Yushan Liu

An Energy-Stored Quasi-Z-Source Inverter for Application to Photovoltaic Power System

The quasi-Z-source inverter (qZSI) with battery operation can balance the stochastic fluctuations of photovoltaic (PV) power injected to the grid/load, but its existing topology has a power limitation due to the wide range of discontinuous conduction mode during battery discharge. This paper proposes a new topology of the energy-stored qZSI to overcome this disadvantage. The operating characteristic of the proposed solution is analyzed in detail and compared to that of the existing topology. Two strategies are proposed with the related design principles to control the new energy-stored qZSI when applied to the PV power system. They can control the inverter output power, track the PV panels maximum power point, and manage the battery power, simultaneously. The voltage boost and inversion, and energy storage are integrated in a single-stage inverter. An experimental prototype is built to test the proposed circuit and the two discussed control methods. The obtained results verify the theoretical analysis and prove the effectiveness of the proposed control of the inverters input and output powers and battery power regardless of the charging or discharging situation. A real PV panel is used in the grid-tie test of the proposed energy-stored qZSI, which demonstrates three operational modes suitable for application in the PV power system.

An Effective Control Method for Quasi-Z-Source Cascade Multilevel Inverter-Based Grid-Tie Single-Phase Photovoltaic Power System

An effective control method, including system-level control and pulsewidth modulation for quasi-Z-source cascade multilevel inverter (qZS-CMI) based grid-tie photovoltaic (PV) power system is proposed. The system-level control achieves the grid-tie current injection, independent maximum power point tracking (MPPT) for separate PV panels, and dc-link voltage balance for all quasi-Z-source H-bridge inverter (qZS-HBI) modules. The complete design process is disclosed. A multilevel space vector modulation (SVM) for the single-phase qZS-CMI is proposed to fulfill the synthetization of the step-like voltage waveforms. Simulation and experiment based on a seven-level prototype are carried out to validate the proposed methods.

Overview of Space Vector Modulations for Three-Phase Z-Source/Quasi-Z-Source Inverters

Three existing and one extended space vector modulations (SVMs) for the three-phase Z-source/quasi-Z-source inverter (ZSI/qZSI) are investigated. The different switching control patterns, the available maximum shoot-through duty ratio, the maximum voltage stress across the switch versus voltage gain, and efficiency are compared in detail. A total average switch device power taking into account the shoot-through current stress is proposed to evaluate the total stress of power switches. Simulation and experimental results of the prototyped qZSI verify the theoretical analysis. The six parts of shoot-through time intervals will reduce the inductor current ripples, and improve the qZSI efficiency. Also, the maximum voltage stress and total average switch power will benefit from the shoot-through division. However, the division method impacts these performances of qZSI.

A new grid-connected PV system based on cascaded H-bridge quasi-Z source inverter

A new scheme for grid-connected photovoltaic (PV) interface by combination of a quasi-Z source inverter (qZSI) into cascaded H-bridge (CHB) is proposed in this paper. The proposed scheme enables PV string voltage boost to a higher level, and solves the imbalance problem of DC-link voltage in traditional CHB inverters. A multilevel voltage waveform of inverter output is generated by an improved phase shifted sinusoidal pulse width modulation (PS-SPWM) algorithm, which introduces shoot-through states into the conventional zero states to control qZS-CHB module. The effective control schemes are proposed to regulate the maximum power point tracking (MPPT) of each string, and control the DC-link voltage of each H-bridge, respectively. Grid injected power is controlled corresponding to the proportionality factors of each PV string output power. A 1.5 kW system is built in MATLAB/SIMULINK, and the simulation results verify the proposed novel multilevel PV interface inverter and its control principles.

Z-Source\/Quasi-Z-Source Inverters: Derived Networks, Modulations, Controls, and Emerging Applications to Photovoltaic Conversion

The current and emerging investigations on Z-source/quasi-Z-source inverters (ZSIs/qZSIs) are presented. The ZSI/qZSI fulfills buck/boost capability in single-stage topology and overcomes the limits of traditional voltage source inverters (VSIs). Hence, high reliablity, high efficiency, and low cost can be achieved. The recently derived impedance networks with improved voltage gain are discussed. The classic and newly proposed modulations and feedback controls for ZSIs/qZSIs are compared. Finally, the emerging applications and trends of ZSIs/qZSIs to photovoltaic (PV) power generation are discussed. This study offers a comprehensive and systematic reference for the future development of the high-performance ZSI/qZSI.

An effective control method for three-phase quasi-Z-source cascaded multilevel inverter based grid-tie photovoltaic power system

The quasi-Z-source cascaded multilevel inverter (qZS-CMI) presented many advantages over conventional CMI when applied in photovoltaic (PV) power systems. For example, qZS-CMI provides the balanced dc-link voltage and voltage boost ability, saves one-third modules, etc. However, current research studies only disclosed control of single-phase qZS-CMI-based PV power systems, and there was no literature related to control of three-phase qZS-CMI-based PV power systems. In this paper, for the first time, three-phase qZS-CMIs control is proposed and demonstrated for application to PV power systems. The models of PV-panel-fed qZS H-bridge module and qZS-CMI-based PV power system are built to accurately design control algorithms for each module and the whole system. The proposed control method includes the distributed maximum power point tracking for each module, dc-link peak voltage balance control for all modules, and grid-tie control for the whole system; moreover, a new multilevel space vector modulation method is proposed for the three-phase qZS-CMI. Simulation and experimental results on a test bench with a three-phase seven-level qZS-CMI-based PV power system verify the proposed control and modulation methods.

Comprehensive Modeling of Single-Phase Quasi-Z-Source Photovoltaic Inverter to Investigate Low-Frequency Voltage and Current Ripple

The second harmonic (2ω) power ripple of single-phase quasi-Z-source (qZS) photovoltaic (PV) inverter highly affects the whole systems design and performance. The topologys 2ω ripple model is very important to analyze qZS PV inverters 2ω voltage and current ripple. The existing models did not consider the PV-panel dynamic and terminal capacitors, which causes the theoretical results apart from the truth. In this paper, a comprehensive modeling for single-phase qZS-PV inverter is proposed, where the 2ω ripple model of the qZS-PV inverter system with a real PV source is established and discussed without and with a PV terminal capacitor. The influences from qZS inductance and capacitance, and PV-panel terminal capacitance to the 2ω voltage and current ripple are investigated using the built model. The system parameter design method is proposed to mitigate this ripple. Simulation and experimental results validate the proposed 2ω ripple model and parameter design method.

Control System Design of Battery-Assisted Quasi-Z-Source Inverter for Grid-Tie Photovoltaic Power Generation

A control strategy for the quasi-Z-source inverter (qZSI) with a battery-based photovoltaic (PV) power conversion system is proposed. A battery-assisted qZSI can buck/boost PV panel voltage by introducing shoot-through states, and make full use of PV power by the energy-stored battery paralleled to the quasi-Z-source capacitor. A dynamic small-signal model of the battery-assisted qZSI is established to design a closed-loop controller for regulating shoot-through duty ratio and managing the batterys energy storage. A modified space vector modulation (SVM) technique for the qZSI is applied to achieve low harmonics, high voltage utilization, and high efficiency. A P-Q decoupled grid-tie power injection is fulfilled with the maximum power capture from PV panels and the unity power factor. The validity of the proposed PV system is proved by experimental results, showing an efficient method for the energy-stored PV power generation.

Quasi-Z-Source inverter based PMSG wind power generation system

In view of the buck and boost capabilities of quasi-Z-Source inverter (qZSI), an application of qZSI to a permanent magnet synchronous generator (PMSG) with wind power generation system (WPGS) is presented. The traditional space vector PWM (SVPWM) technique is modified to satisfy the shoot-through characteristic of qZSI. The closed-loop control of shoot-through duty cycle is designed to obtain the desired DC bus voltage. The DC-link boost control and AC-side output control are presented to reduce the impacts of disturbances on grids. Further, some significant simulations are done in MATLAB / SIMULINK. The simulated results verify the validity and superiority of the proposed control strategies, indicating a significant method to enhance the anti-interference performances of wind power generation systems.

Modeling and SVPWM control of quasi-Z-source inverter

In todays renewable energy power generation systems, inverters have become key devices connected between energy sources and power grids. The newly presented quasi-Z-source inverter (qZSI) has the unique advantages of boosting DC bus voltage without additional boost link, lower component ratings, as well as constant DC input current compared with the traditional Z-source inverter. To better design the control schemes of this novel topology, the small-signal model is deduced using state variables perturbance mathematical analysis. By this model, the voltage-current closed-loop control of shoot-through duty cycle is designed. The space vector PWM (SVPWM) technique is modified to satisfy the particularity of qZSI. Simulation results certify the correctness and effectiveness of the proposed model and control strategies and demonstrate the applicability of qZSI technology to overcome the disturbances from DC link or AC loads in renewable energy power generation systems.