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Microgrid (MG) usually adopts droop control to stabilize its frequency and voltage. However, because the line impedance between distributed generators (DGs) is not proportional, the power sharing cannot be achieved. In order to solve this problem, a finite-time power sharing approach for MG is proposed. Firstly, the secondary controller is constructed which is composed of voltage regulator and power regulators. Secondly, the reference voltage and frequency are calculated by combination with droop control. Finally, the stability of the system is analyzed and the effectiveness of the method is verified by simulation.

A 30-pulse rectifier using passive voltage harmonic injection method at dc link is proposed in this article. The passive voltage harmonic injection circuit has a function to reduce the characteristic harmonics of the input voltage. Through injecting square wave voltage whose frequency is six times of the supply frequency into the dc link of the rectifier, some existing harmonic at the input voltage of the rectifier is reduced, so that the steps number of the rectifier increases from 18 to 30. The equivalent model of the rectifier bridges is established. The switching functions of the harmonic injection circuit and the rectifier bridges are analyzed, the operation modes of the rectifier are obtained, then the formation processes of the input voltage are analyzed, the quantitative relationship between the input voltage of the rectifier and the turn ratio of the injection transformer is obtained. The simulation and experimental results show that the total-harmonic distortion values of the input voltage and input current are reduced after using the harmonic injection circuit. In unbalanced conditions, the harmonic injection circuit still has good harmonic suppression performance.

Tubular linear vernier generator keeps the advantages of high efficiency and low cost of primary permanent magnet linear generator, but it has more flux leakage. First of all, a new type of tubular linear vernier generator is proposed in this paper. The permanent magnet is embedded in the primary teeth by horizontal and vertical magnetization, which effectively solves the problem of more magnetic leakage at the end of the generator and puts forward the concept of modularization, which greatly reduces the cogging force by modularization. Secondly, taking no-load back EMF, waveform distortion rate and cogging force as the optimization objectives, the sensitivity analysis of the optimization variables of the generator is carried out, and the optimization variables with significant sensitivity are selected to build the model based on box Behnken. Thirdly, particle swarm optimization algorithm is used to solve the response surface model and generate a set of Pareto solutions, so as to obtain the optimal structural parameters of the generator. Finally, the cogging force, output voltage quality and other performance of the optimized generator are calculated and analyzed based on the two-dimensional finite element method, and the multi-objective optimization based on the response surface model and particle swarm optimization algorithm is verified the effectiveness of the design method based on modularization.

As a small-scale power system, microgrid (MG) will lose support from the main grid if it switches to islanded mode because of the pre-planned scheduling or unplanned disturbances. To synchronise the frequency and voltage to their reference values, a secondary frequency and voltage cooperative control is proposed in this study. The proposed secondary control can synchronise the frequency and voltage to their reference values in finite time and achieve the active power sharing simultaneously. Moreover, it is suitable for switching communication architecture. The MG is considered as multi-agent systems and the system stability is proved by multi-agent theory and finite-time stability theory. A simulation system is established in Matlab/Simulink environment, and the results show the effectiveness of the proposed controller.

A series-connected 18-pulse rectifier using isolated transformer is proposed in this paper. In order to study the operating characteristics of the proposed rectifier, an 18- pulse isolated transformer is designed. The equivalent model of the rectifier is established based on the structure of the rectifier, by analyzing the switching function of three-phase bridge rectifiers, the operation modes of the rectifier are obtained. The simulation and experimental results show that the proposed rectifier operates under an 18-pulse rectification state, the load voltage and load current keep constant.

Decoupling control of two-motor dual three-phase permanent-magnet synchronous motor (PMSM) series-connected system can be realized by using proper phase transposition rules with a single supply from a six-phase voltage source inverter (VSI), which solved the control problem of two motors in series. Since the traditional two-vector SVPWM strategy, which is commonly used in three-phase system, exists 5 th and 7 th harmonics in x-y plane, making it hard to realize decoupling control of two-motor dual three-phase PMSM series-connected system. Meanwhile, the DC bus voltage utilization of carrier-based PWM strategy is lower. Thus one method, which adopted a TBTSB-SVPWM strategy with two modulators, was proposed in this paper. It ensures the inverter output voltage only contains two kinds of fundamental components required by the two seriesconnected motors with a single VSI, making it possible to realize decoupling control of two-motor dual three-phase PMSM series-connected system. Simulation results indicate that there is no influence on the other motor while the speed/load of any one motor changed, achieving the decoupling control of series-connected system, and the feasibility and validness of the proposed approach are verified.

Decoupling mathematical model in d-q coordinates of dual Y shift 30 degree six-phase permanent-magnet synchronous motor (DYSP-PMSM) is established through coordinate transformations. Direct torque control (DTC) strategy based on switch table is applied to improve the rapidness of the system. The theory of DTC strategy of multi-phase PMSM is elaborated. After analyzing the impact of voltage vectors on stator flux and electromagnetic torque, appropriate voltage space vectors are selected to obtain the best switch table of DTC. Simulation results show that electromagnetic torque can follow the change of load torque and the speed remains constant when load disturbance appears, the rapid dynamic response and strong robustness to parameter are achieved.