Managing Voltage Instability in Older Power Grids

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

Major Grid Failure in South-East Europe: A Wake-Up Call for Power System Resilience On June 21, 2024, a severe grid incident in South-East Europe triggered widespread blackouts across Albania, Bosnia & Herzegovina, Montenegro, and Croatia, disrupting the interconnected Continental Europe power system. What happened? 1. The failure began with a short circuit on two 400 kV transmission lines, caused by vegetation proximity, leading to cascading outages. 2. Within minutes, the voltage collapsed, causing the loss of 2,214 MW of generation and a major blackout. 3. The restoration process took nearly four hours, relying heavily on cross-border coordination. Key Lessons for Grid Stability: a. Vegetation Management Matters: Both initial short circuits were caused by inadequate clearance, highlighting the need for better maintenance policies. b. Real-Time System Awareness is Critical: The N-1 security analysis failed to detect voltage instability, underlining the need for improved dynamic monitoring. c. Resilience in High-Renewable Grids: Air conditioning demand accounted for 30-35% of total load, making voltage stability more vulnerable in heatwaves. d. Cross-Border Coordination is Essential: The top-down restoration strategy worked, but slow communication between TSOs delayed recovery. What’s Next? The report recommends: 1. Revising vegetation control policies near high-voltage lines 2. Enhancing real-time grid observability to predict voltage collapses 3. Optimising reactive power compensation to prevent instability 4. Fast-tracking digital grid technologies to improve response times The incident serves as a reminder that grid modernisation must go beyond adding renewable generation, it requires stronger transmission networks, real-time monitoring, and better cross-border coordination. Together with Prof. Aoife Foley, Chair in Net Zero Infrastructure at The University of Manchester, we are working to find innovative solutions to manage power system events like this as we move toward net-zero targets. What do you think? How can power systems better prepare for grid contingencies? #PowerSystems #GridStability #EnergyTransition #NetZero #Blackout #Transmission #GridResilience #VoltageStability

⚡ Capacitor Banks in Power Systems – The Silent Hero of Grid Stability 👉 The Capacitor Bank As electrical engineers, we often focus on transformers, generators, and protection relays — but capacitor banks quietly play a critical role in maintaining system reliability and reducing operational costs. Let’s break it down. 🔹 Why Do We Need Capacitor Banks? Most industrial and utility loads (motors, pumps, compressors, HVAC, induction furnaces) are inductive in nature. Inductive loads: Consume Reactive Power (kVAR) Lower the Power Factor Increase current flow Cause voltage drops Increase system losses (I²R losses) Attract penalties from utilities Capacitor banks provide leading reactive power, which compensates the lagging reactive power of inductive loads. ✅ Result? Improved power factor Reduced line losses Improved voltage profile Increased system capacity Lower electricity bills 🔹 Types of Capacitor Banks Used in Power Systems 1️⃣ Low Voltage (LV) Capacitor Banks Installed in industries Typically 415V / 480V systems Automatic Power Factor Correction (APFC panels) Controlled through contactors or thyristors 2️⃣ Medium Voltage (MV) Capacitor Banks 6.6kV / 11kV / 33kV systems Installed at substations Switched via vacuum circuit breakers Often protected with unbalance relays 3️⃣ High Voltage (HV) Capacitor Banks 132kV and above Used in transmission systems Improve voltage stability over long lines 🔹 Protection of Capacitor Banks – Critical for Reliability Capacitor banks are sensitive equipment and require proper protection: 🔸 Overcurrent protection 🔸 Unbalance protection 🔸 Overvoltage protection 🔸 Inrush current control (reactors) 🔸 Harmonic filtering (detuned reactors) In systems with harmonic distortion (VFDs, UPS, converters), detuned capacitor banks are essential to avoid resonance conditions. 🔹 Real-World Impact in Power Plants & Substations From my experience in power generation environments: ✔ Proper reactive power management reduces transformer overloading ✔ Voltage regulation improves generator stability ✔ System losses significantly decrease ✔ Grid compliance becomes easier Capacitor banks are not just cost-saving devices — they are strategic grid assets #ElectricalEngineering #PowerSystems #CapacitorBank #PowerFactor #ReactivePower #GridStability #Substation #EnergyManagement #PowerPlant #ElectricalProtection #Transmission #Distribution #SmartGrid #RenewableEnergy #EngineeringLife #HighVoltage #IndustrialEngineering #EnergyEfficiency

The reactive-power voltage support a plant is required to provide can turn into a voltage oscillation when the grid connection is weak. Many teams see it for the first time during commissioning, not in the study. Here's the chain. A plant holds voltage by adjusting reactive power on a droop: voltage drops, push in reactive power, voltage recovers. That works cleanly when the grid at the connection point is stiff, meaning a high short-circuit ratio (SCR). When the connection is weak, with SCR down toward 2, the same reactive injection moves the voltage enough that the controller starts reacting to its own output, and the loop can ring instead of settle. The inverter's phase-locked loop (PLL), the part that tracks grid angle, makes this worse at low SCR because voltage and angle estimation stop being independent. A fast PLL then amplifies the ringing. There's no single setting that fixes it. A flatter droop and a slower PLL both add margin, and both cost something real: less voltage support, slower fault response. The right operating point depends on the actual grid strength at the connection, which is why it has to be studied dynamically rather than read off a steady-state curve. IEEE 2800-2022 now expects weak-grid behavior to be demonstrated, not assumed. A study that only shows steady-state reactive capability hasn't checked the case that fails.

Most engineers think voltage collapse starts with low voltage. It doesn't. The voltage can read 0.90 pu, perfectly "acceptable" while the system is already heading toward collapse. Here's what's actually happening: Every power system has a limit on how much load a bus can serve. That limit is the nose point of the P-V curve. Below that point, there is no stable high voltage solution. None. As load grows, voltage slowly sags. Normal operation. The operating point slides down the curve toward the nose. Then reactive support hits its limits. Generators max out their reactive output. Capacitor banks become less effective as voltage drops. The nose point shifts, closer to where the load already is. Now the system is past the limit. The voltage doesn't just sag. It collapses. What determines your margin: → Reactive support- generators on AVR, SVCs, STATCOMs, capacitor banks → Load power factor- more lagging load means a tighter nose and less margin → Tap changer coordination- done wrong, it can actually accelerate collapse → Under voltage load shedding- your last line of defense before the nose is reached The dangerous part: The system gives almost no warning. Operators may see a slow voltage drift and treat it as routine. Then one generator trips, one capacitor bank saturates, and the margin is gone. That is why reactive power planning, voltage stability studies, and Q margins matter far more than most people realize. The grid does not collapse because voltage is low. It collapses because the system runs out of room. Model: lossless Thevenin equivalent, constant power factor load, no AVR dynamics. illustrative. #PowerSystems #ElectricalEngineering #VoltageStability #GridResilience

⚡ 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