A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

MODELING AND ANALYSIS OF A DC POWER DISTRIBUTION SYSTEM IN 21ST CENTURY AIRLIFTERSby Konstantin P. LouganskiThesis submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree ofMaster of Science in Electrical EngineeringDr. Dusan Borojevic, Chairman Dr. Fred C. Lee Dr. Douglas K. LindnerSeptember 30, 1999 Blacksburg, VirginiaKeywords: power distribution system, multi-level modeling, simulation, small-signal analysis, stability, regenerative energy, three-phase system, starter/generator, power converter, actuator, constant power load1

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

MODELING AND ANALYSIS OF A DC POWER DISTRIBUTION SYSTEMIN 21ST CENTURY AIRLIFTERSKonstantin P. LouganskiAbstractA DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of complexity. The subsystem models were implemented in Simulink and combined into the whole PDS model according to certain interconnection rules. Effective modeling of different scenarios of operation was achieved by mixing subsystem models of different levels in one PDS model. Linearized models were obtained from the nonlinear PDS model for stability analysis and control design. The PDS model was used to examine the system stability and the DC bus power quality under bidirectional power flow conditions. Small-signal analysis techniques were employed to study stability issues resulting from subsystem interactions. The DC bus stability diagram was proposed for predicting stability of the PDS with different types of loads without performing an actual stability test based on regular stability analysis tools. Certain PDS configurations and operational scenarios leading to instability were identified. An analysis of energy transfer in the PDS showed that a large energy storage capacitor in the input filter of a flight control actuator is effective for reduction of the DC bus voltage disturbances produced by regenerative action of the actuator. However, energy storage capacitors do not provide energy savings in the PDS and do not increase its overall efficiency.2

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

AcknowledgmentsI would like to thank Dr. Dusan Borojevic for being my advisor and giving me an opportunity to pursue graduate study at Virginia Power Electronics Center (VPEC), presently the Center for Power Electronics Systems (CPES). I appreciate his broad views and enthusiasm in developing this subject, his encouragement and support, and his taking time to guide me through this work. I would like to thank Dr. Fred C. Lee, director of CPES, for serving as my committee member and for putting so much energy into making CPES one of the best power electronics centers for research and education. It was my pleasure and honor to study and work there. I would like to thank Dr. Douglas K. Lindner, the major investigator in this sponsored research, for reviewing this thesis and serving as my committee member. His valuable comments and attention to details helped me make this work better. My special thanks to Mr. Sriram Chandrasekaran, who was always available to provide any help and to share his expertise. I would like to recognize a contribution of Mr. George Korba and Ms. Catherine Frederick from Lockheed Martin Control Systems in Johnson City, New York, who provided actuator models for use in this research. Finally, I give thanks to all my friends and family for their encouragement and support during this period of my life. This work was supported by the Air Force Office of Scientific Research under Grant Number F49620-97-1-0254.iii

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

Contents1. INTRODUCTION ................................................................................................................................................ 1 2. PRINCIPLES AND TECHNIQUES OF MODELING AND ANALYSIS OF A DC POWER DISTRIBUTION SYSTEM.................................................................................................................................. 9 2.1 MULTI-LEVEL MODELING OF POWER DISTRIBUTION SYSTEM COMPONENTS ................................................... 9 2.2 POWER DISTRIBUTION SYSTEM ANALYSIS AND SIMULATION TOOLS ............................................................. 12 3. MODEL DEVELOPMENT FOR POWER DISTRIBUTION SYSTEM COMPONENTS......................... 16 3.1 MODELING OF SUBSYSTEM ELEMENTS ........................................................................................................... 16 3.2 MODELING OF DC-DC SWITCHING POWER CONVERTERS .............................................................................. 23 3.2.1 DC-DC Buck Converter Modeling........................................................................................................ 23 3.2.2 DC-DC Boost Converter Modeling....................................................................................................... 24 3.3 MODELING OF THREE-PHASE SUBSYSTEMS .................................................................................................... 26 3.3.1 Three-Phase Subsystem Modeling Approach........................................................................................ 26 3.3.2 Three-Phase Synchronous Generator Modeling .................................................................................... 29 3.3.3 Three-Phase Boost Rectifier Modeling ................................................................................................. 33 3.4 SWITCHED RELUCTANCE GENERATOR MODELING ......................................................................................... 37 3.5 MODELING OF FLIGHT ACTUATORS ................................................................................................................ 43 3.5.1 Electromechanical Actuator Modeling .................................................................................................. 43 3.5.2 Electrohydrostatic Actuator Modeling .................................................................................................. 47 3.6 DC POWER DISTRIBUTION BUS MODELING .................................................................................................... 49 4. STABILITY ANALYSIS OF A DC POWER DISTRIBUTION SYSTEM................................................... 51 4.1 INTRODUCTION ............................................................................................................................................... 51 4.2 SYSTEM CONFIGURATION AND STABILITY ANALYSIS TECHNIQUES ............................................................... 52 4.3 STABILITY ANALYSIS OF A PDS WITH CONSTANT POWER LOAD.................................................................... 55 4.4 STABILITY ANALYSIS OF A PDS WITH AN ELECTROMECHANICAL ACTUATOR ............................................... 78 5. ANALYSIS OF BIDIRECTIONAL POWER FLOW IN A DC POWER DISTRIBUTION SYSTEM ..... 97 5.1 INTRODUCTION ............................................................................................................................................... 97 5.2 SYSTEM CONFIGURATION FOR BIDIRECTIONAL POWER FLOW ANALYSIS ....................................................... 98 5.3 OVERALL SYSTEM PERFORMANCE CHARACTERISTICS AND METHODOLOGY OF BIDIRECTIONAL POWER FLOW ANALYSIS ............................................................................................................................................................ 102 5.4 EFFECT OF THE INPUT FILTER CAPACITOR ON THE SYSTEM CHARACTERISTICS UNDER BIDIRECTIONAL POWER FLOW CONDITIONS .............................................................................................................................................. 107iv

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

5.5 EFFECT OF THE BOOST RECTIFIER CAPACITOR ON THE SYSTEM CHARACTERISTICS UNDER BIDIRECTIONAL POWER FLOW CONDITIONS .................................................................................................................................. 116 6. CONCLUSIONS ............................................................................................................................................... 123 APPENDIX A. PARAMETERS OF THE DC POWER DISTRIBUTION SYSTEM..................................... 126 APPENDIX B. MATLAB FUNCTION FOR DQ-TO-ABC TRANSFORMATION....................................... 130 APPENDIX C. STATE-SPACE MODEL FOR A SYNCHRONOUS GENERATOR.................................... 132 APPENDIX D. MATLAB CODE FOR STABILITY ANALYSIS OF THE DC POWER DISTRIBUTION SYSTEM.................................................................................................................................................................. 136 D.1 SIMULINK MODELS FOR STABILITY ANALYSIS OF THE PDS ........................................................................ 136 D.2 MATLAB FILES FOR STABILITY ANALYSIS OF THE PDS ............................................................................... 138 APPENDIX E. MATLAB CODE FOR BIDIRECTIONAL POWER FLOW ANALYSIS IN THE DC POWER DISTRIBUTION SYSTEM ................................................................................................................... 145 E.1 SIMULINK MODEL FOR PARAMETRIC SWEEP ANALYSIS OF THE PDS ........................................................... 145 E.2 MATLAB FILES FOR PARAMETRIC SWEEP ANALYSIS OF THE PDS ................................................................ 146 REFERENCES ....................................................................................................................................................... 152 VITA ........................................................................................................................................................................ 155v

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

List of FiguresFigure 1.1 Power distribution system of a transport aircraft....................................................................................... 5 Figure 1.2 Power distribution system architecture...................................................................................................... 6 Figure 1.3 DC bus voltage specifications according to MIL-STD-704E.................................................................... 7 Figure 2.1 Mixed-level modeling concept. ............................................................................................................... 11 Figure 2.2 Interconnection rules for two-port networks............................................................................................ 13 Figure 2.3 Buck converter – partitioning into generic blocks. .................................................................................. 14 Figure 2.4 Simulink block diagram for the buck converter in Figure 2.3. ................................................................ 15 Figure 3.1 Low-pass L-C filter. ................................................................................................................................ 17 Figure 3.2 Simulink model of the L-C filter. ............................................................................................................ 17 Figure 3.3 Alternative Simulink model of the L-C filter based on state-space representation.................................. 18 Figure 3.4 Development of detailed and average models for the PWM switch. ....................................................... 19 Figure 3.5 Detailed and average Simulink models for the PWM switch. ................................................................. 20 Figure 3.6 Buck converter example of simulation with detailed and average models for the PWM switch............. 21 Figure 3.7 Buck converter open loop input impedance............................................................................................. 22 Figure 3.8 Bidirectional buck converter Simulink model. ........................................................................................ 23 Figure 3.9 Boost converter topology......................................................................................................................... 24 Figure 3.10 Equivalent circuit of the boost converter power stage........................................................................... 24 Figure 3.11 Simulink average model for the boost converter power stage. .............................................................. 25 Figure 3.12 Bidirectional closed-loop Simulink model for the boost converter. ...................................................... 25 Figure 3.13 ABC-to-DQ transformation Simulink block.......................................................................................... 28 Figure 3.14 DQ-to-ABC transformation Simulink block.......................................................................................... 28 Figure 3.15 Equivalent circuit of a synchronous generator in dq coordinates. ......................................................... 30 Figure 3.16 Closed-loop Simulink model of the synchronous generator in dq coordinates...................................... 32 Figure 3.17 Power stage topology of the boost rectifier. .......................................................................................... 33 Figure 3.18 Average model of the boost rectifier in dq coordinates. ........................................................................ 33 Figure 3.19 Simulink model of the boost rectifier power stage in dq coordinates.................................................... 35 Figure 3.20 Control diagram of the boost rectifier.................................................................................................... 35 Figure 3.21 Closed-loop Simulink model of the boost rectifier in dq coordinates. .................................................. 36 Figure 3.22 Switched reluctance starter/generator circuit diagram........................................................................... 38 Figure 3.23 Switched reluctance starter/generator discrete average modeling concept, [21]. .................................. 38 Figure 3.24 Switched reluctance starter/generator discrete average model. ............................................................. 39 Figure 3.25 Switched reluctance starter/generator continuous average model. ........................................................ 40 Figure 3.26 Switched reluctance generator phase current waveform, [18]. .............................................................. 41 Figure 3.27 Simulink model of the switched reluctance starter/generator. ............................................................... 42vi

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

Figure 3.28 Electromechanical actuator system diagram.......................................................................................... 43 Figure 3.29 Electromechanical actuator Simulink block diagram. ........................................................................... 44 Figure 3.30 Simulink model for a separately excited dc motor. ............................................................................... 45 Figure 3.31 Simulink model for the mechanical transmission of the EMA. ............................................................. 45 Figure 3.32 Simulink model for the surface dynamics. ............................................................................................ 46 Figure 3.33 Simulink model for the EMA feedback controller. ............................................................................... 46 Figure 3.34 Electrohydrostatic actuator system diagram. ......................................................................................... 47 Figure 3.35 Electrohydrostatic actuator Simulink block diagram............................................................................. 48 Figure 3.36 Hydraulic actuator Simulink model....................................................................................................... 48 Figure 3.37 Three-section dc bus equivalent circuit. ................................................................................................ 49 Figure 3.38 Simulink model for a three-section dc bus. ........................................................................................... 50 Figure 4.1 Power distribution system configuration for stability analysis. ............................................................... 53 Figure 4.2 Boost rectifier control-to-output transfer functions with constant current load....................................... 56 Figure 4.3 Boost rectifier loop gain transfer functions with constant current load. .................................................. 57 Figure 4.4 Boost rectifier output impedance transfer functions with constant current load...................................... 58 Figure 4.5 Boost rectifier control-to-output transfer functions with different types of load. .................................... 60 Figure 4.6 Boost rectifier loop gain transfer functions with different types of load. ................................................ 61 Figure 4.7 Boost rectifier output impedance transfer functions with different types of load.................................... 62 Figure 4.8 Small-signal impedances of the boost rectifier and 100kW constant power load.................................... 64 Figure 4.9 Small-signal impedances of the boost rectifier and 253kW constant power load.................................... 65 Figure 4.10 Small-signal impedances of the boost rectifier and 270kW constant power load.................................. 65 Figure 4.11 Nyquist plots for the PDS with different values of constant power load............................................... 66 Figure 4.12 Transient response of the DC bus voltage for different values of constant power load......................... 67 Figure 4.13 Equivalent small-signal resistance of the DC bus with mixed load....................................................... 68 Figure 4.14 Critical negative resistances of the bus load obtained from the boost rectifier output impedance transfer functions for different load powers..................................................................................................................... 70 Figure 4.15 The DC bus stability diagram. ............................................................................................................... 71 Figure 4.16 Examples of analysis with the DC bus stability diagram....................................................................... 72 Figure 4.17 Nyquist plot for the PDS with the load represented by point B in Figure 4.16. .................................... 73 Figure 4.18 Transient responses of the bus voltage and current for a step load change with 40kW resistive load added to 250kW constant power load. ............................................................................................ .................... 74 Figure 4.19 Nyquist plots for the PDS with loads represented by points C and D in Figure 4.16. ........................... 75 Figure 4.20 DC bus voltage and current transient responses to load changes corresponding to points C and D in Figure 4.16.......................................................................................................................................................... 76 Figure 4.21 Electromechanical actuator system diagram.......................................................................................... 79 Figure 4.22 Electromechanical actuator operating cycle. ......................................................................................... 81 Figure 4.23 Input filter for the electromechanical actuator....................................................................................... 82vii

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

Figure 4.24 Input filter forward voltage transfer function. ....................................................................................... 83 Figure 4.25 Input filter and converter transfer functions at operating point OP1. .................................................... 83 Figure 4.26 Input filter and converter transfer functions at operating point OP2. .................................................... 84 Figure 4.27 Input filter and converter transfer functions at operating point OP3. .................................................... 84 Figure 4.28 Input filter and converter transfer functions at operating point OP4. .................................................... 85 Figure 4.29 Nyquist plots for the EMA and input filter interaction.......................................................................... 86 Figure 4.30 Simulation of the EMA with an ideal DC bus for the whole operating cycle........................................ 87 Figure 4.31 Simulation of the EMA with an ideal DC bus. ...................................................................................... 87 Figure 4.32 Input filter and converter transfer functions. ......................................................................................... 89 Figure 4.33 Nyquist plots for the input filter and converter interaction with ideal and real bus................................ 89 Figure 4.34 Boost rectifier output impedance and the EMA input impedance. ........................................................ 90 Figure 4.35 Nyquist plot for the boost rectifier and the EMA interaction. ............................................................... 90 Figure 4.36 Simulation of the EMA with real DC bus.............................................................................................. 91 Figure 4.37 Input filter and converter transfer functions with Ra   .............................................................. 93  ........................................ 93   ............................. 94   .................................... 94Figure 4.38 Nyquist plots for the input filter and converter interaction with Ra Figure 4.40 Nyquist plot for the boost rectifier and the EMA interaction with Ra Figure 4.41 Simulation of the whole PDS with Ra Figure 4.42 Simulation of the whole PDS with Ra Figure 4.39 Boost rectifier output impedance and the EMA input impedance with Ra ..................................................................................... 95 GXULQJ WKH RSHUDWLQJ F\FOH .......................................... 96Figure 5.1 System configuration for bidirectional power flow analysis. .................................................................. 99 Figure 5.2 System operation without the wind load................................................................................................ 100 Figure 5.3 System operation with the wind load..................................................................................................... 101 Figure 5.4 Settling time of the transients on the DC bus. ....................................................................................... 102 Figure 5.5 Bidirectional energy flow in the system. ............................................................................................... 103 Figure 5.6 System operation without the wind load; Cif = 63mF............................................................................ 108 Figure 5.7 System operation with the wind load; Cif = 63mF. ................................................................................ 109 Figure 5.8 Voltage spikes magnitude vs. input filter capacitance........................................................................... 110 Figure 5.9 Settling time of the transients vs. input filter capacitance. .................................................................... 110 Figure 5.10 Total energy delivered from the engine and regenerated back (J) in the system without the wind load. .......................................................................................................................................................................... 111 Figure 5.11 Total energy delivered from the engine and regenerated back (J) in the system with the wind load. . 111 Figure 5.12 Energy balance (J) for subsystems with ( ) and without (o) the wind load. ....................................... 113 Figure 5.13 Generator losses in the system with ( ) and without (o) the wind load. ............................................. 114 Figure 5.14 Overall efficiency of the system with ( ) and without (o) the wind load............................................ 115 Figure 5.15 System operation without the wind load; Cbr = 350mF. ...................................................................... 116 Figure 5.16 System operation with the wind load; Cbr = 350mF. ........................................................................... 117 Figure 5.17 Voltage spikes magnitude vs. boost rectifier capacitance.................................................................... 118viii

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

Figure 5.18 Settling time of the transients vs. boost rectifier capacitance. ............................................................. 118 Figure 5.19 Total energy delivered from the engine and regenerated back (J) in the system without the wind load. .......................................................................................................................................................................... 119 Figure 5.20 Total energy delivered from the engine and regenerated back (J) in the system with the wind load. . 119 Figure 5.21 Energy balance (J) for subsystems with ( ) and without (o) the wind load. ....................................... 120 Figure 5.22 Generator losses in the system with ( ) and without (o) the wind load. ............................................. 121 Figure 5.23 Overall efficiency of the system with ( ) and without (o) the wind load............................................ 122 Figure D.1 Simulink model for linearization of the PDS........................................................................................ 136 Figure D.2 Simulink model for simulation of the PDS..................................................................................... ...... 137 Figure E.1 Simulink model for bidirectional power flow analysis in the PDS. ...................................................... 145ix

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

Chapter 1IntroductionThe “More Electric Initiative” is becoming a leading design concept for future aircrafts. It assumes using electrical energy instead of hydraulic, pneumatic, and mechanical means to power virtually all aircraft subsystems including flight control actuation, environmental control system, and utility functions. The concept offers advantages of reduced overall aircraft weight, reduced need for ground support equipment and maintenance personnel, increased reliability, and reduced susceptibility to battle damage in military applications [1-7]. Hence, all electrically powered subsystems become parts of an electric power distribution system (PDS), which unites all electrical sources and loads of an aircraft by means of a power distribution bus.A PDS of a more-electric aircraft includes the following elements [2, 6, 7]: internal engine electric starter/generators, integrated power units, solid-state power controllers, electricdriven flight actuators, electric-actuated brakes, electric anti-icing system, fault-tolerant solidstate electrical distribution system, electric aircraft utility functions, electric-driven environmental and engine control. A PDS consists of two independent channels, according to the number of starter/generators in the aircraft. An auxiliary/emergency power unit contains an additional auxiliary starter/generator [6].The generating system includes starter/generators, power control units, and a generator and system control unit [2]. Either three-phase synchronous machines [2] or switched-reluctance machines [7] may be used as starter/generators in a more-electric aircraft. The power control units are used to transform the “wild frequency” AC power produced by the synchronous1

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

generators into 270V DC power [2]. This power is supplied to the DC power distribution bus, which consists of several different sections. The generator and system control unit controls the generators, power control units, and the DC busses. An auxiliary power unit and battery system provide power for starting the engines and emergency back-up.Electromechanical (EMA) and electrohydrostatic (EHA) flight actuators are used in a more-electric aircraft instead of traditional hydraulic actuators with a central hydraulic system [2, 7]. The EMA and EHA employ DC brushless motors powered from the 270V DC distribution bus through DC-AC inverters.Other loads of the PDS include environmental control system loads, utility loads, and avionics. Solid-state DC-DC and DC - AC power converters [2] are used to convert 270V DC power to 115/200V, 400Hz AC power for brushless and induction motor loads and 28V DC power for electronic equipment. According to [22], up to 75% of total PDS load installed in an aircraft will be the constant power type of load.The following reasons motivated the choice of the 270V DC distribution bus [1]: it is a good voltage source for inverters that power motor loads of the aircraft, it is easy to provide uninterruptible power on the bus by using a battery back-up, regenerative power from electrical actuators can be easily returned to the bus.At the same time, there are a number of technical issues related to the choice of the 270V DC bus. Among those addressed in [1] are the following: system stability, power quality on the bus, regenerative power flow.Stability is the most important requirement for a PDS. The issue of stability is closely related to the EMI filter design for subsystems powered through switching power converters. Improper designs of the input filter for such subsystems may result in undesirable interactions [23, 25]. Currently, stability of a PDS in a more-electric aircraft for a particular design is2

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

proposed to be evaluated through computer modeling [1]. Therefore, the importance of development of computer-aided modeling and analysis tools for a PDS cannot be underestimated.Considerable experience is developed in evaluating stability of subsystem interactions in distributed power systems. Usually, the impedance ratio stability criterion suggested in [23] is used to analyze stability of interactions between two interconnecting subsystems. Stability of a spacecraft DC PDS is addressed in [27]. Stability analysis for a system with a source converter and one or more load converters is given in [26] and [28]. A method of defining the load impedance specifications based on the concept of forbidden region is presented in [29]. All these examples covered relatively simple system configurations, with an ideal voltage source, one source converter, and one or more load converters with resistive loads. The examples represented particular case studies rather than universal analysis tools. The results were obtained by tedious analytical developments for a particular system configuration rather than applying computer-aided analysis techniques to easily reconfigurable global system model.An effort in modeling and simulation of a more-electric aircraft power system is presented in [24]. The system simulated included a synchronous generator, a diode bridge rectifier, and a resistive load. A circuit-oriented type of simulation software was used. No stability analysis was performed.It is seen that although some work on modeling, analysis, and simulation of distributed power systems similar to the PDS of a more-electric aircraft has already been done, the issues of stability, DC bus power quality, and regenerative power flow in the PDS of an aircraft have not been addressed comprehensively. Therefore, the following research objectives are proposed: 1. Develop computer-aided analysis tools for modeling and analysis of a PDS in a more-electric transport aircraft. 2. Create nonlinear models of a PDS for large-signal simulations and small-signal analysis. 3. Present examples of using the modeling and simulation tools for analysis of a PDS: stability analysis of a PDS with an electromechanical actuator, stability analysis of a PDS with constant power load and mixed load,3

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

analysis of the DC bus power quality under bidirectional power flow conditions, analysis of ways to optimize regenerative energy flow in the PDS in order to increase its overall efficiency.In order to achieve the research objectives, a representative power distribution system architecture of a 21st century transport aircraft (Figures 1.1 and 1.2) was developed. The electrical energy sources are two main Starter/Generators (S/G) and a Starter/Generator of an Auxiliary Power Unit (APU). The system loads are two flight control electric actuator systems with electromechanical and electrohydrostatic actuators, Environmental Control System (ECS) motor loads, and utility loads. There is also a battery unit, which can work as either source or load. All the PDS components are connected by a 270V DC bus, which consists of two primary busses and an APU bus. Each subsystem is connected to the power distribution bus through a Bidirectional Power Converter (BDC), which may be DC–DC or Three-Phase–DC type. The Electric Load Management System provides system level control and protective functions.A prominent feature of this PDS is bidirectional power flow in the DC bus and in many of its subsystems. Normally, the power flows from the sources (generators) to the loads (actuators, ECS, battery, utility loads). However, in certain modes of operation, some loads can work in regenerative mode, thus supplying electric power to the DC distribution bus. For example, a flight control actuator works in regenerative mode when it has to slow down a moving flight control surface, or when the surface is being moved by the air flow. Another example is using the battery as an energy source and the APU starter/generator as a motor to start the APU engine. Bidirectional power flow in the PDS becomes possible because all power converters connecting subsystems to the DC distribution bus possess bidirectional power flow capability.The regenerative power phenomenon mentioned above has potential advantages since the power regenerated by one subsystem can potentially be used to power the others or be stored in the PDS for future use. However, it also has potential problems because unused regenerative power may create voltage spikes on the DC bus, which may exceed the limits set by the standard. This effect may happen because a load with sufficient power consumption may not be available4

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

Figure 1.1 Power distribution system of a transport aircraft.Three-Phase – DC Power Converters Internal Engine Starter / Generators Cabin Pressure and Temperature Control BatteriesElectromechanical Actuators (EMA)5Electrohydrostatic Actuators (EHA)AvionicsDC – Three-Phase Power Converters DC – DC Power ConvertersDC Power Distribution BusFigure 1.1 Power distribution system of a transport aircraft.

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

500 kW Electric Load Management System200 kW500 kWLS/GGCUAPU S/GGCURS/GGCU3 Φ L-BDCL-EPCU3-Φ APU-BDCAPUEPCU3-Φ R-BDCR-EPCU270V L-DC busFlight controlUtilityECS270V APU-DC bus (essential bus)270V270V R-DC bus R-DC busUtility ECS Flight control3-Φ BDC 1-Φ BDCSmart Actuatorsnegative impedance avionics3-Φ BDCInduction motorDC-DC BDCElectronically commutated motor 270V battery3-Φ BDCnegative impedance avionics3-Φ BDC 1-Φ BDCSmart ActuatorsElectrohydrostatic actuator6Figure 1.2 Power distribution system architecture.PUAMOTORMOTORPUAElectromechanical actuatorHydraulic linkageECS pumpCoolant pumpECS pumpMechanical linkageSurface deflectionSurface deflectionCargo tempCargo pressureSurface deflectionSurface deflectionLS/G : Left-Starter Generator RS/G : Right-Starter Generator APU : Auxiliary Power UnitGCU : Generator Control Unit EPCU : Electronic Power Control Unit BDC : Bidirectional ConverterPUA : Power Unit Actuator ECS : Environmental Control SystemFigure 1.2 Power distribution system architecture.

A DC power distribution system (PDS) of a transport aircraft was modeled and analyzed using MATLAB/Simulink software. The multi-level modeling concept was used as a modeling approach, which assumes modeling subsystem of the PDS at three different levels of

DC bus voltage (V)330280 27025020000.010.020.04time (s)Figure 1.3 DC bus voltage specifications according to MIL-STD-704E.on the bus at the time of regeneration. Figure 1.3 shows the limits of the DC bus voltage transients set by MIL-STD-704E [8], which specifies power quality characteristics of the DC bus. These transients may affect normal operation of the equipment connected to the bus or even damage it. In order to store the regenerative energy, it is necessary to provide additional energy storage components in the PDS. Since the battery alone charges too slowly to accept transient power spikes generated in certain modes of operation, a significant number of capacitors in the PDS will be needed to fully utilize the regenerative power.Unlike the current PDS designs based on using components that have minimal interactions, the PDS under investigation consists of highly coupled subsystems closely interacting with each other. This approach brings potential benefits, which include an opportunity for optimization of the whole system that will allow resources of one subsystem such as regenerative energy to be shared with the others. The result would be reduction of the overall weight and cost of the aircraft and increase of the efficiency of its power distribution function.7