Aimed at the transient instability of a voltage source converter (VSC) based on phase locked loop (PLL) under weak network conditions and considering the influences of power grid line impedance, VSC reactive power injection and PLL filtering, the transient instability boundary of VSC is comprehensively studied based on the critical voltage in a variety of fault scenarios such as grid voltage sag, frequency fluctuation and three-phase asymmetric fault. Through the analysis of the VSC grid-connected vector diagram in different operation scenarios, the mathematical models of relationships between the grid voltage of VSC grid-connected system and factors (e.g., line impedance and impedance angle, VSC operation power and its power factor, PLL phase-locked error, and grid frequency) in multiple scenarios are established. Then, the transient instability boundary of VSC is indicated based on the critical voltage. Results show that both the line resistance and reactive power injection can directly reduce the critical voltage and improve the stability of the system. The increase or decrease in grid-side frequency will directly lead to the phase lag or lead of PLL and an increase or decrease in line reactance, thus indirectly affecting the critical voltage and further affecting the transient stability of VSC. The PLL pre-filter may cause errors in the phase-locked result, and its phase lag or lead will reduce or increase the critical instability voltage of the system, respectively.

Phase-shifted full-bridge zero-voltage zero-current switching(ZVZCS) converters are favored in high- power DC conversion applications owing to their advantages such as simple structures and high efficiency. However, high-power phase-shifted full-bridge ZVZCS converters still face problems including difficulty in the current reset and severe duty cycle loss. In response to the above issues, a novel phase-shifted full-bridge ZVZCS converter is put forward, which ensures that it can realize zero-current switching over a wide load range by introducing an auxiliary circuit on the primary side to reset the current to zero before the turn-on of lagging-leg switches. At the same time, it can accelerate the commutation speed on the primary side, reduce the duty cycle loss and realize an optimized design of power supply. Based on the analysis of the circuit structure, working principle and characteristics of the proposed converter, a 1 kW experimental prototype was designed to verify its correctness.

Single-phase charging systems usually face the problem of secondary power pulsation. To solve this problem, a single-phase electric drive reconstructed onboard charger (EDROC) system with low voltage ripple is proposed, in which a Buck/Boost active filter is placed in parallel on the output side of a traditional single-phase EDROC system to absorb or compensate the secondary power pulsation, thereby obviously reducing the output voltage ripple of the charging system. First, the topology, working principle and secondary power pulsation generation mechanism of the single-phase EDROC system are analyzed. Second, a single-phase EDROC system with low voltage ripple is put forward by combing the Buck/Boost active filter. Meanwhile, the topology, working principle, and selection of inductors and capacitors of the Buck/Boost active filter are analyzed in detail. Third, a control strategy for the proposed EDROC system is designed. Finally, a 200 W experimental prototype was designed, and experimental results verified the feasibility of the proposed charger.

To solve the difficulty in online life prediction of silicon carbide metal-oxide-semiconductor field-effect transistor (SiC MOSFET) under practical working conditions, a digital implementation method for SiC MOSFET module life prediction based on particle swarm optimization-back propagation (PSO-BP) neural network was proposed. First, the saturation voltage drop of SiC MOSFET was extracted by a saturation voltage drop platform as the temperature-sensitive electric parameter, and a junction temperature prediction scheme based on experimental data was established. Second, a life prediction scheme based on PSO-BP neural network was established by using a power cycling accelerated aging experimental platform to extract the aging characteristic data. Third, the junction temperature prediction scheme and life prediction scheme were transplanted to field programmable gate array to realize the digitization of SiC MOSFET life prediction. Finally, a circuit was designed to verify the proposed method. Experimental results show that the error between the digital junction temperature and real junction temperature was 4.73 ℃, and the percentage of error between the predicted life times and real life times was 4.1%, which proves that the proposed life prediction method is realized digitally and can accurately predict the life times of SiC MOSFET module.

With the increasing penetration rate of renewable energy, carbon emissions are reduced. However, the inherent intermittency and volatility of renewable energy also bring problems such as inertia, security and economy to the power system. The battery energy storage(BES) technology has become one of the important means to solve this problem. Under this background, an autonomous control method for BES system oriented to the active support of grid voltage is proposed based on full-state feedback. First, based on sagging K_v(V_g-v_g) and the virtual capacitor C inertia technology, static power support control and dynamic voltage support control modules are designed, so that the BES system can provide power(static) support and voltage(dynamic) support. Second, the voltage controller and current controller are combined by using the full-state feedback method, which makes the design of the proposed controller more systematic and flexible and reduces the voltage oscillations caused by single-phase ground fault. Third, in order to maintain the stability of state-of-charge(SOC) of BES, a BES SOC controller based on regulatory factors is also designed to further improve the autonomous operation ca-pability of the BES system. Finally, a case study of a 14-node DC system was carried out based on MATLAB and a semi-physical simulation platform, and simulation results verified the effectiveness of the proposed method in the cases of double-support of static power and dynamic voltage and single-line ground fault. With this method, the BES system can be connected to any key node in the grid, and the voltage at the point of common coupling in the grid can be actively supported through the local monitoring of disturbance, which is not affected by disturbance and can be operated and controlled independently. In addition, this method can also prevent the converter from overcurrent during transient low-voltage accidents, so that the autonomous operation capability of BES is realized.

In the impedance measurement process, since the inverter impedance varied widely, the magnitude of injection disturbance cannot be evaluated in advance. Therefore, it is necessary to adjust the disturbance energy adaptively. The impedance measurement device of disturbance voltage injected in series is taken as the research object, and an adaptive adjustment strategy of disturbance voltage based on disturbance current feedback is proposed. The magnitude of disturbance voltage is adjusted by detecting the responding disturbance current in real time, thus realizing the adaptive adjustment of disturbance energy. Both the disturbance voltage and responding disturbance current are controlled to be within 10% of the steady-state point of the system under test. The effectiveness of the proposed control strategy was verified by hardware-in-the-loop simulations in real time.

Compared with the traditional silicon(Si) devices, the gallium nitride(GaN) devices have lower parasitic parameters, a faster switching speed and a smaller on-resistance, which will easily lead to the phenomenon of continuous oscillation during their switching-on process and further result in the circuit instability. Therefore, it is necessary to suppress this phenomenon in practical circuits. Under this background, a negative conductance model of a bridge circuit under the conventional driving scheme is established at first, and the oscillation stability of the circuit is analyzed. Then, by adding optimization to the conventional driving scheme, the corresponding negative conductance model is established. The optimization schemes of series damping represented by changing the resistance and adding ferrite beads and those of parallel low impedance represented by adding RC snubber are selected, respectively. With this model, the influence of adding the driving optimization schemes on the oscillation stability of the circuit can be identified, and the changes in the stability before and after the addition were verified by experimental results, providing a reference for the driving circuit to select its appropriate driving optimization scheme.

Aimed at the problems of a traditional LLC resonant converter in wide voltage applications such as a wide switching frequency range and a poor voltage regulation performance, a voltage doubling two-phase parallel resonant converter is proposed. There is a parallel double half-bridge LLC structure on the primary side of this converter, and a bidirectional switch is introduced into the full-bridge rectifier network on its secondary side to form a reconfigurable voltage doubling rectifier network. Fixed frequency control is adopted during operation. The lower half-bridge on the primary side changes the input voltage of the resonant tank by changing the duty cycle, while the upper half-bridge works with a fixed duty cycle. The rectifier network on the secondary side realizes full-bridge and voltage doubling hybrid rectification under the bidirectional switch, which can achieve 4 times of voltage gain. At the same time, this converter has a good soft switching performance, its voltage gain is independent of magnetizing inductance and load, and a larger magnetizing inductance can be selected to reduce the switch-off loss and conduction loss. Finally, the feasibility of the proposed converter was verified by simulation and experimental results.

With the large-scale network entry of electric vehicles (EVs), their disorder charging further increases the load peak-valley gap, which has a negative impact on the stable operation of power system. A two-stage optimization scheduling strategy which takes into account the EV load and the energy storage system of batteries is proposed. First, an orderly charging scheduling model for EV is established, which aims at minimizing the absolute peak-valley gap between user charging cost and load. The improved particle swarm optimization algorithm is used to solve this model to avoid peak charging. Second, an optimal scheduling model of peak-shaving and valley-filling for the energy storage system is established with an objective of minimizing the variance of load and the combined cost of energy storage life, which is solved by the improved Harris Hawks optimization algorithm to reduce the peak-valley gap of load. In addition, the optimization results are evaluated and analyzed based on the evaluation index of peak-shaving and valley-filling. Finally, a simulation experiment is carried out with the measured load power of one power network as an example. Results show that under the proposed two-stage optimization scheduling strategy, the peak load decreases by about 147 kW, the valley load increases by about 223 kW, and the peak-valley gap deceases by 46.73%, indicating that this strategy can effectively improve the load curve, alleviate the pressure on power supply during the peak load period and ensure the safe and stable operation of power grid.

With the continuous advancement of medical technology, implantable medical devices (IMD) are increasingly applied in clinical practice. Since the traditional battery-powered method will bring additional tissue damage and surgical costs to patients, the use of wireless power transfer (WPT) technology to power IMD will become a trend in the future. However, how to design a high-efficiency IMD-WPT system in a limited space is very challenging. To this end, the performance characteristics of five WPT technologies suitable for IMD are compared. Then, the magnetic resonance WPT technology is taken as an example to introduce the key issues in the design of a magnetic resonance IMD-WPT system. Finally, the application status of part of the magnetic resonance WPT technologies in some typical IMD is combined, and the research direction of IMD-WPT technology in the future is discussed.