Brazil’s power system is undergoing one of the most rapid transformations among large electricity markets, driven by the accelerated expansion of wind and solar generation. This transformation has delivered clear environmental and economic benefits, but it has also introduced new technical challenges for planning and operating the country’s vast transmission network. Among the most critical of these challenges is maintaining voltage stability in regions with a high concentration of variable renewable energy (VRE) sources, most of which are connected to the grid through inverter-based resources (IBRs). When compared to large synchronous generators, IBRs, in general, cannot withstand high currents during short circuits, which increases the susceptibility to generation outages in case of contingencies. Therefore, a comprehensive understanding of the new phenomena that arise with the increased insertion of IBRs has become essential.
For this, Brazil’s energy research company, Empresa de Pesquisa Energética (EPE), published the Technical Note EPE-DEE-NT-035/2025 titled, “Methodology for diagnosis and allocation of dynamic reactive power compensation solutions in the Sistema Interligado Nacional (SIN)” in June 2025. It presents the methodology adopted by EPE for diagnosis and decision-making regarding the allocation of variable reactive power compensation equipment within the long-term expansion planning of the SIN or national interconnected system. It includes a real-world case study encompassing the transmission networks of Rio Grande do Norte, Ceará, and Piauí. It seeks to strengthen the resilience, security and sustainability of SIN in scenarios of high VRE penetration.
The technical note would also act as a guide for transmission expansion decisions, support regulatory discussions, and ensure that investments in reactive power resources are technically justified, economically rational and aligned with Brazil’s long-term energy transition.
Figure 1: Power system with high insertion of IBR’s
Note: PAC – Ponto de Acoplamento Comum or common coupling point; UFV – Usina Fotovoltaica or solar power plant; EOL – Eólica or wind energy
Source: EPE
Background: Rising VRE and new system dynamics
Brazil has seen a sharp increase in wind and solar photovoltaic (PV) capacity, particularly in the Northeast region. Even though historically SIN has relied heavily on large synchronous generators, such as hydropower plants, the growing share of IBR-based generation has altered the dynamic behaviour of the system.
IBRs differ fundamentally from synchronous machines in the way they provide inertia and reactive settings, which may limit reactive current injection under fault conditions. While IBRs require electrical robustness at the connection point for safe operation, the electrical system also demands the contribution of the IBRs connected to its network for better performance during disturbances. This important relationship impacts both operational safety and the optimisation of the electrical system’s expansion. Hence, regions with high VRE penetration can experience reduced voltage support during contingencies, increasing the risk of voltage instability and collapse.
These risks became more evident following the August 15, 2023 event, when a disturbance triggered by the automatic shutdown of the 500 kV Quixadá–Fortaleza II line cascaded through the system. The event resulted in the interruption of 23,368 MW of load—approximately 31 per cent of the system’s total demand at that moment. Following this event, the Operador Nacional do Sistema Elétrico (ONS) or National System Operator adjusted several wind and solar plant models in the electromechanical transient database. The adjustments, intended to better align simulation results with observed system behaviour, effectively reduced the reactive power contribution of these plants during faults. Subsequent studies revealed a deterioration in simulated system performance, particularly in high-renewable scenarios, exposing vulnerabilities that had not been apparent under previous modelling assumptions.
Importance of minimum technical requirements
EPE, in its technical note, has emphasised that continuous improvement of minimum technical requirements for connecting IBRs to SIN is an important dimension. During voltage and frequency deviations, Brazilian requirements allow disconnection more readily under non-nominal conditions, in comparison to operators such as Canadian Alberta Electric System Operator (AESO), US-based Electric Reliability Council of Texas (ERCOT), and the UK’s National Energy System Operator (NESO), which require plants to remain connected for extended periods. In addition, Brazil’s requirements for reactive power contribution decrease linearly with active power generation.
Proposed methodology
The methodology developed by EPE represents a structured, multi-step process designed to guide decision-making for strategic allocation of reactive power compensation solutions. The framework comprises five interconnected steps.
Figure 2: Flowchart of the proposed methodology for reactive power allocation
Source: EPE
- Step 1: Scope definition– Establishing clear objectives
The methodology begins with a precise definition of study objectives and identification of representative scenarios. Each objective implies different study priorities and success criteria. Given the objective in question, the time horizon is determined. The study then identifies the most severe load and generation scenarios for the network under study. For networks with undervoltage problems, high load and dispatch scenarios that cause high loading in the region of interest are selected. For networks with overvoltage problems, light load scenarios and sufficient power flow are chosen to allow the switched-off switchable reactors to operate.
- Step 2: Static analysis– Preliminary identification of vulnerable points
In this step, studies are carried out to evaluate the static safety of the system. In addition, candidate buses for reactive power compensation and short-circuit power allocation are preliminarily listed. Considering the scenarios defined in Step 1, operating points are obtained with optimal exploitation of the voltage control resources existing in the system, using optimal power flow tools. From these points, buses with lower steady-state voltage profiles are identified. Further, transmission lines with higher loads are identified, verifying if they are above the surge impedance loading (SIL).
- Step 3: Dynamic analysis – Validation through simulation
The third step represents the methodology’s analytical core, where preliminary assessments face rigorous validation through dynamic performance evaluation. This step involves comprehensive electromechanical transient simulations subjected to severe contingencies. Using the candidate buses identified in step 2 as reference points, the methodology prescribes applying perturbations at these locations. Perturbation types include N-1 and N-2 contingencies, along with energisation and rejection events. Substations exhibiting the highest voltage drop rates, deepest voltage sags, or slowest recovery are flagged as priority locations for variable reactive power support.
- Step 4: Verification of stopping criterion
The fourth step implements a decision gate, wherein it is verified if the proposed solutions demonstrate adequate performance across all representative scenarios identified in step 1, not just under the most favourable conditions. If all the studied conditions are satisfied, the process concludes. Otherwise, it advances to step 5 for solution studies.
- Step 5: Solution studies—Strategic allocation and sizing
In this step, reactive power compensation equipment is allocated and sized accordingly, aiming to maximise the systemic benefit. Then, the process returns to step 3 to perform new dynamic simulations. This refinement continues until the stopping criterion is satisfied.
Application of the methodology in a practical case study in the SIN
The application of the methodology focuses on the transmission network of the states of Rio Grande do Norte, Ceará, and Piauí. This region was chosen based on the observations during the August 2023 disturbance, which justified the need for further evaluation. It was also chosen due to the planned inclusion of a new direct current transmission link in the same area, which is currently under study. Relevant characteristics of the region include the massive presence of IBRs and the reduced presence of synchronous machines.
Following the August 2023 incident, the electromechanical transient database – critical for analysing power system stability, particularly long-term dynamic behaviour – has been revised to better align simulation performance with field observations during the disturbance. The revisions also aim to prevent wind and solar power plant models from producing overly optimistic responses during contingencies. The assessment concluded that there is scope for significant modifications in the performance of the SIN once all its wind and solar power plant models are validated.
To support the study of solutions for reactive power compensation, robustness indices were calculated for selected buses in the Northeast region of the electrical system. At the initial stage, a uniform operating point was adopted for variable renewable generation, assuming capacity factors of 60 per cent for wind farms and 30 per cent for solar PV plants, to map the region’s most sensitive areas. Subsequently, the dynamic analysis applied higher capacity factors to increase stress on the area of interest, based on the sensitive points identified.
The methodology prescribes calculating both the short-circuit ratio (SCR) and the multi-infeed SCR (MISCR) for candidate buses. These calculations utilise three-phase short-circuit powers and multi-infeed interaction factors obtained from electromechanical transient analysis programmes.
Based on the results, regions most suitable for the deployment of synchronous compensation were identified, given the technology’s ability to increase short-circuit levels in specific parts of the system. Within Rio Grande do Norte, Ceará, and Piauí, the substations at Jaguaruana II, João Câmara III, Ceará Mirim II, Morada Nova, Açu III, and Quixadá emerged as the most suitable locations for implementing the solutions. These substations recorded the lowest robustness index values for the region.
Importantly, the methodology recognises that these indices should be computed across multiple operating conditions. The Northeast study calculated both “nominal” indices (based on installed IBR capacity) and “dispatch” indices (based on actual generation levels in the studied scenario). This dual calculation reveals an important nuance: system robustness varies with operating conditions, and a bus that appears adequately strong under low renewable generation may become vulnerable during periods of high generation.
Following the identification, through static analysis, of locations requiring increased reactive power support, two scenarios were evaluated under dynamic conditions with high renewable generation – Scenario 1, characterised by high solar generation and Scenario 2, characterised by high wind generation. Both scenarios assumed high power flow through the North-Northeast interconnection, deliberately stressing the system to identify vulnerabilities unlikely to appear under normal conditions.
The initial assessment of the electrical performance revealed limited reactive power support in the evaluated region and a rapid occurrence of voltage collapses. One of the most critical contingencies analysed was the single-phase short circuit at the 500 kV Quixadá substation. During the fault, several plants showed reduced contribution to voltage control, while some absorbed reactive power before resuming injection. However, the analysis indicated that even after adding synchronous compensators at Açu III and João Câmara III substations, MISCR values at these locations remained low. This suggests either that the reference threshold may need to be revised lower or that additional support could be necessary. This interpretive ambiguity underscores the importance of dynamic validation within the methodology.
Recommended solutions in the Northeast region
Following the assessment, EPE recommended the installation of four 300 MVA synchronous compensator units at three 500 kV substations – Ceará Mirim II substation (1 unit), Morada Nova substation (2 units), and Quixadá substation (1 unit). These substations were identified as critical nodes within an area of high VRE concentration and export constraints. Dynamic simulations showed that installing synchronous compensators at these locations significantly reduced the risk of voltage collapse under severe contingencies. This solution involved an estimated investment of BRL571.6 million, with allocation of BRL147.6 million each to Ceará Mirim II and Quixadá substations and BRL276.5 million to the Morada Nova substation.
The proposed solutions have been harmonised with the network expansions already foreseen in EPE’s other ongoing studies on increasing the Northeast Region’s export capacity, delivering assured benefits to the electrical system. Notably, in addition to providing a greater safety margin for SIN operation, this solution may also help reduce generation curtailment in the short-term.
Conclusion and the way forward
The study evaluated performance under both high-solar and high-wind scenarios, confirming that the proposed solution maintains adequate margins under diverse operating conditions. It also emphasised the importance of using a database containing officially validated and accurate models of wind and solar PV power plants. Further, it identifies opportunities to enhance the methodology, including the use of more advanced, efficient, and faster computational tools to simulate daily power ramps, particularly the rise and fall of solar PV generation, which must be balanced by dispatchable generation. More accurate and robust solution planning will be possible following the update of EPE’s Database for Electromechanical Stability Studies of the SIN. These updates will replace the adjusted models, where reactive current injection during faults was limited, with models validated in coordination with generation agents, reflecting the currently limited but more realistic reactive power contribution during faults.
As Brazil advances its energy transition, the principles set out in EPE’s technical note are expected to play a central role in balancing security, reliability and sustainability across the SIN.
