This thesis presents two new topologies of Hybrid Active Power Filters (HAPF). Advances in power semiconductor devices on one hand and appearance of fast and sophisticated microprocessors on the other hand have caused a fast growth in power electronic loads. These loads typically have non-sinusoidal waveforms at their input terminals which result in the injection of harmonics into the grid. Harmonics have adverse effect on the network components like energy loss, interference and nuisance operation of protective devices. Power filters are one of the most common methods employed to mitigate harmonics at their source. Among different type of power filters, HAPFs are composed of both passive and active elements and are an attractive solution to mitigate harmonic pollution. They are less expensive than Active Power Filters (APF), and at the same time do not have the drawbacks associated with Passive Power Filters (PPF). Although converter rating in HAPF is smaller than that of AF, the research is still underway to further reduce the rating of active part in a HAPF. In this thesis, a survey is carried out on different HAPF topologies. Simulation and theoretical analysis are presented with the focus on converter rating in different topologies. Among various topologies proposed for HAPF, the one with series combination of active and passive elements in parallel with the load is more common (HAPF type-2). This topology is used to study the effect of passive component on HAPF performance. It is shown that large values of converter’s gain improve the compensation performance almost independent of the passive filter performance alone. In most studies carried out for the reduction of fundamental component in the active part of an HAPF, some fundamental current still flows in the active part without any contribution in harmonic compensation. This fundamental current in turn increases the current rating of the active part. In this thesis, two new topologies are proposed in which the fundamental current through the active part is eliminated. In first topology this objective is achieved by introducing another branch which generates a fundamental current equal to the main branch. By using the MMF balancing feature of a three-winding transformer, the equal fundamental current in two branches cancel out each other, leading to zero fundamental current in the tertiary winding where the active part (i.e. converter) is connected. In the second proposed topology eliminating the fundamental current is achieved by using a parallel resonance circuit tuned at the fundamental frequency. It is shown that the passive elements of fundamental resonance circuit are fairly smaller than that of similar configuration known as injection type HAPF. Theoretical analysis is presented which explains the operation of proposed topologies and simulations are presented to validate theoretical results. It is proved that both of the proposed topologies are able to eliminate the fundamental current of converter. Different comparisons are presented to investigate the advantages and disadvantages of various topologies. The comparison is made based on harmonic compensation performance, ratings of the converter and the size of the passive elements. Keywords: Hybrid Active power filter, Active Part Rating, Passive Part, Fundamental Current