Owing to its obvious advantages such as high control flexibility, no commutation failure and strong dynamic reactive power support capability, the voltage sourced converter based high-voltage direct-current（VSC-HVDC） transmission technology has been widely applied in scenarios including point-to-point transmission, back-to-back interconnections and DC grids. As a core piece of equipment in VSC-HVDC transmission engineering, the VSC valve achieves AC/DC energy conversion through frequent switching of power electronic devices. In this paper, the key design requirements for VSC valves in different application scenarios are systematically summarized by combining with practical experiences accumulated in engineering, the commonly used power devices and VSC valve topologies in VSC-HVDC transmission engineering are compared and analyzed, and their development trends are projected. In addition, different schemes for two typical application scenarios in the future are also compared, providing reference for the applications of VSC-HVDC transmission technology in high-voltage, large-capacity and long-distance transmission scenarios.

The state of health（SOH） estimation for sodium-ion batteries is crucial for their safe and efficient applications, which is also a key to large-scale energy storage implementations. However, sodium-ion batteries exhibit usage-induced degradation with unclear mechanisms and are sensitive to operating conditions and environmental factors, posing a challenge to the accurate SOH estimation. In this paper, a data-driven SOH estimation method for sodium-ion batteries is proposed. The charging data is correlated with capacity degradation, and variance filtering, grey relational analysis and recursive feature elimination are integrated for feature selection. In addition, four machine learning methods including multiple linear regression, support vector machine, Gaussian process regression and error back propagation neural network are applied to formulate the corresponding estimation methods. Test results reveal that the root mean square errors for the four methods are all less than 1.6%, with Gaussian process regression showing an error rate below 0.8%, indicating a precise SOH estimation for sodium-ion batteries.

With the increasing proportion of new energy power generation, it is necessary to design a high-power converter with a high efficiency and a wide input range. In this paper, a Sigma converter composed of a Buck-boost LLC converter and an LLC based on DC trsnsformer（LLC-DCX） converter is proposed based on the existing research, which can meet the requirements of a high efficiency and a wide input range. The Buck-boost LLC converter can effectively decrease the number of components and reduce loss through bridge arm integration. However, all the input current flows through its Buck-boost inductor, which will cause a lot of loss in high-power scenarios. Therefore, the Sigma structure and the concept of partial power transmission are adopted to reduce the loss caused by the Buck-boost LLC converter. Finally, experimental results demonstrated the superior voltage regulation characteristics and high efficiency of the proposed topology.

Owing to environmental protection and transportation convenience, the proportion of electric bicycles（E-bikes） is increasing. However, the safety and convenience of charging has become a bottleneck for the wide applications of E-bike. Therefore, the wireless charging for E-bike has become an alternative development direction in the future. The current status of wired charging of E-bike is briefly described, the wireless charging technology for E-bike is introduced, and the research status both at home and abroad is reviewed. Then, the relevant standards and industrial status of wireless charging for E-bike are presented. Finally, the key problems and development trend of wireless power transmission technology for E-bike are discussed, thus providing technical reference for the research and development of E-bike wireless charging.

With the increasing demand for electric bicycles（E-bikes）, there exist safety risks in the traditional wired charging technology. The wireless charging technology for E-bike has a great development space owing to its advantages such as safety, reliability and flexibility. However, the design of a wireless charging device for E-bike which can meet the needs of consumers and then gradually replace the traditional wired charging device is still a serious challenge. The wireless power transmission technology for E-bike is analyzed from the aspects of circuit topology, magnetic coupler, constant current and constant voltage control, and foreign object detection methods. In addition, the research direction of this technology in the future is pointed out.