For ultra-large-scale offshore wind bases, to achieve cost-effective and efficient transmission from offshore wind farms (OWFs) to distant load centers, this study proposes a transmission system that integrates multiple offshore low-frequency AC (LFAC) links and onshore high-voltage DC (HVDC) link. To integrate between multiple offshore LFAC links and onshore HVDC link, low-frequency flexible line commutation converter (LF-FLCC) is introduced, which comprises of LCC and low-frequency auxiliary MMC (A-MMC) in series connection. The LF-FLCC offers benefits of low cost, compactness, and DC fault clearance capability by utilizing LCC, while attains flexibility through A-MMC such as AC voltage support, DC power regulation, and active compensation. The LF-FLCC also possesses independent black-start capability through an auxiliary diode rectifier (A-DR). Through voltage boost and energy conversion by the LF-FLCC, the offshore wind power can be aggregated onto the DC bus, facilitating a continuous transmission to load centers through onshore HVDC links economically and efficiently. Based on proposed system, this study proposes the normal operation, black-start scheme, and DC fault clearance strategy. Besides, this paper analyzes the capacity requirement of the A-MMC. The effectiveness of the proposed system is verified in Matlab/Simulink using a scenario of a 10 GW large-scale offshore wind power base.

Transmission-line icing is a serious threat to the security safe and stable operation of power systems. The icing monitoring of transmission lines is of significance for the prevention of freezing disasters in power systems. However, the lack of effective ice detection methods has brought great trouble to the safe operation for 10 kV overhead transmission lines in freezing weather. To tackle this problem, a method for detecting ice thickness for 10 kV overhead transmission lines based on ultrasonic detecting technique is proposed in this paper, in which the distance from the ultrasonic sensor to the line changed by transmission lines ice-coating and the distance difference before and after ice-coating detected by ultrasonic being the ice thickness. An ultrasonic detection platform for ice thickness on the steel rod to simulate the line icing is established. The calibration experiment with the effect of distance, temperature, and wind velocity on the ultrasonic is carried out, and the experiment with ice thickness on the steel rod detected by the ultrasonic is carried out. Furthermore, the effect of ice shape on the detection is studied, and the reason for the large error is analyzed when the number of bulges on the ice surface is large. The results show that the relative error of ice thickness obtained by the method proposed in this paper is less than 10% under conditions of uniform ice and relatively smooth ice surface, compared with vernier caliper measurements. An improved method for ice thickness detection using the trough time as the ultrasonic echo signal time is proposed when the number of bulges on the ice surface is large, and the relative error value of the improved method is verified to be less than 5%.

This paper proposes an energy optimization management method of interaction between the smart energy hub (SEH) and users in the energy system. Internet of Things (IoT) and energy storage are also considered in the system. First, the models of the SEH and users are constructed. Then the profit models under the influence of energy storage are discussed. Next the Stackelberg game model is applied in this paper, where the SEH acts as the leader and the users are the followers. A double-layer algorithm is proposed to solve the problem. Finally, a case is studied to verify the correctness of the method. The results show that the method can be used for effective energy management in integrated energy system and IoT plays an important role in information exchange.

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.

Converter valves based on the modular multilevel converter (MMC) topology are the core of the voltage source converter-based high-voltage direct current transmission system, with press-pack insulated gate bipolar transistor (PP IGBT) modules being the key components. Studying the long-term aging failure mechanism of the PP IGBT under actual MMC operating conditions is of great significance for improving the safe and reliable operation of the transmission system. In this article, an electric–thermal–mechanical coupled multifield model of the PP IGBT device is built to analyze the influence of junction temperature and pressure on stress and strain distribution, identifying the weak points. Then, based on the alternating current (ac) power cycling tested rig, which equivalently reproduces the working conditions of the MMC submodule, the influence of different junction temperatures and pressures on long-term aging failure is studied, revealing the evolution of key characteristic parameters during the aging process. Two main failure modes are identified under ac power cycling tests. One mode is caused by overvoltage short-circuit failure due to the blocking voltage drop, and the other is induced by open-circuit failure due to uncontrolled gate current. Finally, the evolution of microscopic morphology and static characteristic parameters over the entire lifetime during the power cycling test are obtained, emphasizing the differences between ac and direct current (dc) power cycling tests. The results show that there are significant differences in overheating failures between ac and dc power cycling tests, highlighting the critical role of ac conditions in failure analysis and reliability assessment. The study of the long-term failure evolution mechanism of the PP IGBT device under actual MMC working conditions aims to guide operational management, control protection, and device optimization design for the reliability of MMC systems.

When offshore wind farms are connected to a hydrogen plant with dedicated transmission lines, for example, high-voltage direct current, the fluctuation of wind speed will influence the efficiency of the alkaline electrolyzer and deteriorate the techno-economic performance. To overcome this issue, firstly, an additional heating process is adopted to achieve insulation for the alkaline solution when power generated by wind farms is below the alkaline electrolyzer minimum power threshold, while the alkaline electrolyzer overload feature is used to generate hydrogen when wind power is at its peak. Then, a simplified piecewise model-based alkaline electrolyzer techno-economic analysis model is proposed. The improved economic performance of the islanded green hydrogen system with the proposed operation strategy is verified based on the wind speed data set simulation generated by the Weibull distribution. Lastly, the sensitivity of the total return on investment to wind speed parameters was investigated, and an islanded green hydrogen system capacity allocation based on the proposed analysis model was conducted. The simulation result shows the total energy utilization increased from 62.0768% to 72.5419%, and the return on investment increased from 5.1303%/month to 5.9581%/month when the proposed control strategy is adopted.