Huiqing Wen

Analysis and Evaluation of DC-Link Capacitors for High-Power-Density Electric Vehicle Drive Systems

In electric vehicle (EV) inverter systems, direct-current-link capacitors, which are bulky, heavy, and susceptible to degradation from self heating, can become a critical obstacle to high power density. This paper presents a comprehensive method for the analysis and comparative evaluation of dc-link capacitor applications to minimize the volume, mass, and capacitance. Models of equivalent series resistance that are valid over a range of frequency and operating temperature are derived and experimentally validated. The root-mean-square values and frequency spectra of the capacitor current are analyzed with respect to three modulation strategies and various operating conditions over practical ranges of load power factor and modulation index in EV drive systems. The modeling and analysis also consider the self-heating process and resulting core temperature of the dc-link capacitors, which impacts their lifetimes. Based on an 80-kW permanent-magnet (PM) motor drive system, the application of electrolytic capacitors and film capacitors has been evaluated by both simulation and experimental tests. The inverter power density is improved from 2.99 kW/L to 13.3 kW/L, without sacrificing the system performance in terms of power loss, core temperature, and lifetime.

Nonactive Power Loss Minimization in a Bidirectional Isolated DC–DC Converter for Distributed Power Systems

Originated from analyzing nonactive power loss, a novel optimization method and modulation solution for bidirectional isolated dual-active-bridge (DAB) dc-dc converters are proposed in order to achieve high efficiency in a wide operating range. A comprehensive nonactive power loss model is developed, including both the nonactive components delivered back to the source and from the load. This paper points out that when the minimum nonactive power loss is achieved, zero-voltage soft switching can be naturally fulfilled. The optimal phase-shift pair obtained by the proposed method can keep low values of both root mean square (RMS) current and circulating power. Rather than using ideal power flow analysis, the nonactive power loss model directly embodies practical nonideal factors, including device voltage drops. Based on the analysis, an extended dual phase shift is proposed, and different operation cases are analyzed with comparison of performance indices. Experimental tests verify the theoretical analysis and show effectiveness of the proposed approach to achieve nonactive power loss minimization and efficiency improvement.

An Improved MPPT Method for PV System With Fast-Converging Speed and Zero Oscillation

Maximum power point tracking (MPPT) is essential for photovoltaic (PV) systems to ensure the highest power output of PV arrays under any environmental condition. Comparing to other techniques, the Beta method shows advantages in terms of tracking speed, steady-state performance, and simple implementation. However, the conventional Beta can further be improved by minimizing oscillations around the maximum power point under a steady state and an increasing tracking speed in response to rapid changing of irradiance or temperature. An improved Beta-parameter-based MPPT method is proposed in this paper to achieve the above-mentioned objectives. An adaptive scaling factor is introduced and utilized in the MPPT mechanism, which enhances the tracking speed and is easily applied for any PV power system. Furthermore, the proposed method can identify and maintain the middle point of the three-level perturbations, which eliminate the oscillations at a steady state. The control mechanism is not limited by specific operating conditions and illustrates superior performance over traditional methods with regards to transient response and steady-state performance, which contributes to effective solar power harvesting. Followed by theoretical analysis, the simulation and experimental evaluation validate the claimed advantages of the proposed MPPT solution.

An Improved Beta Method With Autoscaling Factor for Photovoltaic System

Maximum power point tracking (MPPT) is essential to improve the energy yield of solar energy systems. However, conventional MPPT algorithms show obvious problems such as the conflict of the steady-state oscillations and dynamic speed, and the clash of high computational burden and accuracy. Originated from the beta method, which shows the advantages of fast tracking speed in the transient stage, small oscillations in the steady-state, and medium complexity of implementation, this paper proposed an improved beta method to further improve the overall performance, especially for practical applications. Instead of manually tuning key parameters such as the range of β parameter and scaling factor N for different operating conditions, an autoscaling factor is used, which make the method easier in practical implementation and suitable for wider conditions. The meteorological data of two distinct locations are used to verify that the β parameters derived from photovoltaic (PV) modules are valid for one whole year under different environmental conditions. A PV system with the proposed MPPT method was built in MATLAB/Simulink, and different indices such as the rise time, the setting time, and the tracking energy loss are used to evaluate the performance of various MPPT algorithms. Finally, two experimental tests were carried out, including the indoor test with solar array emulator and the outdoor test with an actual PV module, respectively, to show the effectiveness of the proposed MPPT algorithm.

Bidirectional dual-active-bridge DC-DC converter with triple-phase-shift control

A novel triple phase-shift (TPS) control strategy to extend the soft-switching operating range of the isolated bidirectional dual-active-bridge (DAB) dc-dc converter and achieve high efficiency in a large operating range is proposed. Controlled by the TPS scheme, four typical operating modes are identified with respect to the characterization of phase-shift groups. The power characteristics of operating modes are presented in terms of reactive power and rms current. The comparative analysis of the peak value of inductor current and efficiency for TPS and conventional PS (CPS) control are also presented. The improvement of efficiency is determined by the proper selection of operating modes and phase-shift group. Experimental results show good agreement with theoretical analysis.

Design and optimization of laminated busbar to reduce transient voltage spike

The presence of loop inductance causes significant surge voltage due to hard-switching operation of voltage source inverters. This generally requires high-voltage rating of switching devices, which results in additional cost and power loss. This paper presents a surge voltage model to study busbar stray inductances in Electric Vehicle systems. The model demonstrates that the inductance of commutation cells contributes to significant voltage surge, resonance and electromagnetic interference. Based on modeling, the design tradeoff of different busbar structures can be analyzed by finite elements methods (FEM), which are based on Maxwell equations. The optimal design is proposed to reduce stray inductances of interconnection busbars for electric vehicle drive systems. Finally, the proposed design and optimization are evaluated by both simulation and experimental tests.

Current-Fed High-Frequency AC Distributed Power System for Medium–High-Voltage Gate Driving Applications

A current-fed high-frequency alternating-current (HFAC) distributed power system is presented in this paper for medium- or high-voltage gate driving applications. The distributed gate driving modules (GDMs) at the high-voltage side are isolated from the low-voltage-side current-source power supply through magnetic coupling of current transformers (CTs), which are assembled by a high-voltage cable carrying HFAC and passing through the center of toroid magnetic cores. The centralized high-frequency current-source power supply is designed and composed of a two-stage resonant inverter and a double

Comparative evaluation of DC-link capacitors for electric vehicle application

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Photovoltaic Modified β-Parameter-based MPPT Method with Fast Tracking

-CL resonant network. The resonant network is parameterized by considering the tradeoff between the quality factor and component stress. Different magnetic core samples are tested and compared to construct the CTs for improving transmission efficiency. A 400-W prototype was built and experimentally evaluated with two load conditions, including resistive loads and practical GDMs. It showed significant improvements with regard to sinusoidal current output, system stability, and efficiency. Finally, the proposed current-fed HFAC power supply system was successfully applied to a practical solid-state fault current limiter, demonstrating the advantages of multiple outputs (

Estimating power losses in Dual Active Bridge DC-DC converter

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