Multi-Terminal DC Networks: System Integration, Dynamics and Control

Abstract

When large amounts of electricity need to be transported for long distances, or when underground or submarine cables are involved, using direct current high-voltage transmission systems is more efficient and cost effective than using traditional high-voltage alternating current transmission. Therefore, the main thesis objective is to study to what extent can multi-terminal dc networks provide an optimal platform to foster the integration of remotely located renewable resources, with particular focus on the integration of offshore wind farms in the North Sea. In this thesis, five main challenges were identified before high-voltage multi-terminal dc networks – which can promote the inclusion of remotely located renewable sources while strengthening the existing ac power system networks – can finally become widespread: system integration, power flow control, dynamic behaviour, stability and fault behaviour. These challenges are investigated through a comprehensive literature review, a series of detailed simulation models, and an experimental laboratory setup of a three-node multi-terminal dc network. A novel strategy to control the power flow in multi-terminal dc networks – called Distributed Voltage Control – was developed in this thesis. In the experimental setup, three voltage-source converters were successfully operated in a parallel-radial multi-terminal dc network with a symmetric monopolar configuration. Lastly, a real-time digital simulator was used to emulate the behaviour of an offshore wind farm. Real measurements from the Dutch offshore wind farm Egmond aan Zee were used to validate the Distributed Voltage Control strategy, which successfully controlled the power flow inside the three-terminal low-voltage dc network with high overall precision, while providing the complete system with a fast dynamic response.