Control strategies for the More Electric Aircraft starter-generator electrical power system

The trend towards development of More Electric Aircraft (MEA) has been driven by increased fuel fossil prices and stricter environmental policies. This is supported by breakthroughs in power electronic systems and electrical machines. The application of MEA is expected to reduce the aircraft mass and drag, thereby increasing fuel efficiency and reduced environmental impact. The starter-generator (S/G) scheme is one of the solutions from the MEA concept that brings the most significant improvement to the electrical power generation system. A S/G system is proposed from the possible solutions brought by the MEA concept in the area of electrical power generation and distribution. Due to the wide operating speed range, limited controller stability may be present. This thesis contributes to the control plant analysis and controller design of this MEA S/G system. The general control requirements are outlined based on the S/G system operation and the control structure is presented. The control plants are derived specifically to design the controllers for the S/G control scheme. Detailed small signal analysis is performed on the derived plant while taking into consideration the aircraft operating speed and load range. A safe range for the controller gains can then be determined to ensure stable operation throughout the S/G operation. Adaptive gain and a novel current limit modifier are proposed which improves the controller stability during S/G operation. Model predictive control is considered as an alternative control strategy for potential control performance improvements with the S/G system. The technical results and simulations are supported by Matlab®/Simulink® based models and validated by experimental work on a small scaled drive system

Modulation Limit Based Control Strategy for More Electric Aircraft Generator System

Vector based control strategies have been extensively employed for drive systems, and in recent times to the More Electric Aircraft (MEA) generator based systems. The control schemes should maintain the bus voltage and adhere to the generator system voltage and current limits throughout a wide speed range. Typically, the current limit is prioritised first due to ease of implementation and simple control structure. As a result, the voltage limit can be exceeded due to change in operating conditions or disturbance factors. In flux weakening regions, this may affect the controllability of the power converter and lead to generator system instability. In this paper, an alternative control strategy has been investigated to address this drawback. The proposed control scheme refers to the modulation index limit which is the ratio between the power converter input and output voltages as the voltage limit. The control scheme uses a dynamic limit for the generator reference voltages such that the modulation index limit is adhered. Furthermore, a controller is introduced to address the lack of current limit of the proposed control scheme. The linear open loop plant is derived for the bus voltage and current limit controllers and verified against their equivalent non-linear counterparts. They are used to evaluate and design the controllers for stable operation. The performance of the proposed control scheme is then compared with a state of the art existing control method. Simulation results showed superior modulation index limit throughout and short duration stator current overshoots when operating at current limit. Overall, the proposed control strategy showed to be a suitable alternative control scheme for the MEA generator system

Flux weakening control of electric starter-generator based on permanent-magnet machine

The paper presents control analysis and design for a Permanent Magnet Machine (PMM) operated in Flux-Weakening (FW) mode for an aircraft electric starter-generator application. Previous literature has focused on FW control of PMMs in motoring (starting) mode, however the system stability and control in generating mode has been inadequately studied. The paper reports detailed, rigorous control analysis and design for a PMM based aircraft electric starter-generator operated in flux-weakening mode. It is shown that an unstable area of operation exists. A novel control scheme which eliminates this instability is proposed. The key analytical findings of the paper are verified by experimental investigation. The paper therefore concludes that the presented technique is able to ensure system stability under all modes of operation. Furthermore, it is noted that the findings of this work are also valuable for any two-quadrant PMM drive with frequent change between starting and generating regimes under current-limiting operation

Control design for PMM-based generator fed by active front-end rectifier in more-electric aircraft

The future aircraft electrical power system is expected to be more efficient, safer, simpler in servicing and easier in maintenance. As a result, many existing hydraulic and pneumatic power driven systems are being replaced by their electrical counterparts. This trend is known as a move towards the More-Electric Aircraft (MEA). As a result, a large number of new electrical loads have been introduced in order to power many primary functions including actuation, de-icing, cabin airconditioning, and engine start. Therefore electric power generation systems have a key role in supporting this technological trend. Advances in modern power electronics allow the concept of starter/generator (S/G) which enables electrical engine start and power generation using the same electrical machine. This results in substantial improvements in power density and reduced overall weight. One of the potential S/G solutions is to employ a permanent magnet machine (PMM) controlled by active front-end rectifier (AFE). Operation of the PMM as a generator at wide range of speed that is dictated by the engine and electrical loads connected to the aircraft bus require careful design of the controllers. Corresponding plant models are derived and verified with simulations using developed models in Matlab/Simulink. The relevant controllers are designed based on the derived plants and operating points. The controllers are tested with Simulink models and experimentally using a scaled prototype of the investigated generator system

Stability assessment of a high speed permanent magnet machine based aircraft electrical power system

Starting an aircraft engine with an electrical machine has been one of the major trends for future aircraft. This paper studies the stability of a permanent-magnet machine (PMM) based aircraft starter/generator (S/G) system. Using control-to-output transfer functions, the stability analysis of this S/G system is thoroughly studied. The impact of the key parameters including the control parameters is analysed. Simulation and experimental results support the analytical result

Flux weakening control of Permanent Magnet Machine based aircraft electric starter-generator

This paper presents control analysis and design for an aircraft electric starter-generator system based on a Permanent Magnet Machine (PMM) operated in Flux-Weakening mode (FW). The focus is on detailed stability analysis which helped to discover an intrinsic instability when operating the PMM in FW mode. An adaptive voltage magnitude controller, including a variable current limit, is proposed and shown to guarantee stable operation. The analytical findings are verified by experimental investigation

Efficiency Focused Energy Management Strategy Based on Optimal Droop Gain Design for More Electric Aircraft

Due to the substantial increase of the number of electrically-driven systems on-board More Electric Aircraft (MEA), the on-board Electric Power Systems (EPS) are becoming more and more complex. Therefore, there is a need to develop a control strategy to manage the overall EPS energy flow and ensure the operation of safety-critical systems (which are electrical loads) under different operating scenarios, and to consider EPS losses minimization, exploiting the thermal capability of generators, different load priorities, as well as available batteries with their charging and discharging schedules. This paper presents an Energy Management (EM) strategy that considers the aforementioned objectives. The optimal droop gain approach is employed as a power-sharing method to minimize the total EPS losses in MEA. A Finite State Machine (FSM) has been used to implement the control strategy to realize the EPS reconfiguration operation. The proposed EM strategy is implemented and simulated using Matlab/Simulink and Hardware In the Loop (HIL) under the different operational scenarios such as normal operations, failure of one of the power generation channels, and failure of all power generation channels. The proposed EM method has shown its capability to efficiently manage the EPS under different operating conditions to reduce the overall system losses