Shaojun Xie

Improved Z-Source Inverter With Reduced Z-Source Capacitor Voltage Stress and Soft-Start Capability

This paper proposes an improved Z-source inverter topology. Compared to the traditional Z-source inverter, it can reduce the Z-source capacitor voltage stress significantly to perform the same voltage boost, and has inherent limitation to inrush current at startup. The control strategy of the proposed Z-source inverter is exactly the same as the traditional one, so all the existing control strategy can be used directly. A soft-start strategy is also proposed to suppress the inrush surge and the resonance of Z-source capacitors and inductors. The operation principle of the proposed topology and comparison with the traditional topology are analyzed in detail. Simulation and experimental results are given to demonstrate the new features of the improved topology.

An Optimized Transformerless Photovoltaic Grid-Connected Inverter

Unipolar sinusoidal pulsewidth modulation (SPWM) full-bridge inverter brings high-frequency common-mode voltage, which restricts its application in transformerless photovoltaic grid-connected inverters. In order to solve this problem, an optimized full-bridge structure with two additional switches and a capacitor divider is proposed in this paper, which guarantees that a freewheeling path is clamped to half input voltage in the freewheeling period. Sequentially, the high-frequency common-mode voltage has been avoided in the unipolar SPWM full-bridge inverter, and the output current flows through only three switches in the power processing period. In addition, a clamping branch makes the voltage stress of the added switches be equal to half input voltage. The operation and clamping modes are analyzed, and the total losses of power device of several existing topologies and proposed topology are fairly calculated. Finally, the common-mode performance of these topologies is compared by a universal prototype inverter rated at 1 kW.

Transformerless Split-Inductor Neutral Point Clamped Three-Level PV Grid-Connected Inverter

Characterized by low leakage current and low voltage stress of the power device, a neutral point clamped three-level inverter (NPCTLI) is suitable for a transformerless photovoltaic (PV) grid-connected system. Unfortunately, the shoot-through problem of bridge legs still exists in an NPCTLI, so its operation reliability is degraded. An improved three-level grid-connected inverter is proposed based on the NPCTLI and the dual-buck half-bridge inverter (DBHBI), and which avoids the shoot-through problem. The proposed topology guarantees no switching-frequency common-mode voltage and no shoot-through risk. Furthermore, the freewheeling diode of bridge legs of the DBHBI can be removed taking into consideration the unity power factor of grid current, and a straightforward topology is thus derived. The new topology is referred to as split-inductor NPCTLI (SI-NPCTLI). The operation mode, common-mode characteristic, and control strategy are analyzed. Finally, both the simulation and the experimental results of a 1-kW SI-NPCTLI prototype verify the analysis.

Dynamic Voltage Equalization for Series-Connected Ultracapacitors in EV/HEV Applications

Energy-storage systems (ESSs) play an important role in electric vehicle (EV) and hybrid EV (HEV) applications. In the system, an ultracapacitor is preferred for high power buffer and regenerative braking energy storage because it has the advantages of high power density, long life cycles, and high efficiency. While in the high-voltage application, the ultracapacitors are employed in series, and the voltage unbalance issue must be taken care of. This paper presents a novel circuit for equalizing a series ultracapacitor stack, which is based on a dc-dc converter. The proposed voltage-equalization circuit derives energy from the series ultracapacitor stack and transfers them to the weakest ultracapacitor cell. The equalizer balances the whole stack by sequentially compensating the weak ultracapacitor cells. Unlike previous methods for battery-storage systems, which include complex circuit detecting and comparing the voltages of capacitor cells, the novel equalizer can realize autonomic voltage equalization without voltage detection and comparison, and it is more efficient with the soft switching method, which is a benefit for high-power applications in EV/HEV. The simulation and experiment results validate the feasibility of the proposed equalization circuits.

Z-Source AC–AC Converters Solving Commutation Problem

A new family of Z-source AC-AC converters with buck-boost ability are proposed, including four switches single-phase structure and six switches three- phase structure. New commutation strategies for these converters are proposed and safe commutation can be achieved without snubber circuit. Analysis, simulation and experimental results on the voltage-fed Z-source AC- AC converters verified the analysis. These converters have merits such as less conduction and switching loss, less devices, therefore high efficiency and reliability can be achieved.

Pulsewidth Modulation of Z-Source Inverters With Minimum Inductor Current Ripple

This paper proposes the pulsewidth modulation (PWM) strategy of Z-source inverters (ZSIs) with minimum inductor current ripple. In existing PWM strategy with single-phase shoot-through, the shoot-through time interval is divided into six equal parts, therefore the three phase legs bear the equal shoot-through time interval. In this manner, the allotment and arrangement of the shoot-through state is easy to realize, but the inductor current ripple is not optimized. This causes to use relatively large inductors. In the proposed PWM strategy, the shoot-through time intervals of three phase legs are calculated and rearranged according to the active state and zero state time intervals to achieve the minimum current ripple across the Z-source inductor, while maintaining the same total shoot-through time interval. The principle of the proposed PWM strategy is analyzed in detail, and the comparison of current ripple under the traditional and proposed PWM strategy is given. Simulation and experimental results on the series ZSI are shown to verify the analysis.

A Novel Soft-Switching Multiport Bidirectional DC–DC Converter for Hybrid Energy Storage System

A novel multiport isolated bidirectional dc-dc converter for hybrid battery and supercapacitor applications is presented, which can achieve zero voltage switching for all switches in the whole load range. The bidirectional power flow between any two of the ports is free, and the circulating power is low for the well matching of the transformer voltages of all time regardless of the voltage variations of the battery and supercapacitor. Moreover, the current ripples are greatly decreased by interleaved control, which is good for battery and supercapacitor. The converter topology and the operation principle are introduced. Detailed analysis on soft-switching of all switches is given. On the basis of theoretical analysis, the principle and method for parameter designing are provided. A hybrid energy management strategy combining bus voltage control and energy management of the energy storage devices is proposed and the control scheme is presented. Moreover, detailed parameter design of a prototype converter is given for a 380-V dc-bus microgrid lab system. Effectiveness of the control strategy, correctness of the analysis on soft-switching, and the parameter design methods are verified by the simulation and experimental results.

An Improved

This paper proposes an improved Z-source inverter topology. Compared to previous Z-source inverter topology, it can reduce the Z-source capacitor voltage stress greatly, and has an inherent limitation to inrush current. The control strategy is exactly the same as the previous one, so all existing control strategies can be used directly. Soft-start strategy is also proposed to avoid the inrush current and resonance between the Z-capacitors and the Z-inductors. Comparison between the proposed and previous topologies is also presented. Analysis, simulation, and experimental results are presented to demonstrate the new features of the improved topology.

Z

This paper proposes a novel single-phase Z-source inverter topology with input and output sharing the same ground. It is simple in structure and only utilizes two switches while keeping the voltage transfer ratio the same as for the full-bridge inverter. One-cycle control is adopted. Highly efficient constant-frequency control can be achieved without an accurate model of the converter. The unique feature of the proposed topology is verified by operating principle, simulation and experimental results. This topology can be used in distributed power systems where dual grounding is needed.

-Source Inverter

Multipulse converters are suitable for high-power application with the merits of low switching frequency and perfect harmonic performance. But less controllability and poor regulation lead the restriction on its application. A bidirectional pulsewidth modulation (PWM) converter based on multipulse structure is proposed in this paper, which has the same perfect harmonic performance with very low switching frequency. A special sequential sampling space vector modulation technique, which has the sampling sequence from the lagging module to the leading module, is proposed to make the converter controllable like conventional PWM converters. The harmonic performance and linear regulation capability are analyzed theoretically. The converter is modeled in detail, and an instantaneous feedback control strategy with phase delay compensation and decoupling control is also proposed. The controller parameters are optimized to get high dynamic performance with adequate phase margin and gain margin. A 3-kVA prototype is built, and the simulation and experiment results validate that the proposed converter is quite suitable for high-power conversion.