Accurately predicting the remaining useful life (RUL) of lithium-ion batteries is of significance for improving the safety of working environment and the reliability of equipment. To improve the stability and accuracy of RUL prediction, a battery RUL prediction method based on the combination of denoising technology and hybrid data-driven model is proposed. First, the original data is decomposed by variational mode decomposition, and the noise components are filtered by the analysis of correlation. The residual error is combined with the components which have a strong correlation to complete the sequence reconstruction process. Second, with the combination of Tent chaotic mapping, sine cosine algorithm and Levy flight strategy, the sparrow search algorithm (SSA) is optimized, and the optimal weight threshold of extreme learning machine (ELM) is obtained. Finally, the improved SSA-ELM model is trained by using the smoothed denoised data, and the RUL prediction is completed. The NASA data sets are used to verify the effectiveness of the proposed method. Experimental results show that the average absolute error and root mean square error of the prediction result obtained using this method are controlled within 1.58% and 2.14%, respectively, indicating that this method has a high robustness and a high prediction accuracy. Therefore, the proposed method can be applied to battery RUL prediction.

At present, the physical parameters of a lithium battery cycle life model are difficult to obtain, and the parameter identification process needs a lot of experimental data and a long test time. In addition, it is difficult and expensive to simulate the cycling effect of lithium-ion batteries. On this basis, in order to explore the electrical stimulation of lithium-ion battery aging (due to cycling) and its effect on the battery capacity and internal resistance, a novel cycle life model of lithium-ion battery is proposed. First, a simple physical equation is established based on the fatigue theory and equivalent cycle counting. The parameter identification process is simple, requiring only a small amount of data in the battery data table and a limited (or short) cycle test. The proposed model is general and can represent the effects of common cycle life factors such as depth-of-discharge, temperature and C rate. Finally, two kinds of lithium-ion batteries (i.e., LFP-LiFePO₄ and NMC-LiNiMnCoO₂) are used to verify the model. The simulation results are close to the actual situation, and the error is within 1.5% compared with the experimental results.

For an LCL-type inverter connected to weak grid, the appearance of grid impedance often results in a decrease in the phase margin, serious distortion of grid-connected current and even system instability. To solve this problem, an improved grid-connected current control strategy is proposed, in which a multi-resonance controller is introduced in the voltage feedforward loop to suppress the voltage background harmonics and a phase compensator is added to the current feedforward loop to improve the system's phase margin, so as to avoid the risk that the resonance peak of the multi-resonance link intersects with the -180° line. Theoretical analysis and simulation results show that the proposed strategy can effectively suppress the harmonics of LCL-type grid-connected current, improve the current quality and enhance the stability of the grid-connected system.

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.

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.

The cascaded H-bridge is considered as one of the most suitable topologies for photovoltaic (PV) power generation. Aimed at the problems of the traditional three-phase cascaded H-bridge PV inverter such as a large capacitor volume, a short service life, inter-phase power mismatch and a complex control communication system, a novel modular three-phase PV inverter and its distributed control strategy are proposed based on the principle of magnetic flux cancellation. First, the basic structure of the proposed modular topology is introduced. Then, the basic principle of magnetic flux cancellation power decoupling and the influencing factors of double-line frequency voltage ripple are analyzed in detail, and a distributed control strategy is proposed to suppress the double-line frequency voltage ripple and ensure the balance of three-phase output power. Finally, the correctness of theoretical analysis and the feasibility of the proposed control strategy were verified by simulation and experimental results.

Temperature sensitive electrical parameter method has characteristics such as strong online capacity, non-invasiveness, and rapid response, so it has become a research hotspot at present. The on-state voltage drop is taken as a temperature sensitive electrical parameter, and an online monitoring method for IGBT junction temperature is studied based on the on-state voltage drop. First, the data of on-state voltage drop, collector current, and junction temperature of IGBT is obtained through the double-pulse test circuit. Then, based on the measured data, a three-dimensional mapping representation model of IGBT collector current, junction temperature, and on-state voltage drop is constructed. Finally, a novel on-state voltage drop sampling circuit was designed, and an online monitoring experimental of IGBT junction temperature was conducted. Experimental results verified the accuracy and validity of the obtained three-dimensional junction temperature representation model.

To solve the problem of performance degradation in automotive lithium-ion batteries at low temperatures, a self-heating method based on electric vehicle traction motor and inverter reconfiguration was developed, in which the traction motor windings were utilized as energy storage units to realize AC heating of batteries at low temperatures without additional hardware. First, a detailed mathematical description of AC heating topology was given, and the analytical relationship among the battery voltage, current and heating control parameters was obtained. Then, an adaptive fuzzy PI controller was designed to regulate both the heating current and the charge/discharge voltages of batteries dynamically, so that the heating rate can be guaranteed while avoiding the aging of batteries. In addition, in order to relieve the mechanical vibration and noise from the traction motor during internal heating, a torque ripple canceling scheme based on the clamped rotor position was also proposed, thus ensuring the passenger comfort and the motor durability. Experimental results demonstrate that under the proposed strategy, the tested batteries warmed up from-20 °C to above 0 °C within 403 s without permanently damaging the battery life.

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.

When a grid-connected inverter (unit) is connected to a weak grid, the wide range of grid impedance variation may lead to system instability. To solve this problem, a centralized active damping device is configured in parallel at the point of common coupling (PCC) to simulate the external characteristics of damping resistance, thus realizing the suppression of resonance between the grid-connected inverter and grid. In this paper, an adaptive adjustment method for virtual impedance value based on active damping device is proposed, which not only ensures the system stability, but also minimizes the current flowing in the active damping device. At the same time, a current harmonic reference compensation method is proposed, which can reduce the influence of current closed-loop on the virtual impedance characteristics and further improve the damping effect. A 5 kW grid-connected inverter and a 1 kW active damping device were built in a laboratory to verify the effectiveness of the proposed control scheme.