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.

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 scale expansion of a subsea observation network, the stability of its high-power power supply system has attracted attention. First, the impedance models of key parts in the subsea power supply system are established. Considering the characteristics of high power electronic penetration rate, multi-bus cascading and adjacent bus interactive coupling of the subsea DC power supply system, the stability and influencing factors of the system are explored by using the step-by-step analysis method. The analysis result shows that the integral parameter of the controller is the dominant parameter that leads to the instability of the Buck converter, and the proportional parameter of the controller is the dominant parameter that results in the instability of the junction box subsystem. Both an increase in the impedance parameter of the optoelectronic composite cable and a decrease in the inductance parameter are beneficial to improving the system stability. The simulation results based on the PLECS simulation software verify the stability analysis results.

As the penetration rate of renewable energy resources in a new power system continues to rise while the proportion of traditional thermal power units continues to decline, the new power system faces severe frequency control problems. Distributed battery energy storage systems (BESSs) provide an effective way to solve these problems. On this basis, a robust load frequency control (LFC) method for distributed BESSs based on sparse communication network is proposed. To suppress the uncertainties related to system operation, a two-tier model predictive control (MPC) is designed to improve the response characteristics of BESSs, thus improving the performance of LFC. To minimize the area control error, the proposed method can satisfy various operating physical constraints of the system. In addition, the influence of communication delay on the performance of frequency modulation participated by BESSs is also considered, and a fuzzy coordination control device is designed to coordinate BESSs and the traditional generator, so that the mis-operation of the traditional generator under the condition of long delay can be avoided. Finally, simulation results show that the response capability and frequency modulation effect of distributed BESSs are better than the traditional methods under parameters such as different values of capacity, rated power, charge and discharge coefficient, state-of-charge and time constant.

Aimed at the problems of DC bias and frequency variation in a weak grid, a modified inverse Park transform phase-locked loop (IPT-PLL) technology suitable for single-phase grid-connected inverters is proposed. First, the α component after Park transform is used as a reference voltage in the phase detector to solve the problem of DC bias in grid voltage, and an orthogonal component is constructed by the method of 1/4 fundamental periodic delay. Second, the fractional-order delay is approximated by Lagrange interpolation polynomial to reduce the calculation error of delay caused by frequency variation, and the design method for PI regulator is theoretically analyzed. Finally, experimental results show that the modified IPT-PLL proposed has a strong frequency adaptivity, and it can significantly suppress the interference of DC bias in grid voltage. In addition, its dynamic and static performances are satisfying.

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.

For a hybrid cascade H-bridge inverter with a DC-side voltage ratio of 1:1:2, if the hybrid carrier disp-osition modulation strategy is adopted, the problem of output power imbalance in the low-voltage unit will occur al-though there is no current backflow phenomenon and the harmonic performance of output voltage is good. To solve this problem, the power imbalance is analyzed at first. Then, an improved hybrid modulation strategy is proposed, under which the high-voltage unit performs step wave modulation and the low-voltage unit performs PWM, and two low-voltage units adopt different processing methods for the modulation wave. The good harmonic performance of output voltage is kept, the number of triangular carriers is reduced, and the control process is simplified, with a frequency doubling effect. Third, this strategy is optimized, and the switching signal of low-voltage unit is logically calculated, which can solve the power imbalance problem of low-voltage unit in two carrier cycles. Finally, the feasibility was proved by simulation and experimental results.

The magnetic integrated coupling technology is introduced into a high-gain converter, and a magnetic integrated Boost converter based on diode clamping is proposed. Through the theoretical analysis of the working principle and performance characteristics of the proposed converter, it is shown that this converter can reduce its volume and inductance current ripple based on the advantages of the original converter, such as a high voltage gain and a simple control strategy. In addition, the voltage gain of the novel converter is further improved by using the switched capacitor technology, the voltage stress of the switch and diode is further reduced, and the energy conversion efficiency of the converter is effectively improved. Finally, the findings were verified by simulation and experimental results.

The traditional fault diagnosis methods for power transformers cannot detect the power faults accurately or ensure their normal operation. Therefore, a fault diagnosis method for power transformers based on wavelet packet transform and support vector machine (SVM) is proposed. For the power signal collected from a power transformer, the improved minimum noise fraction (MNF) transform denoising is used to denoise, and the noise matrix is estimated by the weighted neighborhood mean method. After the estimation, the improved MNF transform is used to effectively realize image dimensionality reduction and denoising, extract the signal characteristics, and divide the signal into low- and high-frequency part by means of wavelet packet transform to obtain the wavelet packet energy feature vector. The obtained wavelet packet energy feature vector is input into an SVM classifier, and the output results from the SVM classifier are used to realize the state recognition and fault diagnosis of power transformer. Experimental results show that the proposed method can effectively diagnose the faults in the power transformer, such as iron core short-circuit, coil interlayer short-circuit, bushing-to-ground breakdown, coil insulation resistance drop and bushing-to-bushing discharge, and the fault diagnosis accuracy was higher than 98.5%.

Harmonic and electromagnetic interference (EMI) filters are two important output filters used to sup-press the harmonic distortion and EMI noise in grid-connected inverters. Harmonic and EMI filters are combined by planar magnetic integration to reduce the volume and weight. Through the selection of an appropriate magnetic core, the common mode and differential mode inductors are integrated into the same core by drawing PCB planar coil. To integrate the discrete capacitors and further realize the planar magnetic integration of EMI filter, the dielectric is inserted into the PCB and the layer connection mode is reasonably planned. A symmetric LCL filter is used to replace the traditional asymmetric structure of magnetic integration. Furthermore, by designing the air gap in the center pillar of the magnetic core and reasonably arranging the planar windings, the inductors of LCL harmonic filter are also integrated into the same magnetic core unit to form an LCL-EMI planar magnetic integrated filter. A gallium nitride single-phase inverter platform was built, and the LCL-EMI filter with planar magnetic integration was experimentally analyzed to verify the feasibility of the planar magnetic integration method.