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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.

Leakage inductance is a key parameter of high-frequency transformers because it directly determines the maximum transmission power of dual active bridge DC-DC converters using phase shift control. Precise control of leakage inductance is essential for the design of high-frequency transformers. This paper provides an analytical method that can evaluate the magnetic energy including the interturn magnetic energy by dividing the winding into linear area and circular area. The method is verified by a FEM model based on weighted algorithm and the results show that it can accurately evaluate the leakage inductance of high-frequency transformer.

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.

Electric vehicles are the new energy vehicles with the least exhaust emissions. Therefore, the development of electric vehicles has been valued by many countries. The charging station is the energy supply center for electric vehicles. Whether the charging station is built well determines the degree of popularization of electric vehicles. This paper studied the parallel operation control of modules which are based on the common DC bus in charging station. In the vehicle-to-grid (V2G) mode, when the output sides of the rectifier modules are connected in parallel, all DC/DC charging modules will supply power to the DC bus. Whether the modules can achieve current-sharing is a key factor that affects the reliable operation of the system. The I/U droop control is utilized to achieve current-sharing of all the parallel modules. Consequently, the complicated and less reliable high-speed communication can be eliminated, which improved the system reliability greatly. Simulation in MATLAB verified the feasibility of the strategy.

With the increase of the transformer capacity and converter switching frequency, the transformer efficiency, stray parameters, insulation and heat dissipation should be considered when designing a transformer. Based on the transformer losses and stray parameters calculations, this paper focuses on the transformer optimization considering the transformer losses, insulation and leakage under constrained conditions using NSGA-II algorithm. A 220V/3.52kV, 3.52kW/20kHz transformer prototype was built and the experimental results verified the correctness of the calculation and the validity of the proposed optimization method.

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.

Compared with the conventional high voltage DC power supply, the high voltage pulse power supply can significantly improve the dust removal efficiency of electrostatic precipitator and reduce the energy consumption. This paper presents a new type of high voltage pulse electrostatic precipitator power supply topology, the symmetric cascaded structure of the low voltage side which can significantly reduce the voltage stress of switching device, rectifier circuit, clamp circuit and resonant capacitor, thereby significantly reducing the cost. This paper analyzes the steady-state operating principle of the high voltage pulse power supply and the working conditions during flashover while the expressions of the output voltage and current of the transformer primary side are both deduced and verified by simulations. Finally, the experimental verification is carried out by a pulse power supply with a peak voltage of 100kV.

This paper focuses on the relevant content of the MVDC-connected photovoltaic system based on the modular cascaded structure. This paper proposes a new topology for the DC grid-connected photovoltaic system with a novel capacitor voltage balancing control strategy, including the module input power ratio feed forward and module capacitor voltage closed-loop control. The proposed control strategy is independent from the realization of the photovoltaic maximum power point tracking function of the first stage. 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 and AC systems are complex as the electrical distances among DC converter stations are relatively short. Multi-infeed interaction factor (MIIF) can effectively reflect the interaction among DC systems. However, present methods to obtain the interaction factors are through simulations which are unclear in physical meaning. The paper theoretically analyzes recent analytical expressions based on impedance matrix to calculate the MIIF and proposes a new way to express the MIIF based on the admittance matrix. From both expressions, it is clear that how the equivalent impedance of AC system and coupling impedance can affect MIIF. PSCAD simulations validate the effectiveness and the correctness of the proposed expressions and some useful conclusions are drawn.