In this series of short technical articles, we look at various topics that illustrate the complexity of renewables as a concept within the ongoing energy transition. Here, PSC UK’s Carlos Ferrandon-Cervantes, Dereck Dombo, Abraham Alvarez and James Wilson look into the complex interplay of factors influencing transient stability, particularly in systems with a high proportion of renewable generation.
Power systems are complex, interconnected networks that perform the vital task of generating, transmitting, and distributing electrical power to consumers. These systems are designed to operate in a stable manner under a range of conditions, ensuring that electricity is continuously available, safe, and reliable. The area of power systems that analyzes if a system is stable after a disturbance is known as power system stability analysis. One aspect of power system stability, known as transient stability, is particularly challenging. Transient stability relates to the ability of the power system to maintain synchronism when subjected to a severe disturbance, such as a short circuit or a sudden large change in electrical load. The failure to maintain transient stability can result in significant disruptions and even system-wide blackouts .
Transient stability analysis
Transient stability analysis is a significant part of the broader power system stability study. It deals with the system’s behavior in the immediate aftermath of a large disturbance, evaluating whether the system can return to a steady-state operation. This process involves assessing the dynamic responses of generators to the disturbance and their interactions with the rest of the system.
One of the main goals of transient stability analysis is to determine the critical clearing time for a given disturbance, which is the maximum time the disturbance can last before the system loses synchronism. This analysis requires the use of mathematical models and high-speed computer simulations due to the highly dynamic and nonlinear nature of the power system . Within power system stability analysis, we can differentiate for example if we are analyzing a small disturbance or a large disturbance. In this article, we address the large disturbance problem, which is identified by the fact that the event is not possible to be linearised ; hence numeric methods are needed in order to solve an RMS (Root-Mean-Square) time domain simulation problem.
Renewable energy sources and power systems
The integration of renewable energy sources (RES), especially solar and wind into power systems has been growing steadily during the last decade globally . This growth is driven by a variety of factors, including environmental concerns, technological advancements, and economic incentives, but also the available resource for each power system. While traditional power plants, like coal and gas-fired plants, use synchronous generators, most RES use Converter-Interfaced Generations (CIGs). CIGs convert the DC power produced by the renewable sources into AC power compatible with the grid. This approach is quite different from traditional generators, particularly concerning the interaction with the system’s inertia.
In conventional synchronous generators, the rotating mass of the generator contributes to the system’s inertia, which helps resist changes to the system’s frequency. However, CIGs, lacking a physical rotating mass connected directly to the grid, do not inherently contribute to system inertia. This difference is key when considering the impact of RES on power system stability, particularly the transient stability.
Impact of renewable energy on transient stability and the role of fast frequency response
The large-scale integration of RES and CIG poses unique challenges for transient stability analysis in power systems. A prevalent strategy to mitigate the low inertia challenge introduced by these sources is the implementation of Fast Frequency Response (FFR), a service that quickly injects power into the grid when frequency deviations occur.
However, the application of FFR brings certain implications for the transient stability of power systems. According to a study , FFR, despite effectively arresting frequency excursions, can reduce transient stability, particularly when delivered electrically close to critical generators. These critical generators are defined as those operating with high relative angle displacement and low synchronous kinetic energy, i.e., the ones that can be found in so-called “weak systems” since a significant share of conventional generation has been decommissioned already.
We have run a simulation on Single Machine Infinite Busbar (SMIB) where two scenarios have been analyzed for the same fault in a 220 kV busbar of the system. One where a Battery Energy Storage System (BESS) is providing FFR electrically close to a synchronous generator, and one where a BESS is “far” from it. The results can be seen in Figure 0‑1 and Figure 0‑2.
Figure 0 1 SMIB system – SGs close to BESS
Figure 0‑2 SMIB system – SGs far from BESS
We have added one BESS using WECC standard models and tuning (1), and two synchronous generators (3,4), where generator 4 has the same parameters as generator 3, but the inertia has been reduced by 90%. The generators’ data has been obtained by the example cases in the PSS®E libraries. In the case where generators are electrically close to a 220 kV busbar, and a three-phase fault is applied to the 220 kV common busbar, the angle of generator 4 has slightly a wider separation from generator 3, when it is analyzed from the electrically “far” case. In the “far” case, generators 3 and 4 have been set 50 km from the 220 kV common busbar. Although this is only a mock model, and this model it is not exactly a “weak” system, further studies suggest that rapid power injections near these critical generators could lead to an increase in angle divergence, thus reducing transient stability. This unexpected effect of FFR underlines the importance of a strategic approach to placing FFR injections within the system.
These results emphasize the need for a comprehensive approach when designing and implementing FFR schemes in power systems. It’s not only about considering frequency response but also understanding the complex interplay of factors influencing transient stability, particularly in systems with a high proportion of renewable generation.
Conclusions and Future Outlook
The rapid transformation of power systems, focusing on integrating RES and CIGs, brings opportunities and challenges. More often we will see low-inertia power systems as we strive to build sustainable energy solutions and transient stability will still be critically relevant, with the caveat that the dynamic responses will be faster.
Implementing mitigation measures like FFR, designed to arrest frequency excursions, requires careful placement and execution. The unexpected effect of FFR reducing transient stability, particularly when delivered close to critical synchronous generators, highlights the intricate interplay of factors influencing system stability.
Moreover, traditional methods for transient stability analysis, developed in an era dominated by synchronous generators, may not be fully equipped to handle the unique dynamics of power systems heavily populated with CIGs and RES . The development of new analytical tools and methodologies capable of capturing these dynamics is a challenge that researchers and engineers need to address.
Looking ahead, it’s clear that as our power systems continue to evolve, so too must our approaches to transient stability analysis. Increased grid heterogeneity and decentralization, along with the need for real-time or near-real-time stability analysis, present new and complex problems requiring innovative solutions.
The field of transient stability analysis is poised for significant development and innovation in response to these challenges. Ongoing research is critical to understanding these complexities and developing methodologies that ensure the continued reliability and stability of our power systems as they transition toward a sustainable future.
Transient stability analysis in an era of renewable energy and low-inertia systems is complex, but with ongoing research, careful consideration, and continuous innovation, in PSC we are well-equipped to navigate these challenges and power a sustainable future.
 P. Kundur, “Power System Stability and Control,” New York: McGraw-Hill, 1994.
 J. Machowski, J. W. Bialek, and J. R. Bumby, “Power System Dynamics and Stability: With Synchrophasor Measurement and Power System Toolbox,” 2nd Edition, Wiley, 2020.
 V. A. Venikov, Transient Phenomena in Electrical Power Systems. 1st ed. Vol. 24. D. W. Fry and W. Higinbotham, Eds. Elsevier, 1964. [E-book] Available: Elsevier. ISBN: 9781483222714
 IEA Report on Renewable Energy [Online] https://www.iea.org/reports/renewable-electricity
 Z. Zhang, S. Asvapoositkul, and R. Preece, “Impact of Fast Frequency Response on Power System Transient Stability,” The 17th International Conference on AC and DC Power Transmission (ACDC 2021), Online Conference, 2021, pp. 13-18, doi: 10.1049/icp.2021.2436.
 Sajadi, A., Kenyon, R.W. & Hodge, BM. Synchronization in electric power networks with inherent heterogeneity up to 100% inverter-based renewable generation. Nat Commun 13, 2490 (2022). https://doi.org/10.1038/s41467-022-30164-3.
The complexity of renewables, Part 1 – PSC Consulting
The complexity of renewables, Part 2 – PSC Consulting
The complexity of renewables, Part 3 – PSC Consulting
The complexity of renewables, Part 4 – PSC Consulting
The complexity of renewables, Part 5 – PSC Consulting
The complexity of renewables, Part 6 – PSC Consulting
The complexity of renewables, Part 7 – PSC Consulting
The complexity of renewables, Part 8 – PSC Consulting
The complexity of renewables, Part 9 – PSC Consulting