Electricity markets demand that available transmission capacity should not produce bottlenecks and thereby limit the free competition. The transmission capacity of the system should be sufficient in all market situations and the existing system must be used as efficiently as possible. Still, reliability should not be endangered. Available transmission capacity is determined by network's physical properties. In practise this limit can not be reached because of deterministic N-l security requirements. The N-l planning method demands that the network should remain usable during any single fault. It is possible that even if the deterministic transmission capacity during the limiting fault is not high enough for the peak load, it could be sufficient if the fault occurs during a lower load level. It is also possible that fault statistics show that faults occur quite seldom during peak load. It is thereby possible that probabilistic planning shows that there is unused transmission capacity left in the transmission system. The aim of this paper is to propose a simple but practical probabilistic planning method to assess the economic risks of more efficient utilisation of existing networks.
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