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In recent years, renewable energy sources have been integrated on a large scale in power systems all around the world to address the environmental sustainability concerns. With conventional thermal generators being phased out, large offshore wind power plants present a viable alternative to provide blackstart services for power system restoration. In this paper, by means of simulations, grid-forming wind turbines are shown to successfully energize the offshore transformer and the HVDC export link in a controlled manner, to ultimately supply the onshore grid. Two methods for energizing the offshore network have been compared:the prevalent hard-switching approach and the more complex soft-start method. Additionally, control has been implemented to mitigate the significant transients in the export link associated with pre-charging of the onshore converter. It is shown that soft-start can provide faster energization with smaller transients compared to hard-switching. Moreover, the sensitivity analyses performedin this study illustrate the impact of pre-insertion resistor design and voltage ramp-up rates on transients during hard-switching and soft-start, respectively. The results presented in the paper also show that a separate controlled pre-charging stage of the onshore converter from its DC terminals is essential for the safe energization and operation of the export link.The manuscript has been accepted for publication in IET Renewable Power Generation.

High volume integration of renewable energy like large offshore wind power plants (OWPP) has pushed the modern power system closer to its critical dynamic stability limits due to its inherent variability & availability uncertainty, decoupling due to power electronics interface and system-expansion constraints. Thus, an improper handling of certain partial failures can easily lead to severe chain reactions, which ultimately increases the risk of wide area blackouts due to stronger linking of national power systems. In contrast to conventional blackstart units (BSU), large OWPPs can provide fast and fully controlled, high power, environment-friendly blackstart capability. Moreover, grid-forming wind turbines (WT) need not only wait for network restoration to complete like traditional grid-following WTs, but can also participate & support the network energization & load restoration process in the earlier stages, thus reducing the overall impact of a blackout. Additionally, blackstart-able WTs can produce power to sustain themselves and avoid risk to their health, especially when offline for long durations. This minimizes the use of diesel generator and provides cost benefit. Since, grid-forming is a necessary capability for WTs to support the blackstart restoration process, this paper aims at presenting a conceptual classification of the various existing grid forming control strategies. The different control techniques that will be studied are virtual synchronous machine based (VISMA, VSM), power synchronization control (PSC, Synchronverter), Distributed PLL-based, direct power control (DPC, enhanced DPC) and Synchronous machine matching. Furthermore, the different control techniques will be implemented in an aggregated wind farm model for comparison in the scope of blackstart restoration and islanding operation using OWPPs. The main aim of this study will be to ensure controlled network energization and stable load recovery of the offshore collector grid for durations sufficiently long enough that OWPPs be considered as BSUs by TSO for early participation in power system restoration.

During the recent years, there has been a significant penetration of offshore wind power into the power system and this trend is expected to continue in the future. The North Sea in Europe has higher potential for offshore wind power; therefore, the North Seas Countries' Offshore Grid initiative was formed among nine North Sea countries. They agreed on closer energy cooperation to enable development of an efficient and economic offshore grid infrastructure for advantages, interconnectors based on the voltage source converter based high voltage DC (HVDC) transmission system is being used to exchange power between different countries, and different synchronous areas. It is very likely that they will then be combined with offshore wind power plant (OWPP) connections in the North Sea, transforming it in a multi terminal DC (MTDC) grid and, therefore, in a fully meshed offshore DC grid in near future. However, increased penetration of offshore wind power into the power system poses several challenges to its security. This thesis deals with two main research challenges, (1) Develop, and analyze the coordinated control strategies for AC voltage and reactive power control in the cluster of OWPPs connected to common offshore HVDC station, (2). Develop, analyze, and test the control strategies for ancillary services from OWPPs to the AC grid, mainly fast primary frequency control from OWPPs. Moreover, the impact of wind speed on the frequency control from OWPPs is also studied in this thesis. The main results of this research work show that the OWPPs in the HVDC grid can participate in fast primary frequency control of the power system by using the proposed frequency control methods. Also, wind speed has a significant impact on the frequency control, particularly at below rated wind speeds. The proposed methods for AC voltage and reactive power control can improve the steady state and dynamic AC voltage profile of the offshore AC grid with cluster of OWPPs connected to common HVDC station, while minimizing the active power losses in the offshore AC grid. The research work is carried at the Technical University of Denmark (DTU) in the Department of Wind Energy and it is funded by the People Programme (Marie Curie Actions) of the EU FP7/2007-2013/ under REA grants agreement no. 317221, project title MEDOW.

High volume integration of renewable energy like large offshore wind power plants (OWPP) has pushed the modern power system closer to its critical dynamic stability limits due to its inherent variability & availability uncertainty, decoupling due to power electronics interface and system-expansion constraints. Thus, an improper handling of certain partial failures can easily lead to severe chain reactions, which ultimately increases the risk of wide area blackouts due to stronger linking of national power systems. In contrast to conventional blackstart units (BSU), large OWPPs can provide fast and fully controlled, high power, environment-friendly blackstart capability. Moreover, grid-formingwind turbines (WT) need not only wait for network restoration to complete like traditional grid-followingWTs, but can also participate & support the network energization & load restoration process in the earlier stages, thus reducing the overall impact of a blackout. Additionally, blackstart-able WTs can produce power to sustain themselves and avoid risk to their health, especially when offline for long durations. This minimizes the use of diesel generator and provides cost benefit. Since, grid-forming is a necessary capability for WTs to support the blackstart restoration process, this paper aims at presenting a conceptual classification of the various existing grid forming control strategies. The different control techniques that will be studied are virtual synchronous machine based (VISMA, VSM), power synchronization control (PSC, Synchronverter), Distributed PLL-based, direct power control (DPC, enhanced DPC) and Synchronous machine matching. Furthermore, the different control techniques will be implemented in an aggregated wind farm model for comparison in the scope of blackstart restoration and islanding operation using OWPPs. The main aim of this study will be to ensure controlled network energization and stable load recovery of the offshore collector grid for durations sufficiently long enough that OWPPs be considered as BSUs by TSO for early participation in power system restoration.

In the recent years, the worldwide grid integration of renewable energy sources (RES) has grown rapidly. However, the grid-integration of large volume of renewables like wind and solar, interfaced by power electronics converters, is changing the dynamics of the power system and creating challenges to maintain reliability, robustness and stability of electricity supply. Morevoer, the stronger inter-linking of networks poses a greater risk of wide-area blackouts. With the growing share of wind power in the grid and conventional generators being phased out, large offshore wind power plants (OWPP) with state-of-the-art wind turbines (WT) can provide services of blackstart and power system restoration (PSR) in the future. This paper provides an overview of the technical requirements of blackstart units and the current capabilities of WTs, based on grid-codes, studies on ancillary services and recent blackout experience. Finally recommendations to the functional requirements of WTs are made, to make OWPPs capable of blackstart and help facilitate PSR in the future.

High volume integration of renewable energy like large offshore wind power plants (OWPP) has pushed the modern power system closer to its critical dynamic stability limits due to its inherent variability & availability uncertainty, decoupling due to power electronics interface and system-expansion constraints. Thus, an improper handling of certain partial failures can easily lead to severe chain reactions, which ultimately increases the risk of wide area blackouts due to stronger linking of national power systems. In contrast to conventional blackstart units (BSU), large OWPPs can provide fast and fully controlled, high power, environment-friendly blackstart capability. Moreover, grid-forming wind turbines (WT) need not only wait for network restoration to complete like traditional grid-following WTs, but can also participate & support the network energization & load restoration process in the earlier stages, thus reducing the overall impact of a blackout. Additionally, blackstart-able WTs can produce power to sustain themselves and avoid risk to their health, especially when offline for long durations. This minimizes the use of diesel generator and provides cost benefit. Since, grid-forming is a necessary capability for WTs to support the blackstart restoration process, this paper aims at presenting a conceptual classification of the various existing grid forming control strategies. The different control techniques that will be studied are virtual synchronous machine based (VISMA, VSM), power synchronization control (PSC, Synchronverter), Distributed PLL-based, direct power control (DPC, enhanced DPC) and Synchronous machine matching. Furthermore, the different control techniques will be implemented in an aggregated wind farm model for comparison in the scope of blackstart restoration and islanding operation using OWPPs. The main aim of this study will be to ensure controlled network energization and stable load recovery of the offshore collector grid for durations sufficiently long enough that OWPPs be considered as BSUs by TSO for early participation in power system restoration.

This paper presents two methods for frequency control of onshore AC grids from offshore wind power plants (OWPPs) connected to a multi-terminal DC (MTDC) grid. The first method is based on communicating the onshore frequency to the OWPP and the other voltage source converters (VSCs) in the MTDC grid. The second method is based on a coordinated strategy between the VSCs in the MTDC grid, where the onshore frequency is replicated at the offshore grid using additional control blocks implemented locally at the VSCs of the MTDC grid. The proposed control methods are first verified through simulations on a test set up with an OWPP connected to a three-terminal DC grid using DIgSILENT PowerFactory and then validated experimentally on a laboratory scaled three-terminal DC grid. The simulation and experimental results prove that, with the proposed control strategies, OWPPs and the VSCs in the MTDC grid can participate in frequency control and support the onshore grid frequency stability.