This version is available at https://strathprints.strath.ac.uk/62289/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the Strathprints administrator: strathprints@strath.ac.ukThe Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output.This paper is a post-print of a paper submitted to and accepted for publication in IEEE Transactions on Power Delivery and is subject to Institution of Electrical and Electronic Engineering Copyright. The copy of record is available at IEEE Xplore Digital Library.Abstract-A distributed PLL-based frequency control is proposed in this paper for offshore wind turbine converters connected with diode-rectifier based high-voltage-direct-current (HVDC) systems. The proposed control enables a large number of wind turbines to work autonomously to contribute to the offshore AC frequency and voltage regulation. The proposed control also provides automatic synchronization of the offline wind turbines to the offshore AC grid. Stability of the proposed frequency control is analyzed using root locus method. Moreover, an active dc voltage control of the onshore modular multilevel converter (MMC) is proposed to ride-through onshore AC fault, where the onshore MMC converter quickly increases the dc voltage by adding additional submodules in each phase, in order to rapidly reduce wind farm active power generation so as to achieve quick active power re-balance between the offshore and onshore sides. Thus the overvoltage of the submodule capacitor is alleviated during the onshore fault, reducing the possibility of system disconnection. Simulation results in PSCAD verify the proposed control strategy during start-up, synchronization and under onshore and offshore fault conditions. Index Terms-diode-rectifier based HVDC, distributed PLLbased frequency control, offshore wind power integration, onshore AC fault, synchronization of wind turbine converters I. INTRODUCTION N Europe, onshore wind farms have already been well developed, and more attention has been drawn to offshore sites. Many large offshore wind farms will be developed in the ...

Offshore AC fault protection of wind turbines (WTs) connecting with diode rectifier unit based HVDC (DRU-HVDC) system is investigated in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during offshore AC fault transients and quickly provide fault current for fault detection. Considering different fault locations, the fault characteristics during symmetrical and asymmetrical faults are presented and the requirements for fault detection are addressed. A simple and effective offshore AC fault protection solution, combining both overcurrent protection and differential protection, is proposed by utilizing the developed fast fault current providing control. To improve system availability, reduced DC voltage of the DRU-HVDC system is investigated, where one of the series-connected DRUs is disconnected and the onshore modular multilevel converter (MMC) actively reduces DC voltage to resume wind power transmission. The proposed scheme is robust to various offshore AC faults and can automatically restore normal operation. Simulation results confirm the proposed fault protection strategy. Index Terms-diode rectifier unit based HVDC (DRU-HVDC), fault protection, HVDC transmission, offshore wind farm, symmetrical and asymmetrical AC faults.

His research interests include HVDC transmision system, grid integration of renewable power, power electronic converters, and energy conversion.

This paper analyzes the control and operation of offshore wind farms connected with diode rectifier based HVDC (DR-HVDC) system. A small-signal state-space model of the offshore wind turbines (WTs) connected with DR-HVDC system is developed to design the WT Q-f droop control. The use of WT P-V and Q-f control during individual WT active power variation is clearly clarified. In order to reduce the interaction between WT active power and reactive power, an angle feedforward control is proposed where an additional phase shift is directly added to the WT output voltage based on the WT's active power output. The effectiveness of the proposed control on improving dynamic response and reducing active and reactive power interaction is verified by frequency-domain analysis and time-domain simulations in PSCAD/EMTDC. Index Terms-diode rectifier based HVDC, interaction of WT active and reactive power, small-signal analysis, wind turbine control This work was supported by the European Union's Horizon 2020 research and innovation programme under Grant 691714.

This paper investigates the operation of offshore wind farm connected by parallel diode-rectifier based HVDC (DR-HVDC) and HVAC links. A secondary voltage control is proposed to control the offshore AC voltage amplitude by regulating the DC voltage of the DR-HVDC link. A secondary frequency control and a phase angle control are proposed to adjust the reactive power reference in the primary control, which synchronise the offshore point of common coupling (PCC) frequency and phase angle to those of the HVAC link. Such secondary voltage control, frequency control and phase angle control enable seamless transition from DR-HVDC mode to parallel mode. A tertiary power control scheme is further proposed to control the active power flow distribution between DR-HVDC and HVAC links through the regulation of PCC phase angle. To ensure smooth transition from HVAC mode to parallel mode, a virtual DC power control is proposed to control the virtual DC power at zero prior to the connection of the DR-HVDC link. A small-signal model of the parallel system is developed and the stability analysis is carried out for the proposed control scheme. Simulation results in PSCAD/EMTDC verify the proposed control under normal and fault conditions.

DC fault ride-through operations of the offshore wind farm connecting with diode rectifier unit (DRU) based HVDC link are presented in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during DC faults and to provide fault current. This contributes the control of the offshore AC voltage, which does not drop to zero but is remained relatively high to facilitate fast system recovery after clearance of a temporary DC fault. The WT converters operate on current limiting mode during DC faults and automatically restore normal operation after fault clearance. The full-bridge based modular multilevel converter (MMC) is adopted as the onshore station and its DC fault current control ability is explored to effectively suppress the fault current from the onshore station around zero, which reduces semiconductor losses and potential overcurrent risk of the MMC station. Simulation results confirm the robustness of the system to DC faults.Index Terms-DC fault protection, diode rectifier unit based HVDC (DRU-HVDC), full-bridge submodule, modular multilevel converter (MMC), offshore wind farm.I.

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