The reliability and quality of power supply are often affected by the occurrence of faults in power systems. To minimise the impact of faults, power utilities need to locate and isolate the faults as quickly as possible so that repair works and restoration of the faulty network can be done quickly, thus minimising the number of customer-minutes lost.

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The aim of the work presented in this thesis is to improve the operational reliability of a power module and increase the efficiency of its associated cooling system by integrating the design of the cooler as part of the module. Power modules are increasingly used in a variety of applications ranging from aircraft and mass transport systems, to motor control and power conversion in the home. Reliability of the power module is very important in aerospace applications where the highest levels of safety and robustness are required while keeping the volume and mass of the module as low as possible. Certain parts of the power module such as the solder layer beneath the silicon device and the substrate are prone to failure with thermal cycling. The layer of thermal grease between the baseplate of the module and the heatsink significantly increases the thermal resistance between the electronic devices and the coolant fluid. The power module can be constructed so that some of the interfaces within the module which are prone to failure are improved or completely removed from the assembly greatly reducing the thermal resistance from junction to ambient. The research identified cooling methods which are able to cope with the increasingly high heat fluxes produced by power electronic devices. Jet impingement cooling was selected for testing and further development. An initial series of tests confirmed that liquid jet impingement can be used to generate high heat transfer coefficients for the efficient cooling of power modules. Results from experimental tests showed that directly cooling the substrate tile with jet impingement resulted in the devices being cooled more effectively compared to the commonly used serpentine coldplate and a direct-baseplate cooled jet impingement system. It was postulated that more efficient cooling can be achieved by targeting the hotspots on the substrate beneath each device with a carefully designed impingement array. A test apparatus was constructed to test a variety of jet impingement arrays to confirm the hypothesis. A second test apparatus was constructed to characterise the performance of the jet arrays in more detail using a thermal imaging camera to monitor the surface temperature of a single device. An optimal jet configuration was found for the efficient cooling of a single device. The work concluded that an improvement in efficiency and reliability can be gained by constructing power modules with integrated jet impingement arrays direct-substrate cooling the hotspots beneath the devices.

A huge body of work has been published in recent times in the area of multi-phase machines and drives. Many aspects of these drives have been analysed, such as reduction of torque pulsations, increased reliability and fault tolerance, improved power sharing capabilities and possibilities for realisation of series-connected multi-motor drives with supply coming from a single multi-phase voltage source inverter (VSI). Various pulse width modulation (PWM) schemes have been developed for multi-phase machines with concentrated and distributed windings, utilising both carrier-based PWM and space vector PWM (SVPWM) approaches. However, no systematic analysis has been performed in order to determine properties of multi-phase PWM in general, and to establish close correlation between carrier-based PWM and space vertor PWM, for multi-phase VSIs. This thesis presents an analysis and development of multi-phase PWM schemes for sinusoidal output voltage generation with two-level muhi-phase VSIs, which are suitable for multi-phase machines with distributed windings. Therefore, attention is paid to the elimination of low order harmonics. The scope of the thesis has been narrowed down to the continuous PWM schemes and operation in the linear region of the modulation only. Both multi-phase carrier-based PWM and SVPWM schemes are considered, and, in particular, five-phase, seven-phase e-phase systems are addressed in detail. Thus, a strong link between these two different approaches is established, allowing for an easier comparison of the features offered by each method. All PWM schemes are practically implemented in a DSP and experimentally verified through extensive experimentation on the custom-built multi-phase VSI. In addition to the methods of sinusoidal output voltage generation, achieved by means of the synthesis of the reference in only the first plane of the multi-phase system with simultaneous zeroing of voltages in all the other planes.