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Glossary · Energy industry & energy transition

Flow-Based Market Coupling

flow-based market coupling market coupling electricity market Interconnector Cross-border interconnector capacity loop flow European electricity market

Flow-Based Market Coupling (FBMC) is a procedure of European electricity market coupling. Cross-border electricity trading is released according to how it is expected to affect the meshed transmission grid. Trading capacity is therefore not a simple figure per border. It is a range of permissible combinations of net positions of the bidding zones that must not overload certain lines, cables and transformers, even under assumed outages. This range is called the Flow-Based domain.

The term belongs to market coupling, i.e. the joint calculation of electricity prices and trade flows across several European bidding zones. In the day-ahead timeframe this takes place in the Europe-wide Single Day-Ahead Coupling (SDAC). The power exchanges (NEMOs) bring bids together, the transmission system operators supply the grid limits, and a common algorithm maximises overall economic welfare within these limits. FBMC describes precisely this interface between the market algorithm and grid physics.

The technical basis is the behaviour of a meshed alternating-current grid. Electricity does not follow the contractually agreed trading path from country A to country B. It distributes itself across many parallel paths according to impedances. An additional trade from France to Germany therefore loads not only lines at the German-French border. It can also affect network elements in Belgium, the Netherlands, Austria, the Czech Republic or Poland. Simple bilateral limits therefore reflect only to a limited extent what the grid can actually bear.

From border capacity to network constraint

In the coordinated NTC approach (Net Transfer Capacity), a maximum value is set for each border and direction, for example 2,000 megawatts. This is easy to understand, but it assumes that a trade can be attributed predominantly to one border. The grid operators must therefore allocate the mutual interactions in advance to the individual borders by means of flat-rate safety deductions.

In FBMC, the transmission system operators instead determine critical network elements together with associated outage situations (N-1), so-called CNECs (Critical Network Element with Contingency). These can be overhead lines, cables or transformers, in each case in both flow directions. For each CNEC, the extent to which a change in the net position of a bidding zone affects its load flow is calculated. These sensitivities are called Power Transfer Distribution Factors (PTDF). They are dimensionless, namely megawatts of line flow per megawatt of change in net position. A PTDF of 0.15, for example, means that of 100 MW of additional export, around 15 MW flow across this element. In the Core region, CNECs whose largest PTDF for exchanges between two zones is below 5 percent are removed from the calculation. Elements that barely react to cross-border trade should not limit trade.

The second parameter is the Remaining Available Margin (RAM), stated in MW. It describes how much additional market-driven flow is still permissible on a CNEC. Roughly speaking, a reliability margin and the expected reference flow are subtracted from the maximum permissible loading (Fmax). Adjustments through cost-free remedial actions, such as switching operations or tap positions of phase-shifting transformers, as well as increases to prescribed minimum margins, are added. Reductions made during validation by the grid operators are deducted again. Each CNEC thus yields a linear inequality: the sum of PTDF times net position must not exceed the RAM. All inequalities together form the Flow-Based domain. Within this multidimensional space, the algorithm searches for the welfare-maximising result.

This optimisation does not always result in electricity flowing from the cheaper to the more expensive zone. Because trade directions can relieve each other on critical elements, a welfare-maximising result can occasionally also include exchanges from a more expensive to a cheaper zone. These are then referred to as non-intuitive flows.

Taking the Core region as an example, the process runs as follows: the calculation begins in the afternoon two days before delivery (D-2). It is based on individual grid models of the transmission system operators with forecasts of generation, load, topology and planned outages, supplemented by Generation and Load Shift Keys. The models are merged into a common grid model and calculated by regional coordination centres. After validation, the domain is handed over to SDAC by 10:00 on the day before delivery (D-1) and published via the auction office JAO. The details of this calculation process are described in the entry Flow-Based Capacity Calculation.

The relevant unit is the megawatt, because the network constraint limits the power transmitted simultaneously. By contrast, what is traded are energy quantities per market time unit. Since the delivery day of 1 October 2025, this unit in SDAC has been 15 minutes instead of one hour in all European bidding zones.

Where the procedure applies

In the day-ahead market, FBMC was introduced in 2015 in Central Western Europe (CWE). On 8 June 2022 (delivery day 9 June), the procedure was extended to the entire Core capacity calculation region, including Germany/Luxembourg, France, the Benelux states, Austria, Poland, the Czech Republic, Slovakia, Hungary, Romania, Slovenia and Croatia, among others. Since 29 October 2024 (delivery day 30 October), Denmark, Finland, Norway and Sweden have also used FBMC in the day-ahead market. The legal basis is the CACM Guideline, Regulation (EU) 2015/1222. The Core calculation is a coordinated flow-based capacity calculation pursuant to Article 20(2) thereof. At other borders and in other timeframes, NTC and ATC values continue to be used. This applies, for example, to continuous intraday trading in Core, where at the start in 2022 ATC capacities were derived from the Flow-Based domain.

Legal minimum capacity: the 70 percent rule

Since 1 January 2020, Article 16(8) of the EU Internal Electricity Market Regulation (EU) 2019/943 has prescribed minimum values for the capacity available for cross-border use. In the flow-based approach, 70 percent of the capacity of critical network elements, both internal and cross-zonal, must be made available for cross-zonal trade. The benchmark is the capacity that respects the operational security limits taking outages into account. The remaining 30 percent can be used for reliability margins, loop flows and internal flows. Member States with structural congestion could, under Article 15, submit action plans with a linear increase; the target was to be reached no later than 31 December 2025. In addition, there are temporary derogations under Article 16(9). In the Core calculation, the requirement is implemented as a raising of the RAM per CNEC. In addition, a minimum margin of 20 percent of Fmax applies there for exchanges within Core.

Distinction from physical flow, line and grid operation

Market exchange flows are not the same as physical flows. If Germany imports electricity from France in the market result, this describes supply relationships and net positions, not the path of individual electrons. FBMC builds this difference into capacity allocation but does not replace real-time operation. Between the day before and delivery, wind and solar forecasts, power plant availabilities and grid conditions change. Grid operators handle remaining congestion operationally, for example with redispatch, countertrading or topological measures. Frequency and voltage control lie outside the procedure in any case.

An interconnector is a physical connection between grids or bidding zones. FBMC, by contrast, is a calculation procedure for an area with many such connections. The technical cross-border interconnection capacity of a line is also not identical to the tradable capacity. A tie line may have ample reserves while another critical element, even a domestic one, limits trade.

Significance for price formation and grid security

The practical benefit lies in linking price formation and grid security. An overly cautious procedure withholds capacity. Price differences then arise even though the grid could accommodate certain trade combinations. An overly generous procedure shifts congestion to grid operation and increases redispatch costs. FBMC distinguishes between trade directions that load critical elements heavily and those that load them hardly at all or even relieve them. The aim is a stronger convergence of prices in the region.

The connection to loop flow becomes clear here. Loop flows arise when trade within a zone flows through neighbouring grids because of the grid topology. They are not misconduct by individual market participants. They result from zonal market design, a meshed grid and unevenly distributed generation and load. In FBMC they enter the reference flow and thus reduce the RAM. The procedure makes them visible but does not eliminate them. With the 70 percent rule, the law limits how much capacity may be reserved for them.

Bidding zones and responsibilities

FBMC presupposes a zonal market model. Within a bidding zone such as Germany/Luxembourg, a uniform price applies in the day-ahead market, even if physical congestion exists between the wind-rich north and the load centres in the south. Such internal congestion then shows up in redispatch costs, grid reserve, curtailment or, indeed, in the capacity calculation. Neighbouring countries often criticise that internal congestion restricts trade.

Responsibilities are distributed. Transmission system operators supply grid models, CNECs and remedial action potentials. Regional coordination centres calculate the domain, JAO publishes it, and the power exchanges clear the market in the common algorithm. The national regulatory authorities and ACER approve the methodologies and monitor the minimum capacities. Bidding zone configuration, grid expansion and market design are decided politically.

Typical misunderstandings

  • FBMC does not automatically mean more trade. Usually, existing grid capacity is used in a more differentiated way. In individual hours or quarter-hours, however, a particular direction may be more strongly limited than a bilateral value would suggest.

  • FBMC is not exact. The procedure is based on forecasts from the day before or even two days earlier and on simplified assumptions, such as linear PTDFs and shift keys. It increases consistency with grid physics but does not make operational interventions unnecessary.

  • Complex does not mean arbitrary. The results are based on approved methodologies. The domains, CNECs and validation adjustments are published. Transparency nevertheless remains a permanent task, because market participants must be able to understand why capacity was limited.

Flow-Based Market Coupling thus describes neither an everyday electricity flow nor the capacity of a single line. It is a calculation procedure that ties European electricity trading to the physical limits of the meshed transmission grid. This shows that the electricity market and the grid are closely linked through concrete calculation rules and responsibilities.

Sources

Related glossary entries

Edited by eCMS on 12.11.2026 05:05

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.