Use observed max flow, not NTC, as the transmission constraint for backcasting

The backcasting toolbox needs a per-border transmission constraint to feed into Balmorel’s XKFX (exogenous transmission capacity) input for a historical year. We chose the maximum hourly cross-border flow reported by ENTSO-E for that year (Observed Max Flow) rather than the grid operator’s rated/commercial Net Transfer Capacity (NTC).

Backcasting validates whether Balmorel’s simulated behaviour reproduces what actually happened in a specific historical year — including whatever mix of technical, market, maintenance, and social/political constraints limited flow that year, not what was theoretically possible. Observed Max Flow is a lower-bound proxy for the true constraint (a border may have been more constrained than NTC in some hours, but a flow that was actually observed proves the true constraint was at least that large), and it comes from data already being pulled for backcast validation, avoiding a second ENTSO-E dataset (NTC/day-ahead transfer capacity). This choice is specific to backcasting; future-scenario capacity expansion work should use NTC or planned capacity data instead, not this pipeline.