Congestion management: The system operators challenge to balance transmission transfer capacity with an acceptable security level

The paper discusses the challenges and possible solutions to balance the increasing need of transfer capacity with an acceptable security level. The liberalised market system calls for flexibility in power exchanges to enhance the competition at the same time as there is an increasing need for someone to supervise the overall system operation. Operating the power system closer to technical limits makes it necessary to have efficient methods to deal with cases with insufficient transfer capacity. Major techniques discussed in this paper to handle this are the market splitting, locational marginal pricing and the recently proposed flow-based market coupling. Basic principles, possibilities and limitations of these techniques as well as suitable instruments to mitigate the limitations are addressed. A discussion of which market models these techniques are suitable for, is included. The paper further discusses the aspects related to defining appropriate transfer levels under different security criteria and the possibility to make corrective actions during emergencies. Moving from a preventively secure system operation to a correctable secure operation will significantly increase the transfer capacities and give the market some of the requested capacity, but at some risk for disconnection of loads and components