The power control mechanism of cascaded island microgrids (MGs) composed of dispatchable and non-dis-patchable distributed generations (DGs) is complex, and the voltage and frequency regulation is difficult. Aimed at these problems, a novel decentralized master-slave power distribution control method is proposed in this paper to realize the voltage and frequency regulation and power distribution of the cascaded island MGs including second-class DGs. The control methods for master and slave DGs are designed, respectively. The master DG is responsible for adjusting the fre-quency and voltage of MG. Meanwhile, the slave DGs in MPPT mode can run under certain bus voltage, which can max-imally use the output power from non-dispatchable DGs and perform automatic power reduction under the light load condition, thereby effectively guaranteeing the safety and stability of the system. Finally, the effectiveness of the pro-posed method is verified by simulation and experimental results.

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%.

Based on the fact that inductance and capacitance are of fractional-order, the nonlinear dynamic characteristics of a fractional-order Boost converter are studied. The predictor-corrector model of the Boost converter is established using the predictor-corrector algorithm of fractional-order calculus. On this basis, the bifurcation diagrams with the reference current, input voltage and orders of capacitance and inductance as bifurcation parameters are obtained. The period doubling bifurcation and chaotic behaviors of the fractional-order Boost converter are studied, and its nonlinear dynamic behavior is compared with that of an integer-order Boost converter at the same time. Results show that under certain operating conditions, some nonlinear phenomena such as bifurcation and chaos will appear in the fractional-order Boost converter with changes in some circuit parameters. Under the condition of the same circuit parameters, the parameter stability domains of integer-and fractional-order converters are different. Compared with that of the integer-order converter, the parameter stability region of the fractional-order converter is smaller, which more truly reflects the nonlinear dynamic characteristics of the Boost converter.

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.

Due to the duty cycle constraint on the traditional Boost converter, its applications to high-voltage-gain power supply are limited to certain degree. In this paper, a DC-DC converter with high voltage gain based on an isolated Boost converter and Cockcroft-Walton voltage multiplier cell (VMC) is studied, and its working principle and characteristics are analyzed. This converter achieves a conversion with an ultra-high step-up ratio by integrating the isolated Boost converter with the VMC. Compared with the traditional Boost converter, this topology has a high voltage gain in a low duty cycle, a low voltage stress of active switching device, and a simple control circuit with one single switch. Finally, a 35 W prototype with an efficiency of 89.5% was built to achieve a high step-up conversion from 24 V to 1 000 V, and the theoretical analysis results was verified by experimental results.

A soft-switching DC-DC converter with low current ripple and high gain is proposed, which can be applied to new energy generation systems. Based on the conventional interleaved Boost converter, the proposed converter can achieve high gain by introducing a coupled inductor, diodes and a capacitor Boost unit. The coupled inductor transmits energy during the entire switching cycle, thus improving the utilization rate of magnetic core. The input Boost stage works in an interleaved mode, and the current ripple of the two-phase inductor can cancel each other, so as to obtain a lower input current ripple. Due to the existence of leakage inductance of the coupled inductor, the reverse recovery problem of rectifier diodes are alleviated. Meanwhile, an active clamp circuit is adopted to absorb the leakage inductance energy, thereby achieving the zero-voltage soft-switching of all switches, restraining the turn-off voltage spike of switches, and improving the converter's conversion efficiency. The working principle, circuit characteristics and soft-switching realization method of the converter are analyzed in detail. Finally, a 200 W experimental prototype was built to verify the theoretical analysis.

The two-switch Buck-Boost converter has been widely applied in step-up and step-down scenarios. However, it usually operates under hard switching conditions in the existing various control and modulation modes. In addition, its interleaved control circuit is usually complicated. A three-switch interleaved Buck-Boost circuit with co-directional coupling inductor and its control method are given based on the characteristics of co-directional coupling inductor. First, the coupling process of the coupling inductor during the switching process is analyzed under a large coupling coefficient, based on which the circuit's fundamental operating principle is given in detail. Then, it is concluded that the extended duty cycle and soft switching of Boost-side power switches can be achieved in the discontinuous self-induction current mode, so as to avoid the synchronous and current-sharing circuits in the two-phase interleaved control circuit, thus obviously simplifying the control circuit. Finally, simulation and experimental results verified the analysis results.

Aimed at the poor voltage regulation performance and large circulating current of the traditional frequency-controlled LLC resonant converter in wide voltage applications, a six-switch dual-resonant LLC converter is designed, which is a hybrid combination of two full-bridge LLC resonant converters sharing one bridge arm. Compared with the traditional frequency control, fixed-frequency phase-shift control is employed in this converter to adjust the output voltage and reduce the switching frequency range. According to different connection modes of two transformers, the converter has two topological forms. When the two transformers are connected in series in the forward polarity, the converter's gain range is 0-1, which can achieve an ultra-wide output voltage range. When the two transformers are connected in series in the reverse polarity, the gain range is 1-2, and the circulating current loss during the working process is small. Under the two topological forms, the ZVS turn-ON of the primary switch tube and ZCS turn-OFF of the secondary diodes can be achieved, respectively. Finally, the validity of the research in this paper was verified by Simulink simulations and experimental results.

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.

Multi-level converters are widely applied in DC microgrids because of their capability to reduce the voltage stress on switches and the volumes of filtering inductors and filtering capacitors. Since the flying-capacitance voltage and output voltage of a three-level Buck converter are coupled, the converter is a nonlinear system with strong coupling of multi-input and multi-output. To solve this problem, a decoupled backstepping sliding mode control method for inverse system is proposed in this paper. The inverse system method is used to decouple the output voltage control and flying-capacitance voltage control, and the backstepping sliding mode control method is used to ensure the stability and robustness of output voltage. The flying-capacitance voltage is balanced at 1/2 of the input voltage by the state feed-back control. Simulation and experimental results show that the proposed control strategy can achieve satisfying steady-state and dynamic characteristics of flying-capacitance voltage and output voltage.