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A dc-link voltage balancing control strategy for quasi-Z-source cascaded H-bridge (qZS-CHB) inverter photovoltaic (PV) power system is proposed by using multidimensional pulse-width modulation (MD-PWM) technique. The qZS-CHB PV system usually employs proportional-integral (PI) regulators based closed-loop control methods to balance dc-link voltages, combining with distributed maximum power point tracking and grid-tie power control. These require significant control computations and hardware resources for multiple modules based qZS-CHB system. The proposed MD-PWM for the qZS-CHB inverter performs shoot-through behavior based on geometric representation of the output voltage ranges of each H-bridge in coordinate axes. All possible switching sequences of the qZS-CHB inverter are taken into account to choose the most suitable sequence that can balance dc-link voltages of qZS modules in the qZS-CHB PV power system. When compared with the state-of-the-art of voltage balancing control methods, the proposed control strategy has advantages: 1) The computation is low because there is no PI controller, while the stability of the whole system is improved; and 2) there is a reliable balancing capability with fast regulation of dc-link voltages, owing to no extra controller parameters and handling the voltage balance in each control cycle. Simulation and experimental results of qZS-CHB inverter based grid-tie PV power system verify the proposed dc-link voltage balancing control technique. [ABSTRACT FROM AUTHOR]

A discrete-time average model-based predictive control (DTA-MPC) is proposed for a quasi-Z-source inverter (qZSI). As a single-stage inverter topology, the qZSI regulates the dc-link voltage and the ac output voltage through the shoot-through (ST) duty cycle and the modulation index. Several feedback strategies have been dedicated to produce these two control variables, among which the most popular are the proportional–integral (PI)-based control and the conventional model-predictive control (MPC). However, in the former, there are tradeoffs between fast response and stability; the latter is robust, but at the cost of high calculation burden and variable switching frequency. Moreover, they require an elaborated design or fine tuning of controller parameters. The proposed DTA-MPC predicts future behaviors of the ST duty cycle and modulation signals, based on the established discrete-time average model of the quasi-Z-source (qZS) inductor current, the qZS capacitor voltage, and load currents. The prediction actions are applied to the qZSI modulator in the next sampling instant, without the need of other controller parameters’ design. A constant switching frequency and significantly reduced computations are achieved with high performance. Transient responses and steady-state accuracy of the qZSI system under the proposed DTA-MPC are investigated and compared with the PI-based control and the conventional MPC. Simulation and experimental results verify the effectiveness of the proposed approach for the qZSI. [ABSTRACT FROM PUBLISHER]

The battery energy-stored quasi-Z-source (BES-qZS)-based photovoltaic (PV) power generation system combines advantages of the qZS inverter and the battery energy storage system. However, the second-harmonic ( \text2\omega ) power ripple degrades the system's performance and affects the system's design. An accurate model to analyze the \text2\omega ripple is very important. The existing models did not consider the battery, or assumed a symmetric qZS network with $L_{{1}}= L_{{2}}$ and $C_{{1}}= C_{{2}}$, which limits the design freedom and causes oversized impedance parameters. This paper proposes a comprehensive model for the single-phase BES-qZS-PV inverter system, where the battery is considered and there is no restriction of $L_{{1}}= L_{{2}}$ and $C_{{1}}= C_{{2}}$ . Based on the built model, a BES-qZS impedance design method is proposed to mitigate the \text2\omega ripple with asymmetric qZS network. Simulation and experimental results verify the proposed \text2\omega ripple model and impedance design method. [ABSTRACT FROM AUTHOR]

Active ripple decoupling technique of the single-phase inverter is a popular topic to minimize the dc-link capacitance. However, the existing control methods are based on tracking sinusoidal or predetermined voltage waveforms of the compensation capacitor, assuming the inverter outputs are pure sinusoidal voltage and current. Therefore, the performance of existing methods degrades when the inverter output voltage and current are not purely sinusoidal. Furthermore, the limited dynamic performance threatens the safety of dc-link capacitor when the load changes. This happens, because the inrush ripple power is injected into dc link with small capacitance and the dc-link voltage will suddenly rise up when the ripple power is not buffered during transients. In this paper, a direct instantaneous power predictive control is proposed for the decoupling circuit to buffer ripple power of the single-phase inverter, which combines instantaneous ripple power control with model predictive control to overcome the issues above. The proposed method tracks the instantaneous ripple power rather than voltage or current waveforms. In this way, it can fully buffer all ripple powers in the system even for distorted output voltage and current of the inverter and enables the full utilization of storage capacitor. In addition, model predictive control makes the proposed method have fast dynamic and perfectly compensate ripple power during transients and steady states. The buck-type active ripple decoupling circuit is chosen to implement the proposed method after comparing with another typical decoupling topology. The proposed method is also compared with conventional method using proportional-integral-resonant regulator to track the predetermined capacitor voltage waveform. Experimental tests verify the theoretical analysis and the proposed control method. [ABSTRACT FROM AUTHOR]

A battery-energy-stored quasi-Z-source cascaded multilevel inverter (qZS-CMI)-based photovoltaic (PV) power generation system combines advantages of a qZS inverter, a CMI, and a battery energy storage system. However, unbalanced battery state of charge (SOC) between cascaded H-bridge inverter modules will degrade an entire system's performance and shorten battery lifespan. This paper proposes a control method to balance battery SOCs of all modules, no matter the intermittent states of each module's PV power. The method is on the basis of battery SOCs, SOC limits of each module, and the total power injected into the power grid. Simulation and experimental results verify the proposed control method that ensures identical SOCs for the battery-energy-stored qZS-CMI PV system. [ABSTRACT FROM PUBLISHER]

This paper is the continuation of Part I in which a quasi-Z-source modular cascaded converter (qZS-MCC), comprising front-end isolated qZS half-bridge dc–dc converter submodules (SMs), for dc integration of high-power photovoltaic (PV) systems is proposed. The qZS-MCC-based PV system features modular structure, high-voltage dc collection of PV power, simple control with a unified and constant duty cycle for the front-end isolation converter of all SMs, and low qZS impedance due to no double-line-frequency pulsating power. Here, control scheme of the qZS-MCC PV system integrated into the dc collection grid is investigated. Dynamic model of the system is established for controllers design and time-domain transient simulation. Experimental tests are carried out on the downscaled prototype as a proof-of-concept of the proposed control and modeling, demonstrating the validity of the proposed approaches. [ABSTRACT FROM PUBLISHER]

A quasi-Z-source modular cascaded converter (qZS-MCC) is proposed for dc integration of high-power photovoltaic (PV) systems. The qZS-MCC comprises series-connected front-end isolated qZS half-bridge (HB) dc–dc converter submodules (SMs). With the front-end isolation, the qZS-MCC achieves high-voltage dc capability, while maintaining modularity and PV panel grounded. The post-stage qZS-HB handles the PV voltage and power flows, dc-link voltage balance, and output-series power integration. Whereas, the front-end isolation converters of all SMs perform a constant duty cycle, lowing the control complexity. There is no double-line-frequency power flowing through the dc-side PV panels, qZS inductors, and qZS capacitors in the qZS-MCC, so small qZS impedance is possible compared to the existing qZS cascaded multilevel inverter. The configuration, operating principle, power loss evaluation, and passive components design of the proposed system are investigated in this part of the paper. The system control, modeling, and corresponding verifications are stated in Part II of this paper. [ABSTRACT FROM PUBLISHER]

Through coupling the impedance network between the grid and the conventional indirect matrix converter (IMC), the LC-filter-integrated quasi-Z-source (qZS) IMC overcomes the 0.866 voltage gain limitation of the conventional IMC and also avoids the input filter that is required to mitigate current harmonics of the conventional qZS matrix converters. This paper further investigates the voltage boosting and LC filtering function of the LC-filter-integrated qZS IMC. The voltage gain, the filtering function, and qZS network parameters design are presented using a small-signal model and circuit analysis. Simulation and experimental results validate the built model, the voltage gain analysis, and the parameters design of this type of qZS IMC. The input current of the LC-filter-integrated qZS network is compared to the conventional Z-source and qZS IMCs to investigate the integrated LC filtering capability. The experimental results verify that the LC-filter-integrated qZS network provides the necessary filtering function. Thus, the traditional input filter can be eliminated, which reduces the cost, power loss, volume, and weight for the overall system, when compared with the other conventional topologies that require the input filter, even those with impedance-source networks. [ABSTRACT FROM PUBLISHER]

The second harmonic pulsating power flows through the dc side of a single-phase quasi-Z-source inverter (qZSI), which requires bulky capacitor banks and inductors to suppress low-frequency ripple of dc-link voltage and inductor currents in the passive ripple reduction way. However, the resultant huge qZS network seriously deteriorates the system reliability, efficiency, volume, weight, and cost. This paper proposes an active-filter-integrated single-phase qZSI to transfer low-frequency power ripple directly from ac load to active filter's ac capacitor, so that low-frequency power ripple does not present in dc side anymore and constant inductor currents and constant capacitor voltages are ensured. Thus, much small qZS impedance is employed to only smooth high-frequency ripple and active filter's capacitor supports ac voltage (large ripple allowed) with small capacitance. The operation principle, parameter design method, and modeling and control strategy of the proposed topology are investigated. Comparative evaluation, simulation, and experimental results verify the proposed new topology system. [ABSTRACT FROM PUBLISHER]

The active power filter (APF) that consists of a half-bridge leg and an ac capacitor is integrated in the single-phase quasi-Z-source inverter (qZSI) in this paper to avoid the second harmonic power flowing into the dc side. The capacitor of APF buffers the second harmonic power of the load, and the ac capacitor allows highly pulsating ac voltage, so that the capacitances of both dc and ac sides can be small. A model predictive direct power control (DPC) is further proposed to achieve the purpose of this new topology through predicting the capacitor voltage of APF at each sampling period and ensuring the APF power to track the second harmonic power of single-phase qZSI. Simulation and experimental results verify the model predictive DPC for the APF-integrated single-phase qZSI. [ABSTRACT FROM PUBLISHER]

The single-phase quasi-Z source inverter (qZSI) topology has recently attracted attention for single-phase grid-tie photovoltaic (PV) applications. However, due to the inherent second-harmonic power flow in single-phase systems, a large qZS network is required to reduce the second-harmonic component of currents and voltages on the dc side. Minimization of the qZS network remains an open issue. This paper proposes a technique that minimizes the qZS capacitance and inductance of the single-phase qZSI topology by employing dc-side low-frequency current ripple damping control. Through analysis of power flow, a second-harmonic power model is derived and the ripple power is analyzed for minimization of the qZS network. A current ripple damping control is proposed to ensure suppression of second-harmonic power flow through the inductors. Simulation and experimental results verify the theoretical analysis, damping control, and the proposed design minimization of the qZS network for the single-phase topology. [ABSTRACT FROM PUBLISHER]

A hybrid pulsewidth modulated single-phase quasi-Z-source grid-tie photovoltaic (PV) power system is proposed. The hybrid pulse-width modulation (HPWM) combines the pulse-width modulation (PWM) and the pulse-amplitude modulation (PAM). The PWM works when the ac output voltage is lower than the dc source voltage; otherwise, the PAM operates the single-phase quasi-Z-source inverter (qZSI). The HPWM leads to the reduction of power loss, and the quasi-Z-source capacitance and inductance. An effective control strategy is proposed for the new PV power system to manage the maximum power point tracking (MPPT) of PV panel, grid-tie power injection, and dc-link voltage. A grid-tie current controller, combining a repetitive controller and a proportional-resonant regulator, achieves strong harmonic suppression, fast dynamic, and zero tracking error. A 500-W prototype is built to verify the new system. Power loss estimation and impedance design are detailed. Experimental tests validate the HPWM, new PV power system with higher efficiency, and the related control method. [ABSTRACT FROM PUBLISHER]

The second harmonic $(2\omega)$ power ripple of single-phase quasi-Z-source (qZS) photovoltaic (PV) inverter highly affects the whole system's design and performance. The topology's $2\omega$ ripple model is very important to analyze qZS PV inverter's $2\omega$ 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 $2\omega$ 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 $2\omega$ 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 $2\omega$ ripple model and parameter design method. [ABSTRACT FROM PUBLISHER]

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-CMI's 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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

The quasi- $Z$ source (qZS) cascaded multilevel inverter (CMI) (qZS-CMI) presents attractive advantages in application to photovoltaic (PV) power system. Each PV panel connects to an H-bridge qZS inverter (qZSI) to form a power generation module. The distributed maximum power point tracking and all modules' dc-link peak voltage balance can be achieved. However, it is the same with the conventional CMI that the second-harmonic ( $2\omega$) voltage and current ripples exist in each qZSI module. It is crucial for a qZS-CMI to design the reasonable qZS network parameters to limit the ripples within a desired range. This paper proposes an analytic model to accurately calculate the $2\omega$ voltage and current ripples of each qZSI module. A qZS impedance design method based on the built model is proposed to limit the $2\omega$ ripples of dc-link voltage and inductor current. Simulated and experimental results through using the designed 1.5-kW prototype validate the proposed analytic model and the design method. Furthermore, this paper analyzes all of the operating states for a qZSI module and calculates the power loss. The measured efficiency from the prototype verifies the theoretical calculation, and the qZS-CMI-based grid-tie PV power system is tested in practical. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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 panel's 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 inverter's 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. [ABSTRACT FROM PUBLISHER]

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 battery's 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. [ABSTRACT FROM PUBLISHER]