| Active filters are electronic devices that absorb the harmonic contents in power systems, which are produced by nonlinear loads or apparatus such as High-voltage-direct-current (HVDC) systems. This thesis investigates the basic control philosophy of active filter systems. According to their control strategies active filters can be of two types: the direct control and the indirect control. The direct method detects harmonics in the loads and injects current through an active filter to cancel the harmonics. On the other hand, the indirect method senses the harmonics in an AC network, and injects harmonic currents using feedback control to reduce the harmonics. Both analytical and simulation methods are used for the study. The ideal compensation is analyzed based on circuit theory. Both the direct and the indirect control approaches are investigated by numerical simulation. The effect of time delay on the performance of the direct method is studied in detail by both the theoretical analysis and the numerical simulation technique. By implementing a numerical simulation example, it has been demonstrated that the direct method is not robust enough when the time delay in the control circuits is considered, and the indirect method is more reliable. All the simulations in the thesis are performed by the power system simulation tool PSCAD/EMTDC. |
