The smooth switching between grid-connected and islanded microgrid operation modes and the stability of system frequency are important guarantees for the safe and stable operation of a master-slave microgrid system. Combined with the operating characteristics of the microgrid system, an off-grid switching method based on phase angle switching is proposed, and a pre-synchronization control module is added to make sure that the grid phase is quickly tracked and compensated when the islanded mode is switched to the grid-connected mode, thereby solving the problem of shocks in the output voltage and current from master and slave inverters in switching. To ensure the frequency stability of the microgrid system, a V-F frequency control strategy based on fuzzy droop control is proposed and further applied to the smooth switching based on phase angle switching, so that the system frequency is basically maintained at 50 Hz and the problems of large oscillations in voltage and current and frequency shocks in switching are avoided. Finally, an experimental platform was built to verify the effectiveness and stability of the proposed method.

Aimed at the problem that the converter current ripple and electromagnetic interference (EMI) noise will increase due to the increasing switching frequency of a three-level neutral point clamped (NPC) converter, a variable switching frequency modulation strategy based on current ripple prediction is proposed to reduce the current ripple, harmonic noise and EMI noise of the three-level NPC converter. According to the requirements of current ripple, the switching cycle and sampling cycle are calculated to synthesize the latest switching cycle and form feedback, so as to realize variable frequency modulation. The random cycle is distributed around the expected cycle, so that the harmonic noise and electromagnetic noise of the converter are more evenly distributed in a wide frequency band. As a result, the electromagnetic noise of the converter is reduced, and the output inductance current ripple is improved. The relevant simulation and experimental results verified that compared with those under the traditional modulation strategy, the common mode noise was reduced by about 20 dB/µV under the proposed modulation strategy, the differential mode noise was reduced by about 10 dB/µV, and the amplitude of output inductance current ripple was also reduced accordingly.

In order to improve the DC voltage gain and reduce the electrical stress, a novel high voltage gain DC-DC converter based on Z-source is proposed. Theoretically, the ratio of output voltage to input voltage can reach (2-D)/(1-2D). Compared with the traditional diode capacitor filter Z-source DC-DC converter, the proposed topology can provide a higher DC voltage gain at the same duty cycle. It has lower voltage stress and inductance current stress when the DC voltage gain is the same. In addition, the input port and output port of the proposed DC-DC converter share the common ground, which helps to reduce the electromagnetic interference of the system. On this basis, the steady-state principle and characteristics of the proposed DC-DC converter are introduced, and the parameter design and theoretical efficiency calculation are also carried out. Finally, an experimental prototype with a power level of 200 W was fabricated, and experimental results proved the feasibility and superiority of the proposed circuit topology.

Owing to their merits including continuous input and output current, high efficiency and high power density, non-isolated Superboost converters are widely applied in spacecraft power systems. However, the switching loss of the device will increase in a scenario with a high step-up ratio, resulting in a decrease in the converter efficiency. To solve this problem, a zero-voltage switching pulse-width modulation (ZVS-PWM) Superboost converter with low voltage stress is proposed. By introducing a resonant tank, the main switch can be turned on or off under ZVS, and the auxiliary switch can be turned on under zero current switching and turned off under ZVS. Besides, all the diodes are operating under soft-switching. As a result, the switching loss is reduced effectively, and the converter efficiency is improved without increasing the voltage and current stress of the main power device. The operation principle, soft-switching conditions and device stress are analyzed in detail, and the state-space averaging approach is used to estimate the steady-state and dynamic characteristics of the proposed converter. In addition, its feasibility was verified by a prototype with 100 kHz and 400 W.

A novel low delay and low power consumption low-to-high level shift circuit with a logic correction function is proposed, which uses a low delay level shift circuit and a low power consumption level shift circuit to work in parallel. After the logic is corrected, the low level between 1 V and 1.5 V is converted to a high level of 5 V, so this circuit can be widely applied in GaN driver circuits. Based on the 0.5 µm BCD process, 1.5 V power supply low voltage and 5 V power supply high voltage, the circuit is verified at 5 MHz. Results show that although the layout area of this circuit increases as a whole, the rise and fall delays are reduced to 2.3 ns and 1.8 ns, respectively, with a total power consumption current of only 11 μΑ.

The energy storage battery pack consists of a large number of low-voltage battery cells. The states-of-charge (SOC) of cells are different due to the difference in their characteristics, which causes some cells to be over-discharged during the charging process. To avoid this phenomenon, a equalizing charger based on multi-winding transformer and cascaded double-voltage rectifiers is proposed, and its characteristics are compared with those of a centralized passive equalizing charger. Its operation principle is analyzed, and its equivalent circuit is obtained. The buffer inductance of the novel equalizing charger and the turn ratio of the transformer are also optimized. A control strategy of single voltage-loop is put forward, which can guarantee the charging at two stages, i.e., constant-current and constant-voltage. In addition, the influence of the voltage difference between cells on the transformer operation is analyzed. Simulation results show that the proposed charger can achieve a high charging efficiency while satisfying the requirement of voltage equalization between cells.

According to the demand of enterprises which produce UPS, a condition based maintenance(CBM) management system of UPS based on extended Kalman filter(EKF)-Markov is designed. Under the permission of users, the status data of online position and real-time operation of the equipment is visualized by using the geographic information system. Compared with the traditional post-maintenance scheme, the weighted method is used in data preprocessing to model the CBM of the data-driven collected information and reduce differences caused by different types of data. The EKF is used to eliminate the influence of noise on the sampling results, and the average error of state-of-charge(SOC)predicted using the algorithm is 0.434 3%. Combined with the Markov decision process, the UPS battery state is analyzed, the health management and CBM strategy in charge-change mode is implemented, and the maintenance time is reduced by 57.12% on average. Results show that compared with the traditional maintenance, the state prediction and health management system can improve the maintenance efficiency and accelerate the transformation from traditional planned maintenance to CBM mode.

To address the difficulty in predicting the state-of-charge (SOC) of a Li-ion battery pack, an SOC prediction model based on kernel extreme learning machine (KELM) optimized by the improved sparrow search algorithm (ISSA) is proposed. First, Logistic chaotic mapping is introduced to improve the standard SSA and acquire the best population individuals. Second, the improved algorithm is used to optimize the kernel function parameter S and penalty coefficient C of KELM to create an ISSA-KELM prediction model. The simulation is carried out utilizing the historical data from an energystorage device, and the results predicted by ELM, KELM and ISSA-ISSA-KELM models were compared and analyzed. In addition, the robustness of the model was verified using data under other working conditions. Results show that the root mean square error and mean absolute error of predicted SOC decreased to 2.06% and 1.54%, respectively. The proposed model improved the prediction accuracy, and its convergence, generalization and robustness were also satisfying.

To maximumly protect the medium-voltage motor in a back-to-back medium-voltage motor driving system without transformer based on modular multilevel converter (MMC) from the influence of asymmetric grid faults and switching actions, a control strategy for minimizing the common-mode voltage of the front-end transformerless grid-connected MMC is designed. The common-mode voltage caused by the asymmetric grid fault can be canceled by the MMC counterpart voltage, and the switching ripples caused by the switching action of the MMC can be suppressed by arranging the arm-voltage pulses end-to-end. In addition, the influence of MMC common-mode voltage suppression on the single-phase power deviation is analyzed, and the feedforward control is proposed accordingly. Tests were carried out using a grid-connected MMC prototype system, and experimental results verified that the maximum common-mode voltage of the grid-connected system under severe asymmetric grid conditions can be reduced to 1/3N of its original value by the proposed control strategy, where N is the per-arm submodule number. Meanwhile, the unity power factor, constant DC voltage, and balanced single-phase power were also realized.

With the rapid development of new energy technology, the performance of DC-DC converters continuously increases. In this paper, a novel high-gain DC-DC converter is proposed, which is improved based on the quasi-Z-source topology. Owing to the use of the topology in which three capacitors discharge together to the load, a higher voltage gain is obtained while the voltage stress of capacitors is reduced. The proposed converter has advantages of the traditional quasi-Z-source converter such as simple control, continuous current and small current ripple. The working principle for this converter is analyzed. In addition, its performance was verified through simulation experiments and prototype experiments.