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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 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]

Highlights • An advanced controller applied to multilevel converter. • An improved active power filter application. • Compensating Harmonics as well as linear load feeding. • Improved dynamic response behavior while having the previous controllers advantages. • Full Experimental results provided. Abstract Multifunctional DGs and active power filters have become a mature technology in recent years, so in this paper, an optimized current control method for a multilevel converter is proposed. The control method will overcome harmonic current tracking inefficiency of previous control methods in online harmonic compensation applications in microgrids. This control method is applicable for grid-connected inverter-based multi-functional Distributed Generation (DG) converters. It could also be used in active power filter applications which need high-speed reference tracking ability. Having the advantages of current control methods like hysteresis band control, proportional-integral (PI) and proportional-resonant (PR) control methods, the proposed approach overcomes disadvantages of these methods especially in harmonic reference tracking as it will be discussed in detail. The main advantages of this method are the simplicity of implementation, calculation delay compensation and its fast response to changes. The power electronic circuit, operating principles, two-horizon predicted switching states of multilevel inverter, experimental results and applications of this control method will be discussed in the paper. For studying the feasibility of the control method, an experimental prototype is tested in a microgrid platform. [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]

This paper presents a novel approach to the modeling and control of AC microgrids that contain spinning machines, power electronic inverters and energy storage devices. The inverters in the system can adjust their frequencies and power angles very quickly, so the modeling focuses on establishing a common reference frequency and angle in the microgrid based on the spinning machines. From this dynamic model, nonlinear Hamiltonian surface shaping and power flow control method is applied and shown to stabilize. From this approach the energy flow in the system is used to show the energy storage device requirements and limitations for the system. This paper first describes the model for a single bus AC microgrid with a Hamiltonian control, then extends this model and control to a more general class of multiple bus AC microgrids. Simulation results demonstrate the efficacy of the approach in stabilizing and optimization of the microgrid. [ABSTRACT FROM AUTHOR]

This paper characterizes the bidirectional feature of the transformer-based quasi-Z-source inverter (trans-qZSI) obtained by adding a controllable power device to the input of the circuit. Different arising operation modes and states are analysed and an AC small signal model is proposed accordingly. A controller was designed using the proposed model, and an experimental validation platform was implemented. After the computational and experimental model validation, it was analysed how and when bidirectional Z topology benefits from bidirectional feature, apart from obvious inverse power conversion operation. Robustness increase and fault tolerance of the bidirectional trans-qZSI topology were characterized and validated experimentally for different current demand decrease case studies: when an open circuit is produced in one output phase or when the current demand decreases due to normal operation requirements. The paper characterizes and demonstrates, computationally and experimentally, that it is the bidirectional feature that provides the system with the robustness needed to protect against these kind of failures or operation modes. Not having the additional IGBT could cause the system to enter the discontinuous conduction mode (DCM) zone and destabilize the system’s currents and voltages. In that case the voltage is not able to follow the control reference, which could ultimately lead to the destruction of the components. [ABSTRACT FROM AUTHOR]

• Transformerless SqZSI with common-ground and minimum leakage current features is an attractive and cost-effective option as a PV microinverter. • SqZSI injects second harmonic and DC components under certain conditions, which is undesirable in terms of published power-quality standards. • The experimental study on power quality improvement of this family of inverters (SqZSI) is currently limited. • Closed-loop compensation methods based on well-known linear control techniques e.g. PI & PR controllers can mitigate extensively the power quality issues. The transformerless single-phase semi-quasi-Z-source inverter (SqZSI) provides several advantages over classical PWM converters. Reduced or eliminated leakage current, high power density, and low cost are important features making this topology an attractive choice for PV micro-inverter applications. Two drawbacks of this topology are DC and second harmonic injection in the output AC port, which in some cases prevent the fulfillment of standard requirements. To alleviate this power quality issue, four novel harmonic mitigation algorithms based on PI and PR controllers are proposed, analyzed, and experimentally verified with the 90 W SqZSI in this paper. Moreover, a modified modulation reference waveform strategy is introduced. Consequently, the converter performance, in terms of total harmonic distortion (THD V) is achieved at about 1.27% under full power load. Also, in this paper, the non-ideal state-space model of the SqZSI with the effect of the equivalent series resistance of inductors is developed and the generation origin of DC and second harmonic component is analyzed based on the obtained model. The achieved results confirms the significant THD reduction by more than 75%, the DC component suppression, the second harmonic mitigation, and the power quality improvement. [ABSTRACT FROM AUTHOR]

This paper focuses on improving the dynamic response of autonomous microgrids (MGs) by proposing a grid-forming inverter controlled as a virtual synchronous generator (VSG), in combination with a supercapacitor (SC)-based energy storage system (ESS). By this arrangement, the MG-forming VSG is designed to react only in transitory regimes, the steady-state load being distributed to other MG-supporting inverters spread within the MG. In this way, the MG-forming VSG can maintain its full power reserve capacity for dynamic response. The paper details the control solution for the MG-forming inverter, including the VSG and SC-ESS control. The control method for the MG-supporting inverters that allow achieving the proposed control approach is also described. To prove the concept, the paper includes simulation results and experiments accomplished on a complex laboratory MG system based on three parallel inverters, one being controlled as MG-forming VSG, while the others operating as MG-supporting inverters. [ABSTRACT FROM AUTHOR]

• Analysis of the transient stability boundaries of a stand-alone inverter-based microgrid during current limitation employing three different frequency droop controls. • The conventional P-f and Iact-f droop controls cannot guarantee the microgrid stability for all possible operating conditions. • The use of a variable defined as the equivalent conductance is proposed to carry out the frequency droop leading to the G-f droop control. • The G-f droop control provides the same satisfactory performance irrespective of the operating conditions, ensuring always the transient stability of inverter-based microgrids. Currently, inverter-based stand-alone microgrids are gaining interest due to the advantages of obtaining energy from renewable sources. To manage the operation, these microgrids include storage systems connected in parallel to the PCC through electronic inverters that are controlled as voltage sources in order to support the frequency and voltage at the PCC. For the purpose of ensuring P and Q sharing among inverters and also the synchronization stability of the microgrid, droop control is widely used, achieving a satisfactory performance in normal operation. Nevertheless, in the presence of overloads or short-circuits, the inverters must limit the current for self-protection, thereby modifying the performance of the system that then becomes prone to suffer transient stability problems. In this paper, first the performance of the inverter-based stand-alone microgrids with the conventional P - f and I act - f droops is analyzed, obtaining the stability boundaries during current limitation. In order to always ensure the synchronization stability of the system, this paper then proposes the G - f droop that consists in employing the equivalent conductance seen by each inverter for its frequency droop control. Furthermore, as this variable always correctly represents the inverter power angle, the system dynamics are not affected by the operating conditions. The theoretical results have been validated by means of simulation and Hardware-In-the-Loop results, showing the superior performance of the proposed G - f droop. [ABSTRACT FROM AUTHOR]

Highlights • A protection analysis tool for distribution networks with a high embedded generation is presented. • The tool determines the fault response of embedded generators with high convergence success. • The tool automatically simulates and assesses the pertinent faults for protection adequacy. Abstract The protection design philosophy of distribution networks is predicated on the inherent radiality that exists in such networks. However, the increased presence of inverter-interfaced embedded generation is compromising the radial nature of distribution networks. Hence, the increasing importance of accurately modelling the fault response of inverter-interfaced embedded generation is becoming apparent. Presently, the impacts of large penetrations of inverter-interfaced embedded generators on protection adequacy are determined through time-domain modelling. This paper proposes a tool for determining a snapshot of the expected protection response of distribution networks with inverter-interfaced embedded generation. The tool automatically simulates all fault types at the extremities of each protection zone and compiles the data to generate a report that showcases important information from a protection adequacy perspective. The tool can simulate fault responses in orders of magnitude faster than time-domain analysis using a load-flow like algorithm. This algorithm is designed to have a high convergence in fault scenarios and also approximate the fault behaviour of inverter-interfaced embedded generators. Finally, the tool investigates whether a new embedded generator installation may require an upgrade of network infrastructure or an amendment of protection settings. The tool is implemented using MATLAB and compared with time-domain simulations for verification purposes. [ABSTRACT FROM AUTHOR]

Highlights • Active users with PV systems and batteries are considered in a LV distribution grid. • A strategy for voltage regulation ancillary service provision is proposed. • The proposed scheduling algorithm exploits the grid nodal sensitivity coefficients. • A way for determining the most convenient tariffs is proposed. • Numerical results are presented and discusses for a LV radial distribution grid. Abstract In recent years, renewable energies and storage systems are bringing a significant contribution to power system. Due to the high penetration of non-predictable power sources, especially in the distribution networks, new challenges related to the integration and management of these new resources also combined with storage systems arose in power system research. Accordingly, the aim of this paper is to propose an innovative scheduling method for voltage regulation in low voltage networks. A simple type of contract for ancillary services that foresees the participation of active users endowed with a storage system coupled with a PV system is considered. The method allows to regulate the voltage in a distribution system by using a combined PV and storage system. The voltage regulation is achieved by setting the value of the inverter reactive power on the basis of sensitivity coefficients of the considered node, taking into account the capability curve constrains, as required by new grid codes. A possible tariff policy is also proposed for the ancillary service and for the operation of the realization of the scheduling algorithm. Simulation results demonstrate the effectiveness of the proposed method for the scheduling of the power system. [ABSTRACT FROM AUTHOR]

Highlights • A full-order switched model is developed for a HF-Link Grid-Connected Inverter. • A full-order Generalized Average Model (GAM) is derived for the Converter. • The developed GAM in the Complex Fourier domain reproduces the mos relevant system dynamics. • The switched and Averaged models are validated with experimental results. Abstract Full-order switched and time-continuous models for a High-Frequency Link Grid-Connected DC/AC converter are presented in this paper. The configuration is DAB-based, therefore its transformer leakage inductor current is purely AC as well as the remaining grid-side voltages and current; representing a challenge to obtain an average or time-continuos model, simpler for simulation tasks as well as for controller analysis. The presented average model is based on the DC and first harmonic terms of the complex Fourier representation of the derived full-order switched model. Simulation results are corroborated through experimental results, showing the validity of the presented full-order time-continuous model. [ABSTRACT FROM AUTHOR]

Highlights • Unbalanced synchronous reference frame control is adopted in a 1-phase stand-alone inverter. • Voltage harmonics are attenuated by multi-frequency unbalanced synchronous reference frames. • The proposed controller exhibits lower THD and faster transient response than a PR regulator. • The proposed method has a greater stability margin and protection functionality. Abstract This paper presents harmonic damping schemes of the single-phase stand-alone inverter. The unbalanced synchronous reference frame technique is selected to control fundamental frequency voltage at the point of common coupling. The output voltage and the inductor current of the inverter are transformed to the unbalanced synchronous reference frame in which the proportional-integral regulators are used to force the error signals in the dq axes to zero. This control scheme does not require an orthogonal signal generator, and it is equivalent to the ideal proportional-resonant regulator. Harmonic regulators, orders 3rd, 5th, 7th, and 9th are added to minimize the voltage harmonics caused by the connected nonlinear load. The proposed controller is compared with the commonly-used multi-resonant controller. Simulation and experimental results of a 1.5-kVA inverter with the proposed control scheme exhibited excellent harmonic damping of the output voltage, caused by a highly nonlinear load and fast transient performance, which were better than the conventional resonant controller. [ABSTRACT FROM AUTHOR]

Highlights • A two-layer collaborative planning for inverter interfaced DGs and BES units is established for enhanced voltage regulation functions in ADN. • The potential of DG inverter over-sizing design at planning stage, the fully utilization mode of reactive power output of DG inverters at operation stage are discussed. • A time-varying ZIP load model is adopted representing the dynamic load characteristics. • The uncertainties of the DG power output are modeled based on the Taguchi robust test design approach. Abstract Active Distribution Networks (ADNs) are featured by large-scale integration of distributed generation (DG) and energy storage. This paper proposes a novel two-layer co-planning method for optimal placement of inverter-interfaced DG and battery energy storage (BES) units towards enhanced voltage regulation functions in an ADN. The outer-later model determines the planning decision of DG units and the inverter sizing, location, and capacities of BES units, respectively; the inner-layer model corresponds to the operation decision which aims to optimally schedule the BES's charging/discharging and reactive power from inverters for voltage regulation support considering the conservation voltage reduction (CVR). The Taguchi's Orthogonal Array Testing (TOAT) is used to select a small number of scenarios to represent the DG power output uncertainties. The load is represented by a time-varying ZIP load model. The proposed model is tested on a modified IEEE 33-bus radial distribution system and the results to validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Highlights • A method is proposed to calculate the optimal oversize of the DG inverters. • The reactive power capability and active power curtailment are considered. • Different control schemes of the inverter are considered in the proposed method. • The effect of OLTC operation on the total costs and inverter oversize is studied. • IEEE 69-bus and IEEE 33-bus distribution systems are used for simulation. Abstract The penetration of renewable distributed generations (DG) is recently being increased in distribution systems. Due to the intermittent nature of these generation units, several technical problems are expected based on the environmental conditions and load profiles, such as voltage rise and increasing losses. A traditional way to alleviate these technical problems is to oversize their interfacing inverters for releasing their capacities to inject/absorb further reactive power. In the literature, the DG inverter is normally oversized by a certain percentage (e.g., 10%) for this purpose. In turn, an optimization-based method is proposed in this paper to calculate the optimal oversize of the interfaced inverter employed in various utility-owned DG types to regulate voltages and to reduce losses with minimum total costs. The proposed method considers the active power curtailment (APC) feature in the DG inverter and the transformer taps. Different control schemes of the interfaced inverter are considered and incorporated in the proposed optimization model. The simulation results demonstrate the effectiveness of the proposed method compared with existing methods. [ABSTRACT FROM AUTHOR]

Highlights • A versatile control strategy is employed for a grid connected CHB inverter. • Control system reduces the current harmonics under asymmetric operating conditions. • The modulation index of each H-bridge cell is calculated from PV system data. • Using the new estimation approach, the number of sensors is reduced from (2n + 2) to (n + 3) • Controller-in-the-loop implementation of control system is presented. Abstract In this paper, the cascaded H-bridge (CHB) multilevel inverter is employed as a grid connected inverter. This kind of inverter has interesting features compared to conventional inverters, including better efficiency, lower THD, lower common mode current injection, and better waveform quality. Using a series connection of H-bridge cells in this topology, a high modularity is achieved and a stepwise voltage waveform is synthesized at the ac side, leading to lower filter size and complexity. This topology processes the electric power in one stage and does not need any step-up voltage transformer for grid connection. The dynamic performance of control system is studied through simulations and is compared with the conventional approach. In addition, a new voltage estimation approach is proposed to reduce the number of voltage sensors compared to existing methods. The simulation results are presented for a 1.5 kW grid connected 7-level CHB inverter and then the hardware-in-the-loop implementation of the CHB inverter is presented to verify the performance of control system. [ABSTRACT FROM AUTHOR]

In order to deliver power from low voltage sources to the distribution level loads, the key element is the interfacing stage, traditionally formed by a dc-ac power converter, an output filter and a step-up transformer. However, transformerless converter topologies naturally present lower cost and size when compared with conventional solutions. As counterpart, transformerless systems need a boost conversion stage to compensate the absence of the step-up transformer. The two-stage converter (dc-dc boost converter plus voltage source inverter) has the step-up stage necessary to connect the low voltage source to the load, at a cost of more complexity and high leakage currents circulating in the system. The complexity issue can be overcome by using one-stage step-up plus inversion configuration such as the Z-source inverter, although the leakage current problem will persist. In order to solve both issues, this paper proposes a three-state three-phase Z-source inverter (TS3ph-ZSI) that presents an intrinsic buck-boost plus inverter characteristic, featuring a common ground between source and load (dual ground configuration), responsible to eliminate the leakage currents in the system. This topology is especially useful for transformerless photovoltaic systems. The theoretical analysis of this topology, including its modeling and control system design, is carried out and validated by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper presents the quasi-resonant DC-Link transformer-less structures whereby the efficiency of the grid-connected PV systems could be improved. Transformer-less PV inverters as a critical part in power conditioning units are more advantageous to the user. However leakage current that is resulted from omitting bulky transformers can compromise their higher efficiency. So the low leakage current and high efficiency are two crucial points in transformer-less PV systems. By incorporating Quasi-resonant DC-Link as a method of soft switching into the H6 structures two new DC-decoupled soft-switched transformer-less PV inverters are proposed by which the switching losses of power devices as well as high di/dt, dv/dt and EMI could be avoided. A new auxiliary circuit is extracted from available structures. Proposed auxiliary circuit features lower voltage stress and saves one gate drive circuitry as well. Using only two auxiliary switches, ZVS turn on/off for high frequency DC bus switches and ZVS turn on/off of the antiparallel diodes of all grid frequency main switches are realized. Soft switching condition for auxiliary switches and their antiparallel diodes are also provided. Finally a 20 kHz 1-kW universal prototype is built and Operation characteristics, soft switching conditions and efficiency improvement of proposed QRDCL transformer-less PV inverters are verified. [ABSTRACT FROM AUTHOR]

This paper presents the comparative computer simulations of a commercial doubly-fed induction generator (DFIG) and an emerging brushless doubly-fed reluctance generator (BDFRG) for grid-connected wind turbines in terms of frequency support based on the inertia emulation and blade pitching de-loading. The BDFRG features the low operation and maintenance cost by using a fractional inverter, and offers the high reliability of brushless structure with a simpler, more compact 2-stage gearbox design while still ensuring competitive performance to its popular slip-ring companion. The implemented benchmark is carefully designed to ascertain the relative capabilities of the two wind turbine generator technologies in providing this ancillary service. The results reveal that in spite of the fundamentally different operating principles, the DFIG and the BDFRG are highly aligned from the viewpoint of power system applications. [ABSTRACT FROM AUTHOR]

This paper establishes two accurate inverter models featuring local voltage control Q(V): a non-linear Simulink model and a small signal model. While the non-linear model can be used to perform arbitrary analyses of transient events, the small signal model’s purpose is to be incorporated in grid models to study small signal stability and possible interactions. Both models are build bottom up from grid filter and PWM to the cascaded power controllers and Q(V) and are designed in dq0 components. High resolution laboratory measurements are used for verification and design parameters are specified to assure reproducibility. The emerging small signal model can be interpreted as a dynamic admittance and therefore easily be included in power system stability analyses. [ABSTRACT FROM AUTHOR]

The paper presents a new configuration of DFIG based wind energy system which belongs to the connection with the DC grid/bus. The presented configuration of DFIG based wind energy systems is kind of stand-alone DC-DFIG based system where: (i) The load replaces the utility grid; (ii) the rotor is connected to the DC grid/bus/micro-grid via rotor side converter. Being a stand-alone DC-DFIG based system, only voltage and frequency regulation is considered here in this research. The rotor side converter provides the controlled amplitude & frequency of the rotor current for regulating the stator side voltage magnitude and frequency. Also, a simple and easy to implement in hardware as well as in software control technique for the rotor side converter, based on root mean square detection of stator/load voltage, is proposed in this study. The proposed configuration is found suitable in the areas having low-to-medium wind speed. The MATLAB Simulink model of 2 MVA DFIG has been considered to evaluate the performance of the system. Also, hardware set-ups comprise with (i) 3.2 kW DFIG (ii) DC shunt motor, and (iii) FPGA Altium NanoBoard3000 controller, are considered to reveal the effectiveness of the proposed configuration and controller in real time environment. [ABSTRACT FROM AUTHOR]

This paper proposes an improved control method for Unified Power Quality Conditioner (UPQC) with unbalanced load. In UPQC, shunt APF is overburdened when it alone supplies total load reactive power. PAC method aims at effective utilization of series and shunt APFs by sharing reactive power burden between the two. In presence of unbalanced load, existing PAC methods can lead to circulation of reactive power between two APFs and thereby result in overloading of UPQC. Also, due to unbalanced compensating currents, DC link voltage contains second order oscillations, which deteriorates source current quality. In this work, a new PAC method is proposed, which avoids circulation of reactive power and unnecessary VA burden on UPQC. To suppress DC link voltage oscillations, proposed control employs a ‘mean block’ (moving average) at output of PI controller, which ensures balanced and harmonic free source currents. Also, a ‘percentage unbalance’ parameter has been proposed to quantify the unbalance in three phase quantities. Performance of proposed UPQC system is tested in the presence of non-linear, reactive and unbalanced loads. Dynamic performance of system is studied during grid disturbances such as voltage sag, swell, and change in load. Real time simulation carried out in Opal-RT validate the effectiveness of proposed method. The electrical circuit of UPQC is simulated with sub-microsecond time step on FPGA based computational engine of Opal-RT for better verification of the proposed control. [ABSTRACT FROM AUTHOR]

The increasing presence of inverter-based distributed generation (DG) units in microgrid application requires control methods that achieve high performance not only during normal operating conditions, but also under unbalanced conditions. These conditions can occur permanently due to distribution of unbalanced loads on the three phases of the microgrid. This paper proposes a robust control strategy for a grid-connected multi-bus microgrid containing several inverter-based DG units. Each of the DG units can supply a combination of balanced and unbalanced local loads. The proposed control strategy employs an adaptive Lyapunov function based control scheme to directly compensate the negative-sequence current components caused by unbalanced loads in some part of microgrid; and a sliding mode based control scheme to directly regulate the positive-sequence active and reactive power injected by DG units to the microgrid. The control method proposed in this paper is shown to be robust and stable under load disturbances and microgrid parameter uncertainties even in the presence of nonlinear and time-variant unbalanced loads. The effectiveness of the presented controller is validated through time-domain simulation studies, under the MATLAB/Simulink software environment. [ABSTRACT FROM AUTHOR]

This paper proposes an N -step ahead model predictive controller for a multilevel asymmetric cascade grid-connected inverter. To this goal, a predictive function controller has been designed based on the Lyapunov theory. First, the equations of the N -step ahead predictive sequence have been derived for the reference current as well as the reference voltage using first-order linear interpolation considering the mean of previous values. Second, the current, voltage, current error, and Lyapunov function have been calculated for the N-samples ahead. Then, the switching scenario has been determined such that it minimizes the derivation of the Lyapunov function. Moreover, the effect of the number of prediction steps on the unwanted harmonic reduction, switching average, grid power injection, and inverter efficiency has been investigated. Finally, The proposed controller is implemented on a 19-level asymmetric cascaded inverter. The results show that by increasing the prediction steps from 1 to 5, the harmonic distortion and the number of switches are reduced. • An N -step ahead model predictive controller for a cascade inverter is proposed. • A predictive function controller has been designed based on the Lyapunov theory. • The proposed scheme is applied for control the power injection in grid-tied mode. • The effect of prediction horizon on the system characteristics was studied. • Simulation and experimental results validate the operation of the proposed control. [ABSTRACT FROM AUTHOR]

This paper presents a decoupled control of grid connected inverter using dynamic online grid impedance measurements for a micro grid application. The proposed controller is implemented in synchronous reference frame (SRF) and controlled using linear PI controllers. The mutual coupling introduced between the d and q control loops due to the transformation into SRF is accurately decoupled using the dynamically measured grid impedance using a feed-forward control. The decoupling allows independent control of active and reactive powers against step changes in the active/reactive power references. The online measurement of the actual impedance and its use further for decoupling is proposed in this paper for making the decoupling accurate inspite of the network configuration being altered like in micro grids. Here the grid resistance and inductance are measured during the operation using a non-characteristic frequency current continuously injected into the grid, and subsequently calculating the impedance using discrete Fourier transforms. The continuous injection of non-characteristic current at 75 Hz avoids the injection of sub-harmonics into the grid during measurements. The control loop is updated periodically with the estimated grid impedance, thus enabling the independent control of active and reactive powers delivered by the inverter. The proposed decoupled controller with grid impedance measurement is tested through simulation studies and hardware experiments. The experiments are conducted with the proposed controller on a scaled down laboratory model of micro-grid with a 1 kVA solar inverter, and the performances are presented for step changes in the power references and the results are presented. [ABSTRACT FROM AUTHOR]

This paper aims at the investigation of an active power filter (APF) comprised of a transformerless multilevel inverter (MLI) for power conditioning in three-phase three-wire distribution network. The inverter topologies used here are three, five, seven and nine-level. The system configuration mainly involves cascaded MLI structure of APF, generation of compensation filter currents based on instantaneous active and reactive current component ( i d – i q ) method and dc-link voltage regulation using a PI controller. Not many papers focus on the regulation of dc-link capacitor voltage. Here we have proposed the implementation of bacterial foraging optimization (BFO) to extract the gains of PI controller. The proposed work provides improved dc-link voltage regulation, quick prevail over current harmonics and reduction of overall source current THD. Adequate MATLAB/Simulink simulation results are presented for the different cascaded MLIs discussed above. Additionally, the performance has been validated in real-time using Opal-RT Lab simulator considering three different conditions of supply i.e., balanced sinusoidal, balanced non-sinusoidal and unbalanced sinusoidal. [ABSTRACT FROM AUTHOR]

This paper presents the power flow control between the main grid and the microgrid in 400 V low-voltage distribution network using series transformerless H-bridge inverter. This series converter is attached directly in series with the line, connecting the main grid and the microgrid with each other. It controls the real power flow depending on the state of both networks and by injecting series voltage that changes the microgrid side voltage. Therefore, the microgrid units will respond directly to the changes of the series converter set points and vice versa. Controlling the power flow in the line using the series converter is investigated by implementing two novel control strategies. The feasibility of the control strategies is validated during both capacitive and inductive operation modes. FFT analysis of the PCC voltage has been investigated to determine the THD caused by the series converter during both modes of operation. Also, system performance with and without P / V droop control and real power flow limit at PCC are also verified during both modes of operation. Several simulation cases have been demonstrated to investigate the feasibility of the series converter concept. SimPowerSystems and PLECS tool boxes of Matlab/Simulink software are utilized to implement the proposed work. [ABSTRACT FROM AUTHOR]

Conventional maximum power point tracking (MPPT) techniques such as Perturb and Observe (P&O) and Incremental Conductance (IncCond) are used in photovoltaic (PV) systems to extract the Maximum Power (MP). These algorithms cannot track the Maximum Power Point (MPP) under a rapidly changing insolation. This paper introduces a modified efficient variable step P&O (VSPO) algorithm as an attempt to solve the aforementioned problem. The VSPO technique divides the PV-array operation region into four operating sectors. With the help of these four sectors, the step-size is changed according to how the sector far from the MPP. The performance of the conventional and variable step algorithms P&O techniques is compared. Three-phase grid-tie inverter (GTI) is used to connect the system to the utility grid. MATLAB/SIMULINK software is used to study the validity of the modified technique. The results demonstrate that the VSPO algorithm attains the MPP faster than the conventional techniques and at reduced oscillation rate. [ABSTRACT FROM AUTHOR]

This paper presents an analytical methodology to estimate the fault location in active distribution networks. State-of-the-art solutions for active distribution networks fault location consider wide-area measurements, which include synchronized measurements obtained by Phasor Measurement Units (PMU). For distribution networks though, the capital cost of such solutions is prohibitive. Most recently, solutions have been proposed without considering synchronized measurements. However, they have some limitations, such as not considering multiple inverter-interfaced Distributed Energy Resources (DER). The solution presented is composed of a load flow based approach, which uses only locally available measurements. A ladder iterative technique is proposed to estimate the system state variables during the fault period. DERs models, which consider various modes of operation and fault conditions are used. An impedance formulation which considers distribution networks inherent characteristics is presented. The proposed methodology is validated on the IEEE 34-nodes test feeder. The ease of implementation of design, formulation of parameters and encouraging test results indicate potential for real-life applications. [ABSTRACT FROM AUTHOR]

Effective energy management of microgrid is essential due to significant growth of renewable energy resources like photovoltaic (PV) systems. Reactive power compensation capability provided by grid-tie PV inverters when PV power is unavailable increases the efficiency of the PVs utilization in microgrids. In this paper, a robust model combining particle swarm optimization (PSO) algorithm and primal–dual interior point (PDIP) method is proposed for optimal energy management of the microgrid, considering VAR compensation mode of the PV inverters. Besides, both the uncertainty of insolation forecast and forced outages of system components (diesel generators and branches) are taken into account. A bi-objective method is employed to detect the worst-case probable 24-h scenario with the severest effects on the system security. Finally, the energy management problem of the microgrid is solved under the obtained worst-case scenario, minimizing the microgrid operation cost, as well as satisfying the physical constraints of the microgrid and insolation limitation of the PVs. Simulation studies have been carried out on a modified version of IEEE 33-bus standard distribution system operating as a microgrid with variable PV generation. The results demonstrate the effectiveness of the proposed robust model for the microgrid energy management. [ABSTRACT FROM AUTHOR]

In recent years, Energy Efficiency Programs (EEPs) have been greatly expanded in multifarious aspects. Here, EEPs is considered as a virtual resource affecting operating expenditures of the power system. On the other hand, under the smart environment, Demand Response (DR) programs are also contemplated as virtual power plants in energy policy decisions ranging from short term to long term. Therefore, in this paper, a novel integrated model of demand in presence of EEPs as well as DR programs has been nominated. The presented model is handled in a Two-Stage Structure to Coordinate the EEPs and DR programs, namely TSSC DR EEP . In the first stage, the level of Energy Efficiency Investment (EEI) is determined over the midterm horizon time. In the next stage, regarding the rate of EEI, a unit commitment problem incorporating EEPs as well as DR programs is handled while the interactions between EEPs and DR have been scrutinized. The proposed problem is implemented in GAMS. To evaluate the capability of EEPs and DR programs coordination, several analysis are carried out on the IEEE 10-unit to confirm the capability of the proposed framework from economic aspect. [ABSTRACT FROM AUTHOR]

This paper investigates the transient stability of a load-rich distribution subsystem during the switching process to islanded mode instigated by a permanent fault. When operating in islanded mode, the subsystem must maintain a generation-load power balance and use at least one distributed generator (DG) to regulate the system frequency and voltage. Therefore, switching control must be executed after the disconnection of the main grid and a strategy which includes a DG coordination method and a single-step load shedding scheme is proposed. Other factors also have a substantial impact on the system transient performance, including the type of subsystem load, the DG penetration level, the fault clearance time and the switching control delay. To perform the study, a distribution subsystem was simulated using PSCAD/EMTDC software, consisting of a mix of synchronous and inverter-based DGs and a combination of static loads and dynamic motor loads. Simulation results show the proposed switching control strategy can effectively ensure successful switching from grid-connected to islanded mode under different fault conditions and DG penetration levels. [ABSTRACT FROM AUTHOR]

In this paper, a hierarchical control for flexible operation of a microgrid is proposed. The structure of the hierarchical control consists of inner, primary and secondary levels. The inner control is used to regulate the output voltage of the inverter which is commonly referred as zero-level. The primary control based on the universal droop control which we improve it to handle both operation modes. It is used to share the active and reactive power accurately with regardless of the output impedance of the inverters. The secondary control compensates the deviation of the microgrid voltage caused by the primary control as well as synchronizes the microgrid voltage with the grid for a smooth transition. Thus, the microgrid can operate either in grid-connected or in islanded mode by using the same control scheme. The small-signal stability of the ameliorated universal droop control is analyzed for both modes, and other levels of control are modeled. Moreover, a technique based on meta-heuristic optimization to design the hierarchical control parameters optimally is introduced. Finally, the simulation was performed on a microgrid that has three voltage source inverters (VSIs) connected in parallel and a local nonlinear load. The results demonstrate the disturbance rejection performance and the flexibility of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Employing the LCL passive filter in inverter-based distributed generation to ensure high quality power delivery comes at the expense of poor performance due to the resonance phenomenon. Literature elucidated several active approaches to mitigate this phenomenon. This paper proposes a sensorless active damping technique that is able to restrain the resonant current component through the estimation of the filter capacitor current using the Kalman filter. The Kalman filter is able to improve the quality of the estimated variables in noisy environments accompanied with measurements and model uncertainties. The effect of LCL filter parameter mismatch on the performance of the Kalman filter is assessed. Stability and convergence analyses are also conducted. An effective tuning method for the grid current controller employing a proportional-resonant controller is adopted. Experimental evaluations were conducted to validate the proposed control strategy. [ABSTRACT FROM AUTHOR]

• Considering the contradiction between PVMFGCI harmonic, reactive power compensation and active grid connection, the dynamic optimization method of power quality control under PVMFGCI working state (considering meteorological conditions) and non working state (at night) is constructed. • Make full use of the remaining capacity of PVMFGCI to optimize harmonic and reactive power, greatly improve the utilization rate of inverter, and reduce the investment and operation cost of power quality control devices (such as APF) of PV grid connected system. • The adaptive ant colony algorithm is used to solve the objective function of the three-level optimization method. The adaptive value of the objective function is the largest and the convergence speed is faster. In this paper, a new harmonic suppression and reactive power compensation strategy based on photovoltaic multi-functional grid connected inverter (PVMFGCI) and a three-layer optimization model based on adaptive ant colony algorithm are proposed for optimal control of power quality of grid connected photovoltaic (PV) power generation system. Here, an improved CPT (Conservative Power Theory) current detection method is used to extract harmonic and reactive current components from the load current, and a new PVMFGCI hierarchical optimization model is developed to optimize the power quality at the common connection point (CCP). The proposed three-layer optimization model is an improved form of the two-layer optimization model, which successfully solves the contradiction between the active power grid connection of the PV grid connected inverter and the power quality control. The main objective of the proposed three-layer optimization model is to meet the requirements of active power output of PV power generation, and at the same time, to utilize the remaining capacity of the inverter to control harmonics and compensate reactive power. However, when the remaining capacity of PVMFGCI is sufficient, PVMFGCI can reasonably allocate the remaining capacity through the three-layer optimization model to meet the optimal power quality at the CCP. In this process, the power quality of the microgrid is improved. PVMFGCI provides reactive power compensation, harmonic control and other power quality problems. In addition, when the remaining capacity of PVMFGCI is insufficient, it only needs to maximize the active power and make the harmonic distortion rate and power factor meet the grid connection requirements, which improves the utilization rate of PVMFGCI. The proposed techniques are modeled and the IEEE 33 bus systems are analyzed and their performances are verified experimentally on a developed prototype, in PV output active power variation condition, imbalance loading condition for linear/nonlinear loads, where it has shown a very good performance. The effect of harmonic control and reactive power compensation at each node of the system is obvious. [ABSTRACT FROM AUTHOR]

• Multi-damping factor design method for PBC of SPVSI with LC filters. • Ensure the stability of islanded SPVSI system under various load scenarios. • Ensure the stability of islanded SPVSI system under LC filter parameter drift. • Adopting a design sequence from the inner to the outer control loop. • Design of control loops with good performance and distinct control bandwidth. With the advantages of fast response, good stability, and strong robustness to filter parameter variations as well as load perturbations, the application of passivity-based control (PBC) in the single-phase voltage source inverter (SPVSI) has received extensive attention. However, as a non-linear control strategy, PBC control has a complex coupling structure containing feed-forward, negative feedback, etc., which poses many difficulties for multi-damping design. In addition, ensuring the passivity of the islanded system under various load conditions or filter parameter drift is still an unresolved issue in the damping coefficient design of the PBC controller. Therefore, for SPVSI systems with LC filters, this paper proposes a multi-damping coefficient design method for PBC controllers based on stability domain analysis. The designed PBC controller can ensure system stability under several load conditions or certain deviations of LC filter parameters. Moreover, the design order from the inner to the outer control loop is adopted according to the distribution of individual damping coefficients in the control loop. As a result, each control loop has a good dynamic performance and distinct control bandwidth. Finally, robustness analysis, simulation, and experimental results verify the correctness and effectiveness of the proposed multi-damping coefficients design method for the PBC controller. [ABSTRACT FROM AUTHOR]

• Analyze the LCL grid-connected inverter in the discrete domain. • The active damping loop with the grid-side currents feedback only. • A co-design method of the active damping loop and current controller is proposed. • Obtain the optimized parameters by introducing the Lagrange multiplier method. Under non-ideal grid conditions, especially the harmonic grid condition, the proportional resonant controller with multiple harmonic resonant controllers (HR-PR controller) is recommended to provide high quality grid currents for the 3-phase LCL-filtered grid-connected inverter. Besides, in order to solve the resonance problem, the active damping loop with the grid-side currents feedback only is preferred. However, parameters of the HR-PR controller and the active damping loop both affect the performance of LCL filter resonance suppression and stability of the inverter, which is analyzed in this paper. Moreover, the stability constraint and the resonance suppression constraint is derived to maintain the good ability of LCL filter resonance suppression and satisfy the stability requirements of the inverter. Considering the wide control bandwidth for the HR-PR controller, a co-design method of the active damping loop and HR-PR controller is proposed in this paper. By introducing the Lagrange multiplier method, the proposed co-design method obtains the optimized parameters for the active damping loop and grid currents controller in z -domain appropriately. A prototype of 3-phase LCL-filtered three-level neutral point clamped grid-connected inverter is established to validate the proposed method. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a new controller for a grid connected power conditioning unit (PCU). The PCU consists of a flying capacitors inverter (FCI) connected to the grid through an inductor and a step-up transformer. A hybrid model of the PCU is derived using a newly improved commuted Bond Graph (BG) method. The derived hybrid model is an explicit standard model valid for all FCI configurations. A weighted real-time predictive controller is proposed and its implementation is made simple using the developed hybrid model. The weighted predictive algorithm transfers the power to the grid by controlling the current with unity power factor and lower harmonic distortions. Modelling using the BG, theoretical analysis, simulation and experimental results are described in this paper. [Copyright &y& Elsevier]

Abstract: This paper presents the analysis and design of a 3-phase 4-leg (3P4L) unified series–parallel active filter (USPAF) with ultracapacitor energy storage (UCES) for improving the power quality in three-phase four-wire (3P4W) distribution system. The series and parallel active filter (AF) of 3P4L USPAF system are realized by four-leg voltage source inverters (VSIs) to a common dc-link capacitor. Due to its high power density, the UCES is well-suited to supply high power for short period of time. The objective of this paper is to enhance the unbalanced voltage sag mitigating capability of the 3P4L USPAF system by adding UCES, directly connected in the dc-link. This is achieved by injecting energy from the UCES to maintain the dc-link voltage constant. Thus, the proposed system is capable of mitigating unbalanced voltage sag with zero-sequence component in the source voltage and compensating harmonic, reactive power and unbalanced current of the load in 3P4W distribution systems. The proposed scheme is validated by an experimental prototype with a 1.93F UCES bank and using dSPACE DS1103 real-time control platform in the laboratory. The experimental results show that the combined system offer improved performance to maintain the load voltage constant at its rated value during unbalanced voltage sag in the supply voltage. [Copyright &y& Elsevier]

The fault current contribution of inverter based DGs (IBDGs) may affect the operation of protective devices present in the system. Hence, it is necessary to consider the presence of IBDGs in short-circuit analysis of distribution system. A short-circuit analysis approach for unbalanced distribution system with IBDG, incorporating different voltage dependent control modes, is proposed in this paper. Comparison of the results obtained by the proposed method with those obtained by the time domain simulation studies carried out using PSCAD/EMTDC software, shows the accuracy of the proposed technique. [ABSTRACT FROM AUTHOR]

Multilevel inverters (MLIs) have been developed to a matured level for generating high quality output voltage, used in numerous applications. Conventional MLIs has been received more attention due to their remarkable use in medium-voltage applications. But use of high number of part counts in conventional MLIs increase the intricacy of circuitry and controlling both results in high expenses and reduced reliability. Therefore, development of newer topologies is an active attention to reduce part count compared to the conventional MLIs. This paper presents an exhaustive review of recently proposed reduced part count MLIs for grid integration application on the basis of selection of components, comparative factors and their suitability. It is aimed at presenting a state of the art on configuration, working and their features to researchers, inventors, and application engineers dealing with multilevel inverters. [ABSTRACT FROM AUTHOR]

Effective fault diagnosis methods can improve the reliability of the two-level inverter used in traction systems. Both the open-circuit (OC) fault and current sensor fault can affect the phase currents of the inverter, which makes it difficult to distinguish them. In this paper, a diagnosis method for OC faults and current sensor faults is proposed, which is based on finite control set-model predictive control (FCS-MPC). First, the cost function is constructed and the inverter is controlled by the FCS-MPC. Then, the actual and estimated values of the cost function are calculated. The difference between them is applied to fault detection. Next, Combine the command signals and current direction, the number of affected sampling points in a period is counted and normalized. Different faults correspond to different percentages and this feature is used to determine fault location. Finally, the hardware-in-the-loop (HIL) platform is built, the feasibility and effectiveness of the proposed fault diagnosis method are verified by the HIL tests. • The finite control set-model predictive control is a promising control strategy for traction inverter. • The residual of cost function is introduced can provides a new idea for fault diagnosis. • The interference between current sensor and transistor faults should be considered to avoid false alarms. • The fault diagnosis method that requires only two current sensors is more suitable for practical applications. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an advanced power converter employs a sinusoidal voltage absolute value tracking buck–boost DC–DC converter in the first power processing stage and a polarity changing full-bridge inverter in the second stage. The proposed power conversion system has the capability of delivering good quantitative and qualitative sinusoidal output current and voltage waveforms with good output voltage regulation. Consequently, the complete voltage regulator system, which is mainly suitable for new energy generation systems as well as energy storage systems, can be constructed compactly and inexpensively without DC link electrolytic capacitor. Also, the paper presents an auxiliary passive resonant circuit for soft switching operation. Simulation results using PSIM software are presented to verify the operation principles and feasibility of the proposed power conversion system. Finally, these results are verified by means of an experimental prototype. [Copyright &y& Elsevier]

The capacity of the voltage source inverter (VSI) and the values for the link filter connected between the injection transformer and the inverter play a crucial in the design of the DVR. In this research paper, new Dynamic Voltage Restorer (DVR) topology has been proposed. The capacity of the voltage source inverter (VSI) and values of the link filter is small that will improve the compensation capabilities for voltage harmonic, swell and voltage sag mitigation under various fault conditions. The new RLC filter is able to eliminate the switching harmonics. The capacity of the dc supply voltage is reduced when the value of inductance is small. The new DVR topology has high efficiency and the ability to improve the quality of voltage. An outline architecture of the RLC filter parameters for the specific model has been presented. The new DVR topology is modeled and simulated using the PSCAD/EMTDC. The control scheme has good control dynamics with minimum transient current overshoot. The simulation results under transient performance are good. [ABSTRACT FROM AUTHOR]

The increasing application of distributed power generation into the power system makes grid interconnection requirements of power quality more and more stringent. The reduction of current total harmonic distortion (THD) of grid-connected inverters to achieve the grid code by increasing of switching frequency in PWM of inverters is one of the popular methods, but this increases the switching loss. The aim of this paper is to propose a PWM technique with the function of variable switching cycle to reduce the current harmonics in grid-connected inverters. The weights of this function are determined by using a genetic algorithm under the constraint of constant switching loss. The simulation and experimental results of the proposed technique as well as the conventional constant technique are given and compared, which validate the performance of the proposed technique. [ABSTRACT FROM AUTHOR]