Wuhua Li

Review of Nonisolated High-Step-Up DC/DC Converters in Photovoltaic Grid-Connected Applications

The photovoltaic (PV) grid-connected power system in the residential applications is becoming a fast growing segment in the PV market due to the shortage of the fossil fuel energy and the great environmental pollution. A new research trend in the residential generation system is to employ the PV parallel-connected configuration rather than the series-connected configuration to satisfy the safety requirements and to make full use of the PV generated power. How to achieve high-step-up, low-cost, and high-efficiency dc/dc conversion is the major consideration due to the low PV output voltage with the parallel-connected structure. The limitations of the conventional boost converters in these applications are analyzed. Then, most of the topologies with high-step-up, low-cost, and high-efficiency performance are covered and classified into several categories. The advantages and disadvantages of these converters are discussed. Furthermore, a general conceptual circuit for high-step-up, low-cost, and high-efficiency dc/dc conversion is proposed to derive the next-generation topologies for the PV grid-connected power system. Finally, the major challenges of high-step-up, low-cost, and high-efficiency dc/dc converters are summarized. This paper would like to make a clear picture on the general law and framework for the next-generation nonisolated high-step-up dc/dc converters.

Design and Analysis of a Grid-Connected Photovoltaic Power System

A grid-connected photovoltaic (PV) power system with high voltage gain is proposed, and the steady-state model analysis and the control strategy of the system are presented in this paper. For a typical PV array, the output voltage is relatively low, and a high voltage gain is obligatory to realize the grid-connected function. The proposed PV system employs a ZVT-interleaved boost converter with winding-coupled inductors and active-clamp circuits as the first power-processing stage, which can boost a low voltage of the PV array up to a high dc-bus voltage. Accordingly, an accurate steady-state model is obtained and verified by the simulation and experimental results, and a full-bridge inverter with bidirectional power flow is used as the second power-processing stage, which can stabilize the dc-bus voltage and shape the output current. Two compensation units are added to perform in the system control loops to achieve the low total harmonic distortion and fast dynamic response of the output current. Furthermore, a simple maximum-power-point-tracking method based on power balance is applied in the PV system to reduce the system complexity and cost with a high performance. At last, a 2-kW prototype has been built and tested to verify the theoretical analysis of the paper.

Improved Transformerless Inverter With Common-Mode Leakage Current Elimination for a Photovoltaic Grid-Connected Power System

To eliminate the common-mode leakage current in the transformerless photovoltaic grid-connected system, an improved single-phase inverter topology is presented. The improved transformerless inverter can sustain the same low input voltage as the full-bridge inverter and guarantee to completely meet the condition of eliminating common-mode leakage current. Both the unipolar sinusoidal pulsewidth modulation (SPWM) as well as the double-frequency SPWM control strategy can be applied to implement the three-level output in the presented inverter. The high efficiency and convenient thermal design are achieved thanks to the decoupling of two additional switches connected to the dc side. Moreover, the higher frequency and lower current ripples are obtained by adopting the double-frequency SPWM, and thus the total harmonic distortion of the grid-connected current are reduced greatly. Furthermore, the influence of the phase shift between the output voltage and current, and the influence of the junction capacitances of the power switches are analyzed in detail. Finally, a 1-kW prototype has been simulated and tested to verify the theoretical analysis of this paper.

Interleaved Converter With Voltage Multiplier Cell for High Step-Up and High-Efficiency Conversion

A novel interleaved high step-up converter with voltage multiplier cell is proposed in this paper to avoid the extremely narrow turn-off period and to reduce the current ripple, which flows through the power devices compared with the conventional interleaved boost converter in high step-up applications. Interleaved structure is employed in the input side to distribute the input current, and the voltage multiplier cell is adopted in the output side to achieve a high step-up gain. The voltage multiplier cell is composed of the secondary windings of the coupled inductors, a series capacitor, and two diodes. Furthermore, the switch voltage stress is reduced due to the transformer function of the coupled inductors, which makes low-voltage-rated MOSFETs available to reduce the conduction losses. Moreover, zero-current-switching turn- on soft-switching performance is realized to reduce the switching losses. In addition, the output diode turn-off current falling rate is controlled by the leakage inductance of the coupled inductors, which alleviates the diode reverse recovery problem. Additional active device is not required in the proposed converter, which makes the presented circuit easy to design and control. Finally, a 1-kW 40-V-input 380-V-output prototype operating at 100 kHz switching frequency is built and tested to verify the effectiveness of the presented converter.

Mode-Adaptive Decentralized Control for Renewable DC Microgrid With Enhanced Reliability and Flexibility

A mode-adaptive decentralized control strategy is proposed for the power management of a dc microgrid with multiple renewable distributed generators and energy storage systems. In the presented solution, the dc bus voltage signal is used not only to enable power sharing among different sources, but also to designate microgrid operation modes and facilitate seamless mode transitions. With this mode-adaptive operation mechanism, a greater control freedom can be achieved than the conventional dc voltage droop control scheme. More importantly, this approach features fully self-disciplined regulation of distributed converters without an extra control center or communication link. Therefore, both reliability and flexibility can be enhanced. Meanwhile, a novel mode definition criterion is also provided to highlight the special characteristics of the dc microgrid which is different from an ac one. Three typical operation conditions are summarized according to which type of sources are dominating the power balance. Finally, the effectiveness of the proposed technique is verified experimentally based on a composite dc microgrid test system.

Topology Review and Derivation Methodology of Single-Phase Transformerless Photovoltaic Inverters for Leakage Current Suppression

Single-phase voltage source transformerless inverters have been developed for many years and have been successful commercial applications in the distributed photovoltaic (PV) grid-connected systems. Moreover, many advanced industrial topologies and recent innovations have been published in the last few years. The objective of this paper is to classify and review these recent contributions to establish the present state of the art and trends of the transformerless inverters. This can provide a comprehensive and insightful overview of this technology. First, the generation mechanism of leakage current is investigated to divide the transformerless inverters into asymmetrical inductor-based and symmetrical inductor-based groups. Then, the concepts of dc-based and ac-based decoupling networks are proposed to not only cover the published symmetrical inductor-based topologies but also offer an innovative strategy to derive advanced inverters. Furthermore, the transformation principle between the dc-based and ac-based topologies is explored to make a clear picture on the general law and framework for the recent advances and future trend in this area. Finally, a family of clamped highly efficient and reliable inverter concept transformerless inverters is derived and tested to offer some excellent candidates for next-generation high-efficiency and cost-effective PV grid-tie inverters.

Transformerless Inverter With Virtual DC Bus Concept for Cost-Effective Grid-Connected PV Power Systems

In order to eliminate the common-mode (CM) leakage current in the transformerless photovoltaic (PV) systems, the concept of the virtual dc bus is proposed in this paper. By connecting the grid neutral line directly to the negative pole of the dc bus, the stray capacitance between the PV panels and the ground is bypassed. As a result, the CM ground leakage current can be suppressed completely. Meanwhile, the virtual dc bus is created to provide the negative voltage level for the negative ac grid current generation. Consequently, the required dc bus voltage is still the same as that of the full-bridge inverter. Based on this concept, a novel transformerless inverter topology is derived, in which the virtual dc bus is realized with the switched capacitor technology. It consists of only five power switches, two capacitors, and a single filter inductor. Therefore, the power electronics cost can be curtailed. This advanced topology can be modulated with the unipolar sinusoidal pulse width modulation (SPWM) and the double frequency SPWM to reduce the output current ripple. As a result, a smaller filter inductor can be used to reduce the size and magnetic losses. The advantageous circuit performances of the proposed transformerless topology are analyzed in detail, with the results verified by a 500-W prototype.

A Family of Interleaved DC–DC Converters DeducedFrom a Basic Cell With Winding-Cross-CoupledInductors (WCCIs) for High Step-Upor Step-Down Conversions

A basic cell with winding-cross-coupled inductors (WCCIs) and interleaved structure is proposed in this paper. A family of DC-DC converters is deduced from the proposed basic cell, which is suitable for high step-up or step-down conversions. The passive-lossless clamp scheme is derived from the active clamp scheme to recycle the leakage energy and to suppress the voltage spikes caused by the leakage inductance. The advantages of the derived interleaved boost converter with WCCIs and passive-lossless clamp circuits are analyzed as an example. The voltage gain is extended and the switch voltage stress is reduced to minimize the conduction losses. The rectifier reverse-recovery problem is alleviated by the leakage inductance. Furthermore, a series of DC-DC converters with WCCIs and passive-lossless clamp circuits are summarized for high efficiency, high power and high step-up or step-down applications. A clear picture is made in this paper on the general law and structure of the WCCIs for DC-DC conversion in high step-up or step-down applications. At last, the simulated and experimental results of a 1 kW 40 V-to-380 V prototype with WCCIs and passive-lossless clamp circuits verify the significant improvements in performance.

An Interleaved Winding-Coupled Boost Converter With Passive Lossless Clamp Circuits

An interleaved winding-coupled boost converter is proposed in order to realize ZCS turn-on condition and to achieve high step-up gain. The voltage gain is extended easily and the voltage stress on the switches is reduced with different turns ratios of the coupled inductors. The output diode reverse-recovery problem is alleviated due to the inherent leakage inductor of the winding-coupled inductors. The simple but effective passive lossless clamp circuits are introduced to limit the voltage stress on the switches when they turn off so that the low-voltage, high-performance devices with low conduction resistor can be used in the proposed converter to reduce the relative losses. The leakage energy is ultimately recovered to the load. The high efficiency of 90.7% at full load and the maximum efficiency of 92.6% are achieved in a 1-kW 40-V-input-to-380-V-output prototype for front-end application. There is more than 10% efficiency improvement with the passive lossless clamp circuits compared to the case with RCD dissipated snubbers. The efficiency of 5% improvement at full load is achieved with the proposed converter with passive lossless circuits compared to the conventional interleaved boost converter.

High-Step-Up and High-Efficiency Fuel-Cell Power-Generation System With Active-Clamp Flyback–Forward Converter

A high-efficiency fuel-cell power-generation system with an active-clamp flyback-forward converter is presented in this paper to boost a 12-V dc voltage into a 220-V 50-Hz ac voltage. The proposed system includes a high-efficiency high-step-up interleaved soft-switching flyback-forward converter and a full-bridge inverter. The front-end active-clamp flyback-forward converter has the advantages of zero-voltage-switching performance for all the primary switches, reverse-recovery-problem alleviation for the secondary output diodes, large voltage-conversion ratio, and small input-current ripple. Furthermore, there are two coupled inductors in the proposed converter. Each coupled inductor can work in the flyback mode when the corresponding main switch is in the turn-on state and in the forward mode when it is in the turnoff state, which takes full use of the magnetic core and improves the power density. In addition, the full-bridge inverter with an LC low-pass filter is adopted to provide low-total-harmonic-distortion ac voltage to the load. Therefore, high-efficiency and high-power-density conversion can be achieved in a wide input-voltage range by employing the proposed system. Finally, a 500-W prototype and another 1-kW converter are implemented and tested to verify the effectiveness of the proposed system.