Can renewables and batteries solve voltage issues?
A technical look at the capabilities of those technologies
Modern power grids must maintain voltage within tight bounds to ensure safe and reliable electricity delivery. Traditionally, large thermal power plants (coal, gas, nuclear) provided essential voltage support via their synchronous generators’ automatic voltage regulators, and grid operators developed operational processes strongly relying on synchronous generators' voltage support. As the grid transitions to renewable energy, many thermal plants are retiring, raising the question: how will TSO/SOs manage voltage now?
As outlined in my previous article, voltage stability is becoming an increasingly pressing issue in the Spanish electricity system, as illustrated in the average hourly chart below. At present, these voltage problems are primarily managed through the energy redispatch of thermal power plants.
In this article, we explore
Why do voltage issues occur (overvoltage vs. undervoltage)?
How were they solved historically?
How can renewable energy sources and battery storage step in to regulate voltage?
To explain those concepts and especially how renewable energy and batteries might solve those challenges, I’ve decided to partner with Dr. Gilles Chaspierre to co-write this article. Gilles has spent the last 8 years solving complex power system stability challenges arising from massive RES integration in power grids.
Please follow his newsletter GridStab; you will be sure not to be disappointed.
Let’s start back with some basics first!
When Do Overvoltage or Undervoltage Issues Occur?
Understanding when voltage problems occur helps explain why reactive power management is so critical in power systems.
Overvoltage Conditions: High voltages occur when reactive power supply exceeds demand during light loads or excess generation. Long transmission lines inject more reactive power than needed (Ferranti effect), especially during off-peak periods when line capacitances and lightly loaded transformers reduce consumption. Distributed solar PV can push rural voltages above limits during midday generation peaks. System operators increasingly face high voltage challenges during periods of low demand, such as sunny or windy times, due to the rise of distributed renewables and the decrease in conventional plants providing reactive power control.
In the following figure, you can see the number of thousands of hours with voltage above 420 and 240 kV in the Spanish network, while the reference is 400 and 220kV. It shows that overvoltage is increasing, especially in periods with high renewable penetration (April, May, June). Unfortunately, the data is not reported for 2024 and 2025, which does not allow us to say further about the trend.
Undervoltage Conditions: Low voltages occur when reactive power demand exceeds supply during high loads or network stress. Heavy inductive equipment like motors and air conditioning consumes large amounts of reactive power, causing voltage drops when local compensation is insufficient. Evening demand peaks create transmission voltage drops as high power flows through inductive lines that absorb reactive power along their path.
In summary, voltage issues arise from an imbalance of reactive power: a surplus of reactive (relative to load) causes overvoltage, while a deficit causes undervoltage. The traditional solution was straightforward: inject reactive when the voltage is low, absorb reactive when the voltage is high, using the resources we will describe now.
How Grid Operators Traditionally Prevent Voltage Issues
Voltage magnitude and reactive power are tightly linked: injecting reactive power at a node raises the voltage, while absorbing reactive power lowers the voltage.
Because reactive power doesn’t travel far efficiently (“VARs do not travel well” due to high losses and voltage drops over distance), grid operators must manage voltage with local resources distributed throughout the network.
Traditionally, Transmission System Operators (TSOs) ensured voltage management by planning and operating a combination of equipment and practices:
Synchronous Generators: Large thermal plants located near load centres
used automatic voltage regulators (AVRs) to adjust excitation and control reactive output. Over-excited generators produce reactive power (boost voltage), and under-excited generators absorb it (reduce voltage). TSOs set voltage schedules (~1.0 pu) for automatic real-time regulation.
Reactive Reserve: Grid planning maintained reactive power reserves below generator limits for contingencies. Capability curves define reactive capacity at various loads. System operators ensured N-1 contingency compliance, keeping voltages within 0.95–1.05 per-unit ranges.
Shunt Devices: Capacitor banks at substations provide reactive power during high load (raise voltage). Shunt reactors absorb excess reactive power under light load conditions, countering Ferranti effect overvoltage. Devices switched in blocks as conditions changed.
Transformer Tap Changers: An on-load tap changer is a live switch that inserts or bypasses small sections of a transformer winding to change its turns-ratio.By nudging that ratio up or down in tiny steps, it automatically holds the customer voltage steady when grid voltage or load drifts.
FACTS Devices: SVC (Static Var Compensator) and STATCOM (Static Synchronous Compensator) rapidly adjust reactive power output in cycles to stabilize voltage.
Synchronous Condensers: Synchronous machines without prime movers provide voltage support by adjusting excitation to absorb/produce reactive power. In addition to FACTS, it reacts instantaneously to voltage deviation (no control delay). It can provide a large amount of short-circuit power and inertia (especially when coupled to a flywheel).
Using the above tools, TSOs historically kept voltages within the required range.
Undervoltage conditions (i.e., low voltage) were handled by adding reactive power locally – e.g. increasing generator excitation, switching on capacitors.
Overvoltage conditions (i.e., high voltage) were fixed by absorbing reactive power locally – e.g., reducing generator excitation, switching on reactors– or tapping down transformers to pull voltage down.
In essence, the grid operator maintained a balance: not only balancing watts for frequency, but also balancing VARs to hold voltage. This approach worked well in the era of centralised generation: Traditionally, reactive power services were provided by large generators and transmission-owned equipment, meaning the voltage support was built to the presence of big power plants and utility devices.
But as we retire the old thermal plants and introduce more renewables, what will perform this balancing act?
Are renewables and batteries the new voltage guardians?
In theory, yes. Nothing prevents renewable plants or battery systems equipped with modern inverters from supplying reactive power and helping to balance and regulate it.
The real hurdle isn’t physics, it’s policy. Grid codes should require every new inverter-based resource to keep its reactive-power controls active even when its active power set-point drops to 0 MW, and either regulations or financial incentives should compel operators to provide voltage support.
Most inverters installed today are grid-following: they lock onto a strong, stiff voltage waveform that synchronous machines usually supply. As synchronous generation retires, the grid becomes less stiff, reducing the robustness and therefore the reliability of grid-following converters for voltage regulation.
The remedy is to accelerate the adoption of grid-forming (GFM) technology, which involves only a change in the converter’s control strategy. Grid-forming inverters create their own voltage reference, so they remain stable without synchronous generators and offer much more reliable voltage support.
For a deeper dive into grid-following versus grid-forming controls, check this article.
Conclusion
In short, voltage stability isn’t doomed by the retirement of synchronous machines; it just needs new guardians. Wind, solar and battery systems already possess the hardware to supply or absorb VARs.
Still, they must be equipped with grid-forming controls and integrated under grid codes that value voltage support as much as energy. With clear technical standards and market incentives, these inverter-based resources can step seamlessly into the reactive-power shoes once filled by coal and gas plants.
Do that, and the clean-energy grid keeps its voltage tight and its transition on track.
 | A guest post by
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Please be aware that voltage /reactive power is linked to the fault level, which means when there is low fault level the reactive power change will lead to bigger voltage variations, if you have a stronger grid, means higher fault level, the same amount of reactive power will influence the voltage less. In future we will also see more local voltage regulations. To support both renewables and batteries the synchronous condenser solution is VERY important as it can support system strength, MVAr´s and inertia and additional to that being a sink for unbalance and improve power quality.