Rebecca Todd

Improved Energy Conversion for Doubly Fed Wind Generators

There is currently significant interest in offshore wind turbines up to 5 MW. One of the preferred options is doubly-fed slip-ring generators. Turbine manufacturers have proposed an operating scheme for low speeds that is claimed to improve the overall energy extraction from the wind. The aim of this paper therefore is to examine the performance benefits for this new operational mode on a small experimental rig, confirmed by simulation, and to provide a fundamental understanding of the differences in the generator operation in both modes. The paper develops analytical steady-state models to provide this insight and correlates the operating performance with a dynamic real-time generator control scheme and experimental results obtained from a laboratory test machine. A theoretical study on a 2 MW commercial turbine is also undertaken.

Behavioral Modeling of a Switched Reluctance Generator for Aircraft Power Systems

A system-level modeling technique for a switched reluctance generator (SRG) is described for aerospace applications. Unlike existing techniques, this model is very simple and only reproduces the average behavior of the input-output variables that are required for system-level analysis of the aircraft power distribution system. The model is parameterized from the measured generator response, avoiding the need for a detailed knowledge of the equipment structure, which may be unavailable. The modeling procedure is described in detail and validated by measurements on an SRG within an aircraft test facility.

Improved energy conversion for doubly-fed wind generators

There is currently significant interest in offshore wind turbines up to 5 MW. One of the preferred options is doubly fed slip-ring generators. Turbine manufacturers have proposed an operating scheme for low speeds that is claimed to improve the overall energy extraction from the wind. This paper aims to examine the performance benefits for this new operational mode on a small experimental rig, confirmed by simulation, and to provide a fundamental understanding of the differences in the generator operation in both modes. This paper develops analytical steady-state models to provide this insight and correlates the operating performance with a dynamic real-time generator control scheme and experimental results obtained from a laboratory test machine. A theoretical study on a 2-MW commercial turbine is also undertaken

HIL emulation of all-electric UAV power systems

Hardware-in-the-loop (HIL) techniques are used to examine the start-up sequence for an all-electric UAV system powered by a two-spool gas engine with embedded electrical generators on both spools. Start-up begins with the auxiliary power unit charging the high voltage DC bus to enable the high pressure spool switched reluctance starter/generator (SRSG) to spin the gas engine to ground idle speed and ends with the SRSG and the low pressure spool fault-tolerant permanent magnet generator being connected in parallel. HIL methods are used to emulate the behaviour of the gas engine and the loads. The HIL elements compliment the ‘real’ electrical generators and bus system. The start-up sequence is demonstrated on a purpose-built laboratory system to examine system interactions and the feasibility of the operating routine.

Supercapacitor-based energy management for future aircraft systems

A power-based control method is proposed and analysed for a supercapacitor energy storage device. The performance of the energy storage device is examined by simulation and experimentally when operating on a high voltage DC bus with a multi-phase, fault-tolerant generator and a high power, pulsed load such as an actuation or avionic system. In the practical system the energy storage device is emulated using a bi-directional electronic load and a real-time simulation platform. The energy storage device is shown to minimise the DC bus transients and virtually eliminate the torque pulsation on the generator shaft. The system design and performance trade-offs are analysed. The experimental work uses a 70kW generator and 30kW programmable load emulation devices.

Adaptive Rate-Limit Control for Energy Storage Systems

An adaptive energy management control with an integrated variable rate-limit function is described for an energy storage system (ESS). The proposed control protects the primary power source(s) in the system as effectively as possible from sudden load transients within the constraints of the available stored energy. The control can be designed to use the available energy more aggressively during load changes in the low- or high-power regions while offering the lowest possible rate-of-change of the main source power or offers a fixed minimum rate-of-change in power for a given total load and amount of energy. The control design is described in detail and demonstrated experimentally when applied to a supercapacitor ESS within an aircraft test facility.

Single-phase controller design for a fault-tolerant permanent magnet generator

A simple, per-phase controller is described for a five-phase, fault-tolerant, permanent magnet generator. The power conditioning and control system for the generator comprises five independent H-bridge converters operating with sinusoidal PWM and supplying a common high-voltage DC bus. The separate controllers adjust the PWM pattern in each converter to regulate the DC bus voltage over the 3:1 machine speed range, which requires field weakening operation at high speed. Small-signal analysis is used to form a theoretical basis for the control loop design. Practical results are included from a 70 kW, five-phase prototype system to validate the analysis and illustrate the controller performance.

Analysis of SiC MOSFETs under hard and soft-switching

Analytical models for hard-switching and soft-switching SiC MOSFETs and their experimental validation are described in this paper. The models include the high frequency parasitic components in the circuit and enable very fast, accurate simulation of the switching behaviour of SiC MOSFET using only datasheet parameters. The much higher switching speed of SiC devices over Si counterparts necessitates a clear detailed analysis. Each switching transient was divided into four distinct sub-periods and their respective equivalent circuits were solved to approximate the circuit state variables. Nonlinearities in the junction capacitances of SiC devices were considered in the model. Analytical modelling results were close to the LTspice simulation results with a threefold reduction in the simulation time. The effect of snubber capacitors on the soft-switching waveforms is also explained analytically and validated experimentally, which enables the analytical model to be used to evaluate future soft-switching solutions. It was found that the snubber branch can significantly reduce the turn off ringing of the SiC MOSFET in addition to the reduction of switching losses.

Control and development of an electrical systems evaluation platform for uninhabited autonomous vehicles

This paper describes a new control strategy for an aircraft electrical systems evaluation platform. The 100 kVA, experimental facility includes multiple generators and power electronic loads interconnected by a high-voltage dc bus, centrally controlled by a real-time computer and data logging system. The Facility is to allow electrical power management techniques to be developed for future autonomous aircraft. The control of the facility presents important challenges as it has to be flexible, facilitating future expansion, robust, safe, and yet allow the development of advanced control strategies.

Equivalence of generalized state-space averaging with DQ modelling methods in a balanced three-phase system

In most modern power electronics systems constant power loads (CPL) exist, which could have a severe influence on system stability. Conventionally two modelling tools for a system qualitative study are considered - generalized state-space averaging (GSSA) and DQ modelling methods. In this paper their strict mathematical equivalence in the balanced three-phase system is shown and areas of GSSA application are discussed. Experimental and simulation results comparing the switched model and the experiment results together with the comparison of the switched model and the GSSA and DQ models are also presented.