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Lower costs and higher reliability make diode rectifiers (DRs) a promising alternative for connecting offshore wind power plants to HVdc networks. However, such offshore substations lack the grid-forming capability provided by voltage source converters. Hence, the grid-side converters of type-4 (fullconverter) wind turbines (WTs) have been proposed as viable candidates to take over such responsibility, apart from controlling the WT output power. In this work, a common synchronization reference (CSR) is proposed for the synchronization of such converters. Such reference is generated in the offshore HVdc substation and transmitted to the converters through the optical fibers embedded in the power cables. The proposed CSR enables the use of conventional current control for grid-forming WTs. Additionally, a scheme which combines communication-less and communication-based onshore frequency support is proposed and implemented. The proposed controls are validated by simulations under different operational scenarios. [ABSTRACT FROM AUTHOR]

Lower costs and higher reliability make diode rectifiers (DRs) a promising alternative for connecting offshore wind power plants to HVdc networks. However, such offshore substations lack the grid-forming capability provided by voltage source converters. Hence, the grid-side converters of type-4 (fullconverter) wind turbines (WTs) have been proposed as viable candidates to take over such responsibility, apart from controlling the WT output power. In this work, a common synchronization reference (CSR) is proposed for the synchronization of such converters. Such reference is generated in the offshore HVdc substation and transmitted to the converters through the optical fibers embedded in the power cables. The proposed CSR enables the use of conventional current control for grid-forming WTs. Additionally, a scheme which combines communication-less and communication-based onshore frequency support is proposed and implemented. The proposed controls are validated by simulations under different operational scenarios. [ABSTRACT FROM AUTHOR]

High penetration of power electronic devices in generation and transmission networks results in lower inertia in power systems, which can increase the risk of frequency instability. Thereby, the fast frequency support is anticipated to be the most important ancillary service provided by renewable generation systems in the near future. Frequency support from offshore wind power plants (WPPs) connected to high-voltage dc (HVDC) links is a challenging issue because of the complexities in their control and operation. In this paper, some technical challenges regarding to fast frequency support from HVDC-gridconnected offshore WPPs are addressed. Two different methods- communication based and droop based-of implementing such support are explained first, then advantages and disadvantages of each are detailed by means of mathematical and numerical (simulations) analyses. A four terminal HVDC grid, including two offshore WPPs and two onshore ac systems, is used for analyses and simulations. Moreover, a point-to-point HVDC connection is studied to understand how in an HVDC grid converters interaction of controllers can affect the frequency support. [ABSTRACT FROM AUTHOR]

This paper presents a study assessing the actual capability of an offshore wind power plant (offshore WPPs, OWPPs) to provide frequency support (FS) to an onshore network, when connected through a high-voltage direct-current (HVDC) link having a diode rectifier (DR) offshore terminal and a voltage source converter (VSC) onshore terminal. Both primary and fast frequency response (PFR and FFR, respectively) are studied, and both the power reserves from preventive curtailment and the kinetic energy stored in the rotating masses of the wind turbines (WTs) are considered as sources of additional power during onshore under-frequency events. Three methods are considered for overloading the WTs, including the proposed External Reference method, in which the base active power reference can be set externally. The performance of the controls is studied by means of electromagnetic transient (EMT) simulations, for which an aggregated model of the OWPP is used. The results suggest that such OWPPs can in principle provide onshore FS by means of plant-level active power control strategies already developed for OWPPs connected to HVDC via VSCs. Some of the results also suggest that it may be unnecessary to overload the WTs if active power reserves from curtailed operation are available when providing both PFR and FFR. [ABSTRACT FROM AUTHOR]

The low impedance characteristics of DC transmission lines cause the voltage source converter (VSC) in HVDC networks to become electrically closer together and increase the risk of severe interactions between the converters. Such interactions, in turn, intensify the implementation of the grid control schemes and may lead the entire system to instability. Assessing the stability and adopting complex coordinated control schemes in an HVDC grid and wind farm turbines are challenging and require a precise model of the HVDC grid, wind farm, and the controllers. In this paper, a linear multivariable feedback control system (FCS) model is proposed to represent the dynamic characteristics of HVDC grids and their controllers. The FCS model can be used for different dynamic analyses in time and frequency domains. Moreover, using the FCS model the system stability is analyzed in both open- and closed-loop forms. The standard eigenanalysis identifies the modes of only the closed-loop system and detects the pertaining state variables. The open-loop model, in the frequency domain, is a complementary tool that helps to have more intuitive insight into the system stability. A four terminal HVDC grid with two OWPPs and two AC grids is used for simulations and verification of the proposed FCS model. [ABSTRACT FROM AUTHOR]

This paper provides an overview of two technologies for connecting offshore wind power plants (offshore WPPs, OWPPs) to high-voltage direct current (HVDC) networks: voltage source converters (VSCs) and diode rectifiers (DRs). Current grid code requirements for the connection of such power plants are also addressed, and their implications when using such technologies are discussed. [ABSTRACT FROM AUTHOR]