In 2023, both Red Eléctrica (REE) and the Comisión Nacional de los Mercados y la Competencia (CNMC) raised concerns over the Spanish electrical grid, indicating a heightened risk for blackouts due to excessive tension in the system.

Two years prior to the nationwide power failure on April 28, the regulatory body acknowledged REE's assessments in an official resolution published on November 13, which cautioned that elevated tensions could lead to the abrupt disconnection of installations connected to the grid.

The CNMC specified that tension levels were close to or even exceeding permissible limits.

Such disconnections are mechanisms intended to prevent system collapse, automatically disconnecting certain technologies or generation plants in response to excess tension.

This issue culminated in the significant blackout experienced at the end of April.

In response to these warnings, a collaborative working group was established, focusing on implementing technical adjustments among major consumers—referred to as the electro-intensive industry—to manage tension levels and avert future complications.

The group targeted two specific regions: Galicia and Andalucía. The CNMC subsequently validated a pilot project designed to gather further data, which is currently ongoing after its renewal in early 2025.

On the day of the blackout, experts, including Spanish Prime Minister Pedro Sánchez, noted that at 12:33 PM, 15 gigawatts of generation had been disconnected simultaneously, representing approximately 60% of the demand at that time.

In a document from the CNMC released later in 2023, officials indicated that the risk of electrical disturbances could increase if immediate measures were not implemented to rectify the situation.

This assessment emerged after REE reported a tripling of hours with excessive tension between 2021 and 2023, highlighting the most significant incidents occurring in late April and early May—precisely two years before the recent blackout.

In light of these developments, Redeia has been developing solutions since at least 2022 aimed at enhancing supply safety and reducing costs for consumers.

In its summer 2023 report, the company indicated that REE lacked adequate tools to prevent transportation network tension from exceeding acceptable levels, resulting in operational disconnections for safety reasons.

The situation has been exacerbated by several factors, notably a decline in mainland demand that mirrors levels from two decades ago and a significant reduction in industrial demand during off-peak hours.

Redeia also highlighted a substantial increase in the renewable generation mix, which, as of September 2023, does not respond to real-time tension commands, employing fixed power factor ranges that fail to control tension effectively.

Additionally, the diminishing share of synchronous generation (including gas or coal thermal plants, nuclear facilities, and hydroelectric complexes) has hampered tension control during critical periods.

Experts generally agree that the rising share of renewable energy sources, which often operate asynchronously, contributes to the tension problems experienced in the grid.

Errors in managing energy flows during periods of stable demand can lead to safety-related disconnects, comparing the situation to household circuit breakers tripping under overload conditions.

Both the government and authorities at Redeia and CNMC have confirmed the presence of abnormal tension and frequency values before the nationwide blackout.

Investigative groups are currently examining the circumstances surrounding the blackout, but they do not anticipate quick resolutions.

Despite uncertainties regarding the causes of the overvoltage issue, the system operator has dramatically altered its operational strategy, relying less on renewable sources and more on combined cycle gas plants since April 28. The government has acknowledged a newfound commitment to a more cautious approach.

Prior to the national blackout, the CNMC sought to document its findings rigorously, having submitted a draft resolution to the Directorate General for Energy Policy and Mines to gather feedback.

The authority’s observations, first articulated back in July 2022, indicated that the electrical system had undergone significant changes, notably becoming more capacitive, which accounts for an increasing general tension level that occasionally surpasses acceptable limits.

Among the identified causes for the situation were the decrease in electrical demand and the distributed generation rollout, necessitating expanded networks to accommodate this shift.

The CNMC concluded that the intensification of tension control measures was essential, highlighting that even when fully utilizing available tension management tools, the resources were insufficient to maintain tension within acceptable ranges.

To substantiate its warnings, the CNMC referenced an incident from July 24, 2021, when a panel of experts reported a disconnection that temporarily isolated the Iberian Peninsula from the broader continental electrical system.

In the recent blackout, one of the first recorded incidents was the disconnection of the interconnection between Spain and France, which allowed the situation to be contained south of the Pyrenees.

Preliminary findings from Redeia, shortly after the April 28 blackout, ruled out a cyberattack on its infrastructure following assessments conducted in collaboration with national cybersecurity agencies.

In the aftermath of the blackout, Redeia reported massive oscillations in tension and frequency prior to the widespread disconnection, advocating for control of these 'undesired oscillations' in earlier communications.

The absence of mandatory reactive capability was emphasized as a factor that could lead to dangerous voltage oscillations, jeopardizing system safety.