Transformerless Active Harmonic Filtering using Series-Connected Supply Filter Capacitors

In recent years active power filters and harmonic compensators have been widely studied. When compensation is carried out, the switching components are exposed to high voltage stresses. High voltage stresses increase power losses and impose greater demands for switching components, making compensation equipment more expensive. The power losses reduce the harmonic filtering efficiency and increase filtering costs. Practically every switching device has its maximum voltage durability, thus switching bridges cannot normally be used in high voltage systems without transformers. The price, the size, and the power losses have often been drawbacks with the active harmonic filters. Voltage stresses and power losses of the active filter can be decreased using series-connected capacitors between the supply and the converter bridge. These structures are usually called transformerless active harmonic filters. The purpose of the thesis is to study and compare methods to improve the efficiency of the active harmonic filtering and to reduce the power rating of the active filter components using series-connected capacitors between the supply and the active filter. In this thesis, power losses and filtering performances of five different kinds of transformerless active filter topologies are studied. Two of these are current source topologies and three are voltage source topologies. First, transformerless active filters are generally familiarized. After that, PWM bridges, active filter structures, active filter control and current reference generation methods used in this thesis are presented. The operation of the active filter structures are first tested using computer simulations before the prototype implementation. Simulation environment are studied and analyzed in the thesis. After the simulation, prototypes are presented. Power losses, efficiencies, filtering performances and power factors are measured and compared. The filtering performances of the systems with two different kinds of 20 kVA nonlinear load are examined and finally the power losses and efficiencies of the active filters are studied. The comparison is based on experimental tests with the prototypes constructed. The results clearly show the properties of the active filters examined. In addition, the results prove that active filters with series-connected capacitors offer a considerable choice for harmonic filtering when high efficiency is desired and capacitive reactive power is needed