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Leakage inductance is an important parameter in a high-frequency transformer for its influence on power transmission performance and the soft switching of converters. Nowadays, Litz wire is widely used to control winding loss and reach high efficiency. However, it is not an easy task to accurately calculate its leakage inductance. This article presents a detailed model of leakage energy in Litz wire, and proposes an accurate closed-form expression for calculating the leakage inductance in a high-frequency transformer. The skin effect and the proximity effect of Litz wire are taken into account in two-dimensional polar coordinates, and the filling factor of Litz wire is also considered. A global sensitivity analysis is performed to assess the influence of each parameter on the overall leakage inductance. Finally, two transformer prototypes are made to verify the accuracy of the proposed analytical expression.

The construction of AC and DC transmission in China is accelerating. And the grid is getting closer, which will make the grid structure strong. However, the tightly connected power grid is more vulnerable to space weather and HVDC. Now, the researches on transformer DC bias were summarized, and the researching methods and achievements were analyzed. The main reasons and basic theory were introduced, and so did the influences to transformer. The method is generally to establish a corresponding calculation model through actual measurement, simulation and calculation, and limiting, isolating, injecting DC current are main restraining measures.

A novel hybrid resonant zero-voltage zero-current switching three-level converter with capacitive output filter is proposed in this paper, which undergoes half of the input voltage for switches and is suitable for offshore wind farms in medium-voltage dc (MVDC) collection systems. The proposed converter, utilizing two transformers with very different power ratings, adopts insulated-gate bipolar transistors for all six power switches and operates in discontinuous current mode and can achieve zero-current switching for the four main switches and rectifier diodes over the whole load range. Meanwhile, the auxiliary switches with small current rating can realize zero-voltage switching naturally. Hence, the switching loss is reduced, which is vital in MVDC applications. By the trajectory formulas of the resonant current and voltage, the trajectory paths can be achieved, which is helpful to compare the peak and turn- off currents of switches. The design rules of main parameters can be deduced with the trajectory formulas and paths. The influence of the turns ratio of auxiliary transformer and the resonant capacitance on the peak and turn- off currents is discussed in detail. The operation principles of the proposed converter are analyzed and verified by the simulation and experimental results.

In multi-infeed HVDC system, the interactions and influences between DC systems AC systems are complex as the electrical distances among DC converter stations which are relatively short. Multi-infeed interaction factor (MIIF) can effectively reflect the interaction among DC systems. The paper theoretically analyzes the impact factors of MIIF like the electrical distances between two DC converter stations and the equivalent impedance of the receiving end AC system. By applying the Kirchhoff’s current law on the inverter AC bus, the paper deduces the analytical expressions for MIIF. From the expression, it is clear how the equivalent impedance of AC system and coupling impedance can affect MIIF. PSCAD simulations validate the effectiveness and the correctness of the proposed expression and some useful conclusions are drawn.