Overview and development of variable frequency AC generators for more electric aircraft generation system

In more electric aircraft (MEA) system, both ac and dc electric power with multiple voltage levels are required for various aircraft loads. This paper presents an induction generator-based ac/dc hybrid electric power generation system for MEA. In the proposed system architecure, a high-speed induction starter/generator and a low-speed induction generator are installed on the high pressure (HP) and low pressure (LP) spools of the engine, respectively. In generating mode of operation, all of the constant voltage variable frequency ac power is generated by the HP generator while the dc power demand is shared by both HP and LP generators. A control scheme is developed to regulate the ac load voltage and coordinate dc power generation between the two generators. The proposed induction generator based ac/dc hybrid generation system results in reduced hardware requirement compared with both ac and dc primary generation systems.

In this paper, a hybrid excitation synchronous machine (HESM) with magnetic shunting rotor is designed for electric vehicle traction applications with consideration of electromagnetic performance and mechanical strength. The HESM with magnetic shunting rotor is a topology of brushless HESM with extended magnetic bridge to embed field windings. The rotor core extends along the axial direction and the increase of rotor length is inevitable, which reduces torque density. In order to improve torque density, the excitation structure is optimized by built-in field windings and the HESM with one-end built-in field windings is presented. To improve the flux regulation capability and decrease the magnet consumption further, the HESM with dual-end built-in field windings is proposed and investigated. The comparative study of the performances of the original HESM and the HESMs with one/dual-end built-in field windings is carried out by three-dimensional finite element method (FEM). An original HESM prototype and a permanent magnet synchronous machine prototype have been developed and the experimental results, which agree well with the FE-predicted results, confirm the advantages of HESM and the validity of FEM used in predictions in this paper.

In this thesis a detailed study on the battery technologies used in aircrafts in the last five decades is done. A general background of the battery system is studied and the performance of the batteries based on energy densities and low temperature capabilities is evaluated for the deployment in the more electric aircraft (MEA). Evolution of MEA with its power system architecture and load profile is analysed to understand the requirements of the battery system. The electrical load profile of the MEA “Aircraft X” is designed and modeled. Different types of electrical loads such as primary/conventional loads, the environmental control system (ECS) load, anti-icing (wing ice protection) loads, various electrical actuators (aileron, spoilers, rudder) loads and signal communication (RADAR etc.) loads are designed and modeled. Furthermore, starting profile of engine starter generator (ESG), transient stability profile of military standard (MIL-STD-704F) and emergency load profile of the MEA “Aircraft X” are also studied and analysed. Thus, all these electrical loads are combined and an algorithm is successfully obtained to separate the constant and transient loads. An effective battery system is designed for supplying the transient loads by considering the weight saving and cost analysis factors between Li-ion and Ni-Cd batteries. Based on the analysis, Li-ion battery with Lithium ferrous phosphate (LFP) chemistry is designed and proposed for the MEA “Aircraft X”. The designed LFP based battery system is also modeled using the modified shepherd equation wherein the voltage polarization term is added to have a lower complexity and more proximity with the battery model block in MATLAB®. After the design and modeling of the proposed Li-ion battery system, the battery charger (phase shifted high power bidirectional dc-dc “PSHPBD” converter) is designed and modeled. The design of the components of PSHPBD converter is done keeping in view the requirement of maximum power transfer in the MEA environment. Zero voltage switching (ZVS) operation in wide range is also considered for design of the PSHPBD converter for MEA operating parameters. The PSHPBD converter is modeled using the Fourier series based harmonic analysis wherein an optimal harmonic number (h=7) is chosen by considering the proximity and complexity between the modeled converter and switch model. The battery energy storage system “BESS” (consisting of the battery system and the PSHPBD converter) is successfully integrated with the power distribution 270V dc bus architecture of the MEA and tested under different dynamic conditions using three different control techniques. Firstly, a direct estimation based technique is proposed for determining the phase shift of the PSHPBD converter (used as a charger for the batteries). Secondly, a current mode control for the single phase shift control technique for the PSHPBD converter is proposed. The proposed control technique has an inherent capability of current limit. Thirdly, a peak current controller is proposed which provides a limit on the peak current and provides a fast transient response. The advantages of using the proposed peak current control in avoiding the saturation of the transformer core is also reported in the work. In integration, finally the study and analysis of zero current switching (ZCS) inverter for integration of BESS to ESG set in MEA “Aircraft X” is discussed. Furthermore, a control method which can improve the life of the BESS in MEA is discussed in which three different modes of control based on the load estimation are given. Depending upon the load on the BESS, the corresponding control structure is implemented/activated leading to reduction in RMS current flow in the charger circuit and average current flow in the battery system thereby reducing the losses and temperature rise in the BESS, which eventually leads to an increase in the life of the BESS. Finally, a multi-functional battery charger (MFBC) is proposed for integration of BESS to the power distribution network in MEA. A novel control method is devised for suitable transfer and sharing of power between different buses. Based on the placement and applicability of the MFBC, many new dc power distribution architectures for MEA are proposed wherein energy storage system can be integrated into aircraft network leading towards fault tolerant operation and weight optimization in the MEA.

Nowadays, variable frequency AC generation system is widely used on more electric aircraft because of their advantages of simple structure, high power density, and high reliability. However, a variable frequency system has a wide range of operating speeds and frequencies, and its exciter field is designed to be highly saturated at low speed. Under the condition of a saturated exciter field at high speed, the generator can produce a very high voltage, which will cause damage to aircraft utilization equipment, leading to catastrophic failures. Therefore, to protect the safety of the aircraft, transient voltage regulation performance and voltage protection design are critical in the system design of variable frequency AC generation. This paper focuses on the design of voltage regulation and protection of variable frequency electrical power systems that can meet airworthiness requirements.

Renewable electricity generation systems for electric-powered taxis: The case of Daejeon metropolitan city

The aerospace trend is towards the More Electric Aircraft (MEA), which consists in the progressive electrification of the main on-board systems and services. The resulting increase of the aircraft electrical loading involves significant implications for on-board electrical generation systems, realizing the so-called More Electric Engine (MEE). In the MEE concept the electrical generators are integrated inside the main gas turbine engine to generate electrical power. In the paper the magnet losses of a surface mounted Permanent Magnet (PM) generator with non-overlapping fractional-slot concentrated windings, positioned in the front stage of the gas turbine engine, are evaluated. The benefits of the magnet segmentation aimed to the machine rotor losses reduction are investigated by means of a Finite Element Method analyses. The permanent magnet segmentation effects on machine performance and cogging torque are put in evidence too.

By development of more electric aircraft (MEA), it is essential to have an overview of recently presented methods to protect MEA against fault current. This paper gives an overview of possible faults that may be occurs in the electric power system of more electric aircraft. Various functionalities and fault limiting solutions are also covered. Besides, the paper focuses on the main challenges for improving the reliability of the distribution system in MEA based on protection functions such as fuses, circuit breakers, and fault current limiters. Finally, the suggestions are surveyed to enhance electric power system performance of MEA.

A two-stage multi-objective optimization method for envelope and energy generation systems of primary and secondary school teaching buildings in China

Stability of multiple interconnected generation systems are prone to be adversely affected due to the inter-area oscillations among them. This issue becomes more significant when the interconnection strength among the generation systems is stronger. In previous control strategies, the coupling effects have been considered as undesired disturbances that leads to failure in tackling this issue when the generation systems become strongly coupled. Thus, in this paper a new control scheme is presented in this paper to address this strong coupling effect by designing robust decentralized controllers in a sequential way to tackle the load frequency control problem in a three-area generation system connected through tie-lines. Three robust decentralized controllers are designed for the three areas by using the μ-synthesis technique. The model of tie-line coupling among the generation areas is taken into account in the design of sequential controllers. Moreover, the decentralized controllers are designed in such a way that the entire control system is robust against all the parameter variations and uncertainties. Based on the simulation results, the proposed decentralized controllers outperform the conventional independently designed controllers in terms of variations in the coupling strength in the tie-line.

In order to develop a sustainable and reliable hybrid energy generation system model in Southern Libya using concentrated solar power (CSP) with thermal energy storage (TES) to improve the overall efficiency, flexibility and sustainability of the energy generation facilities located in this area, this study has developed an energy generation system that integrates CSP using molten salts as TES with a gas turbine based on the equilibrium conditions defined by the use of sodium nitrate-potassium nitrate (NaNO3-KNO3); essentially, a two-tank molten salt system that can store heat at temperatures up to 565 degrees Celsius. The heat is then captured and produces electricity during non-solar times thereby allowing power generation to continue on demand and ensure continuity of electrical service. The researchers will utilize the System Advisor Model (SAM) to simulate performance of the solar field and the TES, while the Engineering Equation Solver (EES) will be used to conduct a thermal analysis of the thermodynamic processes associated with the Brayton gas turbine cycle. The researchers found that the addition of molten salt TES to a CSP energy system increased thermal efficiency by approximately 22 percent compared to a standard fossil fuel generation system, reduced fuel consumption by 26 percent and reduced carbon dioxide emissions by 18 percent versus a similar thermal generation system. Additionally, when the levelized cost of electricity (LCOE) from the integrated CSP, molten salt TES and gas turbine system was compared with that of traditional fossil fuel generation systems the total LCOE decreased from $0.22/kWh to $0.16/kWh indicating that this hybrid energy generation system provides an economically viable alternative for use in regions that are primarily arid and/or remote such as Ubari in Southern Libya, while addressing both fuel supply and grid stability challenges Keywords: CSP, TES, molten salt storage, Brayton cycle, hybrid energy system, LCOE, CO2, solar thermal.

The growing penetration of generation systems based on renewable energy in electric power systems is undeniable. These generation systems have many benefits, but also many challenges from the technical point of view. One of the biggest problems in the case of solar photovoltaic (PV) and wind energy is the intermittency of the raw material, thus hybrid generation systems that contain both sources are being used to complement electric power generation. To analyze the problems of this type of hybrid generation systems, it is necessary to develop models and test systems that allows to study their dynamic behavior. Reported in this paper is the implementation of a full hybrid PV–wind generation system model in a real-time digital simulation platform, and the development of the electronic converter controls. These controllers were implemented in digital devices (Arduino Due) and connected to the simulation platform to test their performance in real-time. In addition, the procedure followed for the development and implementation of the controllers is presented. The proposed test system can be used in renewable energy integration studies and the development of new control strategies.

In this paper, a novel modeling method for hybrid excitation synchronous machines (HESMs) with radial/axial flux paths via magnetic equivalent circuit (MEC) is proposed. 3-D finite-element analysis (FEA) is essential when analyzing a machine with radial/axial flux paths due to the asymmetry in the axial direction. However, 3-D FEA is computationally expensive and time-consuming, especially during the preliminary design stage. The proposed modeling method is analyzed and validated based on an HESM with magnetic shunting rotor. The fundamental structure and working principle of the HESM with magnetic shunting rotor are introduced. On the basis of 3-D structure, an MEC considering saturation and leakage flux distribution is built and illustrated, as well as field analysis is performed. A 2-D equivalent finite-element model via MEC is built and illustrated. A prototype HESM with magnetic shunting rotor has been designed by the proposed modeling method and developed for experimental verification. With the proposed modeling method, the computational efficiency is significantly improved in the design stage. The proposed modeling method is verified by the good agreement between the predicted and measured results.

Power quality is one of the challenging issues toward future more electric aircraft (MEA). In this paper, the authors presented a solution based on the application of a star-connected autotransformer based 24-pulse rectifier. Analysis and development of the proposed 24-pulse rectifier and the simplified MEA power system structure have been done in MAT-LAB/SIMULINK prior to the experimental implementation. Both simulation and experimental results are presented to validate the developed design procedure and the simulation models. The proposed autotransformer-based rectifier has resulted in elimination of less than 23rd harmonic in the supply current. As a result, the supply current is near sinusoidal in shape with low total harmonic distortion (THD). Moreover, the power factor is also improved to nearly unity in the wide operating range of permanent magnet motor loads in MEA. Thus the proposed 24-pulse rectifier is ideally suited for retrofitting with existing motor drives in MEA.

The More Electric Aircraft (MEA) is a system architecture concept for the aircraft that reduces fuel consumption and environmental load while improving safety, reliability, and maintainability. MEA architecture replaces some of the conventional hydraulic and/or mechanical control system with electric motor-driven system, integrates system power management into the aircraft/engine controls, and optimizes the aircraft geometry by flexibly arranging the accessory devices. The primary challenge to realize the MEA concept is how to manage the heat from these additional power electronic devices. The authors’ group proposed novel cooling system, the Autonomous Air-Cooling System (AACS) which cools the power electronics of the motor devices distributed in the aircraft. In AACS, each power electronic device (e.g. motor controller) is air-cooled by heat sinks connected to compact blowers. This system is very simple and efficient since it re-uses the cabin air and needs no additional coolant. One of the key technologies which realize AACS is an efficient heat sink. In this study, at first the performance evaluation targeting a single-aisle 180-seater aircraft was performed. In the analysis, a plate-fin heat sink was adopted, and the pressure loss and heat transfer was estimated by using empirical correlations. In the analysis, the value of heat generation was assumed from power demand for each operation condition, and the required mass flow rate of cooling air was calculated so as for the enclosure temperature of the power electronics to be 80°C which was the allowable maximum temperature of the motor controller. The effect of the fin geometry on the cooling performance was also examined by varying the geometric parameters (fin height, thickness, and spacing). In order to further enhance the cooling performance without increasing the pressure loss, the water-mist injection to the cooling air flow was adopted and its effect was analytically confirmed. In addition, the effectiveness of the water-mist injection on the cooling performance was verified by performing experiments for a plate-fin heat sink manufactured by a wire electric discharge method.

<div class="section abstract"><div class="htmlview paragraph">Aiming at the global energy optimization of aircraft, the More Electric Aircraft (MEA) concept becomes more interesting for the aeronautical industry. The MEA concept is based on utilizing electric power to drive aircraft subsystems that historically have been driven by a combination of hydraulic, electric, pneumatic and mechanical power transfer systems.</div><div class="htmlview paragraph">The development of the future MEA systems is a challenging task: the system integration is becoming a central topic. In all phases of the system development process, the respective subsystems within the MEA will be treated in a highly integrated manner to achieve optimum efficiency and performance at aircraft and systems level. Concerning the electric network in the future MEA, advanced design and analysis methods based on mathematical models are required to face the potential issues accompanying the MEA. For this purpose, the use of advanced modelling and simulation technologies is a key success factor. This paper gives a comprehensive overview how to effectively deal with some issues raised when designing the electric network of the MEA by using model based approaches. The topics addressed here include network stability, failure analysis and power quality studies.</div></div>