The flexible DC transmission scheme based on modular multilevel converters (MMC) is currently the mainstream solution for offshore wind power delivery. With the ongoing growth in both installed capacity and the offshore distance of wind power projects, the dimensions and weight of corresponding offshore converter stations have increased substantially. These developments present significant economic constraints and engineering challenges, thereby complicating the deployment of large-scale, long-distance offshore wind energy systems.Compared with MMC, diode rectifier units (DRU) offer advantages such as compact size, light weight, low cost, reduced operating losses, and high reliability. Nevertheless, DRUs lack active control capability, and conventional grid-following wind turbines cannot independently support the voltage of the offshore AC network, which severely limits their application in offshore wind scenarios with stringent economic requirements. Grid-forming control of wind turbines is an effective approach to address this issue. Given the widespread application of doubly-fed induction generators (DFIGs) in engineering practice and their relatively low capital costs, this study investigates the implementation of grid-forming control strategies in DFIGs to address the stability challenges of DRU based HVDC transmission systems during fault conditions.First, the mathematical model of the DFIG is established. Then, a suitable control strategy is designed to endow the turbine with certain grid-forming capabilities. Finally, the developed simulation model and control strategy are verified in PSCAD/EMTDC. The results demonstrate that the proposed grid-forming DFIG control strategy can maintain stable offshore AC voltage and frequency under various fault conditions, ensure continuous and reliable operation of the DRU, and achieve fault ride-through. This method provides an effective solution for low-cost, highly reliable offshore wind power DC transmission.

To overcome the application limitations of point-to-point ultra-high-voltage direct current (UHVDC) schemes in large-scale renewable energy integration, this paper proposes a multi-terminal UHVDC system with parallel line commutated converter-modular multilevel converter (LCC-MMC) rectifiers for transmitting power from 10-GW-scale, geographically dispersed photovoltaic (PV) bases. At the sending end, two parallel LCC-MMC rectifier stations realize partitioned grid integration and power export of the PV base; at the receiving end, two inverter stations are connected directly to different load centers. First, the steady-state control strategy of the proposed multi-terminal UHVDC system is introduced. Then, fault ride-through strategies are developed, including current-limiting strategies and an inter-station disturbance-isolation control for sending-end AC faults, and a diode-valve-based selective DC-fault isolation scheme for DC faults in different line sections. Furthermore, dual operating modes and an online mode-switching strategy are presented to accommodate low PV-output conditions. In addition, a techno-economic assessment is performed to quantify the advantages of the parallel LCC-MMC rectifier. Finally, PSCAD/EMTDC simulations verify the effectiveness of the proposed multi-terminal UHVDC system and the associated control strategies.

Due to the negative resistance effect of power electronic devices, power systems with a high proportion of renewable energy face a significant resonance risk. To address this, this paper proposes a resonance-suppression strategy for high-penetration renewable energy systems based on an active amplitude and phase corrector (APC). Firstly, by considering its internal dynamics and complete control loops, the impedance model of the APC is derived. Next, the similarities and differences between resonance stability and harmonic resonance are analyzed using the s-domain and frequency-domain admittance matrices, concluding that resonance suppression for low-damping s-domain modes can be handled in the frequency domain. Then, a supplementary APC control strategy in the abc-frame is proposed, which improves impedance magnitude at specific frequencies while keeping the phase almost unchanged. Finally, the proposed strategy is validated through case studies on an offshore wind power system in Zhejiang Province.

This paper presents a frequency support strategy for the diode rectifier unit (DRU)-high-voltage direct current (HVDC)-based offshore wind power integration system, which coordinates multiple power sources without communication to reduce receiving grid frequency fluctuations. First, based on the deduced DRU's frequency transfer characteristic, a fine-designed ripple carrying frequency information is superimposed on the HVDC link, transferring the onshore frequency to offshore wind turbines (WTs) via the DC ripple and coupled AC harmonic without communication. Second, multiple power sources are utilized for frequency support, including HVDC capacitance and grid-forming WTs combined with energy storage systems, and appropriate sources are activated in the order specified by the designed thresholds. Finally, the effectiveness of the proposed frequency support strategy is verified by simulations in PSCAD/EMTDC.

This paper investigates the overvoltage characteristics and insulation coordination for a medium-frequency Diode Rectifier Unit-Modular Multilevel Converter (DRU-MMC) HVDC system for offshore wind. Switching overvoltages under various faults are analyzed via PSCAD/EMTDC simulations at operating frequencies from 50 Hz to 200 Hz. Results show that a higher frequency increases the maximum overvoltage at several offshore station locations and alters the worst-case fault types. Subsequently, after implementing a surge arrester configuration, simulations were performed for 20 fault scenarios to ultimately determine the required insulation levels for the main equipment. Under the arrester configuration scheme selected in this paper, the equipment’s insulation level is not affected by the increase in the collection system’s frequency.

With the rapid development of large-scale offshore wind farms, efficient and reliable power transmission systems are urgently needed. Hybrid high-voltage direct current (HVDC) configurations combining a diode rectifier unit (DRU) and a modular multilevel converter (MMC) have emerged as a promising solution, offering advantages in cost-effectiveness and control capability. However, the uncontrollable nature of the DRU poses significant challenges for system stability under offshore AC fault conditions, particularly due to its inability to provide fault current or voltage support. This paper investigates the offshore AC fault characteristics and fault ride-through (FRT) strategy of a hybrid offshore wind power transmission system based on a diode rectifier unit DRU and MMC. First, the dynamic response of the hybrid system under offshore symmetrical three-phase faults is analyzed. It is demonstrated that due to the unidirectional conduction nature of the DRU, its AC current rapidly drops to zero during faults, and the fault current is solely contributed by the wind turbine generators (WTGs) and wind farm MMC (WFMMC). Based on this analysis, a coordinated FRT strategy is proposed, which combines a segmented current limiting control for the wind-turbine (WT) grid-side converters (GSCs) and a constant AC current control for the WFMMC. The strategy ensures effective voltage support during the fault and prevents MMC current saturation during fault recovery, enabling fast and stable system restoration. Electromagnetic transient simulations in PSCAD/EMTDC verify the feasibility of the proposed fault ride-through strategy.

This paper analyzes the transient angle stability between synchronous generators (SGs) and grid-forming voltage source converters (GFM-VSCs) considering backward-swing dynamics. The impact mechanism of current saturation control (CSC) on the backward-swing and forward-swing stability is addressed. Based on the second-order rotor motion equation, the conventional energy-based method for transient stability evaluation becomes rough driven by the interactions between SG and GFM-VSC. To overcome the barrier, a forward-time integral (FTI) method is proposed for calculating the critical clearing time (CCT). We discover that the CSC can affect the stable and unstable equilibrium points of the post-fault system, leading to both the backward-swing and forward-swing synchronism instability. Based on the mechanism, an enhanced synchronization control (ESC) is proposed. It combines a modified frequency control with a current-limit strategy to avoid backward-swing instability while enhancing forward-swing stability. Time-domain simulation verifies the theoretical analysis as well as the effectiveness and robustness of the proposed ESC.

DC transmission scheme for medium-frequency grid-forming offshore wind farms is the highly anticipated offshore wind power project. Its rectifier is diode rectifier unit (DRU), which is involved in the issue of harmonics. This paper uses time-domain simulation to investigate the harmonic of DRU based a 12-pulse rectifier configuration under non-ideal operating conditions. The analysis establishes the harmonic profile of DRU with an ideal condition as the reference baseline, against which various non-ideal scenarios are simulated, encompassing both asymmetric commutation reactance and AC supply background harmonics in simulation software PSCAD/EMTDC. The investigation systematically evaluates seven distinct cases: (1) commutation reactance variations (both above and below nominal values), and (2) background harmonic contamination comprising negative-sequence fundamental component, positive- and negative-sequence second harmonics, and positive- and negative-sequence third harmonics. The simulation results quantitatively assess how 1

China’s offshore wind power is rapidly developing towards the direction of “deep-water and far-shore, large-scale, and clustered”. Existing offshore wind power transmission schemes based on the MMC are technologically mature but highly expensive. Although transmission schemes based on the DRU possess economic advantages, they lack AC voltage support and reverse power flow capability. To combine the control performance of the MMC and the economic advantages of the DRU, this paper proposes a heterogeneous DC transmission system for offshore wind power based on the parallel operation of MMC-HVDC and DRU-HVDC, which can realize the clustered transmission of deep-water and far-shore wind power. First, the configuration scheme of the system is introduced, and the basic control strategy is proposed. Secondly, the small-signal model of the system is established, and the small-signal stability analysis is conducted. Then, the control strategies for the system under near-zero power conditions and AC/DC faults are proposed, respectively. Finally, the effectiveness of the proposed topology and control strategies is verified through PSCAD electromagnetic transient simulations.

To enable ultra-long-distance power delivery from renewable energy bases in remote desert and Gobi regions, this paper proposes an ultra-high-voltage direct current (UHVDC) hybrid system featuring a decentralized integration and centralized transmission architecture. On the sending end, two hybrid rectifier stations, each combining line commutated converter (LCC) and modular multilevel converter (MMC) in series, locally integrate dispersed renewable clusters and transfer the collected power to a unified UHVDC backbone. On the receiving end, two inverter stations, each comprising a high-power diode valve in series with an MMC, deliver power to geographically distributed load centers. This architecture enhances transmission-corridor utilization and overall system performance. A steady-state control strategy is developed for the rectifier, employing a differential-mode single-loop control for the MMC. In addition, comprehensive fault ride-through strategies are devised for sending-end and receiving-end AC faults as well as multi-section DC line faults, enabling selective fault isolation while mitigating transient overvoltage and overcurrent stresses. PSCAD/EMTDC simulation results validate the effectiveness of the proposed system and controls.

To address the high cost of offshore wind power transmission, a promising low-cost transmission technology, namely the medium-frequency uncontrolled rectification technology, has been proposed. Compared with flexible DC transmission technology, it offers advantages such as a simplified sending-end system, smaller platform size, lighter weight, and lower costs. However, there are still many challenges to overcome in the practical application of medium-frequency DRU-MMC transmission system in offshore wind power transmission projects, such as overvoltage suppression strategy. This paper studies the overvoltage suppression strategies for the medium-frequency DRU-MMC transmission system, including arrester-based overvoltage suppression strategy, DC energy dissipation devices using half-bridge submodules, and active voltage reduction method on the offshore AC system. Finally, case simulations are conducted in PSCAD/EMTDC to verify the feasibility of the studied overvoltage suppression strategies, followed by comparative analysis.

A fast increasing proportion of renewable energy is the main development trend in today's power systems. The modular multilevel converter (MMC) station adaptable for both strong and weak grids is a potential solution when connecting to AC grids with a high proportion of power electronics. In this paper, a synchronous machine imitation control scheme of the MMC stations is proposed. The proposed control scheme consists of the basic control for normal operation and the additional control for AC fault riding through. With the basic control, the MMC station behaves as a synchronous machine. Then, based on small disturbance stability analysis, the low frequency damping characteristics are analyzed, demonstrating the proposed control's adaptability to both strong and weak grids under small disturbance. For the additional control, its performance under large disturbance is studied by time domain simulation. The time domain simulations are carried out in PSCAD/EMTDC for both the weak and strong grids, and the feasibility of the additional control under AC fault is also verified.

The diode rectifier unit (DRU) based transmission scheme is the technology hotspot in the field of transmission scheme for offshore wind farms. DRU not only realizes the rectification but also brings harmonic problems to the system. Firstly, in this paper, a general mathematical model is built for the multiple rectifier circuit commonly used in DRU, and the frequency and the current harmonics on the AC side are derived theoretically, and the harmonic characteristics of the 12-pulse and 24-pulse rectifier circuits are compared and analyzed. Secondly, the equivalent circuit is constructed for the wind turbine, submarine cable, DRU and AC filters. Thirdly, taking an offshore wind farm integration project as a case, DRU adopts the 12-pulse and 24-pulse rectifier circuits to build a 257-node offshore power system model, respectively. The voltages of harmonics at the point of common coupling (PCC) are calculate under different orders of harmonics, and the corresponding AC filters are designed reasonably. The offshore wind power system with different DRU rectifier without AC filters at PCC is simulated by PSCAD/EMTDC software to verify the accuracy of the harmonic calculation method proposed in this paper. Finally, the harmonics of DRU using different rectifier circuits before and after the installation of AC filters are compared respectively, and the harmonic characteristics and filtering schemes of transmission scheme for offshore wind farms are summarized.

In this study,a novel parallel converter-based hy-brid high-voltage direct current(HVDC)system is proposed for the integration and delivery of large-scale renewable energy.The rectifier uses the line commutated converter(LCC)and low-capacity modular multilevel converter(MMC)in parallel,while the inverter uses MMC.This configuration combines the economic advantages of LCC with the flexibility of MMC.First-ly,the steady-state control strategies are elaborated.The low-ca-pacity MMC operates in the grid-forming mode to offer AC voltage support.It also provides active filtering for the LCC and maintains the reactive power balance of the sending-end system.The LCC efficiently transmits all active power at the rectifier side,fully exploiting its bulk-power transmission capa-bility.Secondly,the fault ride-through strategies of both the AC faults at two terminals and the DC fault are proposed,in which the MMCs at both terminals can remain unblocked under vari-ous faults.Thus,the proposed system can mitigate the impact of the faults and ensure continuous voltage support for the sending-end system.Finally,simulations in PSCAD/EMTDC verify the effectiveness and performance of the proposed system.

The medium-frequency collection and parallel hybrid DC transmission scheme via DRU-MMC for offshore wind power has become a significant option for far-sea wind power development due to its economic advantages. However, existing research primarily focuses on control strategies for this scheme, with limited studies on system overvoltage and insulation coordination. This study first presents the topology and main circuit parameters of the converter station operating at 120Hz. Subsequently, overvoltage fault points are selected, and simulation analyses are conducted under 25 fault conditions using PSCAD/EMTDC software. Based on the simulation results, the overvoltage characteristics of the offshore wind power medium-frequency collection and MMC-DRU parallel DC transmission system are summarized. Finally, a suitable arrester configuration scheme and parameters are proposed for the system. After arrester installation, simulations under the 25 fault conditions are performed to analyze the stresses on different types of arresters, and the insulation levels of the converter station equipment are determined.

Offshore wind power in deep and far offshore areas is one of the important sources of power supply increment along the eastern coastal regions of China. However, with the gradual increase in the grid-connected renewable energy represented by offshore wind power, the trend of “hollowing out” in the power system has become more pronounced. In order to provide solid and reliable frequency support for the onshore main AC power grid, this paper innovatively proposes a grid-friendly control strategy based on the system where offshore wind power is transmitted through a medium-frequency MMC. This paper first introduces the system structure and basic principles, then proposes a communication-less main AC grid frequency support control strategy, and conducts simulation verification in PSCAD. The research results show that this strategy can significantly reduce the frequency deviation of the onshore power grid and enhance the stability and resilience of the grid.