A Study of DTC-Power Electronic Cascade Fed by Photovoltaic Cell-Three-Level NPC Inverter

This paper proposes a high performance three-level inverter Neutral Point Clamped (NPC) structure for photovoltaic system. The proposed configuration which can boost the low voltage of photovoltaic (PV) array, can also convert the photovoltaic DC power into high quality AC power. Attention has been paid to the problem of neutral point potential variation. In this way, a Direct Torque Control (DTC) technique has been applied and the estimated value of the Neutral Point Potential (NPP) is used, which is calculated by motor currents. This control strategy uses the redundancy presented by the inverter for selecting appropriate switching state through a switching table to achieve the control of NPP. This study shows the effect of the stability problem of the DC voltages and good static and dynamic performances were obtained in simulation of the proposed cascade “photovoltaic cell-three-level NPC VSI-induction motor”.

Quasi-Z Source Inverter Control of PV Grid-Connected Based on Fuzzy PCI

The photovoltaic grid-connected inverter is an important interface between the photovoltaic power generation system and power grid.Its high-quality operation is directly related to the output power quality of the power grid.In order to further optimize the control effect of the quasi-Z source grid-connected photovoltaic inverter,a fuzzy proportional complex integral control(PCI)method is proposed for the current internal loop control.This method can eliminate the steady-state error,and has the characteristic of zero steady-state error adjustment for the AC disturbance signal of a specific frequency.The inductance-capacitance-inductance(LCL)filter is adopted in the grid-connected circuit,and the feedback capacitive current is taken as the control variable of the inner loop to form the active damping control method,which can not only effectively suppress the resonance of the LCL circuit,but also significantly inhibit the high-order harmonics in the grid-connected current.Finally,a system simulation model is built in MATLAB/Simulink to verify the superiority and effectiveness of the proposed method.

FCS-MPC Strategy for PV Grid-Connected Inverter Based on MLD Model

In the process of grid-connected photovoltaic power generation,there are high requirements for the quality of the power that the inverter breaks into the grid.In this work,to improve the power quality of the grid-connected inverter into the grid,and the output of the system can meet the grid-connected requirements more quickly and accurately,we exhibit an approach toward establishing a mixed logical dynamical(MLD)model where logic variables were introduced to switch dynamics of the single-phase photovoltaic inverters.Besides,based on the model,our recent efforts in studying the finite control set model predictive control(FCS-MPC)and devising the output current full state observer are exciting for several advantages,including effectively avoiding the problem of the mixed-integer quadratic programming(MIQP),lowering the THD value of the output current of the inverter circuit,improving the quality of the power that the inverter breaks into the grid,and realizing the current output and the grid voltage same frequency and phase to meet grid connection requirements.Finally,the effectiveness of the mentioned methods is verified by MATLAB/Simulink simulation.

A Simulation Research on the Grid-Connected Control Technology of Single-Phase Inverters Based on MATLAB

This paper primarily discusses the main circuit of single-phase inverter circuits.It begins by introducing the research context and the significance of the subject,then discusses the topology of grid-connected single-phase inverter circuits,continues by discussing the control strategy for grid-connected single-phase inverter circuits,realizes a sinusoidal pulse width modulation(SPWM)signal generation circuit and an inverse control algorithm program,and finally ensures good output waveform and fast dynamic response.In view of the hysteresis feature of the grid voltage’s synchronous signal sampling circuit,the acquisition function in digital signal processing(DSP)control chips is applied,and the reasons for the hysteresis phenomenon are thoroughly investigated.The reliability of the SPWM control algorithm is revealed through the results.

The Resonance Suppression for Parallel Photovoltaic Grid-connected Inverters in Weak Grid

Obvious resonance peak will be generated when parallel photovoltaic grid-connected inverters are connected to the weak grid with high grid impedance, which seriously affects the stability of grid-connected operation of the photovoltaic system. To overcome the problems mentioned above, the mathematical model of the parallel photovoltaic inverters is established. Several factors including the impact of the reference current of the grid-connected inverter, the grid voltage interference and the current disturbance between the photovoltaic inverters in parallel with the grid-connected inverters are analyzed. The grid impedance and the LCL filter of the photovoltaic inverter system are found to be the key elements which lead to existence of resonance peak. This paper presents the branch voltage and current double feedback suppression method under the premise of not changing the topological structure of the photovoltaic inverter, which effectively handles the resonance peak, weakens the harmonic content of the grid current of the photovoltaic grid-connected inverter and the voltage at the point of common coupling, and improves the stability of the parallel operation of the photovoltaic grid-connected inverters in weak grid. At last, the simulation model is established to verify the reliability of this suppression method.

Multi-objective Model Predictive Control of Grid-connected Three-level Inverter Based on Hierarchical Optimization

In order to solve the problem of weighting factors selection in the conventional finite-control-set model predictive control for a grid-connected three-level inverter,an improved multi-objective model predictive control without weighting factors based on hierarchical optimization is proposed.Four control objectives are considered in this strategy.The grid current and neutral-point voltage of the DC-link are taken as the objectives in the first optimization hierarchy,and by using fuzzy satisfaction decision,several feasible candidates of voltage vectors are determined.Then,the average switching frequency and common-mode voltage are optimized in the second hierarchy.The average ranking criterion is introduced to sort the objective functions,and the best voltage vector is obtained to realize the coordinated control of multiple objectives.At last,the effectiveness of the proposed strategy is verified by simulation results.

Impedance Modeling and Stability Analysis of PV Grid-connected Inverter Systems Considering Frequency Coupling

Impedance analysis is an effective method to analyze the oscillation issue associated with grid-connected photovoltaic systems.However,the existing impedance modeling of a gridconnected photovoltaic inverter usually only considers the effect of a single perturbation frequency,ignoring the coupling frequency response between the internal control loops of a grid-connected inverter,which severely affects the accuracy of the stability analysis.Hence,a method of impedance modeling and stability analysis for grid-connected photovoltaic inverters considering cross-coupling frequency is proposed in this paper.First,the generation mechanism of frequency coupling in gridconnected photovoltaic inverters,and the relationship between the coupling frequency and perturbation frequency are analyzed.Secondly,a sequence impedance model of grid-connected photovoltaic systems considering the coupling frequency is established by using the harmonic linearization method.The impact of DC bus voltage control strategy on frequency coupling characteristics of a grid-connected photovoltaic system is evaluated,and the impact of a coupling frequency term on system stability is quantitatively analyzed.Finally,the advantages of the proposed method are verified by several simulations.The results show that the proposed impedance model can accurately predict the potential resonance points of the system,and the coupling frequency characteristics will become much stronger with smaller DC bus capacitance or larger bandwidth of the DC bus controller.

New inverter topology for ground current suppression in transformerless photovoltaic system application

An interesting inverter topology is proposed in this paper.It is similar to the typical three-phase full bridge inverter from the topology point of view,but smartly designed for the ground current reduction in single-phase photovoltaic(PV)inverter applications.Theoretical analysis is conducted to clarify the operation mechanism of the proposed topology.Performance evaluation is carried out to verify the effectiveness of the proposed topology for the ground current suppression.

Techno-economic-environmental feasibility study of a photovoltaic system in northern part of Iran including a two-stage multi-string inverter with DC-DC ZETA converter and a modified P&O algorithm

Inverters play a significant role in the configuration of grid-connected photovoltaic(PV)systems.The perturb-and-observe(P&O)algorithm is a common method to derive the maximum power from grid-connected inverters;however,the possibility of losing maximum power due to sudden changes in radiation is a significant drawback of this control strategy.To overcome this barrier,the two-stage multi-string inverter using the ZETA DC-DC converter and a novel P&O algorithm has been proposed to increase the efficiency of these systems.The proposed inverter has been simulated in MATLAB/SIMULINK software.To investigate the performance of the proposed inverter,technical,environmental and economic feasibility studies have been performed for the construction of a 5-kW PV power plant in a northern city of Iran(Sari)using the RETScreen software developed by Natural Resources Canada.On the other hand,most feasibility studies for power-plant construction are based on the concept of inverter peak efficiency,which leads to non-optimal system design due to the short operation duration of the inverter at this value.However,the weighted European efficiency has been used in the feasibility study for more accurate computations.Moreover,the performance of the proposed inverter is compared to that of a two-stage multi-string inverter using a conventional P&O algorithm and the single-stage(central)inverter.The simulation results indicated that the proposed inverter injects 7.6 MW of power into the grid per year.Moreover,it prevents the emission of 88 tons of CO_(2)(over 20 years),which is equivalent to saving 1883.5 litres of gasoline per year.

Impedance Modeling and Stability Analysis of a Three-phase Three-level NPC Inverter Connected to the Grid

The interaction between grid-connected inverters and the grid may cause stability issues,and compromise the reliable operation of the inverters.This study investigates the stability of a three-level neutral point clamped(NPC)inverter connected to the grid using impedance-based methods.Because the impedance model of a three-phase three-level NPC inverter has not yet been reported,this study fills the literature gap by analyzing the influence of three-level DC-side neutral point control on the impedance characteristics.By fully considering the DC bus dynamics and DC voltage control loop,and the current loop and phase-locked loop(PLL),the admittance model of a three-phase three-level NPC inverter is established and verified by simulation.Additionally,in the stability analysis of a threelevel NPC inverter grid-connected system,the frequency coupling introduced by the PLL and DC bus dynamics is included with the help of an established admittance model.The stability of the grid-connected system under different grid short circuit ratios(SCR)and operating power levels is analyzed according to the Nyquist stability criterion.The experimental results revealed that the established impedance model of the three-phase three-level NPC inverter can properly represent the stability of this system.

Single-phase Grid-connected PV System with Golden Section Search-based MPPT Algorithm

Maximum power point tracking(MPPT)is a technique employed for with variable-power sources,such as solar,wind,and ocean,to maximize energy extraction under all conditions.The commonly used perturb and observe(P&O)and incremental conductance(INC)methods have advantages such as ease of implementation,but they also have the challenge of selecting the most optimized perturbation step or increment size while considering the trade-off between convergence time and oscillation.To address these issues,an MPPT solution for grid-connected photovoltaic(PV)systems is proposed that combines the golden section search(GSS),P&O,and INC methods to simultaneously achieve faster convergence and smaller oscillation,converging to the MPP by repeatedly narrowing the width of the interval at the rate of the golden ratio.The proposed MPPT technique was applied to a PV system consisting of a PV array,boost chopper,and inverter.Simulation and experimental results verify the feasibility and effectiveness of the proposed MPPT technique,by which the system is able to locate the MPP in 36 ms and regain a drifting MPP in approximately 30 ms under transient performance.The overall MPPT efficiency is 98.99%.