Regenerative Braking for an Electric Vehicle Using Ultracapacitors and a Buck-Boost Converter

An ultracapacitor bank control system for an Electric Vehicle has been simulated. The purpose of this device is to allow higher accelerations and decelerations of the vehicle with minimal loss of energy, and minimal degradation of the main battery pack. The system uses an IGBT Buck-Boost converter, which is connected to the ultracapacitor bank at the Boost side, and to the main battery at the Buck side. The control of the system measures the battery voltage, the battery state-of-charge, the car speed, the instantaneous currents in both the terminals (load and ultracapacitor), and the actual voltage of the ultracapacitor. This last indication allows to know the amount of energy stored in the ultracapacitor. A microcomputer control manipulates all the variables and generates the PWM switching pattern of the IGBTs. When the car runs at high speeds, the control keeps the capacitor discharged. If the car is not running, the capacitor bank remains charged at full voltage. Medium speeds keep the ultracapacitors at medium voltages, to allow future accelerations or decelerations. The battery voltage is an indication of the car instantaneous situation. When the vehicle is accelerating, the battery voltage goes down, which is an indication for the control to take energy from the ultracapacitor. In the opposite situation (regenerative braking), the battery voltage goes up, and then the control needs to activate the Buck converter to store the kinetic energy of the vehicle inside the ultracapacitor. The measurement of the currents in both sides allows to keep the current levels inside maximum ratings. The battery state-ofcharge is used to change the voltage level of the ultracapacitor at particular values. If the battery is fully charged, the voltage level of the capacitors is kept at lower levels than when the battery is partially discharged. The converter also has an IGBT controlled power resistor, which allows to drop energy when in some extreme situations cannot be accepted neither for the ultracapacitors nor for the battery pack. The car that will be used for future implementation of this experiment is a Chevrolet LUV truck, similar in shape and size to a Chevrolet S-10. This vehicle was already converted to an electric car at the Catholic University of Chile. Introduction Ultracapacitors are a new technology that allows to store 20 times more energy than conventional electrolytic capacitors. Despite this important advance in energy storage, they are still far from being compared with electrochemical batteries. Even Lead-acid batteries can store at least ten times more energy than ultracapacitors. However, they present a lot better performance in specific power than any battery, and can be charged and discharged thousand of times without performance deterioration. These very good characteristics can be used in combination with normal electrochemical batteries, to improve the transient performance of an electric vehicle, and to increase the useful life of the batteries. Fast and sudden battery discharge during acceleration, or fast charge during regenerative braking can be avoided with the help of ultracapacitors. Besides, ultracapacitors allow regenerative braking even with the batteries fully charged. In this paper, an auxiliary ultracapacitor bank, using a Buck-Boost converter, has been simulated. The ultracapacitor has a capacity of 7 Farads, a nominal voltage of 300 Vdc, and a maximum voltage of 360 Vdc. It comprises 144 units in series, each one with 1,000 Farads, and 2.5 volts dc nominal (2,7 volts maximum). The maximum current is 400 amp, and the weight of the capacitor bank is 45 kg. The total weight of the equipment is estimated in 70 kg. The System Proposed The Figure 1 shows a diagram of the ultracapacitor system proposed. The power circuit has two main components: the Buck-Boost converter using IGBT’s, and the ultracapacitor bank. The equipment is connected in parallel to the main battery, which has 26 batteries in series (312 Vdc nominal). The capacitor voltage is allowed to discharge until one third of its maximum voltage (around 120 Vdc), allowing to store an amount of 112 Wh of useful energy. This apparently poor amount of energy allows to have more than 40 kW of power during 10 seconds, which is more than enough time for a good acceleration (or deceleration) without detriment in the battery life. The nominal power of the traction motor is 32 kW, and the peak power is 53 kW. During acceleration, the IGBT T1 is commutated, transferring energy from the capacitor to the main battery. During regenerative braking, the IGBT T2 is operated, moving energy in the opposite direction. Because of the topology of the Buck-Boost converter, the ultracapacitor never reaches voltages higher than the battery pack (self-protection). Figure 1 Ultracapacitor System Buck-Boost Converter