Operation of offshore HVDC systems with grid-forming wind turbines in system energization and DC-fault scenarios

The European Union foresees an installed capacity of offshore wind power of 300 GW by 2050, with 70 GW planned by Germany in the North Sea. Due to the long distances from shore, offshore wind power plant (OWPP) are generally connected via point-to-point high voltage direct current (HVDC) systems to increase efficiency of transmission power to shore. Given these large amounts of power, and due to the reduction in conventional power plants, OWPP have been considered for the provision of system services. In particular, OWPP could provide black-start and power system restoration services following a power system black- or brown-out. However, for an HVDC-connected OWPP to be black start capable, a paradigm shift in the control design is necessary. Current grid-following wind turbine generator (WTG) control relies upon the voltage and frequency provided by the offshore HVDC converter. To eliminate this requirement, the control of the WTG can be adapted providing their own voltage and frequency. Further in the future, multi terminal HVDC (MTDC) systems, interconnecting multiple wind farms through multiple DC links are considered to increase redundancy and availability. To reduce the maximum loss of power, these systems are planned to be protected via DC circuit breakers (DCCB). For current system requirements, it is mandatory that far fault converters do not protectively block due to overcurrent. This is mainly driven by the aspect that, as previously described, the OWPP is dependent on the offshore converter. If the WTG control were adapted and designed for black start capability, temporary converter blocking could reduce protection requirements. This thesis contributes to a holistic understanding of using grid forming wind turbine generator (GFM WTG) in future offshore HVDC systems for different system scenarios, mainly black start and DC-side fault behavior. First, a detailed analysis on functional and technical requirements during the start-up of an OWPP connected via an HVDC system is presented. The required active and reactive power during the start-up process, as well as the control interactions between the WTG converters and the HVDC converter, along with the overall black start capability of the system are analyzed and evaluated. A key outcome of the investigation is that the design parameters such as the number of submodules of the HVDC converters, voltage levels at AC and DC side, and wind park design have a significant impact on the black start process and need to be considered in the design phase. Second, benefits and requirements are analyzed, when black-start capable WTG partially reenergize the HVDC system after a DC-side fault. In a partially selective protected MTDC system, the faulted part of the system is isolated, resulting in a discharge of that system part and a disconnection of the OWPP from the HVDC converter by opening the AC circuit breaker. After maintaining a stable island, the OWPP can then reenergize the HVDC after reclosing the AC circuit breaker. A key take away is that the recovery process crucially depends on the remaining submodule charge and HVDC system post-fault topology. Third, potential to reduce DC-side protection requirements through temporary blocking of the offshore converter in combination with various control strategy combinations is analyzed. After fault occurrence, the offshore converter protectively blocks its submodules temporarily. It is demonstrated that irrespective of the control architecture of the HVDC system, the OWPP can be disconnected and reconnected to the HVDC system while the overall system integrity is maintained. A key take away is that the required DC-side inductance can be significantly reduced compared to the state-of-the art protection designs while fulfilling fault-ride-through requirements on the offshore AC and DC side. Lastly, it is shown that GFM WTGs are capable of flexibly adapting the load flow of the connected HVDC system avoiding DC-side over voltages after DC-side faults. Overall, the findings from this thesis show that GFM WTGs can increase operational flexibility of future OWPPs, while maintaining system integrity. During normal operation, active power provision is identical to the state-of-the-art, while maintaining the ability to quickly adapt to system changes and fault scenarios.