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This paper proposes an improved performance interactive distribution static compensator (DSTATCOM) to address the limitations of conventional current control mode (CCM) and voltage control mode (VCM) operations. The proposed interactive scheme provides smooth transfer of modes of operation while remaining connected in the distribution system. In normal operation, the DSTATCOM operates in CCM to make source currents balanced, sinusoidal, and at a unity power factor. During voltage disturbances, the CCM operation cannot improve load voltage. In that case, DSTATCOM operation is changed to VCM, which maintains a constant voltage across the sensitive loads. Hence, this interactive DSTATCOM ensures continuous, flexible, and robust operation of the load. Moreover, the filter current requirements are reduced in the proposed scheme, which reduces the losses in the filter and feeder, improves inverter efficiency, and requires a reduced-rating inverter for sag mitigation. The principle of operation and control for both operating modes is analyzed. The detailed process of the flexible transfer between the two modes is derived. Deadbeat predictive control algorithms for CCM as well as VCM operation are developed for fast operation during mode transfers. The performance of the proposed scheme is validated by both simulation and experimental results. [ABSTRACT FROM PUBLISHER]

Common-mode voltage (CMV) is one of the most important issues for the power electronics inverters. An interesting space vector modulation of the three-leg current-source inverter has been reported for the CMV reduction recently. However, it has the disadvantages of the bipolar current pulses and modulation index range limitation. In order to solve these problems, a four-leg current-source inverter with a new space vector modulation is proposed in this paper. With the proposed method, the system CMV can be significantly reduced without bipolar current pulses and modulation index range limitation. Finally, the experimental tests are carried out on a four-leg current-source inverter, and the experimental results verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper proposes a technique to suppress low-order harmonics for an open-end winding induction motor drive for a full modulation range. One side of the machine is connected to a main inverter with a dc power supply, whereas the other inverter is connected to a capacitor from the other side. Harmonic suppression (with complete elimination of fifth- and seventh-order harmonics) is achieved by realizing dodecagonal space vectors using a combined pulsewidth modulation (PWM) control for the two inverters. The floating capacitor voltage is inherently controlled during the PWM operation. The proposed PWM technique is shown to be valid for the entire modulation range, including overmodulation and six-step mode of operation of the main inverter. Experimental results have been presented to validate the proposed technique. [ABSTRACT FROM PUBLISHER]

The quasi-Z-source cascade multilevel inverter (qZS-CMI) presents many advantages over conventional CMI when applied in photovoltaic (PV) power systems. For example, the qZS-CMI provides the balanced dc-link voltage and voltage boost ability, saves one-third modules, etc. However, the qZS-CMI still cannot overcome the intermittent and stochastic fluctuation of solar power injected to the grid. This paper proposes an energy stored qZS-CMI-based PV power generation system. The system combines the qZS-CMI and energy storage by adding an energy stored battery in each module to balance the stochastic fluctuations of PV power. This paper also proposes a control scheme for the energy stored qZS-CMI-based PV system. The proposed system can achieve the distributed maximum power point track for PV panels, balance the power between different modules, and provide the desired power to the grid. A detailed design method of controller parameters is disclosed. Simulation and experimental results verify the proposed system and the control scheme. [ABSTRACT FROM PUBLISHER]

This paper proposes an improved half-bridge inverter with wide-input-voltage capability for ozone-driven circuits. The study is motivated because the limited regulation of input voltage often affects the operation flexibility of ozone-driven circuits. Therefore, a modification of a half-bridge inverter along with a multiwinding coil design is proposed in this study, anticipating an increase in the range of input voltage and benefiting the industry applications. To confirm the practicality of this method, a hardware circuit was made with extensive validation. Experimental outcomes support the feasibility of this proposed method for ozone generators, exhibiting a potential of the method extending to other voltage-source applications. [ABSTRACT FROM PUBLISHER]

The aim of this paper is to study the fault-tolerant control strategies when faults occur in paralleled-inverter-fed permanent-magnet-synchronous-motor drives. Different from removing the faulty inverter channels completely, this paper proposes and compares three fault-tolerant control strategies, which fully use all the healthy phase legs, including those in faulty inverters. The three schemes are called “normal channel direct compensation,” “normal channel asymmetric currents,” and “equivalent current value compensation,” respectively. The proposed control schemes not only provide smooth torque but also have smaller copper loss under open-circuit faults. The corresponding principles and design criteria of control are given. Moreover, the proportional-resonant current controller is designed to track the asymmetrical current references of fault-tolerant control and avoid the possible circulating currents among paralleled inverter channels. The experiments are given to verify and compare the performance of the proposed fault-tolerant control schemes. [ABSTRACT FROM PUBLISHER]

A new single inductor dual buck full-bridge inverter is proposed in this paper. The inverter is improved on the basis of a dual buck inverter, and it retains the advantage of a no reverse recovery of body diode. The dc voltage utilization rate can be increased by adopting the voltage commutation bridge instead of the voltage-dividing capacitors in the inverter. In addition, comparing with other dual buck inverters, the inverter has just one filter inductor, which can make the volume and weight of the system decreased observably and improve the integration. The controlling method, combining the high- and low-frequency modulation, is adopted to weaken the complexity of control and improve the reliability and stability of the system. The operating principle, characteristic analysis, simulation and experiment are provided in this paper. A 1-kW prototype is established and tested, and the results of simulation and experiment verify the correctness of the theoretical analysis. [ABSTRACT FROM PUBLISHER]

This paper develops an electric vehicle switched-reluctance motor (SRM) drive powered by a battery/supercapacitor having grid-to-vehicle (G2V) and vehicle-to-home (V2H)/vehicle-to-grid (V2G) functions. The power circuit of the motor drive is formed by a bidirectional two-quadrant front-end dc/dc converter and an SRM asymmetric bridge converter. Through proper control and setting of key parameters, good acceleration/deceleration, reversible driving, and braking characteristics are obtained. In idle condition, the proposed motor drive schematic can be rearranged to construct the integrated power converter to perform the following functions: 1) G2V charging mode: a single-phase two-stage switch-mode rectifier based charger is formed with power factor correction capability; 2) autonomous V2H discharging mode: the 60-Hz 220-V/110-V ac sources are generated by the developed single-phase three-wire inverter to power home appliances. Through the developed differential mode and common mode control schemes, well-regulated output voltages are achieved; 3) grid-connected V2G discharging mode: the programmed real power can be sent back to the utility grid. [ABSTRACT FROM PUBLISHER]

Three-phase inverters subject to droop control are widely used in islanded microgrids to interface distributed energy resources to a network and to properly share loads among different units. In this paper, a mathematical model for islanded microgrids with linear loads and inverters under frequency and voltage droop control is proposed. The model is constructed by introducing a suitable state-space transformation that allows to write the closed-loop model in an explicit state-space form. Then, the singular perturbations technique is used to obtain reduced order models that reproduce the stability properties of the original closed-loop model. The analysis shows that the currents' dynamics influence the stability of the microgrid, particularly for high values of the frequency droop control parameters. It is also shown that a further reduction of the model order leads to a typical oscillator model that is not able to predict the possible instability of the droop-controlled system. Numerical and experimental results demonstrate the validity of the proposed models. [ABSTRACT FROM PUBLISHER]

In this paper, a simplified pulsewidth modulation with switching constraint control scheme is proposed to eliminate the circulating currents for bidirectional paralleled ac/dc converters in grid-tied system. The proposed control scheme can reduce the circulating currents and does not need additional current sensors and communication device among paralleled converters compared with conventional methods. Therefore, the paralleled system cost can be reduced. In addition, the current shaping and sharing between the paralleled converters can be well accomplished so that the overall performance of the paralleled converter system can be raised. Furthermore, the dc, ac, and self-generated circulating currents are clearly analyzed by graphics, and the synchronous circulating currents are first explored. Finally, a prototype system is constructed, and the proposed control scheme is implemented using a Spartan-3E XC3S250E FPGA. Both simulation and experimental results verify the validity of the proposed theory and control scheme. [ABSTRACT FROM PUBLISHER]

This paper presents an experimental implementation of a predictive control strategy for a system made up of two induction machines fed by a six-leg indirect matrix converter. The objective is to evaluate the practical feasibility of this method, considering the large computational cost, and to incorporate available market technologies. This strategy achieves tracking of torque, speed, and flux references on both machines, minimizing the instantaneous input reactive power present in the system. A theoretical explanation of the main concepts that are used in the predictive control strategy is presented, along with comments on the experimental implementation, experimental setup, and results obtained under both balanced and unbalanced grid conditions. [ABSTRACT FROM PUBLISHER]

This paper demonstrates a novel cosimulation architecture that integrates hardware testing using power hardware-in-the-loop (PHIL) techniques with larger-scale electric grid models using off-the-shelf non-PHIL software tools. This test bed for distributed integration enables utilities to study the impacts of emerging energy technologies on their system and manufacturers to explore the interactions of new devices with existing and emerging devices on the power system, both without the need to convert existing grid models to a new platform or to conduct in-field trials. This paper describes an implementation of this architecture for testing two residential-scale advanced solar inverters at separate points of common coupling (PCCs). The same hardware setup is tested with two different distribution feeders (IEEE 123 and 8500 node test systems) modeled using GridLAB-D. In addition to simplifying testing with multiple feeders, the architecture demonstrates additional flexibility with hardware testing in one location linked via the Internet to software modeling in a remote location. In testing, the inverter current, real and reactive power, and PCC voltage are well captured by the cosimulation platform. Testing of the inverter advanced control features is currently somewhat limited by the software model time step (1 s) and tested communication latency (24 ms). These limitations could be overcome using faster modeling and communication within the same cosimulation architecture. [ABSTRACT FROM PUBLISHER]

Low-bandwidth communication channels are used to support the information exchange between a microgrid centralized controller and local controllers in the secondary frequency control of an islanded microgrid. However, the impact of the inherent time delay in these communication channels on the microgrid performance has not been taken into account when the secondary frequency controller is designed. This paper investigates the effect of the communication delays on the secondary frequency control of an islanded microgrid with multiple distributed generators. A small-signal model-based method is introduced for the microgrid to find delay margins below which the microgrid can remain stable. By performing a series of trial studies, the relationships between secondary frequency control gains and delay margins are obtained. A gain scheduling approach is also proposed to compensate the effect of the communication delay on the secondary frequency control. Results from the Canadian urban distribution system have verified that communication delays can adversely affect the microgrid secondary frequency control, and the proposed gain scheduling approach can improve the robustness of the microgrid secondary frequency controller to communication delays. [ABSTRACT FROM PUBLISHER]

A stability study of distribution networks with a high penetration of distributed generators (DGs) actively supporting the network is presented in this paper. A possible way of mitigating the voltage rise caused by DGs is the local control of reactive power. Among the different possible options, the Q(U) control (reactive power as a function of the voltage) is one of the commonly suggested solutions. However, the Q(U) control can, under certain conditions, lead to instability. This paper summarizes the results of a stability study conducted on a single-inverter system and on a multiinverter system. It shows that the requirements for reaching a stable operation can easily be met for integrated systems but could be a significant constraint for systems relying on communication. [ABSTRACT FROM PUBLISHER]

This paper presents a circuit structure with a novel inverter that is applicable to arc welding machines. The output rectifier configuration adopts the current doubler rectifier with two coupled inductors. The proposed inverter topology can keep the energy stored in the coupled inductor until the next commutation interval. Moreover, the conduction loss will be lower, which improve the conversion efficiency of the arc welding machine. The operation principle, control circuit, and inverter design example are also described in detail. Experimental results demonstrate the feasibility and efficiency of the proposed inverter topology. [ABSTRACT FROM PUBLISHER]

Photovoltaic (PV) systems are one of the main actors in distributed power generation. In particular, in urban contexts, the PV generators can be subjected to mismatching phenomena due to the different orientation of the modules with respect to the sun rays or due to shadowing. In these cases, the maximum power point tracking (MPPT) function must be designed carefully. In this paper, architecture, including one dc/dc converter for each PV generator, is considered. The converters' output terminals are series connected to a high-voltage dc bus, where also a bidirectional dc/dc converter managing the power from/to a storage device is plugged. The functional constraints deriving from the dc/dc converters' connection, the mismatching phenomena, the MPPT capabilities of the inverter, connected with its input terminals at the dc bus, are taken into account in order to determine the best operating point of the system as a whole. The real-time constrained optimization problem is solved by using the particle swarm optimization method, which needs the knowledge of the actual current versus voltage curve of each PV generator. The practical impact of this need is also discussed in the paper. The feasibility and the performances of the proposed approach are experimentally validated by using a laboratory prototype. [ABSTRACT FROM PUBLISHER]

The industrial use of inductive power transfer (IPT) systems is becoming widespread, ranging from monorail systems, motors, people movers, and battery charging applications. This paper proposes a 3-D IPT system driven by a multiple-phase power converter. This is aided by spice circuitry simulation of the power converter, finite-element-assisted software magnetic frequency analysis, and via implementation of the proposed system. The proposed system consists of a cubic power transfer primary window generated by a rotating magnetic flux flow path. This rotating field is loosely coupled via magnetomotive force (MMF) induction into a secondary power pick-up. The system has been demonstrated as a low-power battery charging system. [ABSTRACT FROM PUBLISHER]

The energy produced by renewable energy systems must fulfill quality requirements as defined in the respective standards and directives. Improvement of the quality could be achieved through a more accurate estimation of the frequency of the grid's signal that is used to control an inverter. This paper presents an overview of a method for spectrum interpolation and frequency estimation, and a generalized method for very accurate frequency grid estimation using the fast Fourier transform procedure coupled with maximum decay sidelobe windows. An important feature of this algorithm is the elimination of the impact associated with the conjugate's component on the estimation's outcome (i.e., the possibility of designating the frequency even if the signal's measurement time is on the order of 2.5 periods), and the implementation of the algorithm is straightforward. The results of the simulation show that the algorithm could be successfully used for a fast and accurate estimation of the grid signal frequency. The systematic frequency estimation error is approximately 5\cdot 10^-11\ \Hz for a 5-ms measurement window. The algorithm could be used not only for a single sinusoidal signal, but also for a multifrequency signal. This is assuming that the appropriate spectrum leakage reduction (by a time window) will be performed. [ABSTRACT FROM PUBLISHER]

In this paper, a real-time implementation of selective harmonic elimination pulsewidth modulation (SHEPWM) using generalized Hopfield neural network (GHNN) as applied to the single-phase inverter is designed and discussed. Finding the location of the switching instants in the case of SHEPWM involves the solution of a set of nonlinear algebraic transcendental equations. The problem is redrafted as an optimization problem, and it is solved by using GHNN. Though, in principle, the $n$ number of harmonics can be eliminated using the presented methodology, this paper tries for the case of the elimination of the 5th-, 7th-, 11th-, and 13th-order harmonics while retaining the desired fundamental. An energy function is formulated for this problem and the set of ordinary differential equations (ODEs) describing the behavior of GHNN is obtained. A program in KEIL C was developed to solve these systems of ODEs by the Runge–Kutta fourth-order method to give the switching instants for continuously varying modulation indexes $(M)$. A MATLAB simulation was carried out, and an experimental setup was also constructed in order to validate the simulated results. The fast Fourier transform analysis of the simulated output voltage waveform and the experimental output voltage waveform confirms the effectiveness of the proposed method. Hence, the proposed method proves that it is much applicable in the industrial applications by virtue of its suitability in real-time applications. [ABSTRACT FROM PUBLISHER]

In this paper, two new topologies are proposed for multilevel inverters. The proposed topologies consist of a combination of the conventional series and the switched capacitor inverter units. The proposed topologies reduce the number of switches and isolated dc voltage sources, the variety of the dc voltage source values, and the size and cost of the system in comparison with the conventional topologies. In addition, the proposed topologies can double the input voltage without a transformer. There is no need for complicated methods to balance the capacitor voltage. The simulation and experimental results of single-phase 25- and 17-level inverters are given to prove the correct operation of the proposed topologies. [ABSTRACT FROM PUBLISHER]

Multilevel cascade H-bridge (CHB) converters are one of the promising solutions for medium- and large-scale grid-connected photovoltaic power plants. However, there is a lack of a complete study about their operation during voltage sags. This paper proposes a flexible control strategy for the operation of photovoltaic grid-connected CHB inverters during unbalanced voltage sags. The key novelty is that the proposed strategy is able to inject both active and reactive powers to the grid with either balanced or unbalanced currents, while ensuring that all dc-link capacitor voltages remain balanced. The simulation and experimental evaluations of a 9-kVA grid-connected seven-level CHB illustrate and validate the performance of the proposed strategy for the operation of the grid-connected CHB converter during different unbalanced voltage sags. [ABSTRACT FROM PUBLISHER]

Solar energy has been the most popular source of renewable energy for residential and semicommercial applications. Fluctuations of solar energy harvested due to atmospheric conditions can be mitigated through energy storage systems (ESS). Solar energy can also be used to charge electric vehicle batteries to reduce the dependence on the grid. One of the requirements for a converter for such applications is to have a reduced number of conversion stages and provide isolation. The Z-source inverter (ZSI) topology is able to remove multiple stages and achieve voltage boost and dc–ac power conversion in a single stage. The use of passive components also presents an opportunity to integrate ESS into them. This paper presents modeling, design, and operation of a modified ZSI integrated with a split primary isolated battery charger for dc charging of electric vehicle batteries. Simulation and experimental results have been presented for the proof of concept of the operation of the proposed converter. [ABSTRACT FROM PUBLISHER]

In inverter-based microgrids, individual inverters operate usually with their own digital processor. The clocks used to generate the time signals of these processors differ from each other due to clock drifts. This paper analyzes the impact that the drifts of the processors clocks produce on the operation of inverter-based islanded microgrids. Several communication-free secondary control schemes, which avoid well-known problems caused by digital communication networks, are considered. Active power sharing and frequency regulation are the metrics used to evaluate the performance of the control schemes. The study reveals advantages, drawbacks, and practical limitations of the control schemes caused by the clocks drifts. Therefore, it facilitates the selection of the most suitable control scheme for the practical deployment of microgrids. The theoretical results are validated by experimental tests in a laboratory microgrid equipped with three inverters and three digital signal processors driven by autonomous internal clocks. [ABSTRACT FROM PUBLISHER]

In this paper, finite-set model-predictive control (FS-MPC) methodologies for a grid-connected three-level neutral-point-clamped converter are investigated. The proposed control strategies produce fixed switching frequency, maintaining all the advantages of predictive control such as fast dynamic response, inclusion of nonlinearities and restrictions, and multivariable control using a single control loop. The first of the proposed FS-MPC strategies is based on a multiobjective cost function, designed to regulate both the inverter currents and the balancing of the dc-link capacitor voltages. The second FS-MPC strategy is derived from the first one, and it is based on a cost function that regulates only the grid current, with the balancing of the capacitor voltages being realized by controlling the duty cycles of the redundant vectors. The proposed control systems are experimentally validated using a 5-kW prototype. The experimental results show a good performance for both strategies, in steady-state and transient response, with a total harmonic distortion lower than \text2\% for the currents supplied to the grid. [ABSTRACT FROM PUBLISHER]

Due to the global environmental issues and energy crisis, the injection of renewable energy sources (RESs) into the power system is continuously increasing. As the interface between RESs and power grid, grid-tied inverters using MOSFET switches, without traditional line frequency transformers, show some potential advantages, in terms of low cost, high efficiency, and lightweight and small size. Among several proposed configurations, the Aalborg inverter was proposed as a new family of high efficiency MOSFET-switch-based hybrid source inverters. For a conventional “half bridge” type Aalborg inverter, due to the imbalance of two independent dc sources, the input dc energies may not be fully utilized, which may reduce the efficiency of whole system. In order to extract the maximum energy from two independent dc sources, a coupled-inductor-based “half bridge” type Aalborg inverter is proposed in this paper. The principle of operation is presented and demonstrated through equivalent circuits. The effectiveness of a new energy balancing technique has been validated through simulations and experiments on a 110 V/ 500 W prototype. [ABSTRACT FROM PUBLISHER]

In this paper, a dc-link voltage protection (DCVP) control method is proposed to address the dc-link overvoltage issue due to power backfeeding in a parallel uninterruptible power supply (UPS) system. The proposed control method is able to protect the inverter against the excessive dc-link voltage, which increases the system reliability and robustness. Moreover, a current sharing control strategy is proposed by online regulating the virtual resistance of each UPS module. The proposed current sharing control strategy is able to address the circulating fundamental and harmonic currents caused by the line impedance mismatching or power backfeeding issue in the UPS system. In addition, an improved consensus-based distributed controller is proposed to alleviate the overshoot issue during the transient process in voltage amplitude and frequency restoration. Finally, the feasibility of the proposed methods is verified by experimental results from the parallel UPS prototypes. [ABSTRACT FROM PUBLISHER]

This paper proposes a simplified optimum control strategy for a monoinverter dual parallel (MIDP) permanent magnet synchronous motor (PMSM). With reference to the parallel PMSM unified model, two different auxiliary conditions are investigated. The first minimizes the supplied inverter current, and the second maximizes the motor's efficiency. Their formal expressions are derived by the introduction of three new normalized variables, linked to the load conditions. Thus, the optimum tracking considerations are general, and the parameters are independent. Simplified closed analytical forms are found for the optimum tracking, independently under steady-state and transient conditions. The selection of the best solution is then found via a parameter independent look-up table, and imposed through a closed-loop control diagram. The proposed control is compared to the present state of the art via experiments on a 1400-W MIDP PMSM test bench, showing an equivalent performance, with a lower computational burden. [ABSTRACT FROM PUBLISHER]

This paper aims to investigate a disturbance-rejection-based model predictive control (MPC) with two flexible modes (i.e., unconstrained mode and constrained mode) for three-phase inverters with an $LC$ filter. A disturbance observer (DOB) is designed to both simplify the prediction model and achieve the robustness against uncertain parameters. First, the unconstrained mode is designed with a simple horizon-one MPC control law to precisely regulate the output voltage in the steady state. Next, the constrained mode is designed with long-horizon MPC to quickly drive the system to the unconstrained mode by further optimizing the modulation stage. Also, the proposed MPC ensures the robustness and optimality in all operating modes. Especially, the overall closed-loop stability with DOB dynamics is proven by using Lyapunov function. Unlike other MPC methods, the long-horizon MPC in the constrained mode is specialized by finite switching sequences of the sector-based space vector modulation. Comparative studies with the conventional linear quadratic regulator are conducted on a prototype test bed with a TI TMS320F28335 DSP. Then, the proposed MPC method verifies fast dynamic response and advanced voltage regulation (i.e., small steady-state errors and low total harmonic distortion) under various load conditions and uncertain parameters. [ABSTRACT FROM AUTHOR]

Adding the auxiliary switches to the conventional H-bridge inverter is an effective way to eliminate the leakage current for transformerless PV systems, such as H5, H6, etc. Inspired by the newly developed embedded-switch H6 topology, a novel embedded-switch inverter (ESI) is proposed in this paper for three-phase transformerless PV systems. First, the operation principle and characteristics of the proposed ESI are analyzed. Second, the common-mode model of the three-phase ESI is established, based on which the main factors that affect the leakage current are discussed. The finding reveals that the switching states with conventional modulation strategy result in high-frequency common-mode voltage, which is the source of leakage current. In order to solve the problem, a new modulation strategy is presented for ESI to eliminate the high-frequency leakage current. Finally, the time-domain simulation and the experimental tests are carried out. The results verify the effectiveness of the proposed solution. [ABSTRACT FROM AUTHOR]

In this paper, a framework for stability analyses of a typical inverter-based islanded microgrid with two types of nonlinear loads is presented, namely ideal constant power loads (CPLs), which are the loads supplied by tightly regulated power electronics converters, and dynamic CPLs, which are used to represent motor-drive systems with large time constants. The comprehensive dynamic model of the considered microgrid is first developed, based on which a bunch of small-signal models are deduced using Taylor expansion made at different stable operating points. Afterward, eigenvalue-theorem-based stability analysis and parametric sensitivity analysis are successively performed on the obtained small-signal models to verify the stability of the system, predict the system's unstable regions, and identify the effects of parameters on the stability boundaries. In the meantime, the impacts of different kinds of nonlinear loads on the system stability are studied. Hardware-in-the-loop (HIL) real-time simulation platform of a 30-kVA microgrid, which is mainly formed by a 10-kVA photovoltaic (PV) system, a 10-kVA wind energy conversion system, a 10-kVA lithium-ion battery energy storage system, and two CPLs, is established in Typhoon HIL 602 device. The validity of the theoretical results is verified by real-time simulation results. [ABSTRACT FROM AUTHOR]

This paper proposes a novel autonomous control scheme for equal load current distribution between $n$ parallel-connected, LCL-based, three-phase online uninterruptible power supply (UPS) modules. The control is based on regulating the reference voltage of the filter's capacitor of each inverter as a function of its own output current. The control is achieved without exchanging information between the inverters. The load voltage is maintained fixed at the specified reference value. The control method is fast, precise, and robust against load variations. The validity of the proposed method is verified experimentally and by simulation on a system of two parallel inverters under different load conditions, including step-load and nonlinear load. [ABSTRACT FROM AUTHOR]

The parallel three-phase grid-connected converters sharing dc and ac bus can increase the system power level effectively. However, the zero sequence circulating current (ZSCC) will occur inevitably under space vector pulse width modulation or discontinuous pulse width modulation (DPWM). In this paper, the mechanism and the mathematical model of ZSCC are studied. Then, a hybrid PWM (HPWM) strategy is proposed to suppress the ZSCC based on the same zero sequence voltage injected, which is dependent on the common grid information. The dc voltage utilization of HPWM can be extended compared with sinusoidal PWM (SPWM). The number of parallel modules can be infinity theoretically by using HPWM. It is noted that there is no internal commutation among modules (expect the dq current reference). Finally, the feasibility and the effectiveness of the proposed method are verified by experiments. [ABSTRACT FROM AUTHOR]

In this paper, optimal phase advances under fault-tolerant control (FTC) of a five-phase permanent magnet assisted synchronous reluctance motor (PMa-SynRM) have been proposed under different fault conditions. Critical applications where the consistency and safety are the major concerns in automotive and aerospace industries require reliable control systems. The multiphase motor is considered a promising candidate for these applications as it has redundant phases primarily for fault-tolerant operation. However, advanced FTC for a PMa-SynRM with maximization of reluctance torque has been limitedly studied until now. In the conventional approach, to maintain constant magnetomotive force under fault conditions, phase currents of a motor need to increase significantly. However, this will easily saturate the PMa-SynRM resulting in significantly reduced reluctance torque. To overcome this issue, this paper proposes a novel phase current control method that maximizes the reluctance torque with minimum phase current. Here, a phase current control with a novel phase advance has been proposed under various fault conditions. Extensive theoretical and experimental analysis has been carried out to verify the effectiveness of the proposed idea with 5-hp dynamo system controlled by TI DSP F28335. [ABSTRACT FROM AUTHOR]

Traditional model predictive control cost functions, when utilizing long prediction horizons ( $N\geq 10$) in power inverter applications, have demonstrated harmonic performances approaching that of schemes based on offline-generated optimal pulse patterns. However, long prediction horizons often prohibitively increase computational load. A different approach to maximizing harmonic performance is taken in this paper. It is shown that shaping the output voltage spectra using a simple feedback mechanism can significantly reduce the length of the required prediction horizon, facilitating a feasible computational load. When a short-horizon cost function is augmented by the feedback mechanism an excellent tradeoff between harmonic performance and switching loss is achieved, even with a prediction horizon of one. It is also shown that the feedback mechanism can be used to improve input reference tracking. Experimental verification of the proposed modulator is performed on a two-level three-phase inverter. [ABSTRACT FROM AUTHOR]

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

High dc voltage utilization is one of the important requirements in a three-phase inverter. Under some carrier-based pulse width modulations (PWMs), the dc voltage utilization can reach the same maximum as that under the space vector modulation. But the third harmonic component of common-mode voltage (CMV) will be increased markedly while using these carrier-based PWMs in inverters. To improve the situation, this paper proposes that a midpoint-fluctuation carrier (MFC) scheme should be used in the three-phase inverter. The MFC is formed by adding a midpoint-fluctuation signal on the standard symmetrical triangular carrier. By analyzing the linear modulation region of the midpoint-fluctuation signals, a minimum fluctuant signal in the linear modulation region is designed in the paper. Compared with the use of other conventional schemes in the three-phase inverter, the use of minimum MFC scheme has higher fundamental amplitude in the output line voltage and smaller magnitude of the third harmonic in the CMV. The feasibility and validity of the MFC are proved by experiment results in a three-phase inverter. [ABSTRACT FROM PUBLISHER]

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

This paper presents the design, analytical estimation of power losses, and experimental measurement of choke inductors for switched-mode power inverters. The described approach is applicable to a wide variety of circuits such as Class-E inverters and converters. The expressions required to design the magnetic core in the choke inductor using the area–product ($A_p$) method are given. The equations for the dc resistance, ac resistance at high frequencies, and dc and ac power losses are provided for the solid round winding wire. To validate the presented approach, an example class-E zero-voltage switching power inverter with practical specifications is considered. A core with air gap is selected to avoid core saturation in the choke inductor, which conducts dc current and a small ac current. The gapped core power loss density and power loss are estimated using Steinmetz empirical equation. It is shown that the winding power loss due to the dc current is dominant for the given design. A comparison between the area–product $A_p$ method and the preexisting core geometry coefficient $K_g$ method is presented. The measured magnitude of the inductor impedance has shown that the designed choke is useful up to ten times the switching frequency. [ABSTRACT FROM PUBLISHER]

This paper proposes a family of novel flying capacitor transformerless inverters for single-phase photovoltaic (PV) systems. Each of the new topologies proposed is based on a flying capacitor principle and requires only four power switches and/or diodes, one capacitor, and a small filter at the output stage. A simple unipolar sinusoidal pulse width modulation technique is used to modulate the inverter to minimize the switching loss, output current ripple, and the filter requirements. In general, the main advantages of the new inverter topologies are: 1) the negative polarity of the PV is directly connected to the grid, and therefore, no leakage current; 2) reactive power compensation capability; and 3) the output ac voltage peak is equal to the input dc voltage (unlike neutral-point-clamped and derivative topologies, which requires twice the magnitude of the peak ac voltage). A complete description of the operating principle with modulation techniques, design guidelines, and comprehensive comparisons is presented to reveal the properties and limitations of each topology in detail. Finally, experimental results of 1-kVA prototypes are presented to prove the concept and theoretical analysis of the proposed inverter family for practical applications. [ABSTRACT FROM PUBLISHER]

A new topology of a load-commutated silicon-controlled rectifier (SCR)-based current source inverter (CSI)-fed induction motor drive with open-end stator winding is presented in this paper. The proposed topology has a CSI at one side of the stator winding and a capacitor-fed voltage source inverter (VSI) at the other end. The CSI feeds only active power to the motor and the VSI is controlled to provide the reactive power required for operation of the CSI at leading power factor, thereby facilitating natural commutation of SCRs. This topology does not require any interfacing inductor or separate dc voltage source for the VSI. The control scheme enables operation of the motor even at very low speeds without problems such as commutation failure. The experimental verification of the proposed scheme is carried out on an induction motor with open-end stator windings. A digital signal processor (TMS320F28335) is used for implementation of the control algorithm. [ABSTRACT FROM PUBLISHER]

In this paper, a new nonisolated high-voltage-gain boost converter is proposed. The proposed converter is constructed by adding an additional inductor to a conventional three-level-boost (TLB) converter. When compared with the conventional boost and TLB converters, the proposed converter can achieve higher voltage conversion ratio with reduced voltage and current stresses in the switches. In particular, the proposed converter can reduce the losses of active devices by reducing the current of the switch compared to conventional converters, thus, can achieve high efficiency. In addition, the proposed converter automatically balances the output voltages for an unbalanced load without the need for any additional control strategy or auxiliary circuit. A 2-kW prototype converter was built and tested to verify performances of the proposed converter. [ABSTRACT FROM PUBLISHER]

Three-phase cascaded inverter with leakage current reduction for the transformerless PV system is investigated in this paper. The common-mode loop model of the conventional three-phase cascaded H4 grid-connected inverter is established. The relationship among the leakage current, common-mode voltage, and differential mode voltage is analyzed. The inherent reason that the conventional cascaded H4 grid-connected inverter fails to reduce the leakage current is clarified. In order to solve the problem, a novel three-phase cascaded H5 grid-connected inverter and its modulation strategy are proposed. It can significantly reduce the leakage current. Finally, the performance tests of the cascaded H4 and H5 grid-connected inverters are carried out. The results verify the effectiveness of the proposed solution. [ABSTRACT FROM PUBLISHER]

Rising demand for distributed generation based on renewable energy sources (RES) has led to several issues in the operation of utility grids. The microgrid is a promising solution to solve these problems. A dedicated energy storage system could contribute to a better integration of RES into the microgrid by smoothing the renewable resource's intermittency, improving the quality of the injected power and enabling additional services like voltage and frequency regulation. However, due to energy/power technological limitations, it is often necessary to use hybrid energy storage systems (HESS). In this paper, a second-order sliding mode controller is proposed for the power flow control of a HESS, using a four-leg three-level neutral-point-clamped (4-Leg 3L-NPC) inverter as the only interface between the RES/HESS and the microgrid. A 3-D space vector modulation and a sequence-decomposition-based ac-side control allow the inverter to work in unbalanced load conditions while maintaining a balanced ac voltage at the point of common coupling. DC current harmonics caused by unbalanced load and the NPC floating middle point voltage, together with the power division limits, are carefully addressed in this paper. The effectiveness of the proposed technique for the HESS power flow control is compared to a classical PI control scheme and is proven through simulations and experimentally using a 4-Leg 3L-NPC prototype on a test bench. [ABSTRACT FROM PUBLISHER]

This paper presents a method for small-signal dynamic studies of power systems including utility-scale PV energy systems considering the effects of dc-side components. Existing methods commonly model a PV energy system as a dc source connected to a grid-tied inverter. This paper shows that the dc components of a PV energy system can also contribute in the dynamics of the power system in the range of small-signal studies. The paper first develops the model of a PV energy system by merging the submodels of dc- and ac-side components. Then, a method to obtain the augmented model of the power system including a PV energy system will be explained using a study system. The study system consists of a PV energy system in parallel with a synchronous generator connected to a grid. Then, based on eigenvalue analysis, verified by time-domain simulations, it is shown that the dc-link capacitor, front-end converter, and its controller can introduce oscillatory modes in subsynchronous range of frequencies. Furthermore, it is shown that the damping of the sensitive modes can significantly vary with respect to the parameters of the dc-side components. [ABSTRACT FROM PUBLISHER]

To achieve precise voltage value for modern model-based motor control strategies, an accurate voltage distortion compensation method for voltage source inverters is proposed in this paper. The voltage distortion principles of a mosfet-based inverter and an IGBT-based inverter are analyzed and compared. A graphical solution of compensating voltage error is proposed, all the voltage distortion factors are considered. A current observer is designed to overcome time delay, achieve precise current in each switching period for compensation. The proposed method is of high precision, intuitive, and easy to implement. The effectiveness of the proposed method is verified by experiments. [ABSTRACT FROM PUBLISHER]

This paper shows the feasibility of a novel decentralized control scheme for the grid-tied ac-stacked photovoltaic (PV) inverter architecture. The proposed decentralized control scheme with low-bandwidth communications requirements enables a fully distributed PV inverter architecture. Detailed modeling and theoretical analyses will be provided to support the existence of such decentralized control scheme. Simulation and experimental results on a representative laboratory-scale prototype will be presented under symmetrical and asymmetrical conditions as well as 10% grid voltage sag to show the effectiveness of the proposed control scheme in different operating conditions. [ABSTRACT FROM PUBLISHER]

This paper deals with an analysis of a middle-frequency resistance spot-welding (RSW) system with a dc welding current controlled by a pulsewidth modulated inverter, which supplies a welding transformer with a full-wave rectifier mounted at the secondary. Welding transformers in the automotive industry are usually mounted on the arm of a moving robot, so the weight is important. To achieve the same welding power with a large welding current, the transformer's weight can be reduced with a higher PWM switching frequency. This higher frequency allows a reduction in the transformer's iron-core cross section with shorter primary and secondary windings. Unfortunately, the leakage inductances prevent the RSW system from achieving the same nominal welding current at higher frequencies. The frequency-dependent maximum welding current is a characteristic behavior of the RSW system, which can be determined by sophisticated and time-consuming simulations or with the expensive measurements. The third option presented in this paper is determination of an analytical solution, which allows calculation of the maximum welding current as function of frequency or any parameter of the RSW system circuit model. The analytically calculated frequency-dependent function of the maximum welding current was completely confirmed by measurements on an industrial RSW system and by numerical simulations. [ABSTRACT FROM AUTHOR]

Inverter-based distributed generation (DG) system is becoming an attractive solution for high penetration of renewable energy sources to the main grid. DG system should be able to supply power to the local loads whenever necessary even in case of utility power outage. Thus, the inverters in DG systems are expected to operate in both grid-connected and islanded mode, where they are acting as a current source for the ac grid and a voltage-source for the load, respectively. Transition between modes of operation is nontrivial and can cause deviations in voltage and current, because of mismatch in frequency, phase, and amplitude between the inverter output voltage and the grid voltage. Thus, it is necessary to have seamless transition between grid-connected and islanded mode. This paper presents a new control strategy with seamless transfer characteristics for a grid-connected voltage-source inverter using model predictive control (MPC) framework. The main objectives of the proposed predictive controller are: 1) decoupled power control in grid-connected mode, which enables the proposed power electronics interface to provide ancillary services such as reactive power compensation; 2) load voltage control in islanded mode; and 3) seamless transition between modes of operation through proposed synchronization and phase adjustment algorithm. The proposed controller features simplicity to implement since only one cost function should be minimized for all modes of operation, and hence no ambiguity in the control algorithm that could cause mode transition problems. An autotuning strategy for weight factors in MPC cost function is proposed to simplify the weight factor tuning strategy. The stability analysis of the proposed controller is provided. Simulation and experimental results validate the expected performance and effectiveness of the proposed control strategy. [ABSTRACT FROM PUBLISHER]

In this paper, a switching loss-free (SLF) concept for the first six-switches H-bridge inverter (H6-I) topology is proposed. SLF means that its switches are able to operate with soft turn-on and turn-off transitions. In order to implement the SLF goal, a new resonance-trajectory is proposed. Compared with the zero-current-transition H6-I (ZCT-H6-I) topology published in previous literature, the proposed resonance-trajectory can precisely compensate for losses of resonant tanks every switching period. With this intention, an implementing circuit is structured based on the H6-I topology, and its detailed operation principle and performance characteristics are analyzed. As a result, all active switches of the new circuit are switched under zero-current turn-on and zero-current turn-off conditions. Also, the reverse recovery problem of freewheeling diodes is alleviated owing to the zero-current turn-off property of diodes. The SLF target is realized in theory. Finally, experimental results from a 1 kW prototype at 50 kHz switching frequency are provided to verify the effectiveness of the proposed SLF concept in practice. Specifically, the conversion efficiency of the new circuit is over 95% in a wide load range, and there is roughly a 1.5% efficiency improvement compared with the hard-switching H6-I topology. [ABSTRACT FROM PUBLISHER]

The paper presents an innovative concept of applying a combination of well-known selective harmonic elimination (SHE) and selective harmonics mitigation (SHM) methods to shape a spectrum of catenary current harmonics, generated by a traction drive system equipped with a voltage source inverter. Application of this method allows for reducing distorting influence of a modern rolling stock supplied by a 3 kV dc catenary on the railway signaling, command, and control systems. The efficiency of the proposed method is independent of synchronization of a vehicle's inverters and of load balance of traction motors, which makes it more reliable than as it is proposed in the literature. Results of computer simulations were presented and compared with laboratory measurements. The obtained dc-link current spectrum using the proposed SHE/SHM method was compared with spectra received using synchronized and unsynchronized sinusoidal pulse-width modulations (SPWMs). Based on the conducted tests, it was shown that the proposed method of control, which was implemented in a laboratory drive system, is effective in reducing dc side current harmonics in a frequency range of 1300–3100 Hz, which is used by track circuits on railway lines, e.g., in Poland. The technique proposed by the authors allows taking into account any restrictions arising from the technical nature of low-frequency traction drives. [ABSTRACT FROM PUBLISHER]