This paper presents a novel decentralized control for offshore wind farms connected to the onshore grid through a high voltage direct current link by means of a diode rectifier. The proposed control system is implemented in each wind turbine generator system (WTGS). The capacitor placed at the filter of the wind turbine front-end converter is used for the proposed control implementation. Frequency control is achieved by aligning the capacitor voltage vector along a reference axis rotating at the reference frequency. Then, a frequencyreactive power droop control allows the synchronization of all the WTGSs. On the other hand, this droop strategy also leads to total reactive power sharing among WTGSs without relying on communications. An additional secondary frequency control is also implemented to compensate the frequency deviation caused by the droop control. The proposed control system has been validated by simulation and results demonstrate the appropriate performance even during start-up and faults.

This paper presents a direct frequency control for offshore wind farms (OWFs) connected to the diode rectifier (DR)-based high voltage direct current (HVDC) link. The frequency control is guaranteed through reactive power balance at the DR station without a capacitor bank placed at the DR station; while AC-voltage magnitude is clamped by the DR, provided that the HVDC link voltage is imposed by the onshore inverter. The control system is implemented by a voltage source converter (VSC) connected to the DR station. The VSC is also used to compensate harmonic currents, leading to passive AC-filters being also removed from the DR station. The proposed control system provides comparative advantages in terms of DR station volume and OWF installation cost with other centralized controls where besides a VSC, a high-voltage large capacitor bank is needed to be placed at the DR station. A new average-value model (AVM) of the DR system is derived to test the proposed frequency control, while the stability of the system is evaluated through a small-signal analysis. The proposed control system performance and the accuracy of the proposed AVM are verified through a detailed switching model of the system using operation of a cascaded H-bridge VSC.

This paper presents a control of the parallel operation of diode rectifier based high-voltage direct current (DR-HVDC) and voltage source converter based HVDC (VSC-HVDC) links. The two links are connected to offshore wind farms (OWFs). The VSC-HVDC controls the voltage and frequency of the offshore AC-grid needed for the OWFs start-up. The proposed grid forming control is based on controlling the OWFs voltage by controlling the active power balance at the VSC-HVDC AC-bus. When the OWFs inject more power than a certain active power reference established for the VSC-HVDC, the OWFs voltage increases and the DR-HVDC starts conducting. Then, the offshore voltage automatically accommodates the required value for transmitting the OWFs power through the DR-HVDC link while the system frequency is controlled by the VSC-HVDC through satisfying the reactive power balance. The power transmitted through the VSC-HVDC can be set by adjusting the corresponding power command while the DR-HVDC will automatically close the power balance. The main advantage of the proposed control system is that the VSC-HVDC can be sized just to support the system energization; while once the OWFs are in operation, the DR-HVDC will be loaded automatically. Simulation results verify the proposed control system.

This paper presents a novel active power filtering (APF) scheme embedded in a centralised frequency control of an offshore wind farm (OWF) connected to a high voltage direct current link through a diode rectifier station. The APF is carried out by a voltage source converter (VSC), which is connected to the rectifier station to provide frequency control for the offshore ac-grid. The proposed APF scheme eliminates harmonic currents at a capacitor bank placed at the rectifier station. This leads to a significant reduction in the total harmonic distortion of the offshore ac-grid voltage, and thus, to an improvement in the OWF power. Hence, the rectifier passive ac-filter bank is not needed anymore. A new selective harmonic compensation method based on the dynamic phasor (DP) theory is used in the proposed APF scheme which allows the extraction of the phasor form of harmonics in dc-signals. Therefore, the well-known proportional-integral regulators are used for the harmonic current compensation. Moreover, the offshore ac-grid is modelled for the system harmonic analysis using a grid solution based on the DP theory. Finally, a VSC power rating analysis is studied. The performance of the proposal is validated by simulations in both steady-state and transient conditions.