Aimed at the problem of low efficiency of a dual active bridge (DAB) converter in the wide voltage range of single phase shift modulation strategy, a one-sided asymmetric duty modulation strategy is proposed in this paper, which significantly improves the efficiency of DAB converter, especially in the case of light load. First, the principle of one-sided asymmetric duty modulation scheme is described, and two operation modes are obtained according to the relationship of control degrees of freedom. Second, based on the time-domain analysis, the steady-state characteristics in the two operation modes are derived, including inductance current and transmission power. Third, in order to find the optimal combination of control degrees of freedom, the peak-to-peak value of inductance current is selected as the optimization objective, and the optimal one-sided asymmetric duty modulation strategy is obtained by applying the KKT condition. Finally, an experimental platform for DAB converter based on SiC device was built, and experimental results verified the effectiveness of the proposed one-sided asymmetric duty modulation strategy.

Aimed at the voltage distortion and current distortion of power grid with pulse load, a three-level unified power quality conditioner(UPQC) with supercapacitor energy storage is designed. A method based on artificial neural network is proposed to control the series and shunt compensation units, and a method based on double closed-loop PI control is used to control the supercapacitor energy storage unit. The series compensation unit compensates voltage to maintain the stability of load voltage and ensure the power demand of load, while the shunt compensation unit compensates current to maintain the stability of supply current and avoid the continuous and large impact on power grid. The supercapacitor energy storage unit charges and discharges on the DC side to maintain a constant voltage on the DC side and ensure the normal operation on the series and shunt sides. The proposed method eliminates the complicated coordinate transformation process and avoids the phase lag caused by multiple filters. Simulation experiment results show that the proposed topology and control strategy are helpful for improving the power quality of power grid with pulse load.

A shutdown strategy for a low-cost hybrid DC converter based on diode rectifier is studied. The shut-down process of the auxiliary converter is divided into an energy feedback stage and an energy dissipation stage. At the energy feedback stage, part of the energy stored in the capacitors of sub-modules is fed back to grid through the DC transmission line under the active control. At the energy dissipation stage, the resonant capacitor and the capacitors of sub-modules are discharged in sequence through a discharging resistor. The calculation methods for the discharging cur-rent, capacitance voltage, and discharging time are derived, and the design method for the discharging resistor is also given. The capacitors of the auxiliary converter can be discharged quickly and efficiently under the proposed shutdown strategy without a high-voltage and large-capacity discharging resistor, which is conducive to the realization in engineer-ing projects. Finally, a simulation model is constructed in Matlab/Simulink, and the validity of the shutdown strategy is verified by simulation results.

Aimed at the low conversion efficiency and all the load power that needs to be processed by converters at different stages in the two-stage architecture of a DC transformer cascaded PWM converter, a quasi single-stage DC-DC conversion method with partial power active regulation based on a split-sigma structure is proposed. By splitting the output port of the DC transformer into two ports, the power of one port is directly transmitted to load without being processed by the back-stage PWM converter, thus realizing the quasi single-stage power conversion equivalently. In addition, the power capacity and loss of the PWM converter are effectively reduced, and the system efficiency is improved. The principle and circuit implementation method for the split-sigma structure are analyzed in detail, and the principle and characteristics of system output voltage regulation are also studied. One of the circuits is taken as an example, and its working principle, voltage regulation characteristics and design method for key parameters are analyzed. Finally, experimental results verified the effectiveness and correctness of the proposed scheme.

As a commonly used islanding detection method, the active detection method can reduce the non-detection zone by increasing the disturbance intensity, but it will also increase the degree of harmonic pollution to the grid at the same time. Aimed at this problem, a hybrid fuzzy low-harmonic islanding detection method without non-detection zone is proposed in this paper. First, the non-detection zone is judged, and the passive islanding detection method which has no harmonic pollution to the grid is used preferentially. If this zone is within the non-detection zone of the passive detection method, then a fuzzy low-harmonic islanding detection method with fuzzy disturbance control is used to disturb the output current in its second and fourth quarter periods. Finally, the corresponding simulation model is built to verify the effectiveness of the proposed method.

The large grid inductance in weak grid may cause a grid-connected converter to be unstable. Therefore, an impedance model of grid-connected converter is built at first, and the influence of grid inductance on the stability of grid-connected converter is analyzed according to the impedance ratio criterion. Then, aimed at the problem of low adaptability of the grid-connected converter to inductive grid impedance, a virtual impedance control strategy based on band-pass filter is proposed, and the influence of virtual resistance value on the adaptability of grid-connected converter to weak grid is studied. Furthermore, a selection principle for the virtual resistance value is also given. Finally, a system simulation model is built, and simulation results verify the correctness of theoretical analysis and the effectiveness of the proposed control strategy.

Aimed at the problems of complex topologies, high total standing voltage of switches, and unbalanced series capacitor voltages in the existing multilevel inverters, a novel seven-level inverter based on switched-capacitor is proposed. By adopting the phase disposition pulse width modulation strategy, three capacitors are reasonably controlled in series/parallel with the DC source, so that the seven-level output with a triple boost gain is realized. The proposed inverter uses only one single DC source, so it has advantages such as a simple structure, fewer components, high boost gain and self-balanced capacitor voltage. In addition, it does not use the H-bridge to change the polarities of output levels, thus reducing the total standing voltage of switching devices. The topology, working principle, capacitor voltage self-balance, modulation strategy, capacitor parameter and current stress are analyzed, and five aspects including the numbers of switches, diodes and capacitors, the total standing voltage and the boost gain are compared with those of the existing topologies, which fully proves the practicality of the proposed topology. Finally, the feasibility of this topology was verified by experimental results.

In this paper, a Boost converter model in the pseudo-continuous conduction mode(PCCM) of inductive current based on the definition of R-L fractional-order is studied. On this basis, the state-space averaging model of the converter is derived. Then, the expressions for the step-up ratio, DC static operating point, inductive current ripple, and output voltage ripple are derived. Results show that compared with the corresponding expressions derived on the basis of the definition of Caputo fractional-order, the derived expressions for DC static operating point and step-up ratio under the definition of R-L fractional-order are related to both the duty cycle and the orders of the capacitor and inductor. The expression for output voltage ripple is not only related to the order a of fractional-order inductor, but also related to the order ? of fractional-order capacitor. Therefore, the orders of inductor and capacitor obviously affect the DC component of state variables and the stable-state characteristics of factional-order PCCM Boost converter. Finally, a mathematical model and a circuit model of the R-L fractional-order PCCM Boost converter are built in MATLAB/SIMULINK, and simulation results show that the analysis result of the R-L fractional-order PCCM Boost converter model is more stable than that of the other PCCM Boost model, and the corresponding error is smaller.

Aimed at the problem that DC-DC converters with coreless transformers must operate at high frequencies, a DC-DC converter with a coreless transformer based on free decay oscillation is proposed. First, a circuit model in series-series topology with no excitation sources is established, the evolution of the eigenvalue is discussed, and the free decay oscillation behavior of the system is analyzed. Second, the DC-DC converter with a coreless transformer based on free decay oscillation is designed. The switching frequency of this converter can be significantly lower than the operating frequency of the coreless transformer, and the output power from the converter and the operating frequency of the coreless transformer can be described by the eigenvalue of the free decay oscillation system. Finally, an experimental prototype was constructed, which was under constant duty control. The switching frequency of the converter was reduced to one half(103 kHz) and one third (69 kHz) of the operating frequency of the coreless transformer (206 kHz), respectively. The converter efficiency achieved 91.2%, and the transformer efficiency was always higher than 96%.

In a weak grid, due to the existence of grid impedance, the natural resonant frequency of a new energy grid-connected LCL filter will shift, and the traditional active damping control strategy cannot guarantee the system stability. Moreover, as the proportion of new energy power generation in the power system continues to grow, how to reduce the operating costs is a hot topic for research. Therefore, a novel control strategy based on grid-connected current and common coupling voltage feedback is proposed in this paper, which not only provides active damping to suppress LCL resonance, but also reduces the use of sensors. In addition, it has a strong adaptability under wide-ranging changes in grid impedance. Simulation and experimental results show that, compared with that under the traditional control strategy, the practical range of weak grid under the improved strategy increases, the system stability is enhanced, and the capability to suppress harmonics is raised, indicating that the quality of grid-connected current is well improved.