Voltage Stability Applications in Power Engineering

Everyone’s Worried About Insufficient FFR. But What Happens When The Problem Is Too Much…Voltage? As we transition to high inverter-based resources, frequency stability dominates headlines. Yet one of the most disruptive, and silent, threats to modern grids is overvoltage, especially in systems with high solar PV penetration. Voltage instability develops faster and more quietly than frequency instability. While frequency deviations are system-wide and trigger alarms, voltage issues are often more localised and can escalate rapidly, sometimes before system-wide alarms are triggered. What’s happening under the hood? ➤ Most grid-following inverters can exchange reactive power, but without proper headroom, settings, or coordination, they often fail to provide dynamic voltage support during disturbances. ➤ Under light load, long transmission lines behave like capacitors, injecting charging current (the Ferranti effect). ➤ When synchronous generators trip, the system loses critical reactive power sinks, weakening its ability to absorb excess vars. The Result? Rising voltages trigger protection relays, sometimes before frequency deviations begin. Clean Energy ≠ Stable Grid Overvoltage isn’t new, but phasing out synchronous machines (coal, gas, etc.) also removes inertia, voltage damping, and fault ride-through capability. Even if solar isn’t the root cause, the grid may lack the tools to mitigate minor disturbances before they cascade. The key question isn’t just what trips, it’s what stays online that determines whether a voltage cascade unfolds. What do we need now? ● Grid-forming inverters with reserved reactive power headroom and robust voltage control. ● Synchronous condensers for dynamic VAR absorption and system strength. ● FACTS devices (STATCOMs, SVCs) for fast, localised voltage regulation. Updated grid codes addressing overvoltage risks in high-VRE, low-demand scenarios. The gap may lie not in capability, but in implementation strategy, grid code enforcement, and system coordination. Case Study: When Inverters Don’t Trip In the modelling below, I forced a DFIG-based grid-following inverter to remain connected beyond its overvoltage threshold, emulating a scenario where, under low system strength, protection systems respond too slowly to isolate the fault. Rather than tripping offline as expected, the inverter stayed online: → Reactive power surged, → Active power spiked, and → Voltage oscillations spread across the system. This is one of the hidden fragilities of passive inverter behaviour, clean on paper, but unstable in practice when protection systems delay or inverters fail to disengage. The result? Small disturbances can escalate rapidly, turning a local issue into a system-wide event. Have you encountered overvoltage challenges in your grid? How is your region or market adapting its tools and standards to manage this risk? #PowerSystemStability #GridResilience #GridForming #GridCode #VoltageStability #IBR #FFR

While we await a more detailed technical report, we have come to understand that the main issue behind the Iberian blackout was something a bit less “sexy” than frequency stability: voltage stability. Interestingly, the trigger was not the “usual” undervoltage problem, but rather something less common in traditional power systems: overvoltage instability. In simple terms, voltage stability is a question of keeping a balance of reactive power (Q) throughout the system. A surplus or shortage of reactive power can both lead to instability. There are several reasons that can lead to a surplus of reactive power. •) Classic synchronous machines are less capable of operating under-excited (i.e., consuming reactive power, thus behaving like inductors) than overexcited (i.e., generating reactive power, thus behaving like capacitors), due to risks of terminals overheating and angular instability. •) High-voltage lines (and even more so, cables) when unloaded, behave like capacitors. The higher the voltage rises, the more reactive power (Q) they generate, following the relation: Q=V^2/Xc. With increasing distributed generation, the likelihood of high-voltage lines becoming unloaded rises. •) Tap changers in primary substations can also be misleading if not properly coordinated with local power injections, potentially leading to overvoltages. •) And finally, converter-connected resources (solar and wind) that lack proper requirements for reactive power support… simply won’t provide any. •) Add to that the possibility of some units (conventional or not) not fully complying with grid codes, and you have a recipe for a voltage instability. The good news? The fix is relatively straightforward, we don’t need new technologies. If inductive power is lacking, classic shunt reactors can be very effective (or STATCOMs, if you're looking for something more sophisticated). The tricky, and fascinating, aspect of reactive power is that it needs to be available in the right amount, at the right time, and in the right location, since we don’t like reactive power to travel. We prefer 0-km reactive power. Fortunately, voltage (and frequency) stability are key topics in our fall course on integration of wind power in the power system. I am looking forward to September, when a new edition begins. This time, we will have a new story to add to the book. PS: An example of grid code (Energinet) for wind and PV includes specific reactive power requirements for large units (>25 MW). DTU Wind and Energy Systems

After the recent big blackout and repeated voltage and frequency instability, Spain has permanently tightened its grid operation rules -enacted Royal Decree 997/2025 and a revised Operational Procedure 7.4 to enhance grid stability(*). The objective is cristal clear here: Protect system stability ⚡️💡⚠️ I think these implications will go far beyond Iberia. Because this is in contrast to those, not about limiting renewables, but about making high-renewable systems physically more stable, controllable, dispatchable and secure. Based on some FAQs and my personal operational experience, here are some highlighted technical questions and my simplified answers: Q1). What triggered Spain to change its grid operation rules? - The Spanish blackout and repeated voltage and frequency instability showed that a low-inertia, inverter-dominated grid becomes highly sensitive to disturbances and oscillations. Q2). Why is “fixed power factor operation” no longer safe? - Because when active power changes, reactive power automatically changes too, which creates voltage fluctuations that can spread across the grid and destabilize the system. Q3). What does “active voltage control” actually mean? - It means power plants must dynamically regulate voltage, not just follow fixed settings and actively support grid stability in real time. Q4). How are system operators responding to this risk? - By tightening scheduling rules, enforcing ramp-rate limits, activating reserves earlier and requiring dynamic grid support from renewables. Q5). Is this only happening in Spain? - No, as far as I could follow up, Germany and the Netherlands already apply similar measures through advanced voltage control, synchronous condensers and system-strength services. Q6). What is the real lesson from Spain? - High renewable penetration is possible, but only if grid stability, control systems and system strength evolve together with generation technology. Overall, stability(!) should come before megawatts⚠️⚡️💡 A secure energy transition depends on operating the grid within real-time physical limits, not just market signals. Dispatchability, system strength and active control must come first -capacity alone is not enough☘️ (*). https://lnkd.in/eaF9zXkE #Wind #RenewableEnergy #GridStability #EnergyTransition #PowerSystems #RenewableIntegration #EnergyMarkets #FossilFreeFuture

ENSTO-E Final Report on Iberian Blackout Published The European Network of Transmission System Operators for Electricity (ENTSO-E) published their final report on the April 2025 Iberian blackout. While I need more time with it, a quick scan shows that its all about voltage control. Here is a brief overview of the findings and recommendations. I’ve provided a link to the report and I encourage you to review it for yourselves. For me, the message is that voltage control and monitoring need to be top priorities for all transmission system operators as the operational characteristics of the system rapidly evolve with increasing inverter-based resources and, while not part of this incident, large dynamic data centre loads. Here are a few highlights: “The incident evolved through a sequence involving a combination of voltage fluctuations and oscillatory phenomena, leading to widespread generation disconnections in Spain, particularly inverter-based resources, followed by a cascade of overvoltage disconnections and culminating in the loss of synchronism of the Iberian system with the Continental Europe Synchronous Area.” “The increasing penetration of variable renewable and distributed generation, further market integration, broader electrification, and evolving environmental and geopolitical risks place the European electricity system under increasingly challenging operational conditions, requiring higher levels of resilience.” The report makes 23 recommendations, including the following 8 high priority recommendations: Voltage Control: ENTSO-E to develop a guideline of good practice on voltage support means and studies on voltage stability Voltage Control Mode: multiple recommendations including that generators use voltage control mode whenever possible and that TSOs should explore the possibility of a centralised or zonal voltage regulation Operating Voltage Range: TSOs should ensure that the harmonised operating voltage range foreseen at the European level is effectively applied across Europe Voltage Oscillations: Establish a framework to improve the damping of interarea oscillations in the Continental Europe Synchronous Area Voltage Oscillations: A common procedure should be established to create a snapshot common grid model of Continental Europe Synchronous Area promptly after a significant event Dynamic Monitoring & Oscillation Detection: Improve and expand the monitoring detection framework by efficiently using existing PMUs and oscillographs, or, where needed, installing additional PMUs, oscillographs, and power quality monitoring devices Generator Disconnections: Type A power-generating modules should be capable of stable operation without disconnecting from the grid for a voltage-versus-time profile. Restoration: Make realistic black-start tests mandatory and periodic, preferably every 3 years or after major control or protection changes https://lnkd.in/g-i8_cVb

⚡ The official report on the Iberian blackout confirms it was mainly a voltage instability event. The system had already experienced "intense voltage fluctuations" in the days before the incident. Wide-area oscillations prompted the system operator to increase grid meshing and reduce exports to France. These measures, unfortunately, decreased line flows, which paradoxically raised voltages due to the line charging effect, causing power plants to trip on over-voltage. This triggered a cascading failure, worsened by some plants tripping improperly before voltage limits were reached. The main conclusion from the report is a "lack of voltage control resources"; either they were poorly scheduled, or those allocated failed to provide sufficient power, despite an overall adequate generating capacity.   🔦 For the voltage control to be effective, it is important to consider the difference between high R/X and low R/X ratio systems. In high-voltage grids (transmission networks), which typically have a low R/X ratio, voltage magnitude is primarily sensitive to reactive power. Here, the voltage drop can be approximated by ignoring resistance and focusing on the reactive component. This is why traditional grid operators use reactive power to regulate voltage in these systems. Conversely, in low voltage (LV) systems and distribution networks, the high R/X ratio means voltage magnitude is more sensitive to active power injection. In these systems, the effect of resistance is significant, and the voltage drop approximation includes both active and reactive components. For instance, a PV plant can regulate voltage by reducing active power injection or providing negative reactive power, as per standards like IEEE 1547-2018. If reactive power alone is insufficient, active power control, which involves elements such as heat pumps, electric vehicles (EVs), or battery storage, may be necessary.   🪫 A notable point from the Iberian blackout report is the recommendation to "allow asynchronous installations to apply power electronics solutions to manage voltage fluctuations." This indicates that the voltage control capabilities of inverter-based resources (IBRs) were not fully utilised. Although IBRs offer considerable potential, challenges persist, particularly for real-time smart inverter Volt/Var Control (VVC). These include susceptibility to control instability caused by incorrect parameter selection, as smart inverter settings are sensitive to feeder configuration and operating conditions. An inappropriate droop (slope) setting can lead to control instability or voltage oscillations. There is an inherent trade-off between maintaining control stability and achieving accurate set-point tracking, which can cause voltage violations. Additionally, the non-adaptability of droop VVC to changing conditions can hinder deployment. #blackout #renewables #gridmodernization #powerelectronics #gridforming #voltage #cleanenergy