Showing papers on "Inverter published in 2016"

Analysis of D-Q Small-Signal Impedance of Grid-Tied Inverters

Abstract: This paper analyzes the small-signal impedance of three-phase grid-tied inverters with feedback control and phase-locked loop (PLL) in the synchronous reference ( d-q ) frame. The result unveils an interesting and important feature of three-phase grid-tied inverters – namely, that its q–q channel impedance behaves as a negative incremental resistor. Moreover, this paper shows that this behavior is a consequence of grid synchronization, where the bandwidth of the PLL determines the frequency range of the resistor behavior, and the power rating of the inverter determines the magnitude of the resistor. Advanced PLL, current, and power control strategies do not change this feature. An example shows that under weak grid conditions, a change of the PLL bandwidth could lead the inverter system to unstable conditions as a result of this behavior. Harmonic resonance and instability issues can be analyzed using the proposed impedance model. Simulation and experimental measurements verify the analysis.

Z-Source Inverter: Topology Improvements Review

Abstract: One of the most promising power electronics converter topologies is the Z-source inverter (ZSI). The ZSI is an emerging topology for power electronics dc?ac converters with interesting properties such as buck-boost characteristics and single-stage conversion. A two-port network, composed of two capacitors and two inductors connected in an X shape, is employed to provide an impedance source (Z-source) network, coupling the inverter main circuit to the dc input source. The ZSI advantageously uses the shoot-through (ST) state to boost the input voltage, which improves the inverter reliability and enlarges its application fields. In comparison with other power electronics converters, it provides an attractive single stage dc?ac conversion with buck-boost capability with reduced cost, reduced volume, and higher efficiency due to a lower component number. For emerging power-generation technologies, such as fuel cells, photovoltaic (PV) arrays, and wind turbines, and new power electronic applications such as electric and hybrid vehicles, the ZSI is a very promising and competitive topology [1]-[4].

Sensor-Less Five-Level Packed U-Cell (PUC5) Inverter Operating in Stand-Alone and Grid-Connected Modes

Abstract: In this paper, a new mode of operation has been introduced for packed U-cell (PUC) inverter. A sensor-less voltage control based on redundant switching states is designed for the five-level packed U-cell (PUC5) inverter, which is integrated into switching process. The sensor-less voltage control is in charge of fixing the dc capacitor voltage at half of the dc source value results in generating symmetric five-level voltage waveform at the output with low harmonic distortion. The sensor-less voltage regulator reduces the complexity of the control system, which makes the proposed converter appealing for industrial applications. An external current controller has been applied for grid-connected application of the introduced sensor-less PUC5 to inject active and reactive power from inverter to the grid with arbitrary power factor, while the PUC auxiliary dc bus is regulated only by sensor-less controller combined with new switching pattern. Experimental results obtained in stand-alone and grid-connected operating modes of proposed PUC5 inverter prove the fast response and good dynamic performance of the designed sensor-less voltage control in balancing the dc capacitor voltage at desired level.

Analysis and Optimization of Droop Controller for Microgrid System Based on Small-Signal Dynamic Model

Abstract: Droop control strategy enables the microgrid switch between grid-connected and islanded mode flexibly, and easily realizes the “plug and play” function of distributed generation and loads, which has recently aroused great concerns. However, small disturbances may occur during the changing process and eventually yield transient oscillation, thus the focus of microgrid control is how to switch smoothly within different operation modes. In order to improve the dynamic characteristics of an inverter-based microgrid, this paper derived a precise small-signal state-space model of the whole microgrid including droop controller, network, and loads. The key control parameters of the inverter and their optimum ranges, which greatly influence the damping frequency of oscillatory components in the transient response, can be obtained through eigenvalue analysis. In addition, genetic algorithm is introduced to search for optimal settings of the key parameters during time-domain simulation in MATLAB/Simulink. Simulation results verified the effectiveness of the proposed small-signal dynamic model and optimization algorithm, and enhanced the dynamic performance of the microgrid, which can be the reference for parameter design of droop control in low voltage microgrids.

A New Cascaded Switched-Capacitor Multilevel Inverter Based on Improved Series–Parallel Conversion With Less Number of Components

Abstract: The aim of this paper is to present a new structure for switched-capacitor multilevel inverters (SCMLIs) which can generate a great number of voltage levels with optimum number of components for both symmetric and asymmetric values of dc-voltage sources. The proposed topology consists of a new switched-capacitor dc/dc converter (SCC) that has boost ability and can charge capacitors as self-balancing by using the proposed binary asymmetrical algorithm and series–parallel conversion of power supply. The proposed SCC unit is used in new configuration as a submultilevel inverter (SMLI) and then, these proposed SMLIs are cascaded together and create a new cascaded multilevel inverter (MLI) topology that is able to increase the number of output voltage levels remarkably without using any full H-bridge cell and also can pass the reverse current for inductive loads. In this case, two half-bridge modules besides two additional switches are employed in each of SMLI units instead of using a full H-bridge cell that contribute to reduce the number of involved components in the current path, value of blocked voltage, the variety of isolated dc-voltage sources, and as a result, the overall cost by less number of switches in comparison with other presented topologies. The validity of the proposed SCMLI has been carried out by several simulation and experimental results.

Open-Circuit Fault Diagnosis and Fault-Tolerant Control for a Grid-Connected NPC Inverter

Abstract: This paper presents an open-circuit fault detection method for a grid-connected neutral-point clamped (NPC) inverter system. Further, a fault-tolerant control method under an open-circuit fault in clamping diodes is proposed. Under the grid-connected condition, it is impossible to identify the location of a faulty switch by the conventional methods which usually use the distortion of outputs because the distortion of the outputs is the same in some fault cases. The proposed fault detection method identifies the location of the faulty switch and the faulty clamping diode of the NPC inverter without any additional hardware or complex calculations. In the case of the clamping diode faults, the NPC inverter can transfer full rated power with sinusoidal currents by the proposed fault-tolerant control. The feasibility of the proposed fault detection and the fault-tolerant control methods for the grid-connected NPC inverter are verified by simulation and experimental results.

Single-Phase T-Type Inverter Performance Benchmark Using Si IGBTs, SiC MOSFETs, and GaN HEMTs

Abstract: In this paper, benchmark of Si IGBT, SiC MOSFET, and Gallium nitride (GaN) HEMT power switches at 600-V class is conducted in single-phase T-type inverter. Gate driver requirements, switching performance, inverter efficiency performance, heat sink volume, output filter volume, and dead-time effect for each technology is evaluated. Gate driver study shows that GaN has the lowest gate driver losses above 100 kHz and below 100 kHz, SiC has lowest gate losses. GaN has the best switching performance among three technologies that allows high efficiency at high-frequency applications. GaN-based inverter operated at 160-kHz switching frequency with 97.3% efficiency at 2.5-kW output power. Performance of three device technologies at different temperature, switching frequency, and load conditions shows that heat sink volume of the converter can be reduced by 2.5 times by switching from Si to GaN solution at 60 $^{\circ }$ C case temperature, and for SiC and GaN, heat sink volume can be reduced by 2.36 and 4.92 times, respectively, by increasing heat sink temperature to 100 $^{\circ }$ C. Output filter volume can be reduced by 43% with 24, 26, and 61 W increase in device power loss for GaN-, SiC-, and Si-based converters, respectively. WBG devices allow reduction of harmonic distortion at output current from 3.5% to 1.5% at 100 kHz.

Dual-Space Vector Control of Open-End Winding Permanent Magnet Synchronous Motor Drive Fed by Dual Inverter

Abstract: This paper proposes a dual-space vector control scheme for the open-end winding permanent magnet synchronous motor (OEW-PMSM) drive fed by the dual inverter with a single dc supply. Potential zero-sequence current in the open-end winding drive system has to be considered since it causes circulating current in the winding and leads to high current stress of power semiconductor devices and high losses. Zero-sequence current in open-end winding ac motor drives is usually caused by the zero-sequence voltage, and therefore switching combinations which do not produce zero-sequence voltage are used to synthesize the reference voltage in existing methods. But even so, the zero-sequence voltage can also be produced by the dead time of the inverter. In order to suppress zero-sequence current in the OEW-PMSM drive, a dual-space vector control scheme is proposed and a novel dual-inverter space vector pulse width modulation (PWM) with the zero-sequence voltage reference is employed to regulate system zero-sequence voltage in this paper. Compared with existing dual inverter PWM strategies, the novel algorithm build a regulation mechanism for the zero-sequence voltage. The proposed method is compared with the conventional vector control by simulations and experiments, and the results shown that the proposed scheme can suppress zero-sequence current effectively.

A Single DC Source Cascaded Seven-Level Inverter Integrating Switched-Capacitor Techniques

Abstract: In this paper, a novel cascaded seven-level inverter topology with a single input source integrating switched-capacitor techniques is presented. Compared with the traditional cascade multilevel inverter, the proposed topology replaces all the separate dc sources with capacitors, leaving only one H-bridge cell with a real dc voltage source and only adds two charging switches. The capacitor charging circuit contains only power switches, so that the capacitor charging time is independent of the load. The capacitor voltage can be controlled at a desired level without complex voltage control algorithm and only use the most common carrier phase-shifted sinusoidal pulse width modulation strategy. The operation principle and the charging–discharging characteristic analysis are discussed in detail. A 1-kW experimental prototype is built and tested to verify the feasibility and effectiveness of the proposed topology.

Highly Reliable Transformerless Photovoltaic Inverters With Leakage Current and Pulsating Power Elimination

Abstract: This paper presents a transformerless inverter topology, which is capable of simultaneously solving leakage current and pulsating power issues in grid-connected photovoltaic (PV) systems. Without adding any additional components to the system, the leakage current caused by the PV-to-ground parasitic capacitance can be bypassed by introducing a common-mode (CM) conducting path to the inverter. The resulting ground leakage current is therefore well controlled to be below the regulation limit. Furthermore, the proposed inverter can also eliminate the well-known double-line-frequency pulsating power that is inherent in single-phase PV systems. By properly injecting CM voltages to the output filter capacitors, the pulsating power can be decoupled from the dc-link. Therefore, it is possible to use long-lifetime film capacitors instead of electrolytic capacitors to improve the reliability of the PV system. The mechanism of leakage current suppression and the closed-loop control of pulsating power decoupling are discussed in this paper in detail. A 500-W prototype was also built and tested in the laboratory, and both simulation and experimental results are finally presented to show the excellent performance of the proposed PV inverter.

Fault-Tolerant Operation of an Open-End Winding Five-Phase PMSM Drive With Short-Circuit Inverter Fault

Abstract: Multiphase machines are well known for their fault-tolerant capability. Star-connected multiphase machines have fault tolerance in an open circuit. For an inverter switch short-circuit fault, it is possible to keep a smooth torque of a permanent magnet synchronous machine if the currents of the faulty phases are determined and their values are acceptable. This paper investigates fault-tolerant operations of an open-end five-phase drive, i.e., a multiphase machine fed with a dual-inverter supply. Inverter switch short-circuit fault is considered and handled with a simple solution. Original theoretical developments are presented. Simulation and experimental results validate the proposed strategy.

Lyapunov-Function and Proportional-Resonant-Based Control Strategy for Single-Phase Grid-Connected VSI With LCL Filter

Abstract: This paper presents a new control strategy based on Lyapunov-function and proportional-resonant (PR) controller for single-phase grid-connected LCL-filtered voltage-source inverters (VSIs). While Lyapunov-function-based control guarantees the global stability of the system, the PR controller is employed to process the grid current error and determine the inverter current reference. However, it is shown that the conventional Lyapunov-function-based control (CLFBC) together with the PR control cannot damp the inherent resonance of the LCL filter. Therefore, this control approach is modified by adding a capacitor voltage loop so as to achieve the desired resonance damping. In addition, a transfer function from the reference grid current to actual grid current is formulated in terms of the LCL-filter parameters and their possible variations in the proposed control strategy. An important consequence of using the PR controller is that the need for performing first and second derivative operations in the generation of inverter current reference is eliminated. Also, a zero steady-state error in the grid current is guaranteed in the case of variations in the LCL-filter parameters. The computer simulations and experimental results obtained from a 3.3-kW system show that the proposed control strategy exhibits a good performance in achieving the required control objectives such as fast dynamic response, zero steady-state error, global stability, and sinusoidal grid current with low total harmonic distortion (THD).

Small-Signal Analysis of the Microgrid Secondary Control Considering a Communication Time Delay

Abstract: This paper presents a small-signal analysis of an islanded microgrid composed of two or more voltage-source inverters connected in parallel. The primary control of each inverter is integrated through an internal current and voltage loops using proportional resonant compensators, a virtual impedance, and an external power controller based on frequency and voltage droops. The frequency restoration function is implemented at the secondary control level, which executes a consensus algorithm that consists of a load-frequency control and a single time delay communication network. The consensus network consists of a time-invariant directed graph and the output power of each inverter is the information shared among the units, which is affected by the time delay. The proposed small-signal model is validated through simulation results and experimental results. A root locus analysis is presented that shows the behavior of the system considering control parameters and time delay variation.

Enhanced-Boost Z-Source Inverters With Switched Z-Impedance

Abstract: This paper proposes a new topology for an enhanced-boost Z-source inverter (ZSI) with combined two Z-impedance networks. By two Z-impedance networks and low shoot-through duty cycle, the proposed inverter produces high output voltage gain. In traditional ZSIs for high boosting voltage, a low modulation index is required; hence, under these conditions, the output voltage will have low quality with high total harmonic distortion. Compared with the conventional high-boost ZSI topologies, the proposed inverter uses shorter shoot-through duration and a larger modulation index to improve the output waveform quality. Comparison between the proposed topology and previously proposed schemes, in terms of inductor numbers, voltage and current stresses on elements, sizes of inductors and capacitors, efficiency, and switching device product (SDP) factors of diodes, is made, and the results verify the priority of the proposed topology. The operating principle of the proposed topology is analyzed in detail. Both simulation and experimental results verify the high performance of the proposed inverter.

Prediction of Bond Wire Fatigue of IGBTs in a PV Inverter Under a Long-Term Operation

Abstract: Bond wire fatigue is one of the dominant failure mechanisms in insulated-gate bipolar transistor (IGBT) modules under cyclic stresses. However, there are still major challenges ahead to achieve a realistic bond wire lifetime prediction in field operation. This paper proposes a Monte Carlo based analysis method to predict the lifetime consumption of bond wires of IGBT modules in a photovoltaic (PV) inverter. The variations in IGBT parameters (e.g., on-state collector–emitter voltage), lifetime models, and environmental and operational stresses are taken into account in the lifetime prediction. The distribution of the annual lifetime consumption is estimated based on a long-term annual stress profile of solar irradiance and ambient temperature. The proposed method enables a more realistic lifetime prediction with a specified confidence level compared to the state-of-the-art approaches. A study case of IGBT modules in a 10-kW three-phase PV inverter is given to demonstrate the procedure of the method. The obtained results of the lifetime distribution can be used to justify the selection of IGBTs for the PV inverter applications and the corresponding risk of unreliability.

Hardware-Based Cascaded Topology and Modulation Strategy With Leakage Current Reduction for Transformerless PV Systems

Abstract: Leakage current reduction of the single-phase transformerless cascaded H-bridge PV inverter is investigated in this paper. The high-frequency common-mode loop model of a typical single-phase cascaded H-bridge PV system is established. Based on the model, the main factors that affect the leakage current are discussed. The reason why the typical single-phase cascaded H-bridge inverter fails to reduce the leakage current is explained. In order to solve the problem, a cascaded topology based on the H5 inverter is presented, along with a new modulation strategy, which can ensure that the stray capacitor voltage is free of high-frequency components. In this way, the leakage current can be effectively reduced. Finally, a prototype with TMS320F28335DSP + XC3S400FPGA digital control is built. The performance tests of cascaded H-bridge and the proposed topologies are carried out. The experimental results verify the effectiveness of the proposed solution.

Fault diagnosis method based on FFT-RPCA-SVM for Cascaded-Multilevel Inverter

Abstract: Thanks to reduced switch stress, high quality of load wave, easy packaging and good extensibility, the cascaded H-bridge multilevel inverter is widely used in wind power system. To guarantee stable operation of system, a new fault diagnosis method, based on Fast Fourier Transform (FFT), Relative Principle Component Analysis (RPCA) and Support Vector Machine (SVM), is proposed for H-bridge multilevel inverter. To avoid the influence of load variation on fault diagnosis, the output voltages of the inverter is chosen as the fault characteristic signals. To shorten the time of diagnosis and improve the diagnostic accuracy, the main features of the fault characteristic signals are extracted by FFT. To further reduce the training time of SVM, the feature vector is reduced based on RPCA that can get a lower dimensional feature space. The fault classifier is constructed via SVM. An experimental prototype of the inverter is built to test the proposed method. Compared to other fault diagnosis methods, the experimental results demonstrate the high accuracy and efficiency of the proposed method.

An Open-Switch Fault Diagnosis Method for Single-Phase PWM Rectifier Using a Model-Based Approach in High-Speed Railway Electrical Traction Drive System

Abstract: The converter with a single-phase rectifier, a dc-link circuit and a three-phase inverter is widely applied in high-speed railway electrical traction drive system. The fault frequency of single-phase rectifier is higher than that of three-phase inverter. Thus, this paper presents a new and fast model-based approach for open-switch fault diagnosis of the single-phase pulse width modulation rectifier, based on the mixed logical dynamic model and residual generation. It requires no additional hardware but only some measurements and command signals which are available in control system. This diagnosis method is quite suitable for electrical traction application due to the fast diagnosis time, simple structure and high reliability. Experimental results confirm the effectiveness and accuracy of the proposed algorithm. It is shown that such diagnosis method can locate the faulty switch in a few milliseconds which is important to avoid catastrophic consequences.

Leakage Current Elimination of Four-Leg Inverter for Transformerless Three-Phase PV Systems

Abstract: Eliminating the leakage current is one of the most important issues for transformerless three-phase photovoltaic (PV) systems In this paper, the leakage current elimination of a three-phase four-leg PV inverter is investigated With the common-mode loop model established, the generation mechanism of the leakage current is clearly identified Different typical carrier-based modulation methods and their corresponding common-mode voltages are discussed A new modulation strategy with Boolean logic function is proposed to achieve the constant common-mode voltage for the leakage current reduction Finally, the different modulation methods are implemented and tested on the TMS320F28335 DSP +XC3S400 FPGA digital control platform The experimental results verify the effectiveness of the proposed solution

Modular Cascaded H-Bridge Multilevel PV Inverter with Distributed MPPT for Grid-Connected Applications

Abstract: Due to the possibility of providing energy with much less dependence on the fossil fuels, renew-competent energy sources, in specified sun photovoltaic (PV) conversion have received elevated acceptance and progress in latest times. Big benefits of PV panels comprise easy and trustworthy power production and suitability for disbursed iteration. In addition the costs for photovoltaic modules is drastically lowering. To comprehend this issue, a control plan with modulation compensation scheme is likewise proposed. An exploratory three-stage seven-level cascaded H-bridge inverter has been manufactured using nine H-bridge modules (three modules for each stage). Each H-bridge module is associated with a 185-W solar panel. Simulation results are introduced to confirm the practicality of the proposed approach.

Switched-Coupled-Inductor Quasi-Z-Source Inverter

Abstract: Z-source inverters have become a research hotspot because of their single-stage buck–boost inversion ability, and better immunity to EMI noises. However, their boost gains are limited, because of higher component-voltage stresses and poor output power quality, which results from the tradeoff between the shoot-through interval and the modulation index. To overcome these drawbacks, a new high-voltage boost impedance-source inverter called a switched-coupled-inductor quasi-Z-source inverter (SCL-qZSI) is proposed, which integrates a switched-capacitor and a three-winding switched-coupled inductor (SCL) into a conventional qZSI. The proposed SCL-qZSI adds only one capacitor and two diodes to a classical qZSI, and even with a turns ratio of 1, it has a stronger voltage boost-inversion ability than existing high-voltage boost (q)ZSI topologies. Therefore, compared with other (q)ZSIs for the same input and output voltages, the proposed SCL-qZSI utilizes higher modulation index with lower component-voltage stresses, has better spectral performance, and has a lower input inductor current ripple and flux density swing or, alternately, it can reduce the number of turns or size of the input inductor. The size of the coupled inductor and the total number of turns required for three windings are comparable to those of a single inductor in (q)ZSIs. To validate its advantages, analytical, simulation, and experimental results are also presented.

Three-Phase Split-Source Inverter (SSI): Analysis and Modulation

Abstract: In several electrical dc–ac power conversions, the ac output voltage is higher than the input voltage. If a voltage-source inverter (VSI) is used, then an additional dc–dc boosting stage is required to overcome the step-down VSI limitations. Recently, several impedance source converters are gaining higher attentions [1] , [2] , as they are able to provide buck-boost capability in a single conversion stage. This paper proposes the merging of the boost stage and the VSI stage in a single stage dc–ac power conversion, denoted as split-source inverter (SSI). The proposed topology requires the same number of active switches of the VSI, three additional diodes, and the same eight states of a conventional space-vector modulation. It also shows some merits compared to Z-source inverters, especially in terms of reduced switch voltage stress for voltage gains higher than 1.15. This paper presents the analysis of the SSI and compares different modulation schemes. Moreover, it presents a modified modulation scheme to eliminate the low frequency ripple in the input current and the voltage across the inverter bridge. The proposed analysis has been verified by simulation and experimental results on a 2.0-kW prototype.

A Unified Virtual Power Decoupling Method for Droop-Controlled Parallel Inverters in Microgrids

Abstract: The coupling between the active and the reactive power of the droop-controlled inverter is critical in the low-voltage microgrid. The conventional virtual power-based control strategy can decouple the active and the reactive power of the inverter, whereas the active power of the load is probably not equally shared among the parallel inverters in the microgrid. To tackle this issue, this paper proposes an improved virtual power-based control method for the droop-controlled parallel inverters with a unified rotation angle in the power transformation. Meanwhile, the optimal value of the unified rotation angle is rigorously derived through investigating the relative stability of the system by innovatively adopting Routh Stability Criterion. Moreover, the proposed method is enhanced by introducing low-pass filters in the coupling path for further mitigating the coupling between the active and the reactive power of the inverter. Additionally, the stability analysis and parameter design for the proposed method are comprehensively presented. Finally, the effectiveness of the proposed method is validated by the simulation and experimental results.

Analysis, Design, and Implementation of a Quasi-Proportional-Resonant Controller for a Multifunctional Capacitive-Coupling Grid-Connected Inverter

Abstract: The capacitive-coupling grid-connected inverter (CGCI) is coupled to the point of common coupling via a second-order LC branch. Its operational voltage is much lower than that of a conventional inductive-coupling grid-connected inverter (IGCI) when it serves as a multifunctional inverter to compensate reactive power and transfer active power simultaneously. It is a promising solution for microgrid and building-integrated distributed generator systems. A quasi-proportional-resonant (quasi-PR) controller is applied to reduce the steady-state current tracking errors of the CGCI in this paper. The quasi-PR controller generates the voltage reference for use of carrier-based pulse-width modulation, which can effectively reduce output current ripples. The second-order coupling impedance of the CGCI causes its modeling and controller design to differ from that of the conventional IGCI. A comprehensive design method for the quasi-PR controller in a CGCI is developed. The quasi-PR controller is also compared with a proportional-integration current controller. Simulation results are provided to verify the effectiveness of the quasi-PR controller and its design method in a CGCI. The current tracking errors are greatly reduced when the quasi-PR controller rather than the proportional-integration controller is applied. Experimental results are also provided to validate the CGCI as a multifunctional grid-connected inverter.

Finite-Control-Set Model Predictive Control for Grid-Connected Packed-U-Cells Multilevel Inverter

Abstract: This paper presents a finite-control-set model predictive control (FCS-MPC) for grid-tied packed U cells (PUC) multilevel inverter (MLI). The system under study consists of a single-phase 3-cells PUC inverter connected to the grid through filtering inductor. The proposed competitive topology allows the generation of 7-level output voltage with reduction of passive and active components compared to the conventional MLIs. The aim of the proposed FCS-MPC technique is to achieve, under various operating conditions, grid-tied current injection with unity power factor and low total harmonic distortion while balancing the capacitor voltage. Parameters’ sensitivity analysis was also conducted. The study is conducted on a low-power case study single-phase 3-cells PUC inverter and with possible extension to higher number of cells. Theoretical analysis, simulation, and experimental results are presented and compared.

An Efficient and Robust Hybrid Damper for $LCL$ - or $LLCL$ -Based Grid-Tied Inverter With Strong Grid-Side Harmonic Voltage Effect Rejection

Abstract: A high-order ( $LCL$ or $LLCL$ ) power filter with a small grid-side inductor is becoming more preferred for a grid-tied inverter due to less total inductance and reduced costs. In a microgrid, the background harmonic voltage (BHV) may distort the injected currents of the grid-tied inverters. In order to resist the effect of the BHV, a feedforward voltage compensator and a proportional resonant regulator with harmonic compensation are often adopted. However, they still have their own limitations, particularly when there are higher order BHVs at the point of common coupling and when the equivalent grid impedance widely varies due to the different numbers of grid-tied inverters in parallel. Thus, an extra damper should be inserted to keep the system stable. In this paper, the control bandwidth limitation of a multiloop control active damping (AD) method is analyzed and illustrated by the capacitor-current-feedback AD. Based on this, a single-loop current control with a hybrid damper is proposed for a single-phase $LCL$ - or $LLCL$ -filter-based grid-tied inverter. A step-by-step design of the controller method is also introduced in detail. Experiments on a 2-kW prototype fully demonstrate the strong robustness of the stability and the high harmonic rejection ability of the inverter using the proposed control method.

A Single-Phase PV Quasi-Z-Source Inverter With Reduced Capacitance Using Modified Modulation and Double-Frequency Ripple Suppression Control

Abstract: In single-phase photovoltaic (PV) system, there is double-frequency power mismatch existed between the dc input and ac output. The double-frequency ripple (DFR) energy needs to be buffered by passive network. Otherwise, the ripple energy will flow into the input side and adversely affect the PV energy harvest. In a conventional PV system, electrolytic capacitors are usually used for this purpose due to their high capacitance. However, electrolytic capacitors are considered to be one of the most failure prone components in a PV inverter. In this paper, a capacitance reduction control strategy is proposed to buffer the DFR energy in single-phase Z-source/quasi-Z-source inverter applications. Without using any extra hardware components, the proposed control strategy can significantly reduce the capacitance requirement and achieve low input voltage DFR. Consequently, highly reliable film capacitors can be used. The increased switching device voltage stress and power loss due to the proposed control strategy will also be discussed. A 1-kW quasi-Z-source PV inverter using gallium nitride (GaN) devices is built in the lab. Experimental results are provided to verify the effectiveness of the proposed method.

An FPGA-Based Advanced Control Strategy of a Grid­Tied PV CHB Inverter

Abstract: In this paper, an advanced control strategy for grid­tied photovoltaic (PV) cascaded H­bridge (CHB) inverter is proposed. The circuit topology consists of a proper number of power cells (H­bridge configuration) connected in series and supplied by individual PV modules. The adopted control method is a mixed staircase­PWM technique performed by means of a sorting algorithm to determine cells' switching state. The cells' state is related to the need of charging or discharging a particular cell much more than the others by calculating the voltage error at each dc­link (e.g., by considering the difference between the maximum power point tracking (MPPT) reference and the measured quantity). A dedicated PO thus, increasing the power extraction even in mismatched conditions. In order to prove the effectiveness and feasibility of the proposed approach, a set of experiments are performed on a laboratory prototype of a single­phase five­level PV CHB. The control section is implemented on FPGA by using a dSPACE real­time hardware platform; thus, obtaining fully dedicated digital circuits. Experimental results show good performance in terms of MPPT efficiency, total harmonic distortion, and power factor in both normal operation and mismatch conditions.

Fast and Robust Single-Phase $DQ$ Current Controller for Smart Inverter Applications

Abstract: This paper presents a fast and robust $DQ$ current controller to regulate the output power of single-phase grid-connected inverters. The proposed method generates the grid current orthogonal component without introducing any additional dynamics or distortions to the control loop. Moreover, its operation does not depend on the system parameters. The proposed method exhibits improved steady-state and dynamic performances in comparison with the inverters equipped with the conventional orthogonal signal generation techniques. Its improved characteristics make the proposed controller suitable for smart inverter applications, to provide advanced grid functionalities as demanded by recently revised standards. Simulation and experimental results show the feasibility and performance of this control structure.

Sliding-Mode Control of PWM Dual Inverter-Based Grid-Connected PV System: Modeling and Performance Analysis

Abstract: This paper proposes a novel robust and adaptive sliding-mode (SM) control for a cascaded two-level inverter (CTLI)-based grid-connected photovoltaic (PV) system. The modeling and design of the control scheme for the CTLI-based grid-connected PV system is developed to supply active power and reactive power with variable solar irradiance. A vector controller is developed, keeping the maximum power delivery of the PV in consideration. Two different switching schemes have been considered to design SM controllers and studied under similar operating situations. Instead of the referred space vector pulsewidth modulation (PWM) technique, a simple PWM modulation technique is used for the operation of the proposed SM controller. The performance of the SM controller is improved by using an adaptive hysteresis band calculation. The controller performance is found to be satisfactory for both the schemes at considered load and solar irradiance level variations in simulation environment. The laboratory prototype, operated with the proposed controller, is found to be capable of implementing the control algorithm successfully in the considered situation.