Glossary

Power grid glossary: the terms behind HVDC and interconnectors

Short definitions, each linked to the page where the idea does the most work.

Grid engineering has a habit of naming one thing three ways. A line’s ampacity, its thermal rating and its current-carrying capacity are the same limit seen by different people. The entries below settle on one plain definition each and point to where the term matters on this site.

Four families of terms

Most of the entries below belong to one of four families, and knowing which one a word comes from usually tells you what it measures.

Units and ratings. A megawatt is power at one moment; a megawatt-hour is energy over time. An interconnector is rated in megawatts, the most it can carry at once, while its yearly traffic is counted in gigawatt-hours or terawatt-hours. Voltage is given in kilovolts: 400 kV is common for AC transmission in Europe, and the DC links in the directory reach 600 kV on the Western HVDC Link. Ampacity and thermal limit describe the same ceiling from the side of the wire.

Direct current. HVDC has its own vocabulary because the equipment at each end is so different. A line-commutated converter (LCC) uses thyristor valves and needs a strong AC grid at both ends; a voltage-source converter (VSC) uses transistor valves such as IGBTs, can start up against a weak or dead grid and has become the standard for offshore wind. Of the links in the directory, 28 use VSC and 18 LCC. Monopole, bipole and metallic return describe how the conductors are arranged, and multi-terminal means more than two converter stations on one DC system.

Markets. Bidding zone, market coupling, net transfer capacity and flow-based market coupling belong to the trading side. They decide how much of a link’s physical capacity is offered to the market and in which direction power flows, which is why a cable’s rating and its commercial use are not always the same figure.

Who does what. A transmission system operator runs the high-voltage network in its area, a distribution system operator the lower voltages that reach homes. ENTSO-E is the association of Europe’s transmission operators and publishes the ten-year network development plan; ACER, based in Ljubljana, coordinates the national regulators.

Cable and line hardware makes up the rest: XLPE and mass-impregnated insulation, armour, cable joints, insulators and cross-arms. Where an entry points to another page, that page shows the term at work, from a converter station to a line sagging towards the trees.

A

ACER: The European Union Agency for the Cooperation of Energy Regulators, based in Ljubljana. It coordinates the work of national energy regulators, decides on cross-border questions where they cannot agree, monitors the internal electricity and gas markets, and gives opinions on network plans such as the TYNDP. Its full name explains its job: it regulates the regulators' cooperation, not the companies directly.
Active power: The part of electrical power that does useful work, such as turning a motor, heating a kettle or charging a battery. It is measured in watts and its multiples, megawatts and gigawatts. When an interconnector is described as carrying 1,000 MW, the figure refers to active power. Its counterpart, reactive power, moves back and forth without being consumed.
Alternating current (AC): An electric current that reverses direction many times a second, following a regular wave. In Europe the grid alternates at 50 Hz. Alternating current is used across almost all of the power system because transformers can raise or lower its voltage easily, which makes it cheap to distribute. Its weakness is long cables, where much of the current is spent charging the cable itself. HVDC explained
Ampacity: The largest current a conductor can carry continuously without exceeding its safe temperature. For an overhead line the limit is set by how hot the wire may become before it sags too close to the ground or loses strength; for a cable, by what the insulation can tolerate. Ampacity depends on the surroundings, so wind, sunshine and air temperature all change it. Repowering transmission lines
Ancillary services: The services a transmission system operator buys to keep the grid stable, apart from the bulk energy itself. They include frequency reserves, voltage and reactive power control, black start capability and, increasingly, inertia and short-circuit strength. Power plants, batteries, large consumers and HVDC converters can all provide some of them, usually under contracts or market rules set by each operator.
Armour: The layer of steel or other metal wires wound around a subsea cable to protect it from anchors, fishing gear and the strain of being laid from a ship. Armour adds weight and some electrical losses, and its design is a trade-off between strength and cost. Cables in shallow, busy waters are usually buried in the seabed as well as armoured. Subsea power cables guide
Array cable: The medium-voltage cables that connect the individual turbines of an offshore wind farm to each other and to its offshore substation. Turbines are usually strung along the cables in rows or loops. Array cables are shorter and lighter than export cables, but a wind farm has many kilometres of them. HVDC and offshore wind
Automatic frequency restoration reserve (aFRR): A balancing reserve that brings the grid frequency back to its target value after a disturbance, and frees the faster containment reserve for the next event. It is activated automatically by a central controller at the system operator and responds within minutes. Across much of Europe it is now procured and exchanged through a shared platform known as PICASSO.

B

Back-to-back converter: Two HVDC converters built in the same station and joined by a very short DC connection, with no cable or line in between. It is used to link two AC grids that are not synchronised, or to control the flow of power precisely between two that are. The Kriegers Flak Combined Grid Solution between Denmark and Germany uses one. Kriegers Flak
Balance responsible party (BRP): A market participant that is financially responsible for the difference between the electricity it has scheduled and what its portfolio actually produces or consumes in each settlement period. Suppliers, generators and traders are typically balance responsible. Any gap is settled at the imbalance price, which gives every party a reason to forecast carefully and keep its position even.
Balancing: The continuous work of keeping electricity generation and consumption equal, second by second. Because a power grid stores almost no energy, any mismatch shows up at once as a change in frequency. Transmission system operators correct it with reserves that can raise or lower output quickly, and they charge the cost of imbalances to the market parties that caused them.
Balancing service provider (BSP): A market participant, such as a power plant, battery operator, aggregator or large consumer, that offers balancing energy or capacity to a transmission system operator. It must pass a prequalification test to show that its units can respond as quickly and reliably as the service requires.
Bidding zone: The largest geographical area within which electricity is traded at one wholesale price, without taking internal grid limits into account. Many European countries form a single zone; Sweden, Norway, Denmark and Italy are divided into several. Where power cannot flow freely between zones, prices separate. The size of bidding zones is a recurring political argument, especially in Germany. Grid congestion guide
Bipole: An HVDC system with two conductors at opposite polarity, one positive and one negative, relative to earth. Each pole can usually carry about half of the power, so if one fails the other keeps part of the link running. Many of the largest HVDC links are bipoles, since this layout halves the loss of capacity from a single fault. HVDC explained
Black start: Restarting a power system after a total or partial blackout without drawing on the grid itself. A black start begins at plants that can start on their own, usually hydropower or gas turbines with their own supply, and rebuilds the network island by island. Some voltage source converter HVDC links can help energise a dead grid. The Iberian blackout
Busbar: A thick conductor, often a metal bar or tube, inside a substation that connects the incoming and outgoing lines, transformers and other equipment at one voltage level. Circuit breakers and disconnectors sit between the busbar and each connection, so that faulty equipment can be isolated without taking the whole substation out of service.

C

Cable joint: The connection that splices two lengths of power cable together. On long subsea routes the cable is manufactured and laid in sections, and every joint has to restore the insulation, screening and armour of the original cable. Joints are made by hand, often at sea on a laying vessel, and they are among the most common points of failure. Subsea power cables guide
Cap and floor: The regulatory regime used in Great Britain for most new interconnectors. The regulator, Ofgem, sets a maximum and a minimum revenue for the link over a long period. If market income rises above the cap, the surplus goes back to consumers; if it falls below the floor, consumers top it up. The scheme shares risk between developers and bill payers. Power links in Europe
Capacity calculation: The process by which transmission system operators work out how much power can safely flow between bidding zones for each hour or quarter-hour of the next day. It accounts for expected flows, outages and security margins. The result, expressed either as net transfer capacities or as flow-based parameters, is handed to the market coupling algorithm. Grid congestion guide
Capacity factor: The electricity a plant actually produces over a period, divided by what it would produce running at full power the whole time. It shows how much of its rated capacity a wind farm, solar park or nuclear station really delivers. Weather-dependent sources have lower capacity factors than plants that can run on demand, so capacity alone is a poor guide to output.
Capacity mechanism: A scheme that pays power plants, storage or demand response for being available when needed, in addition to what they earn by selling energy. Governments use capacity mechanisms to secure resource adequacy. EU rules require them to be temporary, open to cross-border participation and approved as state aid.
Charging current: The current needed to charge the capacitance of a cable or line every time an alternating voltage reverses. In long AC cables it grows with length until it takes up most of what the cable can carry, leaving little room for useful power. This is the main reason long subsea connections use direct current, where charging happens only once. HVDC explained
Circuit breaker: A switch designed to interrupt current, including very large fault currents, safely and within a fraction of a second. In AC systems the current passes through zero twice in every cycle, which helps the breaker extinguish the arc. Direct current has no natural zero, so DC circuit breakers are much harder to build. Upgrading multi-terminal HVDC
Commutation failure: A fault in a line-commutated HVDC converter when a thyristor valve fails to hand over current to the next valve at the right moment, usually because the AC voltage has dipped. The converter briefly short-circuits on the DC side and power transfer is interrupted. Commutation failures are a known weakness of LCC links connected to weak AC grids. HVDC explained
Composite insulator: An insulator made from polymer materials, typically a fibreglass core with a silicone rubber housing, in place of traditional porcelain or glass. Composite insulators are lighter, resist vandalism and perform well in polluted air. They also make new tower designs possible, such as insulated cross-arms that let a line carry a higher voltage without taller towers. Repowering transmission lines
Conductor: The wire or bundle of wires that actually carries current on an overhead line, usually strands of aluminium wound around a steel or composite core. The conductor's material and cross-section set how much current the line can carry and how much it sags when hot. Replacing it with a better one is the basis of reconductoring. Repowering transmission lines
Congestion: A situation in which the grid cannot carry all the power the market wants to move, because a line, cable or transformer would exceed its limit. Congestion between bidding zones shows up as a price difference. Inside a zone it is handled by redispatch, where the system operator pays some plants to reduce output and others to increase it. Grid congestion guide
Congestion income: The money earned when electricity flows from a cheaper bidding zone to a more expensive one through a congested connection. It equals the price difference multiplied by the flow. Under EU rules it must be spent on keeping cross-border capacity available or on new capacity, or used to lower network tariffs. For merchant interconnectors it is the main source of revenue. Power links in Europe
Connecting DSO: The distribution system operator whose network a generator, consumer, non-TSO interconnector or other grid element is physically connected to. The term appears in the European network codes, which set out what information and technical behaviour the connecting operator may require, and how it must coordinate with the transmission system operator above it.
Connecting Europe Facility (CEF): The EU funding programme for trans-European infrastructure in transport, energy and digital networks. Its energy strand co-finances studies and works for projects of common interest and mutual interest, as well as cross-border renewable energy projects. Grants are awarded through calls managed by the executive agency CINEA. Power links in Europe
Connecting TSO: The transmission system operator that runs the area in which a balancing service provider or balance responsible party operates, and whose balancing terms and conditions that party must follow. The European network codes use the term to make clear which operator's rules apply when a market participant is active across several control areas.
Connection transmission system operator: In the European balancing rules, the single transmission system operator responsible for the area in which a balancing service provider or balance responsible party has to comply with the balancing terms and conditions. In practice it identifies whose rules, prequalification process and settlement a party follows, even if its assets touch more than one area.
Contingency: The loss of a single element of the power system, such as a line, cable, transformer or generator, whether through a fault or a planned outage. System operators keep lists of credible contingencies and check continuously that the grid would stay within its limits if any one of them occurred. This check underpins the N-1 criterion.
Control interaction: Unwanted behaviour that arises when the control systems of nearby converters, such as HVDC stations, wind farms and FACTS devices, respond to each other in ways their designers did not intend. It can lead to oscillations or trips. Studies with detailed models and real-time simulation are used to find and prevent it. Converters from different makers
Converter station: The installation at each end of an HVDC link that converts alternating current to direct current or back again. It contains the valves of thyristors or transistors, converter transformers, filters, cooling, controls and protection. Converter stations are the largest and most expensive part of most HVDC projects, and they are where most of the losses occur. HVDC explained
Converter transformer: A large transformer that connects an HVDC converter to the AC grid and adapts the grid voltage to the level the converter needs. It must withstand the combined stress of alternating and direct voltages and harmonic currents, which makes it more complex than an ordinary power transformer. Its delivery time often sets the schedule of a project. HVDC explained
Critical temperature: The temperature below which a material becomes superconducting and carries direct current without electrical resistance. Conventional superconductors need to be cooled close to absolute zero. Magnesium diboride works at somewhat higher temperatures, which can be reached with liquid helium or cryocoolers, while high-temperature superconductors can operate with liquid nitrogen. Superconducting cables
Cross-border cost allocation (CBCA): A decision by the national regulators concerned on how the cost of a cross-border project of common interest is shared between countries. It usually follows the benefits each country is expected to receive, not simply where the equipment is built. If the regulators cannot agree within the deadline, the decision passes to ACER.
Cross-linked polyethylene (XLPE): A plastic insulation material made by chemically linking the molecules of polyethylene, which makes it tougher and able to operate at higher temperatures. XLPE cables are extruded in one continuous process. They are now standard for AC cables and for most new HVDC cables used with voltage source converters. Subsea power cables guide
Cross-zonal capacity: The transmission capacity made available to the market for trade between two bidding zones, in each direction and for each market time unit. It can be lower than the physical rating of the interconnectors because operators reserve margins for security and for flows that cross the border without being traded, known as loop flows. Grid congestion guide
Cryogenic envelope: The insulated, vacuum-jacketed tube that surrounds a superconducting cable and keeps it at its very low operating temperature. It works on the same principle as a vacuum flask, limiting the heat that leaks in from outside. The cooling fluid circulates inside it, and refrigeration stations along the route remove the heat that does get through. Superconducting cables
Current: The flow of electric charge through a conductor, measured in amperes. In grid documents, current usually means the root-mean-square value of the phase current, a kind of effective average for an alternating wave. The current a line can carry is limited mainly by heat, and the same line can carry more on a cold, windy day.
Curtailment: Deliberately reducing the output of a generator, usually wind or solar, below what it could produce. It happens when the grid cannot take the power, because a line is congested or demand is too low, or when prices turn negative. Curtailed energy is lost. Its volume is one of the clearest signs that a grid has not kept pace with new generation. Grid congestion guide

D

Day-ahead market: The market in which most electricity in Europe is bought and sold for delivery on the following day, in hourly or quarter-hourly blocks. Bids close around midday, and a single algorithm then matches supply and demand across the coupled European zones, taking cross-border capacity into account. The resulting prices are the main reference for the whole wholesale market.
DC circuit breaker: A device that can interrupt direct current quickly enough to isolate a fault on an HVDC line or cable. Because direct current has no natural zero crossing, the breaker must create one, using power electronics, resonant circuits or both. Reliable DC breakers are one of the conditions for building meshed HVDC grids with many terminals. Upgrading multi-terminal HVDC
DC grid: A network of HVDC links connected at more than two points, so that power can flow between several converters and along more than one path. A meshed offshore DC grid could connect wind farms and countries around the North Sea. Building one requires DC circuit breakers, common standards and converters from different makers that can work together. Converters from different makers
Demand response: Changing electricity consumption in reaction to a price signal or an instruction from the grid operator. An industrial plant may pause a process, a cold store may let its temperature drift, or a household heat pump may shift its running time. Demand response adds flexibility to the system without building new generation or storage.
Dielectric: An electrically insulating material that can withstand a strong electric field without conducting. In power cables the dielectric is the insulation between the conductor and the earthed screen, such as cross-linked polyethylene or oil-impregnated paper. How the dielectric behaves under direct voltage, where charge can accumulate, is one of the main design questions for HVDC cables. Subsea power cables guide
Direct current (DC): An electric current that flows steadily in one direction. Batteries and solar cells produce it, and HVDC links use it to carry large amounts of power over long distances, especially under the sea, because a DC cable does not need to be recharged on every cycle. Converting between direct and alternating current requires a converter station at each end. HVDC explained
Disconnector: A switch that opens a visible gap in a circuit so that equipment can be isolated safely for maintenance. Unlike a circuit breaker, a disconnector is not designed to interrupt load or fault current and is operated only when no current flows. Its open position is a guarantee to workers that a section is dead.
Distribution network: The part of the grid that takes electricity from the transmission system and delivers it to homes, businesses and small generators, at medium and low voltages. Distribution networks were built for power flowing one way, down to consumers. Rooftop solar, heat pumps and electric vehicle charging now send power in both directions and strain parts of them.
Distribution system operator (DSO): The company responsible for running, maintaining and developing the distribution network in a given area, including its connections to other networks, and for ensuring that the network can meet reasonable demand over the long term. Some countries have one or a few, others have hundreds of small municipal operators.
Dynamic line rating (DLR): A method of setting an overhead line's capacity from real-time measurements or forecasts of the weather and the conductor itself, in place of fixed, conservative assumptions. Wind cools a conductor and lets it carry more current, so a line often has spare capacity exactly when wind farms are producing most. Sensors, weather models or both provide the data. Repowering transmission lines

E

Earth electrode: A buried or submerged installation that lets an HVDC link pass current into the ground or sea, used by monopolar links with earth return and by some bipoles during a pole outage. Electrodes must be sited and designed to avoid corroding nearby pipelines and to limit effects on marine life. Many newer links use a metallic return conductor. HVDC explained
Electricity balancing platform: A shared European system through which transmission system operators exchange balancing energy, so that reserves offered in one country can meet an imbalance in another. The main platforms are PICASSO for automatic frequency restoration reserve, MARI for manual reserve and TERRE for replacement reserve.
Electricity market time unit (MTU): The period for which prices are set and positions settled in an electricity market. European day-ahead and intraday markets have moved from hourly to 15-minute units, which match the balancing settlement period in most countries and let the market follow fast changes in solar output more closely.
Electrolyser: A device that uses electricity to split water into hydrogen and oxygen. Powered by renewable electricity, it produces hydrogen without direct carbon emissions. Large electrolysers are significant loads on the grid, and because many can vary their output, they may help absorb surplus wind and solar power if they are built in the right places.
Energy highway: A term used by the European Commission for priority cross-border infrastructure corridors. In its European Grids Package of December 2025 it named eight of them, including the Pyrenean crossings between Spain and France, the Great Sea Interconnector to Cyprus, Harmony Link between Poland and Lithuania and the Bornholm Energy Island. Power links in Europe
Energy island: An artificial island or offshore platform that collects power from many surrounding wind farms and sends it on to more than one country through HVDC cables. It combines offshore connection and interconnection in one hub. Belgium's Princess Elisabeth Island and the Danish Bornholm Energy Island project are the best-known European examples. Energy islands guide
ENTSO-E: The European Network of Transmission System Operators for Electricity, which groups the transmission system operators of the EU and several neighbouring countries. It drafts network codes, prepares the ten-year network development plan and adequacy assessments, coordinates operational security and publishes a large transparency platform of grid and market data.
Export cable: The cable that carries power from an offshore wind farm's substation to the shore and on to a grid connection point. Near-shore wind farms usually use AC export cables; farms far from the coast increasingly use HVDC, because long AC cables lose too much capacity to charging current. Export cables are a single point of failure for the whole farm. HVDC and offshore wind
Extra-high voltage (EHV): The highest voltage levels used for transmission, generally from 220 kV upwards. Europe's main transmission backbone runs at 380 kV or 400 kV, with some parts at 220 kV and a few lines and cables above 400 kV. Higher voltages carry the same power with less current, so losses and the number of lines needed both fall. Repowering transmission lines

F

Fault ride-through: The ability of a generator or converter to stay connected and keep supporting the grid during a short voltage dip caused by a fault elsewhere. Grid codes require it because, without it, a single fault could make many wind or solar plants disconnect at once, turning a small disturbance into a large loss of generation.
Flexible AC transmission system (FACTS): A family of power-electronic devices that control voltage, reactive power or power flow on AC networks. Examples include static VAR compensators, STATCOMs and series compensation. FACTS devices let operators push more power through existing lines and steer flows away from congested routes without building new ones. Repowering transmission lines
Flow-based market coupling: A way of allocating cross-border capacity in which the market algorithm models how each trade actually spreads across the meshed grid, instead of treating each border as a separate pipe. It is used in the Core region of central and western Europe and in the Nordic countries. It usually frees more capacity than fixed limits, at the cost of transparency. Grid congestion guide
Frequency: The number of times per second that alternating current completes a cycle, measured in hertz. Continental Europe, Great Britain, Ireland and the Nordic countries all run at 50 Hz. Frequency falls when demand exceeds generation and rises when there is a surplus, so it is the grid's most immediate signal of balance.
Frequency containment reserve (FCR): The fastest balancing reserve, which responds automatically within seconds to stop a change in frequency after a sudden loss of generation or load. It halts the fall or rise but does not restore the frequency to its target; slower reserves do that. It is shared across a synchronous area, and batteries now supply a growing part of it.

G

Gigawatt (GW): A unit of power equal to one thousand megawatts, or one billion watts. Large nuclear reactors and the biggest interconnectors are measured in gigawatts. A gigawatt describes a rate of flow at one moment; the energy delivered over time is measured in gigawatt hours or terawatt hours.
Grid code: The set of technical rules that anyone connecting to a grid must follow, covering how generators, consumers and converters behave in normal operation and during faults. In the EU, network codes adopted as regulations set common requirements, which each operator then details nationally. Grid codes are where new duties such as fault ride-through and grid-forming behaviour are written down.
Grid connection queue: The list of projects waiting for a connection to the transmission or distribution network, usually in order of application. Queues have grown very long in several European countries as cheap wind, solar and battery projects apply faster than networks can be reinforced. Some projects in them are speculative, which makes the queue harder to manage.
Grid-following: The usual control mode of inverters and converters, in which the device measures the grid's voltage and frequency and injects current in step with them. A grid-following converter needs a strong grid to follow. As conventional power plants close, a grid made mostly of grid-following devices becomes harder to keep stable. The Iberian blackout
Grid-forming: A converter control mode in which the device sets its own voltage and frequency, behaving much like a synchronous generator. Grid-forming converters can provide synthetic inertia, support weak grids and help restart a system after a blackout. Several European operators now require or pay for grid-forming capability from new batteries and HVDC links. The Iberian blackout
Gross final energy consumption: The energy supplied to final users in industry, transport, households, services and agriculture, including electricity and heat, plus network losses and the energy the power sector consumes itself. It is the base against which EU renewable energy targets are measured, so a target covers heat and transport as well as electricity. Renewables by country
Guarantee of origin (GO): An electronic certificate proving that one megawatt hour of energy was produced from a particular source, most often renewable. Guarantees of origin can be traded separately from the electricity itself and across borders within Europe. A supplier or company cancels them to show that the electricity it sells or uses is renewable.

H

Hardware-in-the-loop: A test method in which real control equipment is connected to a computer that simulates the power system around it in real time. The controls behave as they would on the actual grid, so engineers can test faults and extreme conditions safely. It is widely used to check that converters from different manufacturers will work together. Converters from different makers
Harmonics: Currents or voltages at whole multiples of the grid frequency, which distort the ideal sine wave. Power-electronic equipment, including older HVDC converters, generates them. Harmonics cause heating and interference, so converter stations include filters to absorb them. Modern modular multilevel converters produce a much cleaner waveform and need far less filtering. HVDC explained
High-temperature low-sag conductor (HTLS): An overhead line conductor designed to operate at much higher temperatures than standard aluminium-steel wire without sagging further. Its core is made of materials such as carbon fibre composite or special alloys that expand very little when hot. Replacing an existing conductor with HTLS can raise a line's capacity substantially while keeping the same towers. Repowering transmission lines
High-voltage alternating current (HVAC): Transmission of electricity as alternating current at high voltage, the standard method for almost all overhead lines and short cables in Europe. It is simple to transform and switch and forms meshed networks easily. Over long cable distances, charging current limits it, which is where high-voltage direct current takes over. HVDC explained
High-voltage direct current (HVDC): Transmission of electricity as direct current at high voltage, typically hundreds of kilovolts. HVDC suits long subsea cables, very long overhead lines and connections between grids that are not synchronised, and it gives precise control of the power flow. The price is a converter station at each end, which is costly and adds losses. HVDC explained
Hosting capacity: The amount of new generation or load that a part of the grid can accept without breaching voltage, thermal or protection limits. Distribution operators publish hosting capacity maps so developers can see where connections are easier. Low hosting capacity is behind many long waits for solar and battery connections.
HVDC converter station: The part of an HVDC system at one location that contains one or more converter units together with buildings, reactors, filters, reactive power devices, controls, monitoring, protection, measurement and auxiliary equipment. It forms the boundary between the direct current link and the alternating current grid on each side. HVDC explained
Hybrid interconnector: A cable or link that serves two purposes at once: connecting offshore wind farms to shore and linking two countries' grids. When the wind is low, the spare capacity is used for trade. The Kriegers Flak Combined Grid Solution between Denmark and Germany is usually described as the first of its kind. Hybrids need special market rules because the wind farms and the traders compete for the same cable. Kriegers Flak

I

IGBT: Insulated-gate bipolar transistor, a semiconductor switch that can be turned both on and off by a control signal. Thousands of IGBTs, stacked in modules, make up the valves of voltage source converters. Because they can switch off at will, VSC links do not depend on the AC grid to commutate and can connect to weak or dead networks. HVDC explained
Imbalance: The difference between what a balance responsible party scheduled and what its portfolio actually produced or consumed in a settlement period. The system operator covers imbalances in real time with reserves and charges the party at the imbalance price. Imbalance prices can be very high at times of shortage, which is a strong reason to forecast wind and demand well.
Inertia: The kinetic energy stored in the rotating masses of generators and motors that turn in step with the grid. After a sudden loss of generation, inertia slows the rate at which frequency falls, buying time for reserves to respond. Wind, solar and batteries connect through converters and provide no natural inertia, although grid-forming controls can imitate it. The Iberian blackout
Insulated cross-arm: A tower arm made of insulating composite material, which holds the conductor without a long hanging string of insulators. Because the conductor hangs closer to the tower, the line takes up less space, or the same tower can carry a higher voltage. It is one of the techniques for raising the capacity of existing corridors. Repowering transmission lines
Insulator: A component that holds a live conductor while preventing current from flowing to the tower, the ground or other conductors. Overhead lines use strings of glass or porcelain discs or polymer composite rods. Their length must suit the line voltage and the local pollution level, since salt or dust on the surface can lead to flashover. Repowering transmission lines
Interconnection target: The EU goal that each member state should be able to import electricity equal to a set share of its own installed production capacity from its neighbours. The target was 10 % for 2020 and is 15 % for 2030. The Commission tracks progress country by country. Several countries on the edges of the continental grid remain below it. Power links in Europe
Interconnector: A transmission line or cable that crosses or spans a border between countries and connects their national transmission systems. Most of Europe's newer interconnectors are HVDC cables under the sea; many older ones are AC overhead lines across land borders. Interconnectors let countries trade electricity, share reserves and help each other in emergencies. Power links in Europe
Interoperability: The ability of equipment from different manufacturers to work together in one system without unwanted interactions. For HVDC it means that converters and their controls from separate suppliers can share a DC grid safely. Without it, any expansion of a multi-terminal system would be locked to the vendor who built the first part. Converters from different makers
Intraday market: The market in which electricity can be traded after the day-ahead auction closes and up to shortly before delivery. It lets generators and suppliers correct their positions as forecasts of wind, sun and demand improve. In Europe it runs mainly as continuous trading across coupled zones, supplemented by intraday auctions.
Inverter: A device that converts direct current into alternating current. Solar systems and batteries connect to the grid through inverters, and in an HVDC link the converter at the receiving end works as an inverter. How the inverter is programmed decides how the plant responds to faults, frequency changes and voltage dips.
Islanding: The separation of part of a power system from the rest, so that it continues to operate on its own. It can be planned, as a defence to protect a healthy area during a large disturbance, or unplanned, as a result of cascading trips. An island must balance its own generation and load, or it will collapse. The Iberian blackout

K

Kilovolt (kV): A unit of voltage equal to one thousand volts. Household supply in Europe is about 230 volts, distribution networks run at tens of kilovolts, the transmission backbone mostly at 220 kV or 380 to 400 kV, and large HVDC links at several hundred kilovolts. For the same power, higher voltage means lower current and smaller losses.

L

Landfall: The point where a subsea cable comes ashore and joins its land section. Cables are often pulled through ducts drilled under the beach and dunes, to protect the coastline and the cable. The landfall and the onshore route to the converter station or substation can be as difficult to permit as the offshore part. Subsea power cables guide
Line-commutated converter (LCC): The original type of HVDC converter, built with thyristor valves that can be switched on but rely on the AC grid voltage to switch off. LCC links carry very large power with low losses and remain in use for bulk transmission. They need a strong AC grid at both ends, absorb reactive power and cannot easily start a dead network. HVDC explained
Load: The electricity drawn from the grid by consumers at a given moment, or a device that draws it. System operators forecast load closely because every megawatt consumed must be generated at the same instant. Load follows daily and seasonal patterns, with peaks on winter evenings in most of northern Europe and, increasingly, summer afternoons in the south.
Loop flow: Power that flows through a neighbouring country's grid because of a trade or generation pattern elsewhere, following the laws of physics, not the commercial contract. A sale from northern to southern Germany, for example, can partly flow through Poland or the Netherlands. Loop flows reduce the capacity neighbours can offer for their own cross-border trade. Grid congestion guide
Losses: The part of the electricity that is turned into heat in lines, cables, transformers and converters on its way from generator to consumer. Losses rise with the square of the current, the reason transmission uses high voltages. HVDC links lose less over long distances than AC, but their converter stations add a fixed loss at each end. HVDC explained

M

Magnesium diboride (MgB2): A superconducting compound of magnesium and boron, discovered to be a superconductor in 2001. It is made from cheap, plentiful elements and can be drawn into wires, and it becomes superconducting at a temperature that can be reached with liquid helium or cryocoolers. It was the material of the superconducting cable demonstration in the BEST PATHS project. Superconducting cables
Manual frequency restoration reserve (mFRR): A balancing reserve activated on instruction from the system operator, usually within about a quarter of an hour, to relieve faster reserves and handle larger or longer imbalances. In Europe mFRR is increasingly exchanged between countries through a common platform known as MARI, so that the cheapest offers can be used wherever they are.
Market coupling: The joint operation of neighbouring electricity markets by one algorithm, which allocates cross-border capacity implicitly by clearing energy bids across all the zones together. Power then flows automatically from cheaper to more expensive zones until the connections are full. Single day-ahead coupling now covers most of the EU and Norway. Power links in Europe
Mass-impregnated cable (MI): A traditional type of HVDC cable insulated with layers of paper soaked in a viscous oil compound, with no pressurised fluid system. It has a long record on subsea links, including very deep and very long routes, and handles polarity reversal, so it suits line-commutated converters. Newer projects often choose extruded XLPE insulation instead. Subsea power cables guide
Megawatt (MW): A unit of power equal to one million watts. It is the usual unit for the capacity of power plants, wind farms and interconnectors: a single large offshore wind turbine at several megawatts, a typical cross-border HVDC cable at several hundred to a few thousand. Energy over time is counted in megawatt hours.
Merchant interconnector: An interconnector built and financed by a private company that earns its revenue from congestion income, not from regulated network tariffs. Merchant links take the market risk themselves and usually need an exemption from certain EU rules on the use of revenues. ElecLink through the Channel Tunnel is a well-known European example. ElecLink
Meshed offshore grid: A network at sea in which wind farms, energy hubs and countries are connected by several links, so power can take more than one route. Compared with radial connections, a meshed grid uses cables more fully and can trade power when the wind is low. It depends on DC circuit breakers, common standards and multivendor interoperability. Converters from different makers
Metallic return: A dedicated low-voltage conductor that carries the return current of an HVDC monopole, in place of sending it through the earth or sea. It avoids the corrosion and environmental questions of earth electrodes at the cost of an extra cable. In a bipole, a metallic return path can keep one pole running when the other is out of service. HVDC explained
Minimum cross-zonal capacity: The EU rule that transmission system operators must make at least 70 % of the capacity of critical network elements available for cross-border trade, after accounting for security margins. It was introduced by the 2019 Electricity Regulation to stop operators restricting exports to solve internal congestion. Several countries use temporary derogations or action plans to reach it. Grid congestion guide
Modular multilevel converter (MMC): The dominant design for modern voltage source converters. Hundreds of identical submodules, each with its own switches and capacitor, are stacked in every valve arm and switched in sequence to build up an almost smooth voltage wave. The result is low losses, very little need for filters and good control over both active and reactive power. HVDC explained
Monopole: An HVDC system with a single high-voltage conductor, returning current either through the earth or sea or through a separate low-voltage metallic conductor. Monopoles are cheaper than bipoles but lose all their capacity if the single pole fails. Many early subsea links were monopoles, and some were later extended into bipoles. HVDC explained
Multi-terminal HVDC: An HVDC system with more than two converter stations connected to the same DC network, so that power can enter or leave at several points. It is the step between point-to-point links and a full DC grid. Italy's SACOI link between Sardinia, Corsica and the mainland is one of the few long-running examples in Europe. Upgrading multi-terminal HVDC

N

N-1 criterion: The planning and operating rule that a power system must remain within its safe limits after the loss of any single element, such as a line, transformer or generator. Every contingency is checked, and flows are kept low enough that the grid can absorb the loss. The criterion is the reason lines normally run well below their thermal limits.
National regulatory authority (NRA): The independent body that regulates energy networks and markets in each country, such as Ofgem in Great Britain, the Bundesnetzagentur in Germany or CRE in France. It approves tariffs, network investment plans and market rules, and decides with its neighbours on the cost sharing of cross-border projects.
Net transfer capacity (NTC): A simple measure of how much power can be exchanged between two zones in one direction, after subtracting security margins from the total transfer capacity. It treats each border as a fixed pipe and ignores how flows spread through a meshed grid. It remains in use on many European borders outside the flow-based regions. Grid congestion guide

O

Offshore bidding zone: A bidding zone created at sea for wind farms connected to a hybrid interconnector or energy hub, so that their power is priced by the flows the hub can actually carry. The idea is to stop offshore generators being paid the onshore price for power that cannot reach shore. Its effect on wind farm revenues is disputed. Energy islands guide
Offshore grid connection system: The complete link between an offshore connection point, such as a wind farm's substation, and the point where it joins the onshore interconnected system. It covers the offshore platform equipment, export cables, landfall and any onshore cable or converter. In some countries the TSO owns it; in others the wind farm developer builds it. HVDC and offshore wind
Offshore substation: A platform at sea that collects power from the array cables of a wind farm and raises its voltage for transmission to shore. For an AC connection it holds transformers and switchgear; for an HVDC connection it also carries a converter, which makes it far larger. Its topside can weigh thousands of tonnes and is installed by heavy-lift vessels. HVDC and offshore wind
Offshore wind farm: A group of wind turbines installed at sea, on fixed foundations in shallower water or on floating platforms in deeper water. Winds at sea are stronger and steadier than on land, so output is higher. The power is gathered by array cables at an offshore substation and sent to shore through export cables. HVDC and offshore wind
Overhead line: A transmission or distribution line whose conductors are carried on towers through the open air, using the air itself as insulation. Overhead lines are much cheaper than underground cables for the same capacity and easier to repair, but they face stronger local opposition. Most of Europe's high-voltage AC transmission network is overhead. Repowering transmission lines

P

Phase: One of the three conductors of a three-phase AC system, or the timing of an alternating wave relative to another. The three phases carry currents offset by a third of a cycle each, which delivers smooth power with less conductor material. The phase angle between two points on a grid also determines how much power flows between them.
Phase-shifting transformer (PST): A special transformer that changes the phase angle between the voltages on its two sides, and so controls how much active power flows along a particular line. Operators use phase shifters on borders to limit unplanned loop flows through their grids. Belgium, the Netherlands, Poland and others have installed them on their interconnections. Grid congestion guide
Point-to-point link: An HVDC connection with a converter station at each end and nothing in between, carrying power from one AC grid to another. Almost all HVDC links built so far are point-to-point. They are simple to control and protect, but they cannot share capacity with other links the way a multi-terminal system or a DC grid can. HVDC explained
Pole: In HVDC, one conductor at a given polarity together with its converter equipment. A monopole has one, a bipole two, positive and negative. Links such as Skagerrak between Norway and Denmark have been extended over the decades by adding poles, and their capacity is often described pole by pole. HVDC explained
Power flow: The pattern in which active and reactive power moves through a network, set by the physics of the grid: impedances, voltages and phase angles. In a meshed AC network power spreads over all available paths, whatever the commercial contracts say. Power flow calculations are the basic tool of grid planning and security analysis.
Power purchase agreement (PPA): A long-term contract under which a buyer, often a company, agrees to purchase electricity from a specific generator at an agreed price. For the developer, a PPA provides the stable revenue needed to finance a new wind or solar plant. The power usually still flows through the public grid, with the contract settled financially.
Power-to-X: A general term for converting electricity into other energy carriers or products, such as hydrogen, ammonia, methanol or synthetic fuels, or into heat. The X stands for whatever is produced. Power-to-X plants can use surplus renewable power and serve sectors that are hard to electrify directly, such as shipping, aviation and parts of heavy industry.
Project of common interest (PCI): A cross-border energy infrastructure project selected by the EU under the TEN-E Regulation because it links the energy systems of member states and supports EU goals. PCIs benefit from faster permitting and can apply for grants from the Connecting Europe Facility. The list is revised every two years. Power links in Europe
Project of mutual interest (PMI): A project under the TEN-E Regulation that connects the EU's energy system with a country outside the Union and meets similar criteria to a project of common interest. PMIs receive comparable permitting and funding advantages. Links to Norway, the United Kingdom and some Mediterranean neighbours have been listed in this category. Power links in Europe
Pumped-storage hydropower: A hydro plant with an upper and a lower reservoir that stores energy by pumping water uphill when electricity is cheap and generating when it is needed. Some energy is lost on each cycle, but pumped storage can hold large amounts of energy for hours or days. It remains the largest form of electricity storage in Europe, with many of the biggest schemes in the Alpine countries and on the Iberian peninsula.

R

Radial connection: A connection that links one offshore wind farm, or one cluster, directly to a single onshore point, with no further links to other farms or countries. It is the simplest arrangement and the usual one today. Its drawback is that the cable sits idle when the wind is calm and cannot be used for trade. HVDC and offshore wind
Rate of change of frequency (RoCoF): How fast the grid frequency moves after a disturbance, measured in hertz per second. The lower the system's inertia, the faster frequency changes after a large generator trips. Protection devices on some older generators disconnect when RoCoF is high, which can make a disturbance worse, so operators set limits and monitor it closely.
Reactive power: Power that oscillates between sources and loads in an AC system without doing useful work, needed to build the magnetic and electric fields of motors, transformers, lines and cables. It is measured in volt-amperes reactive. Controlling reactive power is how operators hold voltage steady; too much or too little can drive voltages out of range. The Iberian blackout
Reactive power compensation: Equipment that supplies or absorbs reactive power to keep voltages within limits, such as shunt reactors, capacitor banks, static VAR compensators, STATCOMs and synchronous condensers. Long AC cables and lightly loaded lines raise voltages and need reactors; heavily loaded lines lower voltages and need capacitive support.
Real-time simulation: Running a detailed digital model of a power system at the same speed as real events, so that physical control equipment can be connected to it and tested. It is used to study wind farm connections, HVDC controls and faults that would be too risky to stage on a live grid. Research laboratories and manufacturers both rely on it. HVDC and offshore wind
Reconductoring: Replacing the conductors on an existing overhead line with new ones that can carry more current, often high-temperature low-sag types, while keeping the existing towers and route. It can add capacity faster and with far fewer permits than building a new line, although towers may need reinforcing and the line must be switched off during the work. Repowering transmission lines
Rectifier: A converter that turns alternating current into direct current. In an HVDC link, the station that feeds power into the DC side works as a rectifier, and the station at the receiving end as an inverter. On most links either end can take either role, so the direction of flow can be reversed. HVDC explained
Redispatch: An instruction from a transmission system operator to generators or large consumers to change their scheduled output in order to relieve congestion. Plants on one side of a bottleneck are turned down and plants on the other side turned up, and the operator pays for the difference. In Germany redispatch costs have run into billions of euros a year. Grid congestion guide
Renewable Energy Directive (RED): The EU law that sets the Union's binding target for renewable energy and the rules around it. The current version, amended in 2023 and often called RED III, requires at least 42.5 % of gross final energy consumption to come from renewable sources by 2030, with sector rules for transport, industry, buildings and heating, and faster permitting. EU renewable rules
Replacement reserve (RR): The slowest category of balancing reserve in the European framework, used to restore faster reserves and cover longer imbalances. Not every country uses it. Where it exists, it is activated some time ahead of need, typically for longer imbalances that faster reserves should not have to carry, and it can be exchanged between operators through the TERRE platform.
Repowering: Upgrading an existing power asset to raise its output or capacity. For a wind farm it means replacing old turbines with fewer, larger ones. For a transmission corridor it means raising its capacity through new conductors, higher voltage, insulated cross-arms or dynamic rating, without building an entirely new line on a new route. Repowering transmission lines
Resource adequacy: The ability of a power system to meet demand at all times, with enough generation, storage, demand response and imports available even in difficult conditions such as a cold, windless week. ENTSO-E carries out a European resource adequacy assessment each year, and national operators run their own.

S

Sag: The vertical distance by which an overhead conductor hangs below the straight line between its supports. Conductors expand as they heat up, so sag increases with current and air temperature. Safety rules set a minimum clearance to the ground and to objects below, which in practice limits how much current a line may carry. Repowering transmission lines
Security of supply: The ability of a power system to deliver electricity to consumers reliably, in the short term through operational security and in the long term through adequate generation and networks. Interconnectors contribute by letting countries draw on each other's resources when their own fall short.
Series compensation: Capacitors installed in series with a long AC transmission line to reduce its effective reactance, so that it can carry more power with less voltage drop. It is used on long lines in Scandinavia and elsewhere. Series capacitors can interact with nearby generators and cause sub-synchronous oscillations, which have to be studied carefully. Repowering transmission lines
Short-circuit current: The very large current that flows during a fault, when a conductor touches earth or another phase. Equipment must be able to withstand it and breakers must be able to interrupt it. The size of the available short-circuit current is also a measure of how strong a grid is at a given point.
Short-circuit ratio (SCR): The ratio between the short-circuit power of the AC grid at a connection point and the rating of the converter or plant connected there. A high ratio means a strong grid that holds its voltage well; a low ratio means a weak grid, where converters can become unstable. It is a standard check when planning HVDC and large wind connections. HVDC explained
Shunt reactor: A large coil connected between a line or cable and earth to absorb reactive power. It counters the capacitive charging of long AC cables and lightly loaded lines, which would otherwise push voltages too high. Offshore AC connections often need reactors at both ends and sometimes midway, one of the costs that makes HVDC attractive at longer distances.
Single day-ahead coupling (SDAC): The European initiative that couples the day-ahead electricity markets of most EU countries and Norway into one auction. A single algorithm calculates prices and cross-border flows for the next day in all participating zones at once, using the available transmission capacity.
Static synchronous compensator (STATCOM): A power-electronic device built on voltage source converter technology that injects or absorbs reactive power very quickly to support grid voltage. It performs well even when the grid voltage is low. Some STATCOMs are paired with energy storage so that they can also supply short bursts of active power.
Static VAR compensator (SVC): A device that controls voltage by switching capacitors and reactors with thyristors, supplying or absorbing reactive power as needed. It is an older and generally cheaper technology than the STATCOM, and many are in service on European transmission grids to stabilise voltages near large loads or long lines.
Sub-synchronous oscillation: An oscillation in a power system at a frequency below the grid frequency, caused by interactions between generators, series-compensated lines and converter controls. If it is poorly damped it can damage generator shafts or trip wind farms and HVDC links. Careful control design and studies are needed wherever power electronics dominate a region.
Subsea cable: A power cable laid on or buried in the seabed, used to connect islands, offshore wind farms and countries separated by water. It must withstand water pressure, currents, anchors and fishing gear, and is heavily armoured. Long subsea power cables almost always use direct current because of the charging-current limit of AC. Subsea power cables guide
Substation: A site where lines and cables meet and where voltage is transformed between levels, flows are switched and faults are isolated. Substations contain transformers, circuit breakers, busbars, protection and control equipment. The largest transmission substations act as the junctions of the national grid.
Superconducting cable: A power cable whose conductor, cooled below its critical temperature, carries current with no electrical resistance. It can transmit very large currents at relatively low voltage in a compact trench. The cooling system and cryogenic envelope add cost and complexity, so such cables have so far been used mainly in pilots and short urban links. Superconducting cables
Superconductivity: The property of certain materials to conduct electric current with zero resistance when cooled below a critical temperature. It was first observed in mercury in 1911. Superconductors used in power engineering include niobium-titanium, magnesium diboride and ceramic high-temperature compounds based on copper oxides. Superconducting cables
Symmetrical monopole: An HVDC configuration with two conductors at equal and opposite voltage from a single converter, positive and negative relative to earth. It avoids earth currents and puts less stress on the converter transformer. Many offshore wind connections and medium-size interconnectors use it, though a fault on either conductor takes the whole link out. HVDC explained
Synchronous area: A region in which all generators run at the same frequency, locked together by AC connections. Europe has several: the continental area, the Nordic area, Great Britain and Ireland. Synchronous areas are connected to each other only by HVDC links. The Baltic states joined the continental area in February 2025. Baltic synchronisation
Synchronous condenser: A large rotating machine, like a generator without a turbine, that spins in step with the grid and produces no active power. It supplies reactive power, short-circuit strength and inertia, the qualities a grid loses when conventional power stations close. Some are new; others are converted from retired generators. The Iberian blackout
Synchronous generator: The type of generator used in most conventional power stations and hydro plants, whose rotor turns at a speed locked to the grid frequency. Its heavy spinning mass provides inertia and its magnetic field supports voltage naturally. As these generators close, their stabilising role has to be replaced by other means.

T

TEN-E Regulation: The EU regulation on trans-European energy infrastructure, revised in 2022. It defines priority corridors, sets the rules for selecting projects of common interest and mutual interest, and speeds up their permitting. The revised version ended support for new oil and gas projects and gave more weight to offshore grids and hydrogen. Power links in Europe
Ten-year network development plan (TYNDP): The plan for the European electricity transmission network prepared by ENTSO-E every two years. It sets out scenarios for future supply and demand, identifies where the grid will need more capacity, and assesses the costs and benefits of candidate projects. It is the main input for selecting projects of common interest. Power links in Europe
Thermal limit: The maximum current a line, cable or transformer can carry before its temperature exceeds a safe level. For overhead lines the concern is sag and conductor damage; for cables and transformers, damage to the insulation. Most capacity ratings on the grid are thermal limits, so cooling conditions matter a great deal. Repowering transmission lines
Thyristor: A semiconductor switch that turns on when triggered and stays on until the current through it falls to zero. Stacks of thyristors form the valves of line-commutated HVDC converters. They handle very high power efficiently, but because they cannot be switched off at will, they depend on the AC grid to commutate. HVDC explained
Transformer: A device that raises or lowers AC voltage using two or more coils around a shared iron core. Step-up transformers at power stations raise voltage for efficient transmission, and step-down transformers bring it to levels that distribution networks and consumers can use. Large power transformers have long delivery times and are hard to replace quickly.
Transmission: The transport of electricity over the extra-high and high-voltage interconnected system on its way to final customers or distribution networks. Transmission does not include selling electricity to customers, which is supply. It is carried out by transmission system operators, who also run the connections that tie national systems together across borders.
Transmission line: A set of towers, conductors, insulators and hardware that carries electricity from one point to another at high voltage. In Europe the transmission backbone runs mainly at 220 kV and 380 to 400 kV, with lines of around 110 to 150 kV forming a regional layer in many countries. Lines can carry large amounts of power over long distances with modest losses. Repowering transmission lines
Transmission network: The electrical network that carries power at high voltage from generators to distribution networks, large consumers and other transmission networks, including its connections to them. Each country defines where transmission ends and distribution begins. The European transmission networks are linked together into synchronous areas.
Transmission system operator (TSO): A company responsible for operating, maintaining and developing the transmission system in a given area, including its interconnections with other systems, and for ensuring the system can meet reasonable demand over the long term. Examples include RTE in France, Statnett in Norway, Elia in Belgium and TenneT in the Netherlands and parts of Germany.

U

Unbundling: The legal separation of electricity networks from generation and supply businesses, required by EU law so that network owners cannot favour their own power plants or retailers. Transmission system operators must be independent of production and supply. Unbundling is one of the foundations of the European internal energy market.
Underground cable: A power cable buried in the ground, used where overhead lines are not wanted or not possible, in cities, protected landscapes or at the land section of a subsea link. Underground cables cost several times more than overhead lines of the same capacity and take longer to repair, but they draw far less opposition. Subsea power cables guide
Uprating: Raising the rated capacity of an existing line or asset, for example by increasing its operating voltage or allowing a higher conductor temperature. Uprating can release capacity from existing corridors without the lengthy permitting of a new line, but it may require new insulators, fittings or clearances. Repowering transmission lines

V

Voltage: The electrical pressure that drives current through a circuit, measured in volts. In grid documents voltage usually means the root-mean-square value of the voltage between two phases. Transmission systems use high voltages because, for the same power, a higher voltage means a lower current and therefore lower losses.
Voltage source converter (VSC): The modern type of HVDC converter, built with transistor valves such as IGBTs that can be switched on and off at will. VSC links control active and reactive power independently, can connect to weak or dead grids and suit offshore wind. They have become the standard choice for new interconnectors and offshore connections in Europe. HVDC explained
Voltage stability: The ability of a power system to keep voltages within acceptable limits at every point, both in normal operation and after a disturbance. Voltage instability can come from too little reactive support, leading to collapse, or from too much, leading to overvoltages that trip equipment. Voltage control was central to the investigation of the Iberian blackout in 2025. The Iberian blackout

W

Weak grid: A part of the network with low short-circuit strength, where voltage changes sharply as power flows vary. Weak grids are typical at the end of long lines, on islands and in areas where large power stations have closed. Converters connected to them need careful control, and grid-forming capability helps.

Questions

What is the difference between HVDC and HVAC?

Both carry electricity at high voltage. HVAC is the alternating current almost every grid runs on; HVDC is direct current, which suits long subsea cables, very long lines and links between grids that are not synchronised, and it lets the operator set the power flow precisely.

What does a transmission system operator do?

It runs, maintains and develops the high-voltage network in its area, including the links to neighbouring systems. RTE in France, Terna in Italy and 50Hertz in eastern Germany are examples that appear on this site.

What is a bidding zone?

The largest area in which electricity trades at one wholesale price without regard to the lines inside it. Many countries form a single zone; Sweden, Norway, Denmark and Italy are split into several.

What is ampacity?

The largest current a conductor can carry continuously without getting too hot. On an overhead line the limit is how far the warm wire sags; on a cable it is what the insulation can bear.