Harmonic Mitigation in Islanded Microgrids by Inverter-Interfaced Distributed Energy Resources

An exciting growth of microgrids market has been witnessing around the world in recent years. The ability of dynamic islanding furnishes microgrids a prominent feature, which not only contributes to a more reliable electricity supply but also provides a systematic way to maximize the benefits of Distributed Energy Resource (DER) for both customers and utilities. However, during islanded operations, microgrids are more sensitive to power quality disturbances, owing to the low short-circuit ratio and limited capacity of DER units. As a consequence, harmonic distortion caused by the proliferation of nonlinear electronic equipment becomes more apparent in islanded microgrids. The recently released IEEE 1547 series standards have outlined the acceptable harmonic voltage and current distortion limits for islanded microgrids.
As the growing penetration of renewable energy sources based DER, their flexible power electronics interfaces offer a number of ancillary services for power quality improvement. There is, consequently, an urgent need to exploit the potential of DER inverters for harmonic mitigation in islanded microgrids. Hence, this thesis concentrates on developing advanced control algorithms for DER inverters in order to provide active harmonic conditioning services, including harmonic voltage reduction, harmonic current filtering and resonance damping in islanded microgrids.
The research work is composed of two main parts. The first part is concerned with building a stable islanded microgrid platform, where the reduction of circulating current in the paralleled three-phase DER inverters is investigated. It is found that the presence of the Negative-Sequence Circulating Current (NSCC) deteriorates the output current quality of inverters and results in the unbalanced voltage distortion at certain buses of an islanded microgrid. In order to effectively reduce the NSCC, an adaptive negative-sequence virtual resistance scheme is proposed and validated via a series of laboratory tests on the built islanded microgrid. The second part addresses the harmonic distortion resulting from nonlinear loads and shunt-connected capacitors via a variety of control algorithms of DER inverters. Under the built islanded microgrid, the autonomous control of DER inverters for harmonic current filtering and resonance damping at the point of connection of DER units, as well as the secondary harmonic voltage compensation strategies at the system-level are developed. Simulation case studies and laboratory test results are presented to confirm the performances of the proposed approaches.