An AC-DC-AC Converter with Smaller DC-link Capacitor for Space Power Distribution System

University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2006 • 106-107

Wajiha Shireen

ABSTRACT—The power conditioning equipment used in a space power system contributes to the total system mass, reliability, and cost. The focus of this research was to reduce the weight and improve the reliability of an AC-DC-AC converter used in a large number of the power conditioning stages in a typical space power system.

The power conditioning stage between the source and distribution or between the distribution and the load efficiently interfaces various source types with different distribution systems and with a wide variety of load requirements in a typical space power system. In addition to providing flexible electric power of high quality, the power conditioning method and the associated equipment (power converters, filters etc.), have a significant effect on the total power system affecting mass, reliability and cost in a space power system. In view of these issues, this research involved the study of a method for reducing the weight and improving the reliability of an AC-DC-AC converter utilized in a large number of the power conditioning stages used in a typical space power system.

The DC link in any AC-DC-AC converter is normally equipped with an electrolytic capacitor, which provides decoupling between the rectifier and the inverter. However, the DC link capacitor is a large, heavy and expensive component. Moreover, the DC bus capacitor is the prime factor of degradation of the system reliability. However, a cost effective and efficient solution is not yet available. For space power distribution systems, these problems may prove to be even more critical.

Objectives
Researchers hope to develop a Digital Signal Processor (DSP)-based modified space vector pulse width modulation (PWM) technique that will allow the use of a smaller DC-link capacitor without affecting the output performance of the converter. The proposed method implemented in AC-DC-AC converter applications will result in the following advantages:

Reduced converter weight and volume.
Significant improvement in system reliability by the use of a smaller link capacitor.
Digital control by a DSP to provide fast transient response, high performance and increased reliability.
A digital controller insensitive to the environment, offering a stable operation under most operating conditions. Moreover, the DSP-based controller, being programmable, can also be easily upgraded or modified to meet the specific system requirements.

Figure 1. A Closed Loop V/Hz Motor Drive System with the Proposed DSP-based Control

Results
Figure 1 shows the block diagram of the experimental set-up used to test and verify the proposed technique. The experimental prototype of the AC-DC-AC converter consists of a diode rectifier, DC-link, and a PWM inverter used in a closed loop V/Hz motor drive. Researchers utilized the DSP-based controller (TMS320F240) to implement the proposed technique, which allowed the use of a smaller DC-link capacitor without affecting the output performance of the rectifier-inverter system. The use of smaller link capacitor introduced a ripple on the dc-bus voltage. The DC-link voltage ripple was sensed and fed as an input to the DSP controller.

Figure 2. Inverter output voltage magnitude when the DC bus voltage varies in the range of 25 V to 34 V (with 47 µF DC-link capacitor).

Figure 3. Inverter output voltage magnitude when the DC bus voltage is varied in the range of 55 V to 110 V.

Figure 2 shows a plot of the recorded experimental data of the inverter output voltage magnitude, with the DC bus voltage varying in the range of 25 V to 34 V (when the DC-link capacitor is 47 µF), both with and without the proposed correction algorithm in effect. Without correction, the output voltage increases as the DC bus voltage is increased, but with the modified SVPWM correction in place the output voltage magnitude is maintained constant (at 12 V) regardless of the DC bus voltage. Figure 3 shows a plot of the recorded experimental data when the DC bus voltage is manually varied over a wider range (55 V to 110 V). The plot in Fig. 3 shows that with modified SVPWM control in effect, the output voltage magnitude is maintained constant (at 26 V) even when the input DC voltage is varied over a wide range.

Publications
Shireen, W., R. Kulkarni, and M. Arefeen. "Analysis and Minimization of Input Ripple Current in PWM Inverters for Designing Reliable Fuel Cell Power Systems," J. of Power Sources 156.2 (2006): 448-54.
Shireen, W. and H. R. Nene. "Active Filtering of Input Ripple Current To Obtain Efficient and Reliable Power from Fuel Cell Sources," IEEE-INTELEC, Intl. Telecommunications Energy Conf., 2006.
Shireen, W. and H. R. Nene. "Control and Design Aspects of Power Electronics Converters using PSpice," J. of Advanced Technology for Learning 3.1, 2006. (Accepted.)
Shireen, W., S. Vanapalli, and H. R. Nene. "A DSP-Based Utility Interactive Inverter for Alternate Energy Systems," IEEE-APEC, 2006; 21st Annual IEEE Applied Power Electronics Conference and Exposition, APEC ‚Äò06. (2006): 1099-03.
Shireen, W., S. Vanapalli, and H. R. Nene. "DSP-Based Inverter Control for Alternate Energy Systems," J. of Power Sources, 2007. (Accepted.)

Presentations
Shireen, W., S.Vanapalli, and H. R. Nene. "A DSP-Based SVPWM Control for Utility Interactive Inverters Used in Alternate Energy Systems," IEEE Applied Power Electronics Conference (APEC) Proc., 2006.

Funding, Proposals, and Grants
Shireen, W. "Development of a Modern DSP-Based Laboratory for Power Electronics Education," National Science Foundation, $74,000. (Project ends September 2007.)
Shireen, W. "Control of Power Converters in a Fuel Cell System Using a Single DSP Controller," University of Houston GEAR Award, $20,000. (Project ended September 2006.)

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