Power systems are undergoing a rapid change in generation mix due to the growth of inverter-based resources (IBRs) such as wind, solar, and battery energy storage. The dynamic behavior of the Bulk Power System (BPS) that has typically been driven by synchronous machines is increasingly influenced or even largely determined by IBRs. Stability challenges related to synchronous generators generally stem from electromechanical phenomena. The focus of stability efforts is therefore to maintain synchronism of the generators and provide sufficient damping of oscillations in the range of 0.2~2 Hz. The dynamic stability simulation tools widely used for Bulk Power System (BPS) studies were initially developed to assess the dynamics of synchronous generators to determine the stability of the BPS. Positive sequence dynamic models used in these stability simulation tools are considered adequate to represent the key dynamic characteristics of synchronous generation and the corresponding system dynamic responses.
With the increasing penetration of IBRs in the BPS, the dynamic nature of IBRs must be considered when assessing system stability. To accommodate the widely used dynamic stability simulation tools, positive sequence models have been developed for IBRs. These tools require fast dynamics like phase lock loop and inner current control to be simplified, and modeled as constants or simple algebraic equations. Such practice is considered acceptable when IBRs are connected to a strong grid with low IBR penetration, where healthy voltage signals can be obtained. However, positive-sequence modelling of IBRs in this way is limited in applicability and accuracy under weak grid conditions, where the fast dynamics of inverter controllers can be critical to the dynamic response of the IBRs and the resulting dynamic performance of the BPS.
System strength from a voltage perspective is typically quantified by metrics such as short circuit ratio (SCR). Due to the limited short circuit capability of IBRs, the continued replacement of synchronous generators with IBRs will lead to a continued reduction in the overall strength of the BPS. In recent years, model revisions (generic and user-defined) and improved controller tuning have been used to mitigate positive sequence model adequacy challenges. While this has allowed study engineers to continue using dynamic stability simulation tools to study the increasing penetration of IBRs under weak grid conditions, a detailed electromagnetic transient (EMT) study should also be considered. EMT studies are able to account for fast IBR controllers without simplification and can be used to verify the adequacy of traditional modelling efforts. Further growth of IBRs could lead to system conditions such that the existing practice of using positive sequence model and simulation tools will no longer be adequate to assess BPS stability.
Conducting a region wide EMT study that includes dozens or hundreds of IBRs is a huge lift compared to running typical dynamic stability simulations. Several barriers need to be addressed in order to promote the mainstream use of EMT studies. First, most planners are well trained with tremendous experience and knowledge in conducting dynamic stability simulation studies. Although they are becoming more common, EMT studies are not widely required and it will take study engineers a significant amount of effort and time to develop such a specialized skillset for these studies. Additionally, the complexity in setting up EMT studies and the related computational burdens limit the number of contingencies and system conditions that can be modelled in a region. In recent years, the advancement of computational hardware and simulation tool capability (such as parallel simulation) has allowed EMT simulation to become more feasible for BPS stability studies. Regional operators, like AEMO[1] and ERCOT[2], have conducted EMT studies for large areas in their own systems with significant IBR penetration. It is not yet considered feasible or common practice to conduct EMT studies daily or even hourly to assist in decision-making for real time operation, although the future is unclear in this respect.
The growth of IBRs in the power system is expected to continue. System operators rely on reliability assessments based on the models and simulation tools to evaluate system behavior and take proper actions to maintain secure operation. Study engineers must be mindful of the limitations of positive sequence model and simulation tools when assessing a system dominated by inverter-based resources. Although EMT studies can analyze the full details of IBRs’ dynamic performance, significant work must be done to overcome the challenges of using EMT studies as normal planning practice for BPS stability assessment and potential operational applications. Industry and researchers must continue to explore and develop advanced tools and techniques that will allow study engineers to assess the system stability of an IBRs dominated system in a timely and accurate fashion. In particular, tools which automate the creation of large system models and offload the burden of analysis from the study engineers are critical; ease-of-use must continue to be a critical objective for EMT software tool developers in the coming years. Model quality and standardization of modeling techniques are also improving but require further effort.
New tools and existing tools are quickly adapting to these needs, helping engineers to assess the system dynamic response of IBRs. These include the use of powerful computers with many CPU cores, and software to help engineers distribute model loads between the cores. Additionally, automation tools have been advancing, like PSCAD and E-Tran/Plus, which aid the engineer in creating network equivalents, maintaining model libraries, automatically setting up faults, contingencies, and monitoring, and analyzing large contingency lists in very large models. However, there is a lot remaining to do on both the software side and in hardware capability, and in the development of engineer capability, as we look forward to a time when our grid is dominated by inverter-based resources.
[1]. Available at https://www.aemo.com.au/-/media/Files/Media_Centre/2017/South_Australia_System_Strength_Assessment.pdf
[2]. Available at http://www.ercot.com/content/wcm/lists/144927/Panhandle_and_South_Texas_Stability_and_System_Strength_Assessment_March….pdf
Shun Hsien (Fred) Huang, Andrew Isaacs, Julia Matevosjana
ERCOT
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