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• The original contribution of the paper is a dq-real domain admittance transfer matrix model of PV-qZ systems. • The study presents some of the the two main origins of instability in multi-terminal AC grid-connected PV-qZ systems. • The study also illustrates the weakness of GNC related to stability assessment and grid partition points. Photovoltaic (PV) power systems are becoming one of the most popular green power systems. Power converters are used in these systems. Among them, quasi-Z-source inverters (qZSIs) are currently on the rise. However, instabilities can appear in multi-terminal AC grid-connected power electronics, such as PV power systems, due to the interaction between power electronic controls and the grid. Modeling of qZSI-based PV power systems is lacking in the literature and stability assessment of grid-connected qZSI-based PV power systems is not yet well understood. Accordingly, the present paper contributes with an s -domain dq -real admittance transfer matrix model of qZSI-based PV power systems derived from their small-signal state-space averaged model. The paper also describes a systematic procedure to study multi-terminal AC grid-connected qZSI-based PV power systems by the nodal admittance method, where the proposed qZSI-based PV power system model is incorporated into the nodal admittance matrix to study stability concerns. Thus, AC grid dynamics in the presence of qZSI-based PV power systems can be investigated using impedance-based stability criteria. The proposed model and the procedure are applied in a stability study of two applications where solutions for improving stability issues are presented. In these applications, the two main causes of instabilities in AC-grid connected PV-qZ systems are analyzed in detail. The study is validated by PSCAD/EMTDC simulation.. [ABSTRACT FROM AUTHOR]

• A mathematical model for inverters' connection transients in weak and strong grids. • An improvement of transients using a phase limiter algorithm. • A set of experiments which validates the proposal. • Analysis of virtual impedance and measurement filter dynamics. This paper derives a mathematical model suitable for analysis of connection behavior of the inverter-based grids and, using that model, it presents guidelines to control grid-forming inverters' rate of change in frequency as function of output power (virtual inertia), or in other words "measured power filters". Furthermore, in order to improve the transient response, an angle difference limiter control is proposed. An angle difference limiter is a control algorithm which is constantly checking the angle difference between the inverter and the grid in the connection point limiting power peaks during the transients. The performance of the angle difference limiter is analyzed in detail and validated with an experimental microgrid consisting of two three-phase inverters and a variable load. The paper analyzes the connection of inverters in weak and strong grids validating the performance of the proposed angle difference limiter. [ABSTRACT FROM AUTHOR]

• The impact of distributed capacitance is eliminated due to the capacitance current can be ignored at the low frequency. • The low-frequency components only exist between the BES and the fault point, and will not flow to the opposite grid. • The least square algorithm is employed to solve the overdetermined equations, improving the accuracy of calculation. The fault current of the photovoltaic (PV)/battery energy storage (BES) hybrid power generation system contains harmonic components, which might cause the maloperation of the distance relay. The time-domain distance protection has attracted more attention recently. However, the distributed capacitance of long transmission lines negatively affects the performance of time-domain distance protection, which hinders its application. To solve this problem, this paper proposes an improved time-domain distance protection based on fault active control. By leveraging the fault active control capability of the BES inverter, a specific low-frequency characteristic voltage is modulated through the phase-locked loop (PLL). By injecting the low-frequency characteristic voltage, the influence of line distributed capacitance can be ignored. Thus, the accuracy of time-domain distance protection based on the equivalent R-L model of transmission lines is guaranteed, which is the first advantage of injecting the low-frequency characteristic voltage. The other advantage is the low-frequency components only exist between the BES and the fault point, and will not flow to the grid on the opposite side of the line. The accuracy of time-domain distance protection has been further improved. In Addition, the faulty line's derivative equations for different fault types are deduced based on fault active control. Finally, the least square algorithm is used to calculate fault distance. The proposed scheme's feasibility and effectiveness are verified through simulations in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

In this paper, the hybrid synchronization based grid forming (HS-GFM) control and coordination strategy are proposed for the inverter and boost converter to provide frequency support. As the main contribution, the inertia power and damping power are designed with HS-GFM based coordination strategy between inverter and boost converter. The inertia power can be obtained from DC capacitor and photovoltaic system in deloading mode independently. Besides, the damping power can be realized from the PV inverter in deloading mode. After that, the small signal models of inverter and boost converter considering grid frequency coupling are constructed with simplified single input single output (SISO) model to guide the parameter selection. Further, the grid synchronization stability, synchronization bandwidth and grid strength influence on the frequency response of the PV inverter can be analyzed with SISO model. Compared with conventional DC voltage based GFM, HS-GFM can improve stability especially in strong grid with slight reduction of synchronization bandwidth while it has the risk of instability in weak grid if parameters are not proper. Compared with phase locked loop (PLL) based control, the proposed HS-GFM control achieves a fast response under frequency excursion with better stability in weak grid. Finally, realtime and offline simulations are carried out to verify the proposed strategy and analysis. • Inertia and damping power are designed with hybrid synchronization based GFM control. • The DC side thevenin equivalence of boost converter is obtained for SISO analysis. • SISO model of PV inverter is built for stability analysis and parameter selection. • Comparisons with conventional GFM and phase locked loop based control are presented. [ABSTRACT FROM AUTHOR]

• Study of PV power plant with quasi-Z-source cascaded H-bridge multilevel inverter under failure. • Control system to provide a suitable DC voltage at the input of the VSI and ensure the grid power delivery. • Control tested from simulation results under different operating conditions and HIL experimental results. • Correct response of the developed control strategy with failure in qZSI module. At the present time, quasi-Z-source cascaded H-bridge multilevel inverters (qZS-CHBMLI) have gained attention in photovoltaic (PV) applications, due to their advantages over traditional multilevel inverters. They allow single-stage power conversion of a PV power plant by independently adjusting the DC voltage in each module; thus, each PV array operates at the maximum power. This advantage permits dispensing with a DC/DC boost converter and reduces the number of modules required. This paper develops a novel control strategy for a qZS-CHBMLI connected to a single-phase system that guarantees proper operation when a fault occurs in a quasi-impedance source inverter (qZSI). The control strategy is employed in a grid-connected qZS-CHBMLI with three modules, each one with a quasi-impedance network, a 4.8 kW PV array and a voltage source inverter (VSI), with H-bridge topology. The purpose of the control system is to deliver a suitable DC voltage at the input of the VSI and ensure the grid power requirements after a fault in a module. The proposed control system is evaluated through a 40 s simulation in MATLAB/Simulink® in two cases: A) analysis of the system behaviour when there is a variation in the operation limit equation; and B) evaluation of the control strategy under dissimilar operating conditions. In addition, an experimental setup based on an OPAL RT-4510 unit and a dSPACE MicroLabBox prototyping unit is employed to validate the results. The results confirmed the correct response of the applied strategy with the failure of a single module. [ABSTRACT FROM AUTHOR]

• A power-decoupled current source inverter (PD-CSI) is proposed for the operation in an unbalanced grid to reduce the current rating of the DC-link inductor. • A model predictive control is proposed for the PD-CSI. It achieves high performance in both stability and rapidity. • A space vector pulse width modulation is proposed for the PD-CSI. It helps to reduce the voltage rating of the ripple storage capacitor. The current source inverter (CSI) is a promising interface between the Photovoltaic (PV) panel and the three-phase AC grid. It boosts the PV panel voltage by a DC-link inductor and converts the maximum available PV power. In an unbalanced grid, double-line-frequency pulsating powers will be generated in the CSI, increasing the rating of the DC-link inductor and impeding the maximum power point tracking (MPPT) operation. To address this issue, a power decoupling (PD) circuit for a three-phase CSI is proposed in this paper. By using the DC-link inductor and active switches multiplex, the proposed PD circuit can reduce the current rating of the DC-link inductor with fewer active switches, and no detrimental power components are introduced to the utility grid. Moreover, to concurrently control multiple objectives without multiple loops, a model predictive control (MPC) method is further proposed for the PD-CSI, achieving a rapid response of PD-CSI. Besides, a space vector pulse width modulation (SVPWM) method is proposed for the PD-CSI to enhance the utilization of DC voltage, helping to reduce the voltage stress of the ripple storage capacitor. The priorities of the proposed topology and the corresponding control and modulation methods are finally verified by comparative experimental results. [ABSTRACT FROM AUTHOR]

• The controller has inherent robustness against the LCL filter resonance and thus removes the need for PD, which produces losses, or AD, which increases the total cost of the system due to the need for extra sensors. • By selecting the controller parameter with a sensitivity analysis over the open-loop transfer function, the closed-loop system shows sufficiently high stability margins. First, the formulation of the open-loop and closed-loop functions is given. • The analysis of the closed-loop transfer function shows the stability of the closed-loop system against the LCL filter inductance and capacitance variations. • The control law is parameterized based on the future reference trajectory over the control and prediction horizons to deal with the steady-state offsets. The approach for the prediction of the future reference trajectory is discussed. • The optimization routine is performed offline, and the control law can easily be implemented in low-cost processors. • To control the power factor precisely, the grid-side currents are selected as the control variables. This paper presents a Generalized Model Predictive Control (GPC) in the natural reference frame for a 3-phase LCL-filter based grid-tied inverter. The proposed controller can track the sinusoidal reference without steady-state phase and amplitude offsets. This is achieved by parameterizing the control law matrices with considering the future reference trajectory. To cope with the LCL filter resonance, traditional controllers need a damping method. The proposed controller has inherent damping against the LCL filter resonance and extra sensors are avoided. The controller parameters are selected by a sensitivity analysis that guarantees the closed-loop stability. The proposed controller provides high accuracy in controlling active and reactive currents, robustness against grid inductance variations, low real-time computation, and a fixed switching frequency. The simulation and experimental results prove the performance of the controller in tracking the grid-side reference currents. [ABSTRACT FROM AUTHOR]

• A generalized control strategy (composed of finite set model predictive control with delay compensation and inverse dynamic model control) is proposed, which improves the equivalent output impedance and phase margin of inverters. • The proposed control strategy can achieve order reduction of inverter's control system, which suppresses the resonance peak of the system and further improves the stability of multi-inverter parallel system. • The high robustness against various parameters of the proposed strategy have been proven through Lyapunov stability and frequency domain analysis. • The proposed control strategy can ensure that the THD value of grid connected current is less than 5% under very weak grid condition and other situations. Under weak-grid condition, the mismatch degree between grid impedance and inverter output impedance can increase, which will lead to inverter instability. Compared with conventional power grid, the parallel resonance of multi-inverter parallel system is more likely to happen. In order to improve inverter stability and suppress multiple-inverter parallel resonance under weak-grid condition, a new generalized control mode for control layer is proposed in this paper, which is based on finite set model predictive control (FCS-MPC) and inverse dynamic model control (IDM). The FCS-MPC with delay compensation is introduced into the current inner loop of the control layer, and the IDM control is introduced into the voltage outer loop, so as to make the transfer functions of current inner loop and voltage outer loop unitized and further reduce the order of the inverter's control system. Order reduction can improve the phase margin of the inverter output impedance, in other words, enhance inverter dynamic performance. Finally, the simulation model of multi-inverter parallel system is built, and by comparing with the traditional control strategies, the effectiveness of the proposed control strategy is verified. [ABSTRACT FROM AUTHOR]