Offshore AC Fault Protection of Diode Rectifier Unit-Based HVdc System for Wind Energy Transmission

Abstract

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.

Analysis and Control of Offshore Wind Farms Connected With Diode Rectifier-Based HVDC System

“…On the other hand, although Q-f droop control is used in [12][13][14][15][16] for WT reactive power sharing, the justification for its use and the design procedure of the Q-f droop gain kq have not been clarified and properly understood. Besides, the impedance between individual WT converter and the offshore PCC is predominately inductive, as the AC cable resistance and capacitance are much less than the combined inductances of WT converter line reactor and WT transformer leakage inductance.…”

Control of Offshore MMC During Asymmetric Offshore AC Faults for Wind Power Transmission

“…Based on the properties exhibited by (14) and (15), the equivalent sequence network for a phase-to-phase AC fault is drawn as shown in Fig. 6.…”

“…Further investigation conducted in [12] reveals that the reactive current injection during faults curtails the delivered active power and the ability to mitigate the oscillating term from DC side, while only has limited impact on the offshore grid voltage during faults. Due to serval constraints posed by the converters, the characterization of negative sequence current for offshore AC grid fault behaviors becomes increasingly important, particularly from the 2 protection point of view [1,13,14]. Unlike two-level VSCs, several studies on the behavior of modular multilevel converters (MMCs) under asymmetric AC faults reveal that a well-controlled MMC does not inject even harmonic currents into DC side thus exhibits no even harmonic voltage ripples in the DC link voltage when it operates with balanced or unbalanced AC currents and voltages [15][16][17].…”

Offshore Wind Power Integration into Future Power Systems: Overview and Trends

“…As DRU can only convert AC to DC [128], a hybrid topology combining DRU and VSC/LCC must be used, introducing the DRU as the offshore rectifier and the LCC/VSC as the onshore converter [129]. The main advantages of DRU-HVDC compared to LCC-HVDC and VSC-HVDC are [130][131][132][133]:…”