Jéssica Santos Döhler: Hierarchical Control and Inverter Functionalities in Microgrids: Modeling, Simulation, and Case Studies

Abstract

As the impacts of climate change intensify, the urgency of achieving carbon neutrality has never been greater. To achieve this goal, many countries are turning to distributed energy resources to reduce their reliance on fossil fuels and accelerate the transition to renewable energy sources. A promising approach is the microgrid, a small-scale, self-sufficient energy system that can operate isolated or in connection with the main grid. With autonomous control of voltage, frequency, and power flow, microgrids enhance reliability and resilience, support faster recovery during disturbances, and improve power quality.

Inverters are central to microgrid operation, as they enable the integration of diverse energy sources while supporting grid stability. They can be classified into three categories: grid-feeding, grid-supporting, and grid-forming. Each fulfills a distinct function, from injecting power into the grid to providing stability during disturbances and even enabling independent islanded operation. Together, these functionalities expand the adaptability and reliability of microgrids across a wide range of scenarios.

The control architecture of microgrids is commonly structured in hierarchical levels: inner, primary, secondary, and tertiary. At the inner level, fast loops regulate current and voltage, ensuring basic power quality. Primary control, typically implemented through droop strategies, allows proportional power sharing among distributed generators without communication. Secondary control restores nominal voltage and frequency, while tertiary control manages power exchange and optimization with the main grid.

Building on this framework, this thesis examines inverter functionalities and hierarchical control strategies across different microgrid configurations. The analysis combines theoretical discussion with practical evaluation through representative case studies. The outcomes were developed and validated using MATLAB/Simulink and PSCAD. The discussion is organized according to the hierarchical control levels, starting with the inner control level and progressing towards the secondary level. The analysis at the levels includes all inverter operation modes, covering cases where inverters operate independently in a single mode, in dual modes, and with a configuration that combines up to three operational modes, illustrating the progressive enhancement of control autonomy and system support capabilities. This approach provides a comprehensive view of how inverter functionalities and control hierarchies interact, highlighting their impact on system performance, adaptability, and stability under different operating conditions.