Kailun Wang

A line commutated converter (LCC)-assisted DR (LCCA-DR) is proposed for the integration of offshore wind farms (OWFs). The LCCA-DR involves connecting a small-capacity LCC in series with the DR on the dc side. This configuration adds a new control freedom for the active power transmission of the high voltage direct current (HVdc) link, allowing for local power balancing control of OWFs. As a result, the wind turbine converters (WTCs) can be designed with classical P/f and Q/V-based grid forming (GFM) control schemes, functioning as virtual synchronous generators (VSGs). In conjunction with the VSG-controlled GFM-WTCs, the LCCA-DR acts as a virtual governor (VG) that effectively regulates the frequency of the offshore ac grid, enabling synchronized operation. The VSG-VG control framework facilitates power decoupling and provides advantages in terms of control performance and stability. Since the adjustment of the HVdc link's active power requires only minimal variation in the dc voltage, the LCC's proportion in the hybrid converter is very small. This ensures that costs, reactive power, harmonic filtering, and losses do not significantly increase compared to traditional pure-DR stations. The effectiveness of the proposed solution is verified through time-domain simulations.

Grid-forming (GFM) converter is a promising interface for integrating renewable energy sources. During grid faults, GFM converters inherently contribute to reactive power support, while current limiting can significantly affect the reactive power characteristics and pose challenges for grid code compliance. In this study, the impacts of virtual impedance (VI)-based current limiting on reactive power characteristics of GFM control are investigated. It reveals that the VI may cause phase offset and deterioration of the natural and instant reactive power support capability, and the subsequent reactive power response faces a coupling with the active power control due to the current magnitude limitation. Leveraging these characteristics, an enhanced fault ride-through (FRT) strategy is proposed. Firstly, the VI impedance angle is recommended to match the equivalent connection impedance, which inherits the natural power support capability of GFM converters and improves instant voltage support capability. Secondly, an FRT strategy based on dynamic active power limitation is proposed for fulfilling the grid code and enhancing the subsequent reactive power support. The effectiveness of the proposed strategy is validated through comparative time-domain simulations.

The connection of offshore wind farms (OWFs) via diode rectifier-based- high-voltage direct current (DR-HVdc) transmission requires the use of wind turbine (WT) converters with grid-forming (GFM) capability. However, active and reactive power coupling and oscillation issues have been revealed in WTs employing reactive power frequency ( Q/f ) droop-based control schemes. This paper proposes a GFM control for OWFs based on remote active power regulation. The proposed scheme remotely regulates the active power of the DR-HVdc link by adjusting the dc voltage of the onshore modular multilevel converter (MMC). As a result, the voltages of offshore point of common coupling and WT converters are freed from being controlled for active power. On this basis, the WT converters can be equipped with classical GFM control schemes that incorporate active power frequency ( P/f ) and reactive power voltage ( Q/V ) droop relationships. The offshore frequency acts as an indication of the OWF active power balance and it is measured and transmitted to the onshore MMC through a communication channel to close the remote active power control. Comparative studies of power coupling characteristics and stability reveal that the proposed control scheme can achieve efficient decoupling control of active and reactive power in WT converters, prevent oscillation risks, and significantly enhance the system's stability. The feasibility and advantages of the proposed control scheme are verified through comparative time-domain simulations.

A new grid-forming (GFM) high-voltage direct current (HVdc) rectifier, named flexible line-commutated converter (Flex-LCC), is proposed in this article to collect the power generated by large-scale islanded or weak-grid-supported renewable energy systems. The Flex-LCC uses a series-connected hybrid valve based on the LCC and full-bridge modular multilevel converter (FBMMC), and adopts a new control. As a result, the Flex-LCC works as a GFM rectifier and retains the same dc voltage and power carrying capacity, dc fault clearing, and power reversal functions as an LCC. Particularly, the Flex-LCC has three salient features: the control of the Flex-LCC is decoupled from the LCC part's firing angle, the internal reactive power allocation is flexible, and the valve-side ac voltage of the FBMMC part has a wide operation range due to the decoupling of dc-port voltage and capacitor voltage. These bring reduced interharmonics, enhanced stability, and a smaller capacity boundary to the FBMMC part for the Flex-LCC. Furthermore, utilizing the reasonable design for the actual capacity and valve-side ac voltage of the FBMMC, the Flex-LCC can effectively balance cost, footprint, and weight, and match the current-carrying capacity between the series-connected LCC and FBMMC. The topology, control, characteristics, and design of the Flex-LCC are analyzed in detail in this article. Simulations and experiments validate the theory.

Increasing the modulation index by utilizing the negative voltage states of full-bridge submodules (FBSMs) can greatly reduce capacitor usage of modular multilevel converters (MMCs), thereby optimizing the cost and volume. The hybrid MMC is composed of half-bridge submodules (HBSMs) and FBSMs, and the capacitor voltages of the two types of submodules (SMs) have different shapes as long as negative voltage states exist. This condition greatly complicates the analysis and design of the energy storage requirement of the hybrid MMC, which utilizes the negative voltage states of FBSMs to boost the AC voltage. A numerical calculation method for solving the capacitor voltages and designing the capacitances of FBSMs and HBSMs is proposed in order to accurately determine the minimum energy storage requirement considering the difference between the energy variations in FBSMs and HBSMs. In the numerical calculation, the energy storage and voltage of the arm are decomposed into FBSM and HBSM parts. According to the physical switching process, the output voltages of FBSM and HBSM parts are determined separately. The one-cycle waveforms of the capacitor voltages are then obtained by numerical integration of the power flows in FBSM and HBSM parts. An iterative solution procedure and the termination criterion that can ensure the accuracy of the obtained one-cycle waveforms are also proposed. Using the numerical integration and iterative solution procedure as the kernel algorithm, the proposed method can accurately analyze the capacitor voltages of the FBSMs and HBSMs and determine the minimum energy storage requirement of the hybrid MMC. Furthermore, the proposed method is applicable for various operating working conditions and various proportions of FBSMs. The simulation results verify the feasibility and accuracy of the analysis and design method.