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Title: The use of multilevel power converter technology in aerospace starter-generator systems
Author: Williams, Richard
ISNI:       0000 0004 2736 5333
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2013
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This thesis investigates how a back-to-back connected, 5-level, diode-clamped multilevel inverter may be applied in an aerospace Starter / Generator (S/G) system. The performance of the proposed system is investigated with both wound field (WF) and permanent magnet (PM) synchronous machines. Control techniques are investigated to improve the waveform quality at the machine side. Also the grid interfacing of the converter is considered to make the system more resilient to AC distribution bus distortion. Aerospace generators are typically low inductance machines owing to the need to regulate the back-emf at the high rated speed, reduce stator losses and to increase power density. However, the driving of low inductance synchronous machines with a two level inverter results in a high level of current ripple at the same frequency as the converter switching frequency. A high level of current ripple increases losses in both the converter and machine necessitating additional heatsinking/cooling. The high ripple also results in increased radiated and conducted EMI. Additional filtering/shielding is therefore required to protect other vulnerable systems in the vicinity. The stability of the converter's controller may be adversely affected by a high level of current ripple. This instability can lead to the increased production of low order harmonic in the current waveform. The effect of interfacing a two-level inverter to an aircraft's AC distribution bus must also be considered. Power quality standards require the converter to draw a high quality sinusoidal current from the grid and introduce minimal distortion onto the voltage waveform. In order to reduce the current ripple in the machine windings and to ensure the distortion at the grid connection is within the governed limit, it is customary to use increased passive filtering and/or raise the converter switching frequency. Both of which incur a weight penalty. This study sought to investigate how a multilevel converter may be used as an alternative approach to this problem. The stepped PWM waveform produced by a multilevel converter offers superior harmonic performance and may therefore require reduced amounts of passive filtering whilst still producing a high quality waveform. The performance of the converter is initially demonstrated experimentally upon a PMSM. This is considered relevant as future 'embedded' S/Gs are likely to be PMSM due to the robust rotor structure and high power density. Performance is verified experimentally using a dynamometer test rig. The dynamometer is controlled to provide a rudimentary model of a gas turbine in both starting and generating mode. It is therefore possible to load the S/G in a similar manner to a real world application and provide realistic testing data. The converter is then applied to a WFSM. WFSMs are the industry norm for aerospace generators and it is this machine topology that is used as the S/G in Trent 1000 engine. Converter performance with a WFSM must therefore be assessed if this study is to be considered relevant. The excitation of the machine is investigated to ensure that the machine may be driven in motoring mode from zero speed and to maximise the available torque throughout the starting sequence. The excitation is then used to regulate the back-emf during generating mode. The same dynamometer test rig is used to provide full system test experimental data. Predictive control is then investigated to obtain a further reduction in waveform distortion in the stator current. Predictive control utilises a mathematical model of the load to predict the converter output voltage required in the next PWM period to minimise the current error. The estimated switching vector may then be applied in the next modulation period. This is shown to provide a substantial reduction in THD throughout both modes of operation. A reduction in THD in a real-world application would provide lower losses in both the machine and converter. The heatsinking/cooling arrangement may be reduced giving a possible weight reduction. In the final stage of this study, the control and synchronisation of the grid connected inverter is investigated. Aerospace power converters are required by the standards to function under distorted grid conditions. In this study a review is conducted into the competing grid synchronisation methods, to determine which is the most suitable to accommodate the heavily unbalanced grid waveforms commonly found on an aircraft distribution bus. A virtual-flux estimator is identified as the most promising technique as it eliminates the three voltage transducers at the grid connection. This not only makes the converter more resilient to grid distortion but may also increase the converter's reliability as three essential components have been removed. The system is assessed in both simulation and hardware. The level of grid distortion applied to the grid waveform is the maximum permissible according to the aerospace power quality standards. The system performance is satisfactory, maintaining synchronisation with the grid with minimal error despite the heavily unbalanced supply.
Supervisor: Stone, David ; Foster, Martin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available