Large-signal Stability Analysis of Droop-controlled Converters for Microgrid Applications

Khalid Hurayb

doi:10.17918/00001688

A microgrid (MG) is described as a set of interconnected loads and distributed energy resources (DERs) that behaves as a controllable entity with respect to the grid. The MG has the ability to operate as connected and disconnected from the grid (i.e., in grid-connected or island mode). The main advantage of the MG is improving the grid's reliability and resilience when disturbances occur. Furthermore, adoption of MGs can be motivated by financial incentives to save infrastructure costs in dense urban settings with fast-growing electricity demand, remote isolated regions, and rough terrain. The technical feasibility of MGs relies on DERs through DC/AC power electronics converter interfaces, such as the conventional droop-controlled inverter. However, these technologies lack inertia, which makes them prone to oscillatory behavior under perturbations. The thesis will address gaps in the literature by investigating the large-signal stability of the conventional droop-controlled inverter operating in grid-connected and island mode by proposing nonlinear dynamic models that include saturable power and voltage controllers, phase-locked loop controller, and power-balancing control strategies. Previous studies ignored the impact of local loads and synchronization dynamics on the large-signal stability of the system. This thesis addressed that by developing a 16th-order nonlinear dynamic model to analyze the large signal stability of the droop-controlled inverter operating in grid-connected mode with local RL loads and phase-locked loop (PLL) controller. This work is important because it allows researchers to build on the model to investigate seamless transition between the operating modes (i.e., grid-connected and islanded modes). In addition, previous studies assumed inverters and the synchronous generators to have similar response behavior under disturbances, which was inaccurate. This thesis is one of the first to introduce saturable power and voltage controllers and study their effects on the stability of grid-connected droop-controlled inverter by implementing the 18th-order nonlinear dynamic models. This is important because it allows the inverter model to capture different physical and software limits that are different from the synchronous generators, which can assist researchers designing more effective protection systems and strategies for MGs. Furthermore, previous studies were unable to utilize the changes in the DC-link dynamics to enhance the conventional droop-controlled inverter performance by balancing the DC- and AC-side active power. In addition, these studies assumed the inverter was connected to an ideal constant DC source without the ability to control its output, which was not an accurate representation of how the DERs operate. The thesis addressed that by analyzing the large-signal stability of the two proposed control strategies that represented by the 17th- and 22nd-order nonlinear dynamic models. The control strategies models utilized the DC-link parameters as input to achieve power-balacing between the DC- and AC-side of the droop-controlled inverter operating in islanding mode. Most importantly, the developed control strategies could be utilized to mitigate DC-link undesired oscillations, which can lead to manufacturing and maintenance cost savings and prolong the inverter life expectancy. Previous studies adopted a large-signal stability analysis method that was feasible for analyzing dynamic models with small number of nonlinear terms. This thesis addressed that by modifying the Takagi-Sugeno (TS) algorithm to be able to quickly produce preliminary ROA estimates for the dynamic models, even when the number of nonlinear terms was large. This is important because it allows researchers to scale up the nonlinear dynamic models to study more complex MGs, and quickly produce preliminary results of time sensitive applications using readily available resources.

Large-signal Stability Analysis of Droop-controlled Converters for Microgrid Applications

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Large-signal Stability Analysis of Droop-controlled Converters for Microgrid Applications

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