In this work, the small signal stability of a two-area four-machine test system and a reduced CIGRE Nordic system is analyzed. The effect on the stability of the system at high penetration levels of non-synchronous generation (NSG) as well as the placement and distribution of NSG are discussed in detail. The focus of this paper is on the behavior of the frequency and damping of the inter-area modes as the system approaches very low levels of inertia. A methodology to identify the inter-area mode frequency based on the system inertia is proposed. The developed theory is compared to the obtained eigen-value results for the small and large test systems, it is shown that the proposed methodology successfully predicts the inter-area mode frequency of the two test bed systems. Time domain simulations of the smaller test system are performed as a point of reference, in order to validate the eigen-value analysis. Additionally, the paper explores the impact of the inertia of grid-forming converters on the behavior of the inter-area mode frequency. Although, the focus of this paper is on the inter-area oscillations of two-area systems; the proposed theory is also applied to a multi-area frequency mode for the large test system and is validated.

Inverter-based generators (IBGs) are becoming popular in modern power systems. When the penetration of IBGs is increasing in power systems, new stability, protection, and monitoring challenges are introduced in the grid. Grid-forming (GFM) control of converters is seen as a promising solution for future power grids to overcome particular stability challenges. Here, the technical challenges of the GFM-based IBGs are reviewed from the point of view of TSOs and academic research. The properties of different GFM methods are studied for different GFM-based IBGs for a single grid-tied IBG and using the IEEE 9-bus test system. Simulation results are provided by using the PSCAD-EMT simulation software. 

This research performs a detailed comparison of fundamental frequency positive sequence (RMS) and electromagnetic transient (EMT) based simulations of a two-area four-machine system with power-electronics based non-synchronous generation (NSG). The RMS based simulations are performed in PowerFactory and the equivalent EMT simulations are performed in PSCAD. Parity between the two-area four-machine model constructed in PSCAD and PowerFactory is rigorously established. The default fully rated wind converter model from PowerFactory with fault ride through is adopted in PSCAD, utilizing the same control strategy. The transient stability of the two-area four-machine model with NSG in PSCAD and PowerFactory is evaluated with respect to one another. Specific RMS modeling limitations are also discussed in this paper.

In this work, a two-area four-machine system with wind power generation is developed in parallel in bothRMS and EMT environments. The fundamental frequency positive sequence (RMS) model is developed in DIgSILENTPowerFactory and the electromagnetic transient (EMT) model, in PSCAD. Specifically, this paper focuses on the systematicdevelopment of the comparable converter model in both environments. To model the wind farm, a PSCAD EMTvoltage source converter model is reconfigured and simplified to replicate the control of the default PowerFactory convertermodel that is used. The performance of the two models is assessed within a simple system, before integrating the windfarm into the two-area system. The selection of the control parameters is chosen in both models to provide a comparableresponse for a step change to the reference power. The default fault ride through (FRT) control is enabled in PowerFactoryand a FRT control strategy is developed in PSCAD to achieve a realistic response. A three-phase short circuit fault on a paralleltransmission line segment and a 100 MW load step change are independently applied in order to compare the dynamicresponse of both models. The specific limitations of converter modelling in RMS simulations are discussed.