Impedance-Based Whole-System Modeling for a Composite Grid via Frame-Dynamics Embedding

TL;DR

This paper introduces a novel impedance-based modeling approach for composite power systems that preserves frame dynamics and scalability, enabling comprehensive analysis of generator-converter interactions in various grid topologies.

Contribution

It presents a frame-dynamics-embedding transformation that allows for whole-system impedance modeling without relying on a global reference frame, applicable to diverse network configurations.

Findings

Identifies instability issues caused by PLL-swing coupling in low-inertia grids.

Provides a scalable impedance modeling method for meshed and radial networks.

Demonstrates improved accuracy and applicability over existing impedance models.

Abstract

The paper establishes a methodology to overcome the difficulty of dynamic frame alignment and system separation in impedance modeling of ac grids, and thereby enables impedance-based whole-system modeling of generator-converter composite power systems. The methodology is based on a frame-dynamics-embedding transformation via an intermediary steady frame between local and global frames, which yields a locally defined impedance model for each generator or converter that does not rely on a global frame but retains all frame dynamics. The individual impedance model can then be readily combined into a whole-system model even for meshed networks via the proposed closed-loop formulation without network separation. Compared to start-of-the-art impedance-based models, the proposed method retains both frame dynamics and scalability, and is generally applicable to various network topologies (meshed, radial, etc) and combinations of machines (generators, motors, converters, etc). The methodology is used to analyze the dynamic interaction between generators and converters in a composite grid, which yields important findings and potential solutions for unstable oscillation caused by PLL-swing coupling in low-inertia grids.