This article describes a hybrid topology of high-voltage direct current (HVDC) for offshore wind farms using a series connection of a voltage source converter (VSC) and six-pulse diode rectifier (6P-DR). In this topology, the offshore side VSC (OF-VSC) acts as a grid-forming converter to maintain the PCC (point of common coupling) voltage of offshore wind farms (WF) and frequency. In addition, the OF-VSC functions as an active power filter to suppress the 5th, 7th, 11th, and 13th order harmonic current components produced by the 6P-DR, making it almost sinusoidal. Due to the 6P-DR being used in the hybrid converter, this new configuration reduces the total cost of the converters and losses, while preserving the power flow to the onshore gird. Compared to the fully-rated converter and hybrid converter based on a 12-pulse diode rectifier, the power loss and cost are reduced, and in addition, the proposed hybrid converter does not require a phase shift transformer nor a high number of diodes. A 200 MW in an HVDC transmission system using the hybrid configuration was simulated in PSCAD. The results show that the system operated correctly and the harmonic components were filtered.

Hybrid HVDC systems have been proposed as an alternative for nominal VSC-Based HVDC for offshore applications. Hybrid HVDC systems consist of an offshore power station composed of the connection of high-power diode rectifiers in series with a fractional power VSC-HVDC. This hybrid configuration allows large power transfer from offshore sites, with the added robustness, simplicity and efficiency of uncontrolled rectifiers. In this research, a robust and fast-acting controller, the Two Degrees of Freedom Internal Model controller (2DF-IMC), is used to control the active power filter features of the fractional-power VSC-HVDC system, resulting in a much faster overall THD reduction in the offshore AC currents in dynamic conditions (i.e. time-varying wind power) when compared with standard active power filter controllers. This improvement is the direct consequence of the fast closed-loop dynamics of the 2DF-IMC controller that do not require filtering stages. Additionally, the increased closed-loop response time did not affect the overall robustness of the control system, thanks to the enhanced disturbance rejection capabilities of the 2DF-IMC configuration.

A key challenge to enable the interoperability of a Multi-Vendor-Multi-Terminal (MVMT) HVDC network is to assess the stability without requiring open sharing of the vendor Intellectual Property (IP) relating to control functions. An analytical criterion is therefore proposed as a first step of this assessment. The criterion is indexed by the margin against loss-ofequilibrium for a MVMT-HVDC network with terminal behaviour of connected converters. Based on a classical control architecture, a static analytical model is established, including relevant parameters within the DC network, its topology and operation. By linearizing the system at 0 Hz, the principle of assessing the singularity of the matrix of extended conductance is proposed and proved with the theorem of implicit function and principle of analytic continuation. Two types of scaler index are proposed and then normalized to indicate the margin against loss-of-equilibrium. The effectiveness of the indices is verified and analysed with simulations in the environments of both Matlab/Simulink and RTDS with pseudo-steady-state and detailed Electro-Magnetic Transient (EMT) modelling, respectively. This approach attempts to represent one MVMT control scheme to support practical specification, testing and demonstration of the first multi-vendor multi-terminal HVDC control system outside of China.