configuration provides flexibility to vary dc link voltage in accordance with frequent charge and discharge operation of the ultra-capacitor. A. Objectives of the paper In [1] a new configuration in which ultra-capacitor connected to the load is proposed. In this topology, as the capacitor is connected directly across the load, the capacitor is made to undergo successive discharge and charge cycle. The capacitor discharges into the load and charges from the battery thereby maintaining its voltage within a hysteresis band. This paper does not discuss the power stored/absorbed by the ultra-capacitor as well as the influence of the hysteresis band on the overall design. As the power depends upon the square of the hysteresis band it is very vital for a designer to understand the relationship between hysteresis band and the power delivered. The present paper is an extension of [1] and it provides some theoretical insight into the factors governing the selection of ultra-capacitor and the hysteresis band. Therefore the main scope of this paper is to provide guidelines for choosing an appropriate hysteresis band which can reduce the frequent charging and discharging of the battery for a given application.

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B. Organization of the paper HESS configuration is introduced in II, various simulation studies are presented in III and inferences in IV followed by concluding remarks in V. II. ULTRA-CAPACITOR CONFIGURATION A. Operation Of Battery/Ultra-Capacitor HESS Configuration

Fig.1 Battery/Ultra-capacitor Configuration [1] The above schematic in Fig.1 depicts the ultra-capacitor based energy system configuration, in which the battery is interfaced with the ultra-capacitor through a bidirectional dc-dc converter that acts as a boost converter in the forward mode and buck converter in the regenerative mode. The ultra-capacitor will support the sudden change in load power demand. If the load demands higher power, the capacitor will discharge and support the demand, and absorbs excess power during regenerative action. Thus it protects the battery against frequent charge/discharge cycles. In this configuration the voltage of the battery can be maintained at a lower magnitude so as to minimize the number of batteries in the series string. B. Control Strategy

Fig.2 Block Diagram of Window Comparator As the capacitor is responsible for the delivery and absorption of power, it is very vital that the capacitor’s voltage is managed for better utilization of the capacitor. Hence a window comparator is used which compares the

capacitor voltage with two upper and lower limits as shown in Fig.2 so as to keep the capacitor voltage within a band. This band influences the effectiveness of the control strategy as will be discussed later in the paper. C. Generation Of PWM Signal For Lower and Upper Switch: The switches of the dc-dc converter need to be operated as per the mode of operation; forward or regenerative action. The scheme of gate signal generation is shown in Fig.3 below.

Fig.3 Block diagram of PWM Sub-system Here current mode is employed for controlling the battery current during forward and regenerative modes. The current in the inductor is sensed and compared with a reference signal produced from the PI controller passed error signal between the capacitor voltage and the reference. The error between the sensed inductor current and the commanded value is again passed through the second PI controller and compared with the ramp signal. The generated signals will be applied to the appropriate switches depending on the mode of operation through pulse steering circuits. III. SIMULATION RESULTS

Fig.4 Complete Simulation Circuit of a Battery/Ultra- Capacitor HESS

The complete system with the control strategy has been implemented in SIMULINK and the schematic is shown above. The simulation’s specifications are given in Table I.

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TABLE I. TYPICAL RATINGS AND CHARACTERISTICS OF COMPONENTS USED IN SIMULATION [1]

Case I: Normal Window of Hysteresis Forward Operation: i) For Load Torque (TL) = 150N-m

Fig.5 Ultra-capacitor Output Voltage, Window Comparator and PWM Output Signal

Fig.6 Ultra-capacitor and DC-DC Converter Output Current In this case study, the window comparator keeps the capacitor voltage between 480 V and 450 V. The compara-tor output and the PWM signals are shown in Fig.5 above. The chosen value for the capacitor is 63F. The Fig.6 shows the output waveforms of capacitor voltage, capacitor current and the dc-dc converter current. From the plots it can be seen that battery is supplying both capacitor and de-manded power from the load.

Reverse Operation: i) For Load Torque (TL) = -150 N-m

Fig.7 Window Comparator and PWM Block Signals The case study pertains to the regenerative mode in which the regenerative load of 150 N-m is considered and a hysteresis band of 480-500 V taken. The upper limit has been increased to 500 V with the intention of storing higher energy in the capacitor, which can be utilized when the demand arises. It can be seen that, when the capacitor reaches the maximum limit, the dc-dc converter is operated as a buck converter with the upper switch getting the PWM signal to charge the battery. Thus the capacitor absorbs the sudden inrush power and protects the battery. Case II: Increased Hysteresis window (400V-480V) Forward Operation i) For Load Torque (TL) = 150 N-m

Fig.8 Ultra-capacitor Output Voltage, Window Comparator and PWM Output Signal

In this case the simulation carried out with an increased window of 400 V – 480 V. The increased window has naturally increased the time taken to discharge the capacitor and thereby increasing the energy stored which may be required for a sudden change in future power.

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Reverse Operation: i) For Load Torque (TL) = -150 N-m

Fig.9 Window Comparator and PWM Block Signals This result as shown in Fig.9 is similar to case I but with higher hysteresis window. Case III: For Reduced Hysteresis Window Control

Fig.10 Ultra-capacitor Output Voltage for Forward Operation

Fig.11 Ultra-capacitor Input Voltage for Reverse Operation The previous two simulation studies have brought out one fundamental flaw with the control strategy in which the capacitor voltage is maintained within a band. This causes frequent charge-discharge cycle resulting in life reduction of ultra-capacitor. The hysteresis window not only caused frequent charge-discharge cycle of the ultra-capacitor but also of the battery. This is the serious limitation of this topology in which the capacitor is directly

connected across the load and made to meet the change in load demand. The desirable option would be to maintain a steady voltage across the capacitor for constant power loading. This causes the power delivered from the battery constant for constant load thereby extending the battery life. Thus considering the above aspects, a simulation analysis has been carried in this case for a very small window of Hysteresis control i.e (480-479.8)V for the forward operation and (499.8-500)V for the reverse operation of the motor load . The simulation results for the same are shown below in figures 10 and 11 respectively. The results show the ultra-capacitor voltage profile with reduced hysteresis window. Even though the profile is favorable from the view point of longevity of capacitor, it will not permit capacitor to discharge in case of sudden load changes. Hence it can be concluded that there is a need for an effective control strategy where the capacitor’s voltage is maintained constant for constant load condition and permitted to change for variable load condition.

IV. SUMMARY OF SIMULATION RESULTS The power delivered, the time for which the capacitor can support the given load, the influence of hysteresis window and the magnitude of capacitance are depicted in the above plots. The graphs of discharging time and output power delivered by the ultra-capacitor for the rated capacitances of 16.67F, 63F and 6.67F are as shown in figures 12 and 14 for the forward mode of operation of the motor for the normal and increased hysteresis window control having voltage windows of (450V-480V) and (400V-480V) respectively. It can be concluded from the plots that the larger capacitor will be able to support a given load for a longer period of time and same is true for the case with increased window that allows the capacitor to discharge deeper so that required power output demand is met.

Fig.12 Ultra-Cap Output Power Delivered Vs Time for Normal Window

of Hysteresis Control Similarly the graphs of charging time and input regenerative power absorbed by the ultra-capacitor for the

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rated capacitances of 16.67F, 63F and 6.67F are as shown in figures 13 and 15 considering the reverse operation of the motor for the normal and increased hysteresis control having voltage windows of (500-480)V and (500-450)V respectively.

Fig.13 Regenerative Power Absorbed by Ultra-Cap Vs Time for Normal Window of Hysteresis Control

Fig.14 Ultra-Cap Output Power Delivered Vs Time for Normal Window of Hysteresis Control

Fig.15 Regenerative Power Absorbed by Ultra-Cap Vs Time for Normal Window of Hysteresis Control

From the plots it can be concluded that by increasing the upper limit of the window comparator, the capacitor will be able to store more regenerative energy from the load and thereby protect the battery from uncontrolled charge dumping from the load.

V. CONCLUDING REMARKS A simulation analysis has been carried out to study the performance of a hybrid energy storage system with ultra-capacitor and battery. In this configuration, the UC is directly connected to the load and interfaced with the battery through a bidirectional dc-dc converter that will control the power fed to the capacitor and also the power delivered from the capacitor. The main purpose of having an energy buffer in the form of UC is that the battery should not be unduly stressed with frequent charging and discharging which will affect the battery life. However, the simulation studies have shown that, having the capacitor directly across the load without proper control strategy will not provide favorable condition for the battery. The favourable condition is when the battery supplies constant load and the capacitor takes care of load variation such as sudden acceleration and braking action. This is possible if the algorithm is flexible enough to maintain the charge on the capacitor with reference to the speed of the vehicle, so that the capacitor is partially charged at higher speed to absorb regenerative power and charged fully at lower speed to meet the sudden demand in power due to acceleration. The simulations have been carried out for various cases in forward and regenerative modes with different capacitances and different hysteresis windows. The smaller hysteresis window will prevent frequent charge-discharge cycles of the capacitor and does not allow the capacitor to deliver or absorb sudden thrust of power. On the other hand, the larger hysteresis window causes charge-discharge cycles but permits discharging of capacitor to meet the sudden demand. Through the simulation studies, selection of capacitance for a given support time is possible.

VI. FUTURE WORK In the battery/ultra-capacitor HESS configuration discussed above, a control mechanism needs to be implemented which according to the speed of operation during either acceleration or deceleration should be able to maintain the charge on the ultra-capacitor for supplying variable load or absorbing the regenerative power for charging the battery. Hence a variable window of the hysteresis control is desired which according to the load requirements and the mode of operation can automatically shift the lower threshold limit of the capacitor voltage conveniently.

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ACKNOWLEDGMENT

The authors thank HOD, EEE Department, Principal and Management of REVA ITM for providing all the facilities for this research work.

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