Evaluation of system strength in a converterdominated power system

Abstract

The steady increase in power electronic-interfaced generation, driven by the continued growth of renewable energy sources, is altering the dynamic behavior of modern power systems. While traditional system dynamics have been dictated by the characteristics of synchronous generators, the increasing penetration of Converter-Based Resources (CBRs) is contributing to a gradual transition toward converter dominated power systems. This transition introduces new challenges to power system stability and raises questions regarding the applicability of conventional system strength metrics, particularly the Short Circuit Ratio (SCR), which has historically been used to assessgrid strength. The aim of this thesis is to evaluate the limitations of the SCR metric in grid strength assessment for converter-dominated power systems and to assess alternative system strength metrics that may be more suitable under these changing conditions. Furthermore, the alternative system strength metrics need to be applicable during grid planning and is therefore restricted to the data available at this stage. The study was structured into three phases: (1) an initial review of the traditional grid strength metric, SCR, supported by Electromagnetic Transient (EMT) simulations; (2) initial evaluation and selection of alternative system strength metrics through comparative evaluation using small-scale case study with EMT simulations; and (3) implementation and evaluation in PSS®E, of the most promising metrics selected in (2), followed by validation of PSS®E results through detailed EMT simulations to determine the most reliable metric for system strength assessment. The results indicate that traditional SCR metric consistently overestimates system strength in systems with high CBR penetration. The preliminary grid strength asessment conducted in phase 2 identified the Available Fault Level (AFL), Equivalent Short Circuit Ratio (EqSCR), and Site-Dependent Short Circuit Ratio (SDSCR) as the most appropriate alternative metrics for evaluating grid strength. While the AFL demonstrated promise in the initial small-scale review, it exhibited inconsistencies in the estimation of system strength during the validation process in phase 3. The results obtained from the EqSCR and SDSCR metrics were closely aligned. This outcome is expected, as both methods are fundamentally similar, differing primarily in the specific computational approaches they employ. For both small-scale (phase 2) and more complex system models (phase 3), the two metrics consistently identified the point of instability. System weakness and instability were observed at EqSCR values ranging from 1.1 to 1.3, and SDSCR values between 1.2 and 1.4. Given their comparable performance, EqSCR was identified as the more suitable metric due to its greater computational efficiency. Based on these findings, this thesis recommends the use of EqSCR for system strength screening in converter dominated power systems.