Offshore wind farm (OWF) is considered as a perfect zero‐carbon energy source for the future power system. However, the growing offshore distance and water depth of OWF make the OWF HVDC transmission technique a more promising solution than HVAC due to higher cost‐efficiency and reliability. In this paper, the current situation of OWF‐HVDC projects is introduced at first. Then, novel converter topologies with the higher power density and cost‐efficiency are presented, including the hybrid modular multilevel converter (MMC), alternative arm converter (AAC), and diode rectifier (DR). Next, several OWF HVDC transmission system topologies are introduced, including terminal‐hybrid, station‐hybrid and all‐DC delivered system. Furthermore, the key technologies for OWF HVDC operation and control are summarized, including grid‐forming control strategy for offshore wind turbines, stability analysis method, corresponding stability enhancement measures and frequency support control strategies. Additionally, the fault ride‐through and protection strategies for different fault locations have been presented. Finally, the main conclusions and prospects for OWF HVDC are summarized.

Highlights:

• Novel converter topologies have been introduced.

• Offshore wind farm HVDC transmission system topologies have been discussed.

• Key operation and control technologies have been presented.

• Fault ride‐through and protection strategies have been illustrated.

• Outlooks of future research direction have been addressed.image

The diode‐rectifier (DR)‐based high‐voltage direct current (HVDC) transmission is regarded as a low‐cost alternative to the HVDC transmission for offshore wind farms (OWFs) integration. However, the DR would generate harmonic voltages in the offshore grid, which greatly affects the power quality. To solve this problem, this paper utilizes a parallel‐connected small‐capacity auxiliary converter to suppress the harmonic voltages and provide black‐start energy for the OWFs. In addition, the submodule branches (SMBs) of the auxiliary converter can be used as part of the HVDC chopper. On this basis, a coordinated grid fault ride‐through (FRT) strategy is proposed in this paper. After a grid fault occurs, the HVDC chopper constructed by the auxiliary converter is able to constrain the DC‐link voltage. At the same time, the OWFs are controlled to reduce the output active power. As a result, the cost of the FRT can be significantly reduced. The proposed harmonic voltage suppression and the FRT method are verified through the simulation model in Simulink.

This paper studies the diode‐rectifier‐based high‐voltage direct current (HVDC) transmission system for offshore wind farms integration using small‐capacity auxiliary converters. The harmonic suppression control for the offshore grid voltages is proposed. Also, the fault ride‐through method of this system is introduced.image

In the process of accessing distributed power sources in microgrid, there will be serious coupling between active power and reactive power, which will lead to significant changes in the inverter outlet voltage and impedance and increase the circulating current phenomenon. The generation of circulating current will accelerate the aging of devices, reduce the inverter efficiency, and threaten the stable operation of the system. In this paper, the mechanism of circulating current generation is analysed, the basic characteristics of circulating current are studied, and an inverter parallel circulating current suppression strategy with adaptive virtual composite impedance and droop control dynamic adjustment algorithm is proposed. The virtual impedance setting can weaken the influence of line difference on system stability, make the system equivalent output impedance resistive, weaken the inductive device characteristics and reduce the circulating current amplitude; the adaptive droop algorithm implantation dynamically correlates the control power with the droop coefficient, enhance the power decoupling accuracy and weaken the circulating current influenced by power changes. The simulation platform and experimental setup established by the large‐scale simulation software PSCAD/EMTDC are used to study the basic scenarios of the inverter under two conditions of equal and unequal capacity, and the rationality of the proposed control strategy is tested.

This manuscript proposes an inverter parallel circulating current suppression strategy with adaptive virtual complex impedance and droop control dynamic adjustment algorithm. The virtual impedance setting can weaken the influence of line difference on system stability. The adaptive droop algorithm implantation dynamically correlates the control power with the droop coefficient, enhance the power decoupling accuracy and weaken the circulating current influenced by power changes.image

Cascaded H‐bridge rectifier (CHBR) is widely used in high‐power fields such as STATCOM and power electric traction transformer (PETT) due to its high efficiency and easy expandability. With the high‐reliability operation requirements of power equipment, the fault‐tolerance capability has gradually become a research hotspot nowadays. In order to improve the reliability of the power supply, cascaded H‐bridge rectifier should restore as soon as possible after a fault. However, the traditional fault‐tolerant strategies have a great effect on the grid side and a long recovery time. This paper proposed a quasi‐hot standby fault‐tolerant control strategy. Compared with the traditional strategies, the quasi‐hot standby fault‐tolerant control strategy has a faster recovery speed after the malfunction, and its transient recovery process has less impact on the power grid. Simulation and experiment results show the effectiveness and good recovery performance of the proposed fault‐tolerant control strategy.

Focusing on the problems of slow recovery speed and grid current distortion of the traditional fault‐tolerant strategies, a quasi‐hot standby fault‐tolerant control strategy is proposed, which can reduce the current distortion after converter failure and have a faster recovery speed, ensuring that the converter could quickly return to normal operation.image