Transmission congestion
The condition where the network cannot carry the cheapest available power to where it is needed, so a more expensive resource runs instead.
Congestion occurs when the transmission system reaches a physical limit and the least-cost dispatch becomes infeasible. The operator must run a more expensive resource on the demand side of the constraint, and in markets with locational pricing the difference shows up directly as a price separation between two nodes.
It is what makes a megawatt a location rather than a quantity. Headroom at one substation is worthless to a load twenty miles away on the wrong side of a constrained corridor, and generation stranded behind one is curtailed rather than delivered. This is why a site selection question for a data center is a network question before it is a real estate one.
Congestion also moves on weather. High temperature raises demand and derates lines and thermal plant at the same hour, which is the coincidence that turns a normally comfortable corridor into a binding constraint - and the reason grid data and climate data belong in one model rather than two subscriptions.
Evidence that keeps pace with the decision.
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