Green & Black-starting HVDC-connected Offshore Wind Power Plants: Grid forming control, Energization transients and Islanding capabilities

Green energy transition is the most urgent need of the hour to curb the rising impact of global warming. The carbon-neutral energy system of the future will have electricity as its backbone, which requires a domination of renewable sources and long distance interconnections. However, such a transformation brings forth a massive penetration of power electronic converters to allow for flexibility in operation that is essential for efficient and cost-effective grid usage.

This paradigm shift will make future power grids very different from the current system resulting in a highly dynamic environment that poses a risk to power system stability. Coupled with the declining strength of the network, it has the potential to increase the impact of severe network disturbances and give rise to challenges for the containment of voltages and frequency excursions, ultimately triggering wide-area blackouts.

Since conventional synchronous generation is being phased out, the responsibility to maintain power system stability and provide network restoration services must be taken up by alternate sources of generation. Being one of the fastest growing renewable energy sources in the world, offshore wind is deemed a prime candidate.

Today wind turbines are controlled as grid following units in that they behave as current sources feeding in maximum power while depending on a stiff external voltage for their stable operation. Although already capable of providing ancillary services like low voltage ride through and fast frequency response, more responsibilities in maintaining stable and robust grid operation requires grid forming control.

This enables wind turbines to create the system voltage and thus not only not wait for completion of the network reconstruction, but also ensure continuity of power supply in an island facilitating bottom-up grid recovery. The main findings from literature review on the potential of wind power in power system restoration and the main technical challenges to overcome to enable their capabilities for facilitating early stage participation, highlight, among others, grid forming control as the key enabler in the interests of both the system operator and the wind farm developer.

The complete energization sequence or ‘greenstart’ of an HVDC-connected offshore wind farm has been simulated in this thesis, sub-divided into target states to study the technical challenges of each separately. Firstly, the energization of the HVDC transmission link including the large transformer and converter submodules is presented.

Potential solutions that allow offshore wind turbines to deal with the demanding transients in a controlled manner while maintaining stable voltage and frequency of the offshore island have been investigated. Then the focus switches to grid forming control. Four of the most common strategies from literature have been implemented in the offshore wind power plant and their transient response has been compared during the different stages of the energization sequence.

This also provides insight into the sensitivities to instability for each, which signifies the importance of tuning the control loops and challenges in adopting to the case of large offshore wind turbine control. Finally, based on the performance above, a grid forming control has been chosen to study the energization transients inside the offshore network, mainly focusing on turbine transformer inrush, synchronization of sequentially starting wind turbines, and voltage control with load sharing during the energization of a string.

Detailed studies have also been performed to investigate the limitations of grid forming wind turbines in energising a large offshore cable network while maintaining stable parallel operation and synchronism with traditional grid following wind turbines. The main findings of this work are seen as a stepping-stone to facilitate the development of grid forming wind turbines for ensuring stable voltage and frequency control with synchronized parallel operation in the face of demanding energization transients of the large offshore network and HVDC transmission link, ultimately providing restoration service to the grid.