Adding damping in power distribution systems by means of power electronic converters

The amount of non-linear and unbalanced loads (such as adjustable speed drives and personal computers) connected to the electric power system is steadily growing. As a result, the power quality i.e. the reliability of the energy supply and the quality of the voltage waveform, may be severely degraded. Especially in the case of resonances, severe voltage distortion can be the result. Both harmonic voltage and current distortion may cause many unfavourable effects on both the power system and the connected loads. Although complete compensation is only possible using true active filters, resistive shunt harmonic impedances (SHI) have been shown to provide a considerable reduction of the harmonic distortion. These impedances can be implemented using active power electronic devices which are controlled to synthesize a resistive load. Benefits are: no need to measure the polluting current of non-linear loads, effective for all harmonics and different resonance conditions, simple control strategy, and possible implementation as a secondary control function of active power supplies and distributed power inverters. In this paper, a practical implementation of a resistive SHI is proposed. A new control strategy for a full-bridge ac-dc converter has been implemented in order to achieve a programmable resistive input impedance for harmonics, independent of the input impedance for the fundamental component. The potential to damp a resonance in a power distribution system is demonstrated using an experimental setup. Providing extra damping into the power system is an interesting feature for power electronic loads as well as for converters used in distributed power generation