Supply stability depends, to a large extent, on a precise and timely electrical voltage controlWe're talking about a set of practices, equipment, and standards that allow voltage to be maintained within defined ranges to prevent problems such as flickering, protection device tripping, overheating, or loss of service continuity. In modern networks with high renewable energy penetration, this control is even more critical because The variability of solar and wind power generation introduces oscillations that demand faster and more coordinated responses.
In this practical guide we review the Key international standards (IEC, EN and IEEE) applicable to voltage and harmonics, how voltage is measured and monitored in transmission and distribution, What technical solutions exist?: from linear and switched voltage controllers to stabilizers, protection relays and AC-DC controllersWe also discussed the implementation of the new PO 7.4 dynamic voltage control service in Spain and the system operator's clarifications.
What is electrical voltage control and why does it matter?
Voltage control consists of regulate, sustain and adjust the tension level at different points in a network or piece of equipment, maintaining it within limits that ensure safety and quality. This function is performed at multiple layers: from the transmission and distribution network to generation plants and critical loads in industry and buildings.
In addition to the classic maneuvers (tap changers on transformers, capacitor/inductor banks), today the system requires dynamic resources that supply or absorb reactive power quickly and accurately. In renewable energy installations, the constant power factor control strategy may fall short, hence the growing relevance of real-time monitoring of voltage settings to dampen rapid variations.
Key supply quality standards and norms
The standards define how to measure, what limits to apply, and what features should the equipment offer? To ensure comparability and legal compliance, it is important to distinguish between groups focused on voltage compatibility levels, current limits, and measurement methods and immunity.
Voltage compatibility (IEC 61000-2-x)This group sets compatibility levels for voltage phenomena in public and private networks, without imposing current limits:
- DIN EN 61000-2-2 | IEC 61000-2-2: compatibility levels at the point of connection to the public low-voltage network (RPC), up to 150kHz.
- DIN EN 61000-2-4; Class 1/2a/2b/3 | IEC 61000-2-4; Class 1/2a/2b/3: for internal system points (IPC) in low and medium voltage networks up to 35 kV.
- DIN EN 61000-2-12 | IEC 61000-2-12: analogous to -2-2 but in medium voltage public network.
- DIN EN 50160.: defines the power quality characteristics of public networks, from low to high voltage, such as nominal voltage, fluctuations, flicker and harmonics.
- IEEE 519: limits of harmonic voltages and currents in supply networks; of Widespread use in the US, Asia, and Arab countries.
Current limits (IEC 61000-3-x)Here the focus is on the harmonics and current fluctuations that the equipment injects into the grid (does not define voltage limits):
- DIN EN 61000-3-2 | IEC 61000-3-2: current harmonic limits for equipment up to 16 A.
- DIN EN IEC 61000-3-12 | IEC 61000-3-12: current harmonic limits for equipment >16 A and <75 A.
- DIN EN 61000-3-3 | IEC 61000-3-3: limits of voltage changes and flickering.
- DIN EN 61000-3-X | IEC 61000-3-X: other standards in the same field that complete the current emission framework.
Measurement methods and immunity (IEC 61000-4-x)This group defines how to correctly measure and test the immunity of equipment to disturbances:
- DIN EN 61000-4-30 Class A Ed. 3 | IEC 61000-4-30 Class A Ed. 3requirements power quality instruments Class A with accurate and reproducible measurements of frequency, flicker, harmonics, etc.
- DIN EN 61000-4-4 | IEC 61000-4-4: immunity to rapid transient disturbances (bursts/EFT).
- DIN EN 61000-4-7 | IEC 61000-4-7: appropriate methods for measuring harmonics and interharmonics in supply networks.
- DIN EN 61000-4-15 | IEC 61000-4-15methodology for measuring flicker and assess the severity of voltage fluctuations.
- DIN EN 61000-4-X | IEC 61000-4-X: set of rules of immunity complementary for different disturbances.
Along with these, the supply quality measuring devices They have additional requirements:
- DIN EN 62586-1 | IEC 62586-1Product characteristics and performance of equipment for measuring, recording and, where applicable, control quality parameters networking.
- DIN EN 62586-2 | IEC 62586-2: test methods for Class A and Class S equipment in accordance with IEC 61000-4-30.
The essence of this framework is clear: IEC 61000-2-x establish voltage compatibility levelsThe 61000-3-x establish limits of current emission of the teams, and the 61000-4-x unify how to measure and immunize to ensure comparable results.
Voltage measurement in transmission networks: operating margins and particularities in Spain
In Europe, operations typically revolve around 420 kV with a safety margin up to 440 kV en very high voltage networksThis upper margin acts as a threshold for automatic disconnection if it is exceeded, preventing damage and serving as an additional safety barrier.
In Spain, the operator has raised the threshold considered normal to 435 kVThis narrows the operating buffer from the optimum point (420 kV) to just 5 kV above before reaching 435 kV, leaving much less clearance up to the 440 kV limit. This narrowing can be problematic when the measurement uncertainty is comparable to that marginbecause small deviations can trigger cascading disconnections.
This practice has been in place since 2010 and was later recognized as specific exception for Spain in the European Generation Requirements Regulation. That said, the operator emphasizes that the The 435 kV limit has been in effect since at least 1998 in Spanish regulations and has been endorsed by recent European regulations, and that structurally reducing it to 420 kV would imply high costs of technical restrictions without guarantees of not exceeding them with current resources.
In recent years, the average level of tensions has not increased, but rather its variabilityAnd therein lies the challenge: controlling the dynamics with enough rapid reactive resources and a greater participation of installations capable of following voltage commands, beyond the classic constant power factor command. This greater variability increases the blackout risks if rapid activation and coordination are not available.
The blackout of April 28: what it taught us
On April 28, 2025, a significant outage occurred in Spain associated with a combination of High renewable generation, low demand, and insufficient voltage control resourcesRenewable energy, due to regulatory design, was unable to provide the necessary support to alleviate the strain at that time. Detailed analysis of the blackout confirms the combination of factors described in this paragraph.
Several factors were identified: scheduled conventional generation was insufficient to meet the observed voltage levels; renewable energy installations were limited in their ability to actively participate in the control; and rapid variations in wind and photovoltaic production directly impacted the voltage, triggering a cascade of gunfire.
According to the available information, the generators complied with current regulations, but the system lacked available and activatable resources to avoid instability. The operator has been detecting similar phenomena on days with high renewable energy penetration and low demand, especially when demand is high. quick response to instructions.
From the operator's perspective, there was no lack of planned reactive capacity; the difficulty lies in its effective activation and response speed in the face of abrupt changes. Furthermore, it is emphasized that groups must be able to contribute or absorb, at a minimum, one 30% reactive power regarding its maximum power, and that there are no exemptions below that legal minimum.
Voltage controllers: definition, types and components
A voltage controller is a device or circuit designed to regulate, stabilize and adjust the tension supplied to a load. Its objective is to maintain the output within defined margins, regardless of variations in the source or changes in the load, protecting sensitive equipment and ensuring reliable operation.
These controllers rely on basic principles (Ohm's Law, watts, volts, and amperes and feedback) and in components such as resistors, capacitors, transistors, diodes, and integrated circuits (linear and switched regulators). The chosen architecture radically changes efficiency, noise, and dissipation.
Linear controllersThey use an active element (BJT or MOSFET) as a “variable resistor” to set a constant output voltageWhen the input increases or the load demands less current, the excess is dissipated as heat. They are simple and quiet (low noise), but inefficient with large differences between input and output.
Switched controllersThey work with a transistor that switches at high frequency, modulating the duty cycle (PWM) so that, after an LC filter, a stable and efficient outputThe inductor stores energy when the transistor conducts and releases it when it cuts off. By operating near saturation or cutoff, losses are reduced and efficiency can exceed that of the transistor. 90%.
Typical components of voltage controllers (linear and switched): voltage reference, error amplifier, control element (BJT/MOSFET), output sensor or resistive divider, feedback comparing output and reference, PWM/oscillator control, LC filter, recirculation diode in non-synchronous topologies and protections (overcurrent, overtemperature, short circuit). Input/output capacitors smooth out curling and transients.
Applications of voltage control and controllers
From consumer electronics to industrial networks, voltage controllers ensure that each piece of equipment receives the appropriate voltagepreventing damage from fluctuations and improving operational efficiency. These are common applications:
- Power of computers, televisions and electronic equipment, ensuring correct voltage to prevent failures.
- Battery chargers (mobile phones, laptops, electric vehicles), adjusting the output voltage to the needs of the battery.
- Telecommunications (base stations, routers), providing stable voltage to maintain signal quality.
- Consumer electronics low power consumption (watches, cameras, toys, household appliances), protecting sensitive components.
- Automotive (lighting, infotainment, auxiliary motors), compensating for battery variations.
- Industrial (motors, automation, machinery control), safeguarding equipment and processes against deviations.
Voltage stabilizers and AC-DC controllers
In environments with unstable networks or critical loads, a voltage stabilizer It maintains a constant supply voltage, correcting any deviation as soon as it is detected. This is essential when even a small fluctuation can cause data loss or damage (laboratories, healthcare, IT, fine processes), and is designed to handle startup spikes, highly reactive loads or great powers.
Meanwhile, the AC-DC controllers They are integrated into AC and/or DC circuits to regulate, filter, convert, and compare signals. They are used, for example, in the regulation of voltage, temperature, motor speed, or volume, being able to choose high-efficiency PWM controllers or synchronous rectifier controllers in high-density AC-DC sources (such as mobile phone chargers).
These controllers cover a wide range of input voltages (approx. -8 V to 60 V) and output currents (around -4 A to 8 A), with multiple variations in packaging, mounting, operating temperatures and output specifications adapted to each application.
Protection relays and commercial solutions for voltage control
Beyond ongoing regulation, protection is key: a electronic voltage control relay It monitors abnormal conditions and triggers in the event of dangerous values. In single-phase and three-phase installations with neutral, these devices detect overvoltages, undervoltages, incorrect sequence, and phase loss, and even offer external trigger inputs and LED signaling.
Example of the features of a voltage control relay: protection against overvoltage above 265 V with tripping times adjusted to severity (approx. 3 s at 300 V, 800 ms at 350 V, 200 ms at 400 V), protection of undervoltage below 160 V (typical time 300 ms), RST sequence error detection in three-phase (trigger ~1 s), external trigger (≤10 ms) and LED indication.
Typical application: protection against neutral break through phase-neutral measurement in both single-phase (RVM model) and three-phase (RV-T/RV-TS), with true root mean square (TRMS) measurement and compact format for modular switchboards, electrical and industrial substations.
REVALCO 1RSQE Three-Phase (AC) Voltage Control. Device designed for monitoring and protection of three-phase systems, with robust materials, 1 switched output relay (NO/NC)assembly in DIN rail EN 50.022Phase-to-phase control, over/undervoltage protection, and phase failure detection. It is ideal for automation and control in multiple scenarios.
Typical reference standards for equipment of this type: EN 55022 (Class B) and the EN 61000-4-x immunity family (including -2, -3, -4, -5, -6, -11), covering everything from electrostatic discharges to radiated and conducted immunity and network variations.
- Representative technical data: 400 V power supply (self-powered between L1-L2) to 50 / 60 Hz, approximate consumption 1,5 W, IP20 protection and Class II insulation.
- Thermal range: operating range of -10°C to +55°CStorage from -25 °C to +70 °C.
- Output relay: 8 A at 250 V~ (NO-NC-C), dimensions 2 DIN modules and weighs around 0,11 kg.
New dynamic voltage control service (PO 7.4) and status in Spain
In recent months, the system operator has enabled the first renewable energy plants to provide a dynamic voltage control service in accordance with the new PO 7.4 (proposed in 2020 and approved by the CNMC in June). The system is ready for these facilities to begin providing the service as soon as they notify us.
To date, around 168 requestsof which approximately 125 correspond to non-dispatchable renewables. 24 facilities are ready to begin testing; the rest either state they cannot follow the voltage instructions or are completing documentation. Priority is given to enabling non-dispatchable renewablesbecause they are the ones that provide new resources to the system, although they have also requested conventional power plants (cycles, hydroelectric) that already have an obligation to provide the basic service.
Benefits for the authorized facilities: dispatch priority and the option to reduce maximum production changeover ramps. To be enabled, they must demonstrate the ability to control voltage in two modes: reactive slogans and tension slogansThis last mode, with real-time tracking, provides flexibility to respond to rapid changes.
Many renewable energy companies that currently operate under the mandate of power factor They already have a regulatory obligation to be technically prepared to follow tension instructions, so an increase in available resources is expected in the short term.
Relevant clarifications from the operator: in recent years there has been no increase average voltage levels Thanks to the commissioning of control elements, although variability has increased, which must be managed by generators with effective voltage controlWork has been underway since 2020 to modify PO 7.4 to expand the volume of resources capable of following high-pressure directives, exceeding pilots and public hearings with divergent positions in part of the conventional generation.
- Required capacities: the power plants providing the service must be able to supply/absorb ±30% reactive power regarding its maximum power.
- There are no known regulatory exemptions that would allow operation by debajo del minimo settled down.
- On April 28th there was no shortage of scheduled reactive capacity; the problem was the no activation when the system required it and slow or insufficient responses.
- The transport limit of 435 kV It has been maintained by regulation for decades; lowering it to 420 kV would imply higher costs. technical restrictions and its fulfillment is not guaranteed with current resources.
- The service using reactive instructions does not provide the temporary flexibility necessary in the face of very rapid variations, hence the impetus to the tension setpoint mode.
This regulatory approach fits within the standards ecosystem: for effective control to exist, measuring equipment must comply IEC 61000-4-30 (Class A) and that the evaluation of harmonics, interharmonics, and flicker be performed using methods of IEC 61000-4-7 and IEC 61000-4-15, while voltage compatibility levels and current emission limits fall within IEC 61000-2-xy 61000-3-xrespectively.
Mastering electrical voltage control involves understanding the regulatory framework (IEC/EN/IEEE), applying comparable and reproducible measurement methodsand deploy technical solutions suited to the challenge: voltage controllers (linear and switched), stabilizers, protection relays and AC-DC controllers, all coordinated with operating strategies that enable renewables to follow voltage targets in real time. With more dynamic resources, faster responses, and Class A metering, the system can operate more safely in a high-demand scenario. renewable penetration and variable demand.
