Stability Analysis of Fully Power Converter-based Microgrids
Sähkötekniikan korkeakoulu | Master's thesis
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Electrical Power and Energy Engineering
AEE - Master’s Programme in Automation and Electrical Engineering (TS2013)
AbstractDue to rising energy demand and climate crisis, distributed energy generation utilizing the renewable energy resource is constantly evolving. Generation near the electrical loads within a deﬁned boundary forms a microgrid. It can be operated by connecting with utility-grid or as a stand-alone system. Solar and wind energy resources use a power-electronic converter to interface with the load or grid. The fast dynamics of the converter is very diﬀerent from the inertial dynamics of the grid with large synchronous machines. Furthermore, low short-circuit capacity, more resistive network and unbalanced loading are few inherent characteristics concerning the operational reliability of the microgrid. This thesis aims to present the various issues with fully power converter-based microgrids in terms of stability and protection. High-power converters with LCL-ﬁlter are simulated under various operating conditions in typhoon real-time simulator. A relay function is used to detect the unstable operating points. High resistance-to-reactance ratio in the low-voltage line forms active-reactive power coupling, making the conventional droop control inaccurate. For a disturbance, droop control allows a steep voltage or frequency deviations which lead to unnecessary protection tripping. Use of virtual inertia control avoids the steep change in the system variables and preserves the stability. Parallel droop-based converters with non-identical parameters or output impedance induce circulating current or reactive power oscillations. Use of virtual impedance control minimizes the circulating current and enhances power-sharing. Phase-locked loop synchronized with a weak grid (high-impedance grid) is unstable on large-signal disturbances. Current-reference saturation limits the converter current for a three-phase balanced fault condition. For higher fault-impedance, the fault current is nearly equal to the load current, which possibly blinds the microgrid protection.
Thesis advisorQvintus, Mikko
power converter-based microgrid, grid-forming, weak grid, droop control, virtual inertia emulation