Modeling Considerations for Grid-forming Capable Devices in Modern Power Systems

Introduction

The vast integration of renewable resources and inverted-based resources (IBRs) continuously transforms modern power grids, giving rise to new dynamics, considerations and potential solutions to issues in the grid. In the last few years, a plethora of IBR driven interactions has been documented across grids worldwide. An important aspect of studying and mitigating such interactions during planning is modeling detail. Those modeling details need to incorporate the wide frequency range and nature of such interactions, which are typically provided by electromagnetic transient type (EMT) models. This paper develops generic EMT models of converter enabled technologies including Type-4 wind turbines and Multilevel Modular Converter (MMC) HVDC. Focus will be drawn in the grid-forming (GFM) capability of each technology. Using the developed models, the GFM mode of operation of these IBR technologies is investigated, and their technology readiness is studied. The effectiveness of these GFM technologies to address grid stability issues is tested in a 18-bus test system. The overall objective is to provide a set of generic GFM EMT models for various IBR technologies that can be used for long term planning studies and highlight potential differences in response, or limitations.

Type-4 WTG GFM Model

Type-4 wind turbine generators (WTGs) are gradually becoming the norm in modern wind farm installations for onshore and offshore applications. These wind parks are often located in weaker parts of the grid thus leading system operators and owners to study the potential of WTGs to operate as GFM and aid with grid strength and stability issues. The developed Type-4 WTG GFM of this paper enables such studies. The model features GFM primary control modes including droop and virtual synchronous machine (VSM) modes. Grid-side converter (GSC) and machine-side converter (MSC) have been represented in detail including corresponding controls. Further, the model incorporates the mechanical parts of the turbine including pitch dynamics and a two-mass torsional model. These modeling details enable the application of the model for offshore and onshore wind integration studies. Extensive simulation tests have been conducted to illustrate that the model gives representative results for a wide range of simulated dynamic events.

Multilevel Modular Converter HVDC GFM Model

HVDC technologies are rapidly being deployed across transmission systems worldwide. The vast majority of these installations are MMC type, allowing for enhanced controllability and reduced harmonic content. The paper develops a model of an MMC HVDC capable of operating as GFM. The developed model features a droop-type GFM primary control implemented withing the power controlling terminal of MMC HVDC. All the relevant energy controls for the MMC arms are also implemented. The paper provides a discussion on potential applications of the developed model for point-to-point connections.

Testing

The performance of the developed models has been assessed using a test 18-bus test system with known stability issues after given contingencies. The impact of each GFM technology on the stability of the system has been assessed. For comparison, this investigation compares the efficacy of GFM WTGs and HVDC with BESS systems, a technology widely used for GFM deployment in modern power systems.