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14 November 2025
What is grid voltage control, and why does it affect blackout risk in Spain?
Voltage control is one of the most critical functions in the electricity system: it ensures grid stability and continuity of supply. In Spain, it has deteriorated in recent years. In this article, we’ll explain the reasons and the possible solutions.
At the heart of Spain’s electricity system, voltage control is a critical function that guarantees grid stability and continuity of supply.
Although it is generally an unfamiliar concept, proper voltage management is essential for maintaining system stability —especially amid the growing penetration of renewable energies.
Over the past few years, Spain has experienced significant voltage fluctuations during periods of low demand and high renewable generation. This is not an isolated problem; it is structural and requires a coordinated response.
What is electrical voltage and why does it matter?
Electrical voltage indicates the ‘force’ with which electricity flows through the grid. If voltage is too low, equipment can malfunction; if it is too high, the risk of serious failures and automatic disconnections increases.
Voltage control depends on balancing two types of energy:
- Active energy: the usable electricity we consume.
- Reactive energy: not directly consumed, but essential for keeping voltage stable and enabling motors, transformers, and inverters to operate.
If reactive energy is lacking, voltage drops; if there's too much, voltage rises. Maintaining voltage within safe limits requires continuously adjusting the amount of reactive energy circulating on the grid. This task falls to the system operator (Red Eléctrica de España, REE), which coordinates the electricity system and issues instructions to generators to adjust their output.
How is voltage measured?
In Europe, the operational standard is 420 kilovolts (kV), with a safety margin up to 440 kV. This means power plants are protected to automatically disconnect if voltage exceeds 440 kV —leaving a 20 kV operational margin (i.e., from 420 kV to 440 kV).
However, in Spain, REE raised the ‘normal’ threshold to 435 kV, reducing the operational margin to just 5 kV. In practice, this does not reduce voltage; it merely labels higher values as ‘normal.’
A 5 kV margin is extremely small —similar to the measurement error itself— making it easier for even minor deviations to trigger cascading disconnections.
This practice has been in place since 2010 and was included in the European Regulation on Requirements for Generators as an exception specific to Spain.
The April 28 blackout: a wake-up call
On 28 April 2025, Spain experienced a nationwide blackout triggered by a combination of very high renewable electricity generation (especially solar PV), low demand, and insufficient voltage control resources.
The system suffered a widespread voltage control failure:
- The scheduled conventional generation was insufficient to respond to elevated voltage levels.
- Renewable generation could not actively participate in voltage control due to regulatory restrictions.
- Rapid fluctuations in wind and solar electricity generation directly affected voltage, triggering cascading disconnections.
Although generators complied with all regulations, the system lacked the resources needed to prevent the failure. In the months that followed, REE continued detecting similar issues, particularly on days of high renewable penetration and low demand.
The core problem stems from the obligation for renewable plants to operate at a constant power factor, which limits their ability to contribute to voltage control.
The regulatory framework
Until June 2025, operating procedure 7.4 (OP 7.4), which governs generators’ participation in voltage control, distinguished between conventional and renewable generation.
- Conventional plants (nuclear, hydro, combined-cycle) were required to maintain voltage within a set range at their connection point. However, many were not equipped to respond to sharp fluctuations.
- Renewable plants operated at a fixed power factor, meaning their reactive energy output was proportional to active production. As a result, any variation in solar or wind electricity generation directly affected system voltage.
This framework, approved in 2000, did not account for modern technologies or today’s high renewable penetration.
Other countries have updated their requirements, mandating all generation —including renewables— to provide dynamic voltage control. The results are clear: stable voltages and no major incidents, even in highly renewable systems.
The June 2025 update of OP 7.4 is a step forward, but implementation will be slow and partial. In practice, the system will continue operating much as it does today through much of 2026 —while other countries have nearly a decade of experience with dynamic voltage control.
What solutions exist?
The key lies in enabling renewables to participate in dynamic voltage control and reinforcing the grid with compensating equipment. The sector advocates a structural, realistic, and EU-aligned approach:
- Dynamic voltage control in all technologies
Enable real-time instruction-based control, as outlined in the new OP 7.4. In many cases, this requires no new investment —just activating functions already available in inverters. - Investment in the grid and technology
Urgent installation of reactors and STATCOMs at critical nodes. - Coherent technical standards and institutional coordination
Maintain Europe’s 420 kV limit and strengthen coordination among REE, CNMC, MITERD, distributors, and generators.
Spain’s voltage challenges are not isolated incidents —they are the predictable result of an outdated model and insufficient investment in system stability. Other countries have shown that the solution works: allow renewables to play an active role in voltage control and equip the system with the necessary technology.
Spain’s energy future isn’t improvised —it requires technical intelligence, market-based criteria, and a long-term national vision. The energy transition brings structural challenges that must be addressed with responsibility and forward-looking planning.