Frequency/Sequence Selective Filters for Power Quality Improvement in a Microgrid

A microgrid is capable of providing highly reliable power supply to sensitive loads like semiconductor fabrication or computer data processing equipment. This is especially required when the sensitive loads are located in distribution systems that also contain several single-phase and/or nonlinear loads. Voltage source inverter (VSI) based distributed energy resources can provide power quality conditioning support. They require filters to accurately detect unbalance and harmonics in the system. The state-of-the-art filtering techniques in three-phase power systems involve employing complex bandpass filters that pass frequencies within a band without much attenuation. Such methods however give good dynamic performance only when there is a wide separation between the desired and undesired frequencies. In power distribution systems containing imbalances and harmonic generating loads, the gap between the two frequencies may be very small, and therefore such methods don´t offer good performance. This paper proposes improved methods of filtering signals in three-phase, three-wire power systems, especially when there is a narrow gap between the desired and undesired frequencies. The proposed techniques contain both complex bandpass and bandstop sections and are designed on three-phase space-vector quantities. Simulation and experimental results are presented to validate these novel filtering concepts.