Transmission congestion occurs when there is insufficient energy to meet the demands of all customers. The term is somewhat misleading, because no actual congestion occurs in the transmission system. These systems don't slow down, and electricity doesn't become blocked or delayed because the transmission system can't be stretched beyond its limits. Attempting to operate a transmission system beyond its rated capacity is likely to result in line faults and electrical fires, so this can never occur. The congestion is actually a shortage of transmission capacity to supply a waiting market, and the condition is marked by systems running at full capacity and proper efficiency which cannot serve all waiting customers.
When congestion occurs in a competitive market, there is a risk of price gouging from utilities that control transmission services. Regulatory bodies are aware of this risk, and most jurisdictions have built safeguards into their free-market regulations to insure that abusive pricing does not occur, and that congestion-related energy cost increases reasonably reflect the extra costs incurred in alleviating the condition. And the only ways the congestion can be alleviated are to tune the system to increase its capacity, add new transmission infrastructure, or decrease end-user demand for electricity.
See also:transmission, grid, transmission congestion contract