Daolian Chen

In this paper, a novel high step-up non-isolated quasi-Z-source DC-DC converter is proposed and analyzed. The converter utilizes both the active switched-inductor and switched-capacitor structure on the basis of the quasi-Z-source network to improve the overall voltage gain. The proposed converter retains the advantages of the traditional quasi-Z-source boost converter, such as continuous input current, common ground between the input and output, and low voltage stress on capacitors. Meanwhile, the voltage gain is significantly improved without changing the maximum duty cycle of the traditional quasi-Z-source boost converter. And the voltage stress of each power switch is greatly reduced. The steady operating principle and the steady characteristics of the proposed converter are discussed in detail. Also, the power loss calculation for non-ideal elements is presented. Finally, a 300 V/300 W prototype is built in the laboratory to verify the performance of the proposed topology, and a comprehensive comparison of the proposed converter with other quasi-Z-source based converters is provided to demonstrate the advantages of the proposed converter.

The 2-D subdomain method (SDM) has proven to be a time-efficient approach suited for determining the magnetic field of permanent magnet synchronous motor (PMSM). However, further application of this method into the field analysis for the line-start PMSM (LSPMSM) is limited by its natural defects including (a) inability to consider the saturation effect of iron material and (b) failure to deal with the rotor eddy currents allowing self-start. In this work, the step-by-step or iterative numerical calculation using the finite difference method (FDM) is embedded into the execution of conventional 2-D SDM, through which an enhanced version addressing all issues as stated above is obtained. The improved SDM avoids limitations of pure analytical method and possesses higher calculation efficiency than that of pure numerical method. Reliability of the presented method is validated by the finite element analysis (FEA) and tests.

According to air-gap field-modulation theory, multiple field harmonics are responsible for high torque density of field modulation (FM) motors. However, abundant field harmonics will undoubtedly increase motor losses, especially iron loss and PM eddy-current loss. Therefore, a harmonic-analysis-based loss minimization (HLM) method incorporated into model predictive torque control (MPTC) for efficiency improvement of FM motors is proposed in this article. First, by analyzing the loss generation mechanism, a harmonic-based online loss calculation model is built to serve for loss minimization control. Then, the reference flux for MPTC can be derived from the proposed HLM method, which avoids complexity of equivalent resistance tuning and improves accuracy of loss calculation compared to the conventional model-based loss minimization method which lacks attention on the harmonic-based loss analysis. Consequently, the HLM-MPTC strategy is finally determined and then applied in a flux-concentrating field-modulation permanent-magnet prototype to verify its validity. The experimental results evidence that the proposed HLM-MPTC can effectively achieve loss reduction and efficiency improvement.

Active power backflow is an inherent problem of three-phase cascaded H-bridge (CHB) PV grid-tied inverters during low voltage ride through (LVRT), probably resulting in no balanced operating point of the system and that the inverter will be shut down and off-grid due to overvoltage fault. Aiming at this issue, this paper first reviews the existing control methods and analyzes their limitations in application, that is, they cannot effectively suppress active power backflow under the scenario of deeper voltage drop and lower output power. On this basis, an optimized LVRT control strategy for cascaded modular medium-voltage PV power generation system is proposed, which adopts different control methods to suppress active power backflow under three types of asymmetric grid drop conditions: single-phase short-circuit fault with ground, two-phase short-circuit fault with ground and two-phase short-circuit fault without ground, enhancing the adaptability of CHB PV grid-connected inverters to different output powers and different drop depths during grid voltage failure, and then improving LVRT capability of the system. Finally, the effectiveness and feasibility of the proposed control strategy are verified by experimental results.

A circuit structure, topological family, and unipolar phase-shift SPWM master-slave power distribution energy management control strategy (EMCS) of the single-stage Buck type multi-input high-frequency (HF) link's inverters with multiwinding time-sharing power supply are proposed in this article, in order to reduce the power conversion stage, volume and weight, and increase conversion efficiency and reliability. The key technologies of this kind of inverter, such as circuit structure and topological family, EMCS, power supply mode (PSM), and steady-state principle characteristics, are deeply studied. The circuit structure is composed of multiple isolated HF inverting bridges, a HF transformer with multi-input single-output, a cycloconverter and an output LC filter. The EMCS is to generate the bipolar tristate multilevel SPWM waves by phase-shift between the right arm and the left arm of multiple HF inverting bridges; the cycloconverter demodulates it into the unipolar tristate multilevel SPWM wave, and performs zero voltage commutation. The experimental results of the designed and developed 1kVA multi-input inverter prototype have shown it has the advantages of single-stage power conversion, HF electrical isolation, high conversion efficiency, small volume and weight, high reliability, stable output voltage, multi-input source master-slave power distribution, and smooth switching of different PSMs.

Miniaturization and low cost contribute to the development and popularization of wireless power transfer (WPT). Reducing the number of power conversion stages can reduce the volume and cost of the WPT converter, and even make it possible to improve the efficiency. A single-stage WPT converter with constant current (CC) and constant voltage (CV) output and its asymmetric modulation method are proposed in this paper. A direct AC-AC converter is applied to the primary side of the WPT converter to directly convert line-frequency AC to high-frequency AC. The applied direct AC-AC converter that can replace the commonly used multi-stage primary converter reduces the number of circuit devices, thereby reducing the volume and cost of primary converter. The proposed WPT converter can realize zero voltage switching (ZVS) and power factor correction (PFC). In order to achieve CC and CV output, a switchable hybrid compensation topology is applied to the proposed WPT converter. A 720-W experimental prototype was manufactured to verify the rationality of the proposed WPT converter and the correctness of the theoretical analysis.

A hybrid computing method for predicting the magnetic field of line-start permanent magnet synchronous motor is presented in this article. The vector magnetic potential (VMP) is selected to be the primary objective of the calculation. For the convenience of expressing the VMP in the polar coordinate system, the topology of each motor component is replaced by the assembly of radial sectors. Along with the topology approximation, the VMPs of the stator and air gap are determined analytically, whereas the finite difference method is used to calculate the VMP of the rotor with the eddy current. Combining the numerical technique with the analytical method not only enables solution efficiency higher than using the former one merely but also keeps away from the inefficacy of using the latter one solely to predict magnetic fields with complex geometry. The presented approach is validated by the finite-element analysis and prototyping tests.

Harmonic injection is commonly used in permanent magnet synchronous motor torque ripple minimization (TRM) of electric vehicle. Traditional inverter switching frequency limits TRM application at high speed due to harmonic modulation accuracy and inverter loss. To achieve TRM in the whole speed range and reduce inverter loss, a novel online calculated low carrier ratio pulsewidth modulation (LCRPWM) is proposed in this article. The carrier ratio determining regular according to the injected harmonics is established first. The relationship between injected harmonics and LCRPWM switching angle is revealed and a set of nonlinear equations is established. Thus, the problem is transformed from solving nonlinear equations into optimization by establishing a cost function. The analytical solutions of nonlinear equations are obtained, which avoids iterative operation and plunging into local minimum. Therefore, the switch angle solution method is suitable for online calculation. The LCRPWM is verified in simulation compared with the traditional regularly sampled space vector PWM, which achieves better results in voltage harmonic injection and flux tracking at a much lower switching frequency. At last, the LCRPWM is used in a test bench experiment that shows good TRM effects. The influence of sideband harmonics on torque ripple is analyzed.