Calculation methods

Elia applies definitions and calculation methods defined by ENTSO-E, the European Network of Transmission System Operators.

Total Transfer Capacity (TTC)

The TTC is the maximum capacity available for the exchange of electricity between grids in neighbouring geographical areas without jeopardising the security of the respective grids and subject to events or new information reported to the system operator by market parties or other system operators.

Calculating TTC involves more than simply adding together the capacities of every interconnector. Electricity flows are distributed unevenly between the individual components of the transmission system. In addition, the N-1 criterion must be taken into account (i.e. the principle that the grid must remain in operation even during the unscheduled loss of an important component, generation unit or connection).

Calculation method

To calculate the TTC between two grids, every system operator defines one or more basic scenarios which correspond to a specific situation that occurred on the grid over the previous years, months or days. These scenarios are used as the basis for performing simulations aimed at assessing electricity flows within the grid, thereby enabling to calculate the Total Transfer Capacity, or TTC.

As Europe’s grids are very tightly interconnected, the system operator also has to take account of the electricity flows likely to flow through their grid without them necessarily being notified in advance. These electricity flows are commonly known as unscheduled flows, loop flows or parallel flows. They pass through the system operator’s grid but the system operator is not informed of such flows (they are unscheduled). They are the result of international transactions conducted outside the system operator’s grid. These transactions are concluded without completely taking account of the physical characteristics of electricity flows.
For example: a transmission contract concluded for generation in France and consumption in Germany will cause an unscheduled electricity flow through the Belgian, Dutch and Swiss grids.

As the calculation day approaches, scenarios are increasingly well defined and the level of uncertainty decreases. This explains why NTC values published on day D-1 can be higher than values published well before day D.


Transmission Reliability Margin (TRM)

The TRM is the minimum reserve that system operators must have available at their connections so that they can help other countries to which their system is directly or indirectly connected, if necessary.

Elia reserves 250 MW at each border to provide mutual international assistance under the rules established by ENTSO-E (formerly the Union for the Co-ordination of Transmission of Electricity, or UCTE). If one or more generation units in one or more countries should unexpectedly break down, the ensuing generation shortfall is immediately and automatically offset by an increase in generation at the power stations belonging to all other interconnected countries. The TRM reserve allows this energy to be transmitted through the Belgian grid and other European grids.


Net Transfer Capacity (NTC)

NTC is the capacity available for commercial transactions.

The NTC value is calculated as follows: TTC – TRM = NTC

The NTC concept is incorporated into the products offered by the system operator – annual, monthly and daily capacities.


Annual cross-border import/export capacity


This is the maximum transaction (for imports or imports) possible between two neighbouring countries. The maximum transaction must be available while complying with grid safety regulations. A heavy storm or any other abnormal event may cause one of the interconnectors, possibly the most powerful one, to trip (i.e. switch off automatically). Despite this, grid safety must be maintained.

Capacity varies over the course of the year, mainly due to the following factors:

  • Season
    During winter, overhead lines are cooled more effectively and can transmit more electricity.
  • Infrastructure maintenance
    Every interconnector must be switched off occasionally for maintenance.
  • Physical flows on the grid
    The Belgian grid is part of continental Europe’s interconnected grid. Flows resulting from sales or purchases between Belgium’s neighbours may pass through our grid and jam our lines. Such flows are in some ways comparable to foreign trucks using our motorways.

Annual capacity is the capacity available throughout the year, even during times expected to be the most restrictive. Click here to find out more about Elia’s calculation method for annual capacity (in French).

Monthly cross-border import/export capacity

Monthly capacity is the capacity available throughout a month, even during times expected to be the most restrictive. Elia applies the following model to calculate monthly capacity (in French).


Daily cross-border import/export capacity

The daily allocation process is based on the Flow-Based methodology. More information on the CWE Flow-Based Market Coupling is available here CWE Flow-Based.


The Central West Europe intraday ATC Capacity Calculation process after Flow-Based Market Coupling

The aim of the Central West Europe intraday ATC Capacity Calculation process (CWE ID ATC CC) after Flow-Based Market Coupling (FBMC) for Transmission System Operators (TSOs) is to have the possibility to release coordinated additional capacity to the market players as a daily process.

The concept of CWE ID ATC CC after Flow Based Market Coupling consists out of different sequential steps:

  1. FB DA (Flow-Based Day-Ahead): TSOs compute capacities for FB DA, based on various inputs. The result is a Flow based domain.
  2. FB DA MC (Flow-Based Day-Ahead Market Coupling): The Power Exchanges (PXs) match the different bids & offers, they have commonly received, taking into account available capacity to perform market clearing
  3. (Initial) ID ATC computation: The Flow Based DA market clearing point is the starting point to define the initial ID capacity limited by the FB DA domain previously computed.
  4. ID Capacity Calculation: TSOs assess the possibility of increasing the capacities that came out of the initial ID ATC computation to provide more ID capacities for a given border in a given direction.

Firstly, TSOs may request an increase of capacity on their own borders for each hourly market time period (MTP) of the following day.
Note: The current maximum increases requests of capacity per border are:
BE <-> FR and BE <-> NL: 200 MW
DE <-> FR and DE <-> NL: 100 MW 

Then, Elia* (as every TSO) assesses increases requests and is responsible for accepting (200MW) or partially accepting (100MW or 50MW) or rejecting (0MW) increase requests, thanks to a local analysis which gives equal ATC increases in all directions.
Finally, the Common Matching Tool 1.0 (CMT) consolidates the assessment results of each TSO per border and per direction by considering the lowest values.

    5. Available ID capacity: capacities provided to the allocation platforms are the final ID ATCs


More information is available via the approval package of the intraday capacity calculation methodology.

* Coreso assesses requests and evaluates the capacity increases on behalf of RTE and a part of the daily process (the period 9-24) for Elia.

More information about the publication timing of Central West Europe intraday available transfer capacity to the market player is available on the JAO website