ANALYSIS OF SUBSYSTEM INTEGRATION IN AIRCRAFT POWER DISTRIBUTION SYSTEMS Sriram Chandrasekaran, Douglas K. Lindner and, Dushan Boroyevich Center for Power Electronics Systems The Bradley Department of Electrical and Computer Engineering Virginia Tech, Blacksburg, VA 24061 Corresponding Author: Douglas K. Lindner e-mail: lindner@vt.edu Presented at International Symposium on Circuits and Systems ISCAS’99, Orlando, Florida, May 31-June 3, 1999 ABSTRACT Stability analysis of a baseline power system architecture for modern aircraft is addressed. Power electronic converters are widely used in modern aircraft power distribution systems. Due to their inherent nonlinear characteristics, instabilities may arise while integrating individual subsystems together. Bifurcation analysis is used to identify the type, multiplicity and stability of system trajectories. The complete bifurcation diagram for the baseline power system is drawn. The dependence of the parameter values on the bifurcation behavior of the baseline system is presented. 1. INTRODUCTION Considerable attention has been paid in recent years to the development of power-by-wire technologies for modern aircraft. As a result, modern aircraft power distribution systems have seen the widespread use of power electronic converters to drive various loads and actuation systems. Power electronic converters are inherently nonlinear systems. The problem of instability that arises due to the integration of these subsystems has been addressed in the past. It is well known that the classical impedance ratio criterion [1] relies on linear analysis techniques in the determination of stability of the interconnected system but it only guarantees local stability in the neighborhood of an equilibrium solution. The application of nonlinear analysis techniques to gain a global understanding of the behavior of the system thus becomes important. However, nonlinear analysis methods do not immediately appeal to the system designer because of their mathematical complexity. In [2] and [3], nonlinear methods were used in the analysis of interaction between an input filter and a regulated power converter modeled as a constant power load. The objective of the paper is to extend this analysis to a baseline power system architecture by studying the bifurcation behavior of the system as a function of a chosen critical parameter. The organization of the paper is as follows: Section 2 introduces the sample power distribution system as a single source-single load system. The source subsystem is a three-phase boost rectifier that feeds the 270V DC bus of the power distribution system. The load subsystem is a regulated DC-DC buck converter with a front-end input filter, which is similar to that studied in [1]. The bifurcation analysis of the input filter-load converter system is presented in Section 3. The bifurcation behavior of the baseline system is then explained in Section 4. Finally, the dependence of the bifurcation behavior of the baseline system on parameter values is presented in Section 5. 2. BASELINE POWER SYSTEM ARCHITECTURE The sample power system architecture shown in Figure 1 is used as the baseline power system in the analysis of subsystem interaction presented in the paper. The three-phase boost rectifier converts the three-phase sinusoidal voltages from the generator (modeled as an ideal voltage source) to the regulated 270V DC required by the bus. The load subsystem represented by Subsystem 2 in Figure 1 is a regulated DC-DC converter with a front-end input filter. The other loads on the DC bus are modeled by a current source, negative impedance (other regulated power converters) or a simple resistance. Rectifier 3-φ -to- DC Input Filter DC-DC Converter Ideal 3-Φ voltage source Load (R, -Z, I o ) Subsystem 1 Subsystem 2 270V DC 2 1 Figure 1. Baseline Power System Architecture The three-phase boost rectifier is represented by its average model in rotating dq-coordinates synchronized with the input line voltages [4]. The load converter is also represented by its corresponding average model. These average models neglect the switching frequency ripple and hence are valid only at frequencies much lower than the switching frequency. The stability analysis of the complete interconnected system starts with identifying the critical subsystem interfaces denoted by (1) and (2) in Figure 1 and analyzing in turn, the stability of each of the interfaces with appropriate terminations. 3. BIFURCATION ANALYSIS The bifurcation behavior of the input filter-load converter interface (denoted by (2) in Figure 1) in subsystem 2 is studied [7,8]. The circuit schematic of the average model of a closed loop DC-DC buck converter with an input filter is shown in Figure 2. However, the regulated DC-DC converter is represented by the average model in contrast to the constant power model used in [1]. The resistance R f , of the input filter is