Reliability of transmission networks: Impact of EHV underground cables & interaction of offshore-onshore networks

For the future, several developments of the power system are expected. The transition towards a more sustainable energy supply puts new requirements on the design and operation of power systems, and the transmission network in particular. Offshore, a transmission grid will be implemented to collect large-scale wind energy and to interconnect national power systems. Onshore, the transmission network needs to be reinforced to transmit the large-scale renewable energy to the main load centers. As all these developments will impact the reliability of transmission networks, it is important to study and quantify these impacts in reliability analysis.
Traditional overhead lines are facing more and more opposition from local society, such that underground extra-high voltage (EHV) cable connections become a promising alternative for future grid extension. As EHV underground cables are a relatively new technology, not much is known yet about their behavior in large transmission networks. Underground cable connections (consisting of cables, joints and terminations) are in general less reliable than traditional overhead lines, mainly because of their much longer repair time. This can negatively influence the reliability of the transmission network as well. In this thesis, the reliability of EHV underground cables and overhead lines are compared, from the level of individual connections to transmission network level. It is studied which factors are of influence and what measures can be taken to improve the reliability.
For offshore grids, network redundancy is a topic of discussion. Implementing offshore redundancy can be costly, but no network redundancy might lead to large capacity outages when connecting large offshore wind farms. In this thesis, the reliability of various offshore configurations is compared and it is discussed what level of redundancy is the most optimal. The consequences of offshore network outages for the operation of the onshore power system are considered as well. Recommendations for the implementation of offshore redundancy are given.
The studies in this thesis show the importance of an integrated reliability analysis, where network design and system operation are combined, to find the most optimal solution. Decisions on network design will have consequences for system operation and vice versa. The studies in this thesis show how the reliability of underground cables and offshore networks can be assessed, which factors are of influence and what measures can be taken to improve power system reliability in the future.