The current practice of power system restoration mainly relies on conventional power plants, which can provide black start in case of a black out using fossil fuels. HVdcconnected offshore wind power plants can, on the other hand, provide fast and environmentally friendly solutions for power system restoration, once their state of the art wind turbines are equipped with the grid-forming capability. In this paper, the background and existing solutions for wind turbine and wind power plant self-energization and onshore grid black start are presented, together with simulation results of an offshore wind power plant sequentially energizing the offshore ac network, offshore HVdc terminal, HVdc link, onshore HVdc terminal, and onshore ac terminal and load. Black start, energization, grid-forming wind turbine control, HVdc transmission, offshore wind energy integration I.

Diode-rectifier-based high voltage dc (DR-HVdc) systems can be a promising low-cost solution for exporting wind power from remote offshore wind farms to onshore power systems. Industrializing the offshore DR-HVdc requires technical maturation, achievable through in-depth studies and pilot experiments. Deployment of offshore DR-HVdc systems may entail a fundamental change of control philosophy in wind turbine (WT) converters from grid-following control to gridforming. This paper proposes a new grid-forming control for DRconnected offshore WT converters. The proposed controller uses two sequential control loops to regulate WTs' active power, and maintain the frequency and voltage of the offshore ac network. The first control regulates the active-power mismatch of each WT into a voltage angle deviation, which leads to a frequency change. The second control adjusts the WT's alternating voltage magnitude to counteract the frequency change. An internal current control loop is used to limit the fault current and eliminate high-frequency resonances in the system. The proposed control is verified by electromagnetic transient simulations, including faultride through, WTs power change, reactive power disturbance, and WTs outage.

Diode rectifiers have been gaining traction as a viable alternative for connecting offshore wind farms (OWFs) to HVdc networks. However, before technical connection requirements compatible with such solutions can be determined, more studies are needed to assess their capabilities to contribute to the secure operation of the networks linked to them. This study assesses the capability of such an OWF to provide support to an onshore ac network by means of primary frequency response (PFR). A semi-aggregated OWF representation is considered in order to examine the dynamics of each grid-forming wind turbine (WT) within a string when providing PFR. Simulation results corroborate that such an OWF can indeed provide PFR, while its grid-forming WTs share the reactive power and keep the offshore frequency and voltages within their normal operating ranges.

Before diode rectifier (DR) technology for connecting offshore wind farms (OWFs) to HVdc is deployed, indepth studies are needed to assess the actual capabilities of DRconnected OWFs to contribute to the secure operation of the networks linked to them. This study assesses the capability of such an OWF to provide communication-less frequency support (CLFS) to an onshore ac network. It is shown that the HVdc link's offshore terminal direct voltage can be estimated from measurements at the OWF's point of connection with the DR platform. Two different methods are proposed for implementing CLFS in the OWF active power controls. In Method 1, the estimated offshore terminal direct voltage is used for estimating the onshore frequency deviation. In Method 2, the actual offshore terminal direct voltage measurement is used instead. Unique features of the provision of CLFS from OWFs connected to HVdc via DRs are highlighted, and the dynamic and static performance of the CLFS control scheme is compared to that of the communication-based frequency support scheme. To assess the impact of parameter estimation errors on the provision of CLFS, a parametric sensitivity study is presented as well, and recommendations are given to increase accuracy.

<div>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:</div><div>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 performed</div><div>in 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.</div>