Line-commutated converter (LCC) is a promising choice for integrating large offshore wind farms. In such a system, grid-forming (GFM) control for wind turbine converters is necessary for establishing the offshore ac grid. However, during severe offshore grid ac faults, the GFM converter is susceptible to overcurrent and transient synchronization stability issues, given its internal voltage source nature. In this study, the offshore ac fault ride-through (FRT) issue is studied for the LCC-HVdc connected offshore wind farms. A unified model for the transient synchronization analysis considering current limiting is proposed. On this basis, key new factors, including current limiting and low emulated inertia are revealed to have detrimental impacts on the reliable FRT. A FRT strategy is proposed to address this problem, and simulations validate the feasibility of the method.

The series-connected hybrid diode rectifier (SCH-DR) based high voltage direct current transmission system (HVdc) for integrating offshore wind farm (OWF) confronts challenges in startup, which comprises of a largecapacity DR and a small-capacity auxiliary modular multilevel converter (A-MMC) in series-connection. To achieve DR’s inputting onto OWF’s grid in zero dc current while maintaining energy balance of offshore system, this study proposes a startup strategy based on coordinated energy balance method. Based on this method, OWF needs to generate certain active power to meet consumption of offshore necessary auxiliary devices. To dissipate the surplus power, the proposed method parallels an energy-consuming resistor on AMMC’s dc side. Hence, through its adjustable dc voltage capability, A-MMC can actively and smoothly regulate the dissipated power, thus realizing the coordinated energy balance. The proposed startup strategy exhibits feasibility and engineering practicality, without much escalating investment cost or space requirements. The effectiveness of the proposed startup strategy is verified in MATLAB/Simulink using a scenario of a 2000 MW/±500 kV SCH-DR-HVdc system.

Conventional modular multilevel converters (MMCs) face significant challenges due to their considerable weight and size in high voltage direct current (HVdc) transmission applications for connecting distant offshore wind farms (OFWs). A 1000 MW OFW is selected as a case study to explore the application of the unidirectional-current hybrid MMC (UC-HYB-MMC) in offshore HVDC transmission systems, aiming to harness its advantages of compactness, lightweight construction, and wide-range dc voltage output. Detailed design of the parameters of UC-HYB-MMC is conducted, and a preliminary design and comparison of the converter valve structure are carried out based on the designed parameters. The design results of the studied case show that compared with conventional HB-MMC, the UC-HYB-MMC scheme reduces the total footprint and volume of the converter valve by approximately 41%. This paper also proposes the control strategy for the HVdc transmission system based on UC-HYB-MMC. A system simulation model is established to simulate the startup process and operation control of the system, validating the feasibility of the HVDC transmission system based on the UC-HYB-MMC with asymmetric wide dc voltage for connecting distant OFWs.

Considering the voltage source characteristics, grid-forming (GFM) control is regarded as an effective method to enhance the voltage support capability of inverters, especially under weak grid condition. However, relatively mature grid-forming control methods such as virtual synchronous generator control typically assume a constant voltage source on the dc side, which is not suitable for the modular multilevel converter (MMC) that controls the dc voltage. To address this issue, a GFM control strategy based on the decoupled control is proposed. This strategy achieves grid synchronization based on the dynamic variation of submodule capacitor voltage rather than the dc voltage, by linking the swing equation of the synchronous generator with the energy dynamic equation of the MMC. Additionally, the energy stored in submodule capacitors can be flexibly adjusted to provide inertial responses for the connected grid. With the proposed GFM control, the MMC can provide voltage and reactive power support under weak grid condition. Meanwhile, the dc voltage can be regulated independently through the decoupled control of MMC. The effectiveness of the proposed control method is verified through the simulation results in MATLAB/Simulink.

The series connection structure of diode rectifier (DR) and modular multilevel converter (MMC) is a promising topology for offshore wind transmission due to its low cost, small footprint, and flexible control capability. But there has been no feasible startup strategy in engineering. And to achieve the startup, the onshore converter has to be able to adjust the dc voltage in a wide range, causing a large usage of semiconductor devices. This paper proposes a novel flexible diode rectifier high voltage direct current transmission system (FDR-HVDC) and its startup strategy for offshore wind farms. On the offshore side, the flexible diode rectifier (FDR) is formed by the series-connection of a small-capacity auxiliary full-bridged (FB)-MMC and a 12-pulse DR. For the onshore converter, an onshore hybrid converter (Ons-HC) is proposed to reduce the power device usage, which consists of a large capacity half-bridged (HB)-MMC and a small-capacity auxiliary FB-MMC in series. The proposed startup method separates the HB-MMC from the ac grid to enable the Ons-HC to have a wide dc voltage range temporarily. Meanwhile, the proposed method uses the dc dynamic braking resistance (DC-DBR) to ensure the capacitor balance of the HB-MMC and the reliable operation of the Ons-HC. For the connection of the DR, this paper proposes to utilize the voltage-limit resistance (VL-R) to achieve a slow dc voltage boost of DR, preventing the adverse impact on the entire system such as the dc surge current, dc over-voltage, etc. The simulation results of a 2000MW/±500kV FDR-HVDC showed that the proposed startup method can effectively realize a smooth and steady startup of the FDR-HVDC.

This study proposed a novel protective action for converter side phase-to-ground fault of FB-MMC-based bipolar HVDC. The faulty converter is kept in operation during fault, and its dc-link voltage reference is set to zero in order to avoid serious overvoltage of the sub-module capacitors. This protective action was verified by simulation. The overcurrent and overvoltage of converter arms are moderate when adopting this protective action.

High-voltage direct current system using modular multilevel converter (MMC-HVDC) is a potential candidate for grid integration of renewable energy over long distances. The dc-link fault is an issue MMC-HVDC must deal with. This paper proposed an enhanced MMC topology with dc fault ride-through capability. By using diode clamp sub-modules, the freewheeling effect of diodes is eliminated and fault currents can be very rapidly extinguished. Since the tripping of circuit breakers is avoided, MMC can immediately restart power transmission for non-permanent faults. The required rated voltage of additional semiconductors is half the conventional semiconductors, resulting in low extra cost. Simulation results using PSCAD/EMTDC have verified the validity of the proposed protection scheme.

Control methods for cascaded STATCOM with star configuration to compensate unbalance load are proposed in this paper, both in transformerless way and transformer-used way. According to the analysis results, when the STATCOM is directly connected to the power grid without transformer, a zero-sequence voltage is injected into its output reference voltage, the amplitude of which is about twice as the system voltage amplitude. When it is integrated into the grid through a DY1 transformer, a zero-sequence current should be injected into in its output current reference, the range of which reaches about twice as the original current needed to be compensated. PSCAD/EMTDC is used to simulate these two conditions and the simulation results show that both methods can compensate the unbalance load effectively.

In this paper, the application performance characteristics of high-frequency isolated bidirectional DC-DC converter (IBDC) based on SiC-DMOS and SiC-SBD samples provided by ROHM SEMICONDUCTOR Inc. are analyzed. The paper gave the basic design procedure of SiC-based converter, and established the mathematical model of power losses by analyzing the switching characteristic of the converter. On this basis, the power loss and efficiency characteristics of the SiC-based converter were analyzed and the related experimental results were presented. Theoretical analysis and experimental results show that the SiC-based converter has better performance than the Si-based converter; it will have a wide application prospect in the future smart electricity network.