Power Quality and Power Sharing Controls of AC Micro Grid Systems

The connection of the nonlinear/unbalanced loads and single-phase Microgrids (MG) to three-phase MGs or power grids brings challenges to the MGs operation and causes power quality problems. Nonlinear loads introduce voltage and current harmonics and the connection of single-phase MGs and loads to three-phase MGs creates unbalanced voltage at the point of common coupling (PCC) of the MGs and poor power-sharing among MGs. Furthermore, the interaction of three-phase grid-connected inverter-based resources (IBR) to the weak grid introduces voltage and current harmonics at the PCC and leads to system instability.

This dissertation presents a hierarchical control scheme for voltage-controlled photovoltaic (PV) inverters with unbalanced and nonlinear loads in MG. With the proposed controller, the voltage distortion at the PCC decreased from6.38 percent to 1.91 percent after compensation, and the unbalanced and harmonic load are shared proportionally among the distributed generations (DG) units. The work was further expanded by proposing a multi-function control scheme and a modified droop control using energy storage system (ESS) to improve the three-phase PCC voltage quality in Multi-microgrids(MMG). Using this scheme, the voltage unbalance factor (VUF) at the PCC is decreased from 4.3 percent to 0.03 percent, while the three-phase PCC unbalanced power is reduced significantly and reactive power is shared proportionally between different rated MGs. Dissertation then tackles the unbalanced MGs problem in distorted weak grids by proposing a PCC voltage feed-forward control method using selective virtual impedance loops (SVIL) based on multiple dual second-order generalized integrators (MDSOGI).

In conclusion, this dissertation focus on improving power quality and sharing in islanded and grid-connected MMGs and investigating the system stability with the proposed methods under abnormal conditions. The effectiveness of the proposed methods are verified using Matlab/Simulink, Opal-RT real-time simulation and experimentally using control hardware-in-the-loop.