Design of Electrical Accumulator Unit (EAU) Using Ultracapacitor

Transcription

1 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition January 2012, Nashville, Tennessee AIAA Design of Electrical Accumulator Unit (EAU) Using Ultracapacitor Thomas Wu and Tony Camarano University of Central Florida, Orlando, FL Jon Zumberge and Mitch Wolff Air Force Research Lab, Wright Patterson, OH Electromechanical actuator (EMA) loads are always highly dynamic for electric aircraft (MEA) applications. Occasionally, the loads need large power to operate and may actually work in the regenerative mode, sending large power back to the bus. This will make generator voltage regulation difficult. In addition, the recessive regenerative energy needs to be stored and used rather than burned through resistors. An electrical accumulator unit (EAU) has been recently proposed, analogous to the hydraulic accumulator unit (HAU) for hydraulic systems. In this paper, the design of an EAU based on an ultracapacitor is presented. A bidirectional boost converter with double arms connects the ultracapacitor and the DC bus. Two PI controllers are used to regulate the PWM duty cycles of the arms, monitoring the DC bus voltage and the ultracapacitor voltage, respectively. Simulations show that the designed EAU can effectively control both the DC bus voltage and ultracapacitor voltage for highly dynamic EMA loads by properly tuning the proportional and integral gains of the controllers Nomenclature V bus = measured voltage from the bus V cap = measured voltage from the ultracapacitor gctrl = gate control signals g 1 = gate control signal for Switch 1 g 2 = gate control signal for Switch 2, complimentary to g 1 g 3 = gate control signal for Switch 3 g 4 = gate control signal for Switch 4, complimentary to g 3 I. Introduction HE motivation behind an electric aircraft (MEA) lies in the increased reliability, improved maintainability, Treduced system weight, and operating costs. One challenge is handling regenerative power from electromechanical actuator (EMA) loads 1-4. It is noted that, for some percent of the time (depending on architecture of the aircraft and implementation of control), the EMA works in the regenerative mode. This makes power management very challenging. Traditionally, the regenerative power is dissipated through resistors which wastes the regenerative energy and requires cooling for the resistors. Instead, the regenerative energy can be stored and reused. Another challenge is that generator voltage regulation becomes difficult because the EMA loads are highly dynamic. For example, the loads will occasionally have a large power demand which may send large regenerative power back to the bus. The electrical accumulator unit (EAU) has been proposed recently in order to properly handle regenerative power and regulate bus voltage 5. The EAU is similar to the HAU (hydraulic accumulator unit) in a hydraulic system. It consists of an energy storage device (ultracapacitor, battery, regenerative fuel cell, flywheel, etc.) and a bidirectional converter. Figure 1 shows a simple aircraft power system model which consists of a synchronous generator, an EMA load and an EAU 6. The EAU stores the regenerative power when any of the EMAs work in the regenerative mode, and sends the stored energy back to the EMAs when they have large power demand. Section II will discuss EAU design. Section III shows simulation results. 1 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

2 Figure 1. A simple power system consisting of a generator, EAU, and EMA. II. EAU Design The SimuLink block diagram of the designed EAU is shown in Fig. 2. It consists of an energy storage device (ultracapacitor), a bidirectional boost converter, a controller, and an EMI filter 6. The EAU controller regulates the voltage of the bus (270 V) and the ultracapacitor (250 V). Figure 2. Block diagram of the electrical accumulator unit (EAU). 2

3 The bidirectional, interleaved boost converter is shown in Fig. 3. In this design, the upper and lower arms are used to control the bus and ultracapacitor voltages, respectively. PWM control signals for the switches are generated from the EAU controller. Switches 1 and 2 are complimentary. Switches 3 and 4 are also complimentary. Figure 3. Bidirectional, interleaved boost converter. Figure 4 shows the details of the EAU controller. Two PI controllers are used to control the PWM duty cycles of the arms. These controllers monitor the DC bus and ultracapacitor voltages, respectively. Figure 4. EAU controller. The EMI filter model is shown in Fig. 5 and is mainly used to filter out the PWM frequency. 3

4 Figure 5. EMI filter model. III. Simulation Results Figure 6 shows the bus voltage, current fed into the EMA, and voltage on the ultracapacitor for the power system model in Fig. 4. The peak EMA power shown here is about 18 kw. The bus voltage can be controlled between 250 and 280 V through effectively tuning of the EAU. The EMA current and voltage on the ultracapacitor are correlated. When the EMA regenerative current reaches the negative peak, the voltage of the ultracapacitor also reaches its positive peak. This demonstrates the ultracapacitor storing regenerative energy. When the EMA current reaches a positive peak, the ultracapacitor voltage reduces, verifying that the EAU releases its stored energy to help the generator provide power to the EMA. Figure 7 shows the duty ratio for switch 1 and 4 (Fig. 3) to control the bus and ultracapacitor voltages, respectively. Figure 6. Bus voltage, EMA current, and voltage on the ultracapacitor. 4

5 Figure 7. Duty ratio of PWM switch 1 to control the bus voltage and PWM switch 4 to control the ultracapacitor voltage. IV. Conclusion A successful design of an electrical accumulator unit (EAU) is presented in this paper. The designed EAU consists of an interleaved, bidirectional boost converter, a PI controller and an EMI filter. The upper and lower arms of the bidirectional boost converter are used to monitor the DC bus voltage (270 V) and the ultracapacitor voltage (250 V), respectively. Simulation results for peak power of 18 kw show the effectiveness of the design. Acknowledgements This work is supported by National Research Council (NRC) senior research associateship. References 1 J. Zumberge, M. Wolff, K. McCarthy, T. O Connel, G. Russel, E. Lucas and E. Walters, Energy Management of an Aircraft Electrical System, 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Nashville, Tennessee, July 2010, AIAA Paper J. Zumberge, M. Wolff, K. McCarthy, T. O Connel, E. Walters, G. Russel, and C. Lucas, Integrated Aircraft Electrical Power System Modeling and Simulation Analysis, Power Systems Conference, Dallas, Texas, November 2010, SAE Paper P. Wheeler, J. Clare, S. Bozhko and A. Kulshreshtha, Regeneration in Aircraft Electrical Power Systems?, Power Systems Conference, Seattle, Washington, November 2008, SAE Paper E. Tajirian, P. Walia, C. Lui, I. Wong, and C. Lee, A Method of Shared Regenerative Power Management, Power Systems Conference, Dallas, Texas, November 2010, SAE Paper J. Wells, S. Iden, M. Amrhein, E. Walters, A. Matasso, A. Page, P. Lamm, Electrical Accumulator Unit (EAU) for the Energy Optimized Aircraft, Power Systems Conference, Seattle, Washington, November 2008, SAE Paper T. Wu, J. Zumberge and M. Wolff, On Regenerative Power management in More Electric Aircraft (MEA) Power System, 2012 IEEE NAECON, Dayton, OH. 5