In order to predict the changing trend of the dissolved gas in the transformer oil accurately and guide the condition-based maintenance of a power transformer, a combined forecasting model based on cross-entropy is proposed and its application is analyzed. First, a combined forecasting model is established on the basis of radical basis function neural network, back propagation neural network, least squares support vector machine with two different kernel functions, and grey model. Then, the weight coefficient of each algorithm is determined by cross-entropy theory. After that, an application method of the combined forecasting model by the pattern of a variable time window is proposed for the purpose of arranging the equipment maintenance in advance. In this way, the increased budget due to emergency maintenance can be alleviated, and potential loss due to lack of timely maintenance can be reduced. Finally, the accuracy of the combination forecasting method and the effectiveness of the application method are verified by a series of examples.

A novel nine-level switched-capacitor multilevel inverter (SCMLI) with reduced capacitance and balanced capacitor voltage is proposed in this paper. Utilizing only two capacitors, the proposed SCMLI can boost the input voltage and realize nine output voltage levels under different kinds of load conditions. The controllable capacitor charging speed in the proposed SCMLI depends on the charging resonant inductor rather than the equivalent resistance of the devices in conventional topologies. The two capacitors can be charged or discharged independently by a DC source when their voltages deviate from the rated voltage. This makes the proposed SCMLI free of the capacitor voltage imbalance. The longest discharging time (LDT) of capacitors is reduced to twice the switching period, which contributes to reduced capacitance in the proposed SCMLI. With the required capacitor voltage ripple, the quality of output waveforms is also very high. A comprehensive comparison with previous SCMLIs in terms of capacitance, the voltage imbalance problem, the output voltage level and so on is conducted to validate the merits mentioned above. The theoretical analysis, the simulation and experiment results are accordant, confirming the feasibility of the proposed SCMLI.

In view of the large neutral-point (NP) voltage fluctuation and the high low-frequency harmonics in the input current of conventional discontinuous pulse width modulation (DPWM) for the Vienna rectifier, a hybrid DPWM (HDPWM) is proposed on the basis of three-level to two-level conversion. The proposed HDPWM combines the modulation degree of freedom, namely the vector synthesis direction and the equivalent zero vector distribution coefficient. Under the two-level modulation unit, the shift angles that satisfy the predefined NP balance requirement for HDPWM are accurately calculated. Compared with the continuous PWM and conventional DPWM, the proposed HDPWM could effectively suppress the zero-crossing distortion, reduce the NP fluctuation and the switching losses. Simulation and experiment results demonstrate that the proposed HDPWM could reduce the NP voltage fluctuation to 1/2 of the conventional DPWM with reduced input current low-frequency harmonics and no zero-crossing distortion.

The inherent neutral-point (NP) voltage fluctuation of Vienna rectifier could have side effects on the input current quality. However, such effects have not been thoroughly analyzed in the literature yet. In this paper, based on the three-level to two-level space vector modulation (SVM) transformation, a novel method to evaluate the influence of the NP voltage fluctuation on the input current quality is introduced, which could optimize the NP control parameters. The proposed method superimposes the NP voltage fluctuation on the basis vectors, thereby establishing the correlation between the NP voltage fluctuation and the square root mean square (S-RMS) of the input current ripple. The detailed model of the NP oscillation under two-level coordinates is presented. By combining the two-level modulation, NP voltage hysteresis control is adopted to simplify neutral point voltage control and suppress NP low-frequency oscillation. Comprehensive influence of the NP compensation coefficient on the input current quality under NP hysteresis control is obtained, and the hysteresis control parameters are optimized by using the proposed evaluation method. Simulation and experimental results verify the correctness and effectiveness of the proposed evaluation method.

For high magnetic field, the pulsed magnets are normally designed with multiple coils. However, the magnetic field produced by the outer coil starts to drop at the moment when the inner coil starts working due to the electromagnetic coupling. A novel scheme is proposed to eliminate the field drop by introducing compensation electromotive force in the coils. A triple-coil pulsed magnet and triple-coil transformer have been designed and manufactured. The inner, middle and outer coils of the magnet are connected in series with the inner, middle and outer windings of the compensation transformer, and are energized with capacitor banks and pulsed generator. Experimental results show that the magnetic field drop reduces from 8.5 T to 1.1 T. With a dual-coil magnet, the peak field of 94.8 T has been achieved. The simulations and experimental results prove the feasibility of the new scheme and the potential to produce 100 T.

Three-level boost (TLB) converters are a promising interface of bipolar DC microgrids. With different power of the positive and negative DC buses, the conventional modulation method used in TLB converters fails to have zero voltage switching (ZVS) for all switches, and the peak current is also high, leading to the risk of inductor saturation. In this article, an optimized modulation (OM) method is proposed to achieve ZVS for all switches even under unbalanced loads. The OM method features the same valley current at both the high and low output power sides, contributing to wider ZVS range as well as smaller peak-to-peak current. The modulation degree of freedom in TLB converters under unbalanced-load conditions is analyzed in detail for the first time, and the advantages of the proposed OM method are compared with its counterparts in terms of the ZVS range, the peak and valley current, and the efficiency. The effectiveness and advantages of the proposed OM method are finally validated by experiment.