Grid-forming wind turbine generators (WTGs) can provide independent and rapid support for grid frequency and voltage, which are the key equipment to construct the new power system. Based on the principle of autonomous sensing of DC-link voltage to grid frequency, the inertial-synchronization control (ISynC) makes the WTGs synchronize with the grid and perform frequency response, which is an effective way to implement grid-forming WTGs. However, when grid faults occur, the stable control of the DC-link voltage cannot be realized due to the limited output power of the grid-side converter (GSC), resulting in the GSC losing the synchronization function. In addition, the transient overcurrent problem of the ISynC under fault conditions is prone to take place, leading to the WTGs out of operation. To solve the above problems, this paper analyzes the impact of grid faults on the ISynC, including the damaged synchronization mechanism of the DC-link voltage and the cause of the transient overcurrent problem. Then, a transient control strategy that comprehensively coordinates grid synchronization, transient overcurrent limitation, and unloading circuit operation is proposed. Finally, the effectiveness of the proposed control strategy is verified using the Bladed+RTDS hardware-in-loop experimental platform.

With the integration of high proportion of distributed generation (DG), voltage violation may occur at the point of common coupling (PCC) of feeders in rural lower voltage distribution networks (LVDN). The DC link, with its advantages of large capacity and low line losses, can connect two or more feeders, facilitating broad area power and voltage regulation. At present, the voltage violation solutions based on DC link can be divided into communication-dependent and communication-less scheme. Given the randomness, rapidity, and real-time nature of voltage violation, communication-dependent solutions typically require fast, real-time communication, leading to high costs and significant resource consumption. Existing communication-less solutions, such as the voltage consensus method, while solving voltage violation, incur substantial power losses due to the need for real-time power interactions. This paper proposes a cooperative AC / DC voltage margin control scheme, which takes the DC voltage information as the medium, and can mitigate the voltage violation of rural LVDN under different operation conditions by coordinating the voltage source converters (VSCs) at the end of the feeders and energy storage system (ESS) without communication. In addition, by reasonably setting different AC and DC voltage thresholds, it can avoid unnecessary power interaction between different feeders and reduce the power loss. Finally, the feasibility and effectiveness of the proposed control strategy are verified by simulation.

The extensive integration of new sources and loads threatens the safe operation of the conventional ac distribution networks. The soft normally open point (SNOP) is proposed as an efficient solution. However, the mainstream back-to-back modular multilevel converter (MMC) scheme requires numerous devices and capacitances, which is not conducive to popularization. Lately, a novel modular multilevel converter with multiple MVac ports (MACP-MMC) has been introduced, offering a significant reduction in the construction cost and volume. Nonetheless, the MACP-MMC has some application limitations: when the MMC's ac output current is minimal, the MACP-MMC's power balanced control submodule has insufficient ability to maintain the internal energy balance, leading to its failed operation. To address this issue, this manuscript presents an enhanced MACP-MMC (eMACP-MMC) that features wide adjustment range. The proposed eMACP-MMC improves the connection structure between the MMC part and the multiport interconnection module part with small cost, thus offering an extra energy exchange path with the circulating current. On the one hand, eMACP-MMC can work as “MACP-MMC mode” under the joint supply or consumption scenario. On the other hand, the internal energy balance can be achieved by actively injecting fundamental-frequency circulating current under the flexible interconnection scenario. Consequently, the eMACP-MMC is suitable for all the application scenario of SNOP. Meanwhile, the circulating current's other degree of freedom is used for eliminate the imbalance between the MMC's arms caused by the former component. Additionally, considering the shortcomings caused by circulating current, the eMACP-MMC's redundant degrees of freedom are used to minimize the injected circulating current. Then, the novel operating principle and the control strategy of eMACP-MMC are researched.

Large-scale offshore wind farms have become the development trend in wind power generation. Long submarine cables are used to collect electrical energy in the collection networks of offshore wind farms. However, the distributed capacitance of cables cannot be ignored, which makes the impedance of the collection network complex and changeable. It is common to encounter high-frequency harmonic resonance (HFHR) problems when cables interact with wind turbine generators (WTGs). The HFHR may threaten the safe and stable operation of wind turbines. In order to solve this problem, firstly, the impedance of a collection network was constructed. Furthermore, the wind farm collection network was divided into the equivalent wind turbine subsystem (EWTS) and the remaining equivalent wind farm subsystem (REWFS). Then, the mechanism of HFHR was revealed based on the impedance stability analysis method. The effects of cable parameters, the number of connected WTG, and the grid impedance on the HFHR of the interconnected system were also analyzed. Finally, the hybrid damping control method, combining active damping control (ADC) and passive damping control (PDC), was proposed to suppress the resonance. A simulation and experiment were performed to verify the effectiveness of the analysis results and proposed control method.

Improving the short-circuit current capability of converters can directly enhance the low-voltage ride-through (LVRT) performance of doubly-fed induction generator (DFIG) based wind turbine systems (WTS). Previously, this approach was limited by power semiconductor modules and costs. Recent advances in materials and modulation strategies have greatly boosted their short-term overcurrent capability (up to 3.0 p.u. for 3 s) with only an approximate 3.3 % increase in the total wind turbine cost. This progress makes it necessary to assess the potential and challenges in its practical applications in LVRT for grid-forming wind turbines. First, the grid-forming control strategy for DFIG-based WTS under symmetrical grid faults is presented and the transient model is established. Second, new boundaries of the WTS are quantitatively examined regarding the controllable region, the power support capability, and the transient stability margin. Compared to before, the enhanced power modules enable wind turbines to better support voltage and expand the stable operation area without activating crowbar circuits. Third, issues of dc-link voltage drops and fluctuations when DFIGs operate at subsynchronous speeds during LVRT are studied. Results indicate that reducing the proportional coefficient of the rotor-side converter current loop and switching the grid-side converter to active power priority mode can solve this problem. Finally, theoretical results are verified by real-time simulations and experimental results.

Implementing grid-forming (GFM) control is increasingly considered as a solution to facilitate power-electronics-dominated power systems. Despite its potential, limited by the thermal stress of power devices, the excessive output current under grid faults needs to be curtailed. The low-level fault current of GFM converters will significantly reduce the fault voltage support capability compared with conventional synchronous generators. Although direct modifications to the physical structure of power devices, i.e., oversizing insulated-gate bipolar transistors, can be effective to some extent, the additional costs and value streams for overdesigning have not been clearly defined. To fill this gap, an active thermal control of GFM converters that utilizes the inherent current tolerance of power devices is proposed in this article. This method enables the online monitoring of junction temperature via a frequency-domain electrothermal model. Moreover, the switching frequency and modulation strategy are dynamically adjusted based on estimated temperature to enhance the short-time fault current output capability during grid faults. The proposed method is verified through comprehensive simulation, and the experiments based on a practical 1-MW converter are conducted to demonstrate the effect of fault current enhancement.