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 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.

The state-of-charge (SOC) balance control of series-connected lithium batteries is of significance to the improvement of battery life. In this paper, a hybrid SOC equalization scheme based on active-passive equalization is proposed against the SOC discreteness of a lithium battery cell, in which the topology of the active equalizer is realized by a multi-winding flyback converter, and the passive equalizer consists of a resistor and a switch connected in parallel at both ends of the cell. The operating principle for the hybrid SOC equalizer is analyzed in detail, and the effect of SOC discreteness on equalization speed is discussed in terms of control strategy. The standard deviation representing the degree of discreteness and the coefficient representing the reasons of discreteness are introduced to realize the fast equalization of SOC under different discrete conditions. The topology of the proposed hybrid equalizer and the corresponding control scheme can optimize energy consumption and balancing speed, and experimental results verify the feasibility of theoretical analysis.

To effectively reduce the inconsistency of series lithium-ion batteries in use, a novel equalization topology with the combination of a Cuk equalizer and a double-layer selector switch is proposed, which can quickly realize the energy transfer between any single cells and improve the equalization speed. According to the characteristics of the open circuit voltage (OCV)-state-of-charge (SOC) curve, piecewise equalization is adopted with voltage and SOC as equalization variables, and a fuzzy logic control (FLC) algorithm is designed to dynamically adjust the equalization current to reduce the equalization time and energy loss. Matlab/Simulink software is used to build a model and conduct simulations. Experimental results show that the energy transfer topology proposed in this paper saves of the equalization time by 22.17% compared with the traditional topology of energy transfer between adjacent cells of Cuk circuit. In addition, compared with the mean difference algorithm, the FLC algorithm improves the time efficiency by more than 30% and the energy efficiency by about 11% under static and charge-discharge conditions. Therefore, the feasibility of the proposed equalization scheme is verified.

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.

Aimed at the problem that the existing incentive pricing compensation mechanism cannot meet the differentiated needs of multiple types of load, a compensation method for interruptible load is proposed in the form of sectional compensatory price. Meanwhile, a two-dimensional alternating function of load transferable time and load transferable power is introduced to establish a transferable load compensation model to quantify the cost of load transfer. A model of multi-type demand response participating in the optimal operation of distribution network considering uncertainties in interruptible load is established. Aimed at the non-convex nonlinearity constraint of the model, it is transformed into a mixed integer second-order cone programming model by the second-order cone relaxation method, which is further solved by the CPLEX solver. In addition, the contribution degree and confidence degree are introduced to evaluate the user responsiveness. Simulation results show that the novel compensation mechanism can more reasonably guide users to adjust the power load, smooth the load curve, and improve the operating economy of distribution network.

A virtual synchronous generator (VSG) can provide inertia and damping for the grid connection of new energy by simulating the characteristics of a synchronous generator. However, a circuit breaking fault may occur in the process of high-frequency switching of power electronic switching devices, which will lead to a serious distortion of output current waveform and affect the safe and stable operation of power grid. In this paper, a VSG fault-tolerant model predictive control strategy based on neutral point voltage equalization is proposed to solve the fault problem of neutral point clamped (NPC) three-level VSG bridge arm. The operation mechanism of NPC three-level VSG single-phase bridge arm after fault is analyzed. The DC-side capacitor forms a virtual bridge arm after the switching device fault, which is reconstructed as a VSG bridge arm fault-tolerant structure. Under fault conditions, a current predictive model is established, and the space voltage vector in a fault state is reconstructed. The neutral point capacitor voltage on the DC-side is introduced into the cost function of fault-tolerant model to reduce capacitor voltage fluctuations and realize the fault-tolerant operation of VSG bridge arm. Experimental results show that the NPC three-level VSG can operate continuously after the switching device fault, which verifies the effectiveness of the proposed model predictive fault-tolerant control strategy and improves the operation reliability of VSG.

The grid-connected operation of an energy storage system is realized by a converter. Due to the low switching frequency of the conventional converter, there exists time delay in sampling and calculation, which will lead to poor transient characteristics of the energy storage system and even instability of the whole power grid. In this paper, model predictive control(MPC) is used to achieve a fast power response of energy storage system and avoid the influence of time delay. A power weight value function is introduced to calculate the optimal output voltage of energy storage converter in the MPC control of active and reactive power. To solve the problem of inaccurate MPC model caused by the parameter deviation of filter inductor, inductance error compensation control is used to improve the model accuracy. Through Matlab/Simulink simulations and experimental results, it is verified that the proposed scheme can improve the transient characteristics of energy storage system and effectively eliminate the influence of error on the MPC control performance.

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.

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.