The interest in the development of offshore wind farms, either for lack of sites with potential interest in land, either by greater resource availability at sea, has been increasing in recent years. Transmission of large amounts of energy and long distances, brought new challenges for transport systems. Initially, were implemented in transmission based systems of high voltage direct current (HVAC), however with the development of power electronics, has developed solutions for the transmission of high-voltage direct current (HVDC) in a much more interesting solution.
Upon the transmission of high-voltage direct current interconnection of offshore productions with Network Transportation Operators (NTO), various technological constraints, such as capacity for independent control of active and reactive power, justify the use of voltage source converters (VSC). At the same time, aspects related to the stability and flexibility of operation has encouraged the development of the concept of Multi-Terminal grids DC (MTDC).
Indeed, the large-scale integration of wind energy can bring a level of stability problems for the electric power system. It’s unpredictability production can cause load imbalances and consequently cause fluctuations in voltage and current of the grids to which they connect.
Therefore, to ensure the safety of electrical power systems, various grid codes were created by NTOs. And thus imposing restricted conditions to connecting a wind farm to transmission grid, so this type of energy production should be able to provide various system services to the network. In particular, provide these grids offshore MTDC of survivability to voltage dips, is the object of study of this thesis.
This thesis then focuses initially on the behaviour of a MTDC grid when the occurrence of a voltage dips on the side of onshore AC grid. The first part consists in defining the principles of operation of an MTDC grid, with reference to the control modes and paper converters VSC-HVDC. The second involves identifying the simulation models of the various components of a Multi-Terminal HVDC network for future implementation in Simulink – MATLAB.
Then a study is made of the problems that arise in a MTDC network, when following voltage dips occurring in onshore AC grid. Once the problem identified, this study presents a strategy for distributed control and operation of MTDC grid that aims to provide the network with the ability to survive voltage dips.
Finally, will be present a case study, in which a VSC based Multi-Terminal HVDC grid, with two converters connected to offshore wind farms and two converters onshore connected with two AC networks continental, will be tested against the different voltage dips with different range and duration.