An investigation of harmonic correction techniques using active filtering
This thesis presents an investigation of techniques used to mitigate the undesirable effects of harmonics in power systems. The first part of this research develops an effective and useful comparison of alternative AC-DC converter topologies. In particular, a full evaluation of the circuit first proposed by Enjeti (known here as the Texas circuit) with a capacitively smoothed output voltage is made, specifically for operation as a 'clean power' supply interface for a variable speed drive (VSD). This mode of operation has not previously been reported in research literature. Simulation and experimental results verify the performance of the circuit and demonstrate that it draws a current with low harmonic content, but the circuit has a number of problems. This part of the research concludes that the six-switch rectifier is the most viable circuit for operation as a supply interface for a VSD due to its bidirectional power flow capability and its excellent versatility of performance. The second part of this research exploits the versatility of the six-switch rectifier and develops the current control strategy for operation of the circuit as a sinusoidal frontend and as a shunt active filter. It is found that the 'traditional' current control method suffers a significant drop in performance when the switching frequency is constrained to 2kHz due to high power levels. The major development in this thesis was an advanced current control strategy, where additional rotating frames of reference are introduced, thereby converting previously oscillatory current values to d.c. values. This is demonstrated to result in vastly improved immunity to disturbances such as supply distortion and a greatly improved steady state performance. In addition, the new controller requires no additional circuitry (apart from current transducers on the load current) and can be applied to an existing sinusoidal front end. Simulation results confirm the operation of the controller with the circuit operating as both a shunt active filter and as a sinusoidal front end. The new controller has been implemented on an experimental rig exhibiting the features of a high power inverter, i.e. low switching frequency and significant device turn-on and turn-off times, and the results confirm the superior performance of the new current controller.