To solve the instability problem caused by time-varying communication delay and uncertain faults in an isolated island AC microgrid, a novel robust hierarchical control method is proposed, which includes cascade current loop, voltage loop, virtual impedance and droop control loop. First, a robust controller based on adaptive backward integral non-singular fast terminal sliding mode control is designed in the current loop to adjust and track the current reference value under unknown bounded uncertainties and external disturbances. Second, the hybrid H2/H∞ control is used in the voltage loop, and the state feedback control law is used to generate the inner loop reference value to increase the robustness of the controller to disturbances, and sufficient conditions are given based on the linear matrix inequalities. Considering the unstable effects of time-varying delay (TVD), a distributed protocol based on consistency is adopted in the second control layer to improve the robustness of the controller against TVD. Third, droop control and virtual impedance loop are used to improve the system's power distribution accuracy. Finally, the performance of the proposed control method was evaluated by hardware-in-the-loop simulation, and its effectiveness was verified. Simulation results show that compared with the existing methods, the proposedmethod has advantages in transient response, steady-state performance and fault crossing capability under large and small signal disturbances.

The state-of-charge(SOC) of lithium-ion battery is an important parameter for the operation and maintenance of a battery management system(BMS), and its accurate estimation is related to the real-time monitoring and safety control of lithium-ion battery. The traditional unscented Kalman filter(UKF) algorithm has the risk of making the covariance matrix negative when estimating the SOC of lithium battery, and the estimation accuracy is not optimal. To solve the shortcomings of this algorithm, a ternary lithium-ion battery is taken as the research object, and a second-order RC equivalent circuit model is established to describe the working characteristics of the battery. Based on the traditional UKF algorithm, a square-root double unscented Kalman filter(SR-DUKF) algorithm with double unscented transformation is proposed, and it is verified under multiple working conditions. Experimental results show that the improved SR-DUKF algorithm can better estimate the SOC of lithium-ion battery based on the second-order RC equivalent circuit. The average errors under HPPC and BBDST conditions are 0.59% and 0.52%, respectively, and the convergence times are 60 s and 110s, respectively, which verifies that the improved SR-DUKF algorithm has a higher estimation accuracy, better convergence and better robustness.

The design of DC-DC converters which are applied to vehicle auxiliary power modules (APMs) is taken as a research object, the topology of a two-stage DC-DC converter consisting of a three-level Boost (TL-Boost) topology and a half-bridge LLC resonant topology is proposed, and its working principle is analyzed. The front-stage TL-Boost topology converts a wide range of input voltage into a stable voltage, ensuring the high-efficiency operation of the back-stage half-bridge LLC resonant topology. The feasibility and correctness of the proposed DC-DC converter were verified by establishing an experimental platform and carrying out relevant experiments.

The traditional linear control method for a Boost converter has a poor dynamic performance and weak robustness to load disturbance. To solve this problem, a fixed-frequency sliding mode current control method based on power balance is proposed. First, the observed value of load current is used to calculate the input power, which is required to maintain the stability of output voltage. Then, the input power of the converter is adjusted by controlling the inductor current, so that the system's state trajectory is restrained on the sliding mode surface that possesses invariance to load disturbance, thus ensuring the system's large signal stability and improving its dynamic performance. Finally, based on the equivalent control principle, the equivalent sliding mode control is achieved through the PWM technique to avoid problems of chattering and unstable switch frequency. Simulations of the Boost converter under the condition of step load change are carried out in Simulink, and the proposed method is compared with the traditional linear control method. Results show that when using the proposed method, the system's dynamic performance is better and its large signal stability under large load disturbances is guaranteed.

Under the background of energy crisis and environmental issues, the high-gain DC-DC converter is indispensable in renewable energy applications. A dual-switch quadratic structure is proposed to enhance the voltage gain of the traditional quadratic boost converter while reducing the current stress of switches. On this basis, by combining the switched capacitors and a coupled inductor, a dual-switch quadratic high-gain DC-DC converter with a couple inductor is put forward. This converter has advantages such as a very high voltage gain, a pair of switches with the same phase, low voltage stress of switches and output diode, and zero-current switching off in many diodes. The operating principle and steady-state performance of the converter are analyzed in detail, including the voltage gain derivation and the voltage and current stresses of components. Finally, a 120 W prototype was fabricated to verify the theoretical analysis and the feasibility of the converter.

Affected by factors such as wind speed and light intensity, wind power generation has characteristics of randomness, intermittence and large fluctuation, so its direct grid connection will cause damage to power grid. To realize a smooth grid connection of wind power and provide safe and reliable power supply to an urban rail transit system, a hybrid energy storage system composed of super capacitors and lithium batteries is proposed as a stabilizing measure. As the traction load of urban rail transit also fluctuates greatly, the hybrid energy storage system not only stabilizes the output of wind and photovoltaic (PV), but also stabilizes the traction load. The wavelet packet decomposition technology is used to decompose and reconstruct the traction load and wind and PV output power signals on multiple scales, the low-frequency wind and PV grid-connected power and medium-and high-frequency components are obtained, and batteries and super capacitors are used to absorb the medium-and high-frequency components, respectively. Aimed at the minimum comprehensive cost of hybrid energy storage system, the state-of-charge and power limit of the hybrid energy storage system are taken as constraints. The differential evolution particle swarm optimization algorithm with shrinkage factor is used to minimize the annual comprehensive cost of

In a closed-loop control system, the aging monitoring method for power devices based on electrical parameters is one of the difficulties in the field of power electronics reliability. The direct torque control (DTC) system of a permanent magnet synchronous motor (PMSM) is taken as an example, and an on-line monitoring method for the aged state of power devices in a power inverter is studied based on the phase diagram of flux linkage and current. First, the aged characteristics of power devices are analyzed, and it is concluded that the on-state resistance will increase due to the aging of bond wires. Second, the relationship between power device bond wires aging and the phase diagram of flux linkage and the relationship between aging and direct axis current and three-phase current peak value are studied, and the aging monitoring methods are proposed accordingly. Finally, through several groups of simulation experiments, it is verified that both the monitoring method based on the phase diagram of flux linkage and the monitoring method based on current can realize on-line monitoring of the aged state of power device bond wires in the power inverter. The monitoring method based on the phase diagram of flux linkage is easy to observe when there are some fluctuations in the system flux, so it is not desirable considering that the power device has already been in a failure state at the same time. In comparison, the monitoring method based on three-phase current can more accurately monitor the aged state of power devices, and its effect is more advantageous.

Compared with the traditional plug-in charging method, it is safer and more convenient to employ an inductive power transfer (IPT) system to charge autonomous underwater vehicles (AUVs). To alleviate the strong magnetic field inside the AUV hull and the dramatic power fluctuation caused by the rotation misalignment of the AUV under the turbulent water, a three-phase IPT system with a novel coupling structure is proposed. The coupler is composed of three transmitting coils and four receiving coils connected in alternating reverse series, which can suppress the central magnetic field and improve the anti-rotation misalignment performance simultaneously. The Maxwell simulation results show that when the AUV hull rotates, the equivalent mutual inductance Meq fluctuation is less than 2%, and the magnetic field of the AUV center always maintains a low level. In addition, to simplify the system analysis, a decoupling method based on a passive component is adopted to decouple the three transmitting coils. A laboratory-scale prototype based on an LCC-S compensation topology was built to verify the feasibility of the system. Experimental results show that when AUV rotated, the output power varied from 536 W to 595 W with a maximum fluctuation of 9.91%. The maximum DC-DC efficiency of the system was 86.28%.

The dual-active-bridge (DAB) converter is a key device in a bidirectional power transmission system. In this paper, its fundamental operation principle and topologies are reviewed at first. Then, four basic modulation strategies for the DAB converter are introduced, including single-phase-shifted, dual-phase-shifted, extended-phase-shifted and triple-phase-shifted strategies. Moreover, the modeling and optimization methods based on these four modulation strategies are compared and analyzed. Finally, some problems faced by practical applications and the corresponding solutions are discussed. Along with the development of DC power distribution, energy storage and distributed energy resources, DAB converters will have broad application prospects.

The light-emitting diode (LED) driver usually needs a large electrolytic capacitor to reduce its low-frequency current ripple. Therefore, the short life of the electrolytic capacitor is an important factor restricting the life of LED driver, and eliminating the electrolytic capacitor is a key to the long-life LED driver. Under this background, a two-switch electrolytic capacitor-less LED drive circuit topology based on flyback converter is proposed. The auxiliary power balance circuit and flyback converter are integrated, and a balance between the instantaneous input power and output power is realized through two switches. The electrolytic capacitor is eliminated, the low-frequency ripple of output current is suppressed, and a high power factor is realized. The working principle of this topology is analyzed in detail, and a control strategy for its circuit topology is put forward. Finally, a 30 W prototype was built, and experimental results verified the feasibility of the proposed topology.