A Purely Ultracapacitor Energy Storage System for Hybrid Electric by rvg11555


									  A Purely Ultracapacitor Energy Storage System for
 Hybrid Electric Vehicles Utilizing a Microcontroller-
            Based dc-dc Boost Converter
                                            Erik J. Cegnar, Herb L. Hess, and Brian K. Johnson
                                               Department of Electrical and Computer Engineering
                                                              University of Idaho
                                                                  Moscow, ID

Abstract— The design and testing of a purely ultracapacitor                    The hybrid design requires the use of an energy storage
energy storage system for the improvement of hybrid electric               system that provides a constant output voltage regardless of
vehicles is presented.      The system utilizes two large                  the stored electrical energy and load requirements. The
ultracapacitor banks for energy storage and a dc-dc boost                  output then appears as a stiff voltage source to the electric
converter that is capable of supplying 8kW for voltage                     motor controller. This behavior is similar to that of a battery.
regulation. The system provides greater roundtrip efficiency               Voltage regulation is accomplished with use of a dc-dc boost
over batteries, improves a vehicle’s ability to recapture energy           converter, which is designed to allow for a widely varying
from regenerative braking, and is controlled and protected by              input voltage from one ultracapacitor bank and a virtually
a microcontroller. The paper presents design considerations,
                                                                           unchanging output voltage.
simulation, and hardware results of the system.
                                                                               A custom, reprogrammable microcontroller board was
   Keywords-ultracapacitor; hybrid electric vehicle; boost                 fabricated to measure system parameters and control the
converter; microcontroller; snubber circuit                                pulse width modulation (PWM) signals to the dc-dc boost
                                                                           converter and the regenerative braking system of the vehicle.
                         I.     INTRODUCTION                               The board utilizes control that can tightly regulate the energy
    Despite improvements in battery technologies, the                      entering the low-voltage and high-voltage ultracapacitor
greatest limiting factor of the hybrid electric or electric                banks shown in Fig. 1. This control ensures that neither bank
vehicle is still the energy storage system. The poor                       will ever exceed its maximum voltage specification.
efficiencies, short life cycles, and low current capabilities of
batteries are some of the more significant problems faced by                                         II.   DESIGN
hybrid-electric vehicle designers today. These unwanted                        The specifications of the energy storage system are as
characteristics limit the performance and desirability of the              follows:
hybrid electric vehicle. An alternative energy storage
technology to the battery is the ultracapacitor [1].                           •   42 volts output
Ultracapacitor technology is superior to batteries in many
                                                                               •   200 A continuous; 600 A max output current
aspects, but also provides some new challenges for the
designer.                                                                      •   600 A continuous; 1000 A max regenerative braking
    The advantages of ultracapacitors over batteries include                       current
the following: long life cycles (>500,000), high efficiencies                  •   200 kJ of usable energy storage
(>90%), and large current/power capabilities over a wide
range of operating temperatures [2]. The two notable                           •   >500,000 cycles
drawbacks to the ultracapacitor are a low specific energy,                     •   70-80% total system efficiency
and wide voltage variations as energy is taken out of or put
into the device.                                                               The ultracapacitor energy storage system (shown in Fig.
                                                                           1) was designed specifically for a vehicle quipped with a
    The energy storage system presented here is designed for               hybrid electric power train. The electric portion of the power
a mild-parallel electric hybrid 2002 model sport utility                   train consists of a series wound dc electric motor that
vehicle (SUV). The design focuses on regenerative braking                  provides assist during acceleration and three large series
as the sole source of energy. The stored energy is then used               connected alternators that recapture energy during braking.
only in acceleration events. This type of use requires a                   Although the energy storage system was designed for this
relatively small amount of energy storage capability and                   particular vehicle, it could be easily scaled to operate in other
large current capabilities for maximizing energy recaptured                types of hybrid electric vehicles and in other situations where
acceleration.                                                              energy storage is required. The schematic of the system,
    University of Idaho National Institute for Advanced Transportation
Technology (sponsor)
                                           Figure 1. Ultracapacitor energy storage system schematic

illustrating the ultracapacitor banks and boost converter, is               process slowly causes the output voltage to drop until the
shown in Fig. 1. The design of the boost converter has                      current drawn from the system falls below 200 A and the
several unique specifications.      First is the previously                 voltage increases until it returns to its set value.
mentioned large voltage swing of the low-voltage
ultracapacitor bank. Equation (1) represents the energy                        The output voltage is regulated by first determining a set
stored in a capacitor. In order to maximize the energy                      point voltage, which is based on the voltage of the low-bank
storage capabilities of the system, the low-voltage, 540 F                  and expressed by
bank must be allowed to swing from a low voltage (ideally 0
V) to its maximum voltage of 25 V. It was also designed to
be able to output 200 A at 40 V to 45 V throughout this large
                                                                                                                V lowbank     3
                                                                                                V setpo int =             + 38 .         (2)
input voltage swing. The significance of this criterion is that:                                                    4         4
1) the converter must have low resistance in the source (the
input inductor, the power semiconductor device, and the                     The set point output voltage is limited to the range of 40 V to
wires that connect them) and 2) the microcontroller that                    45 V. The output voltage is then related to the amount of
controls it must have imbedded information that describes                   energy in the low-bank. This relationship improves energy
the behavior of the converter in both continuous and                        balance as naturally more energy will be transferred to the
discontinuous conduction modes.                                             drive train at a higher output voltage, and less energy will be
                                                                            transferred at a lower voltage. The result is that when the
                                                                            system has more available energy, more energy is used and
                          1                                                 when the system has less available energy, less energy is
                       E = CV 2                               (1)
                                                                                Second, the actual high-bank voltage is compared to the
    The ultracapacitor banks are composed of 2700 F, 2.5 V                  set point voltage and a desired PWM duty cycle is calculated
cells. The capacitors were connected in series and parallel to              that is proportional to the error of the two. The desired
acquire the appropriate specifications for the two banks.                   PWM duty cycle is then compared to a calculated duty cycle
These ultracapacitors have an effective series resistance of 1              limit, which is based on the low-bank and high-bank
mΩ. The resistance was low enough to provide high currents                  voltages. If the desired duty cycle exceeds the calculated
at low input voltages, which gave the dc-dc converter the                   duty cycle limit, the output will be set to the calculated limit.
capability to boost a very low voltage (5-10 V) to high                     Otherwise, the output duty cycle will be the desired duty
voltage (40-45 V). Detailed converter simulation was                        cycle. A graph describing the limits of the converter is
performed using PSPICE to map its behavior over a wide                      shown in Fig. 5.
range of input voltages, various PWM duty cycles, and
operation in continuous and discontinuous conduction                            The controller board utilizes a microcontroller and
modes. The simulation yielded information that was used in                  oversees the operation of the boost converter mentioned
the control of the converter over a wide range of operating                 above and ensures that the ultracapacitors are not
conditions.                                                                 overcharged. It also regulates the temperature of the
                                                                            semiconductor devices by controlling independent fans
    The second unique specification is that the system must                 attached to the heat sinks of each device and can limit the
be able to handle output current transients of up to 400 A for              PWM duty cycle in the event that one of the devices is
several seconds without a significant output voltage drop.                  overheated. Fig. 2 is a block diagram of the controller board.
This criterion is met by having a large output capacitance,
which was chosen to be 150 F. The boost converter is                            The controller board is interfaced to a 4x20 backlit LCD
capable of providing 200 A at the output. In the event that                 display mounted in the dash of the vehicle. It provides the
more than 200 A is drawn from the system, the net current is                driver with system information including: total available
provided by both the low-voltage, 540 F, ultracapacitor bank                energy, boost converter status, PWM duty cycles, and
and the high-voltage, 150 F, ultracapacitor bank. This                      semiconductor device temperatures.
                                            Figure 2. Block diagram of the boost converter controller board

                  III.    DEVICE PROTECTION
    Power electronic circuits often require a means to protect
the switching semiconductor devices against over voltage. In
the case of the dc-dc boost converter, the physical
dimensions are large and the inductor currents are high. This
combination along with a fast turn off time of the IGBT
results in inductive voltage spikes at device turn off [3].
Voltage spikes with high dv/dt are undesirable because they
cause EMI and could exceed the voltage rating of the device.
     The stray inductance in Fig. 1 is due to the large layout
and more specifically the length of the copper bar connecting
the power diode to the large filter capacitor located near the
                                                                                        Figure 4. Oscillogram of the voltage waveform over the IGBT
output terminals of the converter and labeled Cfilter2 in Fig. 1.
This stray inductance causes a large voltage spike at turn off.
This is shown in Fig. 3, which is an oscillogram of voltage                                                   IV.   SIMULATION
over the IGBT during typical operation with no protection. In
order to absorb the energy in the voltage spike, a small filter                      Simulation was used to acquire much of the information
capacitor was placed directly between the collector of the                       describing the behavior of the dc-dc boost converter.
IGBT and the cathode of the power diode, which is labeled                        Although analysis provided some data on the general
Cfilter1 in Fig. 1.                                                              behavior of the converter, simulation was utilized to locate
                                                                                 the limitations of the converter over the range of input
                                                                                 voltages from 5 V to 25 V and over the range of output
                                                                                 voltages from 10 V to 45 V. This process aided in creating a
                                                                                 control algorithm that would ensure safe operation of the
                                                                                 electronic components of the boost converter. Fig. 5 shows
                                                                                 the maximum PWM duty cycle that will allow for safe
                                                                                 operation of the boost converter, which was acquired from

     Figure 3. Oscillogram of the voltage waveform over the IGBT

     In addition to the filter capacitor, a turn-off snubber
circuit was designed to further ensure that the IGBT was kept
well within its safe operating area [4]. The snubber circuit
of Fig. 1 utilizes a fast diode along with a low-loss
metallized polypropylene capacitor. The result of adding the
filter capacitor and snubber circuit is illustrated by Fig. 4,
which shows the voltage over the IGBT after installation. It
is clear that the while the addition of these components
helped to reduce the inductive voltage spike and slowed
down the rate of change of the voltage over the IGBT, the                                      Figure 5. Boost converter limit characterization
addition of the filter capacitor has the added side effect of
                V.     TESTING AND VERIFICATION                                      amps, which for graphical purposes has been scaled down by
     The entire system (including the ultracapacitor banks,                          a factor of four.
the dc-dc boost converter and the controller board) was next                             The energy storage system was then integrated into a
tested in a power lab to acquire data that was compared to                           hybrid electric 2002 model SUV for testing with the
the simulation results for verification of the model. The                            vehicle’s drive train. The truck successfully participated in
testing was performed so that the system would experience                            competition and the system proved to be rugged, reliable and
conditions similar to those in a hybrid electric vehicle.                            stable. Figure 7 shows a graph of telemetry data taken from
Energy supplied to the system from a large dc machine                                the vehicle’s on-board computer system during a city cycle
mimicked the operation of the vehicle regenerative braking                           event. It shows the voltage in the low-bank increasing as the
system, while a resistive load mimicked the vehicle’s electric                       vehicle recovers energy during deceleration. As the vehicle
motor. Fig. 6 is data acquired from testing that illustrates                         begins to accelerate current is drawn from the output of the
how the system voltages behave as current from a resistive                           system. Energy is taken from the low-voltage bank of
load of approximately 0.36 Ω draws it from a nearly full                             ultracapacitors and transferred through the dc-dc boost
state of charge to a nearly depleted state. The figure shows                         converter to the output.
the voltages in volts of each bank and the output current in
                                                                                         The graph shows the vehicle speed in miles per hour, the
                                                                                     voltages of the high and low ultracapacitor banks in volts,
                                                                                     and the current of the electric motor and regenerative braking
                                                                                     system in amps. For graphing purposes, the current shown is
                                                                                     scaled down by a factor of four. The voltage of the high
                                                                                     ultracapacitor bank remains near constant throughout the
                                                                                     operation. The information shown in Fig. 7 is data acquired
                                                                                     while the system is being used at only a fraction of its
                                                                                     potential. The amount of energy put into and taken out of the
                                                                                     system is limited by the vehicle’s electric drive train. Future
                                                                                     plans for the vehicle involve implementing an electric drive
                                                                                     train capable of higher currents to utilize more of the stored

                                                                                                               VI.     CONCLUSION
                                                                                          As a primary storage device, the ultracapacitor has key
                                                                                     advantages over batteries with the drawback of a lower
  Figure 6. Voltage of the high and low ultracapacitor banks and output              relative specific energy.         A system of two large
              current as energy is drawn from the system                             ultracapacitor banks and a dc-dc converter can behave like a
                                                                                     battery with a stiff voltage source with no degradation during

                                        Figure 7. Vehicle telemetry data of the system during deceleration and acceleration
high output currents. A microcontroller-based control                              http://www.maxwell.com/ultracapacitors/support/papers/Ultracapacit
system ensures that the ultracapacitors and boost converter                        ors_and_HEVs.html
operate safely. Simulation provides useful information on                    [2]   E. J. Cowgiallo and J. E. Hardin, “Perspective on ultracapacitors for
                                                                                   electric vehicles,” IEEE Aerospace and Electronic Systems Magazine,
the behavior of the system that otherwise would be difficult                       vol. 10, pp. 26-31, Aug. 1995.
to acquire. Lab testing yielded data that illustrates the                    [3]   M. C. Caponet, F. Profumo, J. Jacobs, and R. W. De Doncker,
behavior of the system under load. On-road testing in a                            “Solutions to minimize conducted EMI in power electronic circuits,”
hybrid electric vehicle proved the system to be rugged and                         Applied Power Electronics Conference and Exposition, 2001 (APEC
reliable. Data acquired during testing are presented.                              ’01). Sixteenth Annual IEEE, vol. 1, 2001. pp. 220-224.
                                                                             [4]   N. Mohan, T.M. Undeland, and W.P. Robbins, Power Electronics:
                                                                                   Converters, Applications, and Design, 2th ed. New York: John Wiley
                         ACKNOWLEDGMENT                                            & Sons, Inc., 1995.
   This project was made possible by funding and support                     [5]   L. Zubieta and R. Bonert, “Characterization of double-layer
from the University of Idaho National Institute for Advanced                       capacitors for power electronics applications,” IEEE Trans. On
                                                                                   Industry Applications, Vol. 36, No. 1, Jan./Feb. 2000, pp. 199-205.
Transportation Technology and Maxwell Technologies.
                                                                             [6]   R. M. Schupbach, J. C Balda, M. Zolot, B. Kramer, “Design
                                                                                   methodology of a combined battery-ultraacapacitor energy storage
                             REFERENCES                                            unit for vehicle power management,” Power Electronics Specialist
                                                                                   Conference, 2003 (PESC ’03). Thirty Fourth Anuual IEEE, vol. 1,
[1]   B. Maher, “Ultracapacitors and the Hybrid Electric Vehicle,” [Online
                                                                                   2003. pp. 88-93.
      Document],     [cited   2003     May     1]     Available     HTTP:

To top