Scope

In recent decades, the increasing integration of renewable-based generation has significantly altered the composition of electric power systems. Despite this shift, conventional synchronous generators remain vital for uninterrupted operation due to their reliability and controllability. As system dynamics evolve, new modelling approaches are necessary to accurately assess stability. This paper presents a small-signal model of a representative transmission network comprising transmission lines, transformers, inverter-based resources, and synchronous generators. The model is used to analyse the impact of synchronous machine parameters (e.g. excitation control settings) on system stability, particularly in the low-frequency range.

Methods

An impedance-based small-signal modelling approach in the positive sequence domain is applied. While only positive-sequence components are modelled, the effects of frequency coupling caused by non-idealities in the grid and by DC sources are taken into account. Analytical expressions are derived for the impedance of each network component and validated using frequency scans in Matlab Simulink simulations. These impedances are then used in conjunction with the Bode stability criterion to assess the interaction between synchronous machines, converter-based generation and the grid, highlighting potential resonance conditions under different operating scenarios.

Main Results

Stability analyses are carried out for different network states and generator excitation control configurations. The study identifies the emergence of undamped resonances in the low-frequency range, which are sensitive to the excitation control parameters of the synchronous machines. These findings suggest that fine-tuning the excitation system could dampen the oscillatory behaviour, contributing to more robust system operation in mixed-generation environments.