A survey on modeling of microgrids - from fundamental physics to phasors and voltage sources (1505.00136v2)

Abstract: Microgrids have been identified as key components of modern electrical systems to facilitate the integration of renewable distributed generation units. Their analysis and controller design requires the development of advanced (typically model-based) techniques naturally posing an interesting challenge to the control community. Although there are widely accepted reduced order models to describe the dynamic behavior of microgrids, they are typically presented without details about the reduction procedure---hampering the understanding of the physical phenomena behind them. Preceded by an introduction to basic notions and definitions in power systems, the present survey reviews key characteristics and main components of a microgrid. We introduce the reader to the basic functionality of DC/AC inverters, as well as to standard operating modes and control schemes of inverter-interfaced power sources in microgrid applications. Based on this exposition and starting from fundamental physics, we present detailed dynamical models of the main microgrid components. Furthermore, we clearly state the underlying assumptions which lead to the standard reduced model with inverters represented by controllable voltage sources, as well as static network and load representations, hence, providing a complete modular model derivation of a three-phase inverter-based microgrid.

• The paper presents a comprehensive survey on microgrid modeling, bridging detailed physics with reduced models for effective control design.
• It explains inverter-based microgrid operations, covering grid-forming and grid-feeding modes to simplify complex inverter dynamics.
• The work employs singular perturbation techniques and phasor transformations to reduce dynamic complexity while preserving key operational stability.

A Survey on Modeling of Microgrids

This paper presents a comprehensive survey on the modeling of microgrids, addressing the spectrum from fundamental physics to phasor representations and voltage source modeling. The authors aim to provide a structured overview tailored for control engineers interested in the intricacies of microgrid applications. This work endeavors to bridge the gap between the detailed physical models to the reduced models used in microgrid control design and analysis.

Fundamentally, microgrids serve as key elements in modern electrical systems, primarily as integrators for renewable distributed generation (DG) units. These microgrids operate either in connection with larger power networks or autonomously, highlighting the necessity for advanced model-based analytical techniques. Through this survey, a comprehensive guideline is presented to simplify the model derivation process and provide consistent assumptions for reduced modeling in microgrids.

Key Contributions and Methodology

The paper is structured to offer clear insights into the modeling process by covering the following key aspects:

1. Basic Concepts and Definitions: It begins with fundamental concepts in power systems, including symmetric AC three-phase signals and the $dq0$-transformation. This groundwork sets the stage for understanding how these transformations aid in the modeling process by converting periodic signals into constant equilibria for simplified control and analysis.
2. Microgrid Components: Details are provided on microgrid constructs, encompassing DG units, loads, and storage devices. The authors emphasize DG units' connection typically through inverters, marking a deviation from traditional synchronous generators (SGs) and indicating why microgrids demand distinct control methodologies.
3. Inverter-Based Microgrid Modeling: The narrative dives deeply into inverter operations, covering grid-forming and grid-feeding modes, alongside their respective control schemes. The grid-forming mode, crucial for voltage stabilization, is theoretically posited as a versatile voltage source, simplifying complex inverter dynamics into a linear control framework.
4. Model Reduction: Using singular perturbation techniques and phasor transformations, the authors translate intricate microgrid dynamics into reduced-order models, which are more accessible for control applications while retaining critical dynamic properties. This includes assumptions conducive to time-scale separations that help isolate fast line dynamics, underscoring their impact on steady-state operational frequency.

Implications and Future Research Directions

This paper's modeling strategy significantly aids in the theoretical and practical realms, offering substantial clarity on how complex systems can be distilled into tractable models for real-world application. The detailed discussion on component dynamics transitioning into phasor and reduced state-representation propounds a structured approach benefiting both researchers and engineers.

Looking forward, the authors recognize multiple areas warranting further exploration:

• Integration of Asymmetric Conditions: Future studies could extend the modeling framework to account for asymmetric operational scenarios, helping refine control strategies under unbalanced conditions—a common challenge in actual deployments.
• Dynamic Phasors and Symmetric Components: The modeling work could further incorporate these constructs to represent diverse operating conditions more realistically, enhancing microgrid reliability and efficiency.
• Advanced Load Models: More sophisticated representations of loads within microgrids remain a pivotal opportunity, given the intricate nature of demand profiles and their influence on stability and control.

In essence, this survey is positioned as a basis for ongoing discourse and advancement in microgrid control and modeling, urging continued innovation in capturing the dynamic and scalable nature of these emergent electrical systems.