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The field-weakening operation of a doubly fed induction generator (DFIG) connected to a dc-link is analyzed in this paper, in order to optimize the efficiency. In the considered DFIG-dc system, the stator feeds a constant-voltage dc link by a diode bridge, and the rotor current is controlled using a voltage-source inverter connected to the same dc link. Since the stator voltage amplitude is imposed by the dc-link, a variation in stator flux magnitude results in a frequency change. However, in this system, a stator frequency variation over a wide range can be accepted, if the rated flux is not exceeded. Thus, the stator flux amplitude can be adjusted through the magnetization current component by the voltage-source inverter and according to the load level, in order to reduce losses in the machine and in the inverter. This paper presents an optimization analysis and simplified formulae determining the optimal reference magnetization current in the control of the system. Conversely to field weakening in conventional drives, in this case, the enabling of field weakening control is not dependent on rotor speed, but depends on the reference torque. The proposed optimal control is validated through simulation and experimental results. [ABSTRACT FROM PUBLISHER]

DC-link capacitors in power electronic converters are a major constraint on improvement of power density as well as reliability. Evaluation of the dc-link capacitors in terms of power loss, ageing, and failure rate will play an important role in design stages of the next-generation power converters. This paper proposes a new evaluation circuit for dc-link capacitors used in a high-power three-phase inverter, which is intended for testing power loss, failure rate, ageing, and so on. The evaluation circuit produces a practical ripple current waveform and a dc bias voltage into a capacitor under test, in which the ripple current is equivalent to that generated by the three-phase inverter on the dc link. The evaluation circuit employs a full-scale current-rating and downscaled voltage-rating inverter for producing the ripple current, so that the power rating of the evaluation circuit is much smaller than that of a full-scale current-rating and full-scale voltage-rating inverter. Theoretical analysis and simulated results verify the effectiveness of new evaluation circuit. [ABSTRACT FROM PUBLISHER]

Multilevel inverters with dodecagonal (12-sided polygon) voltage space vector (SV) structures have advantages like extension of linear modulation range, elimination of fifth and seventh harmonics in phase voltages and currents for the full modulation range including extreme 12-step operation, reduced device voltage ratings, lesser dv/dt stresses on devices and motor phase windings resulting in lower EMI/EMC problems, and lower switching frequency—making it more suitable for high-power drive applications. This paper proposes a simple method to obtain pulsewidth modulation (PWM) timings for a dodecagonal voltage SV structure using only sampled reference voltages. In addition to this, a carrier-based method for obtaining the PWM timings for a general N-level dodecagonal structure is proposed in this paper for the first time. The algorithm outputs the triangle information and the PWM timing values which can be set as the compare values for any carrier-based hardware PWM module to obtain SV PWM like switching sequences. The proposed method eliminates the need for angle estimation, computation of modulation indices, and iterative search algorithms that are typical in multilevel dodecagonal SV systems. The proposed PWM scheme was implemented on a five-level dodecagonal SV structure. Exhaustive simulation and experimental results for steady-state and transient conditions are presented to validate the proposed method. [ABSTRACT FROM PUBLISHER]

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

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

A four-level five-phase open-end winding (OeW) drive topology is introduced in this paper. The drive comprises a five-phase induction machine with open-end stator windings, supplied using two two-level voltage-source inverters with isolated and unequal dc-link voltages, in the ratio 2 : 1. The topology offers the advantages of a modular structure with fewer semiconductor components and has a greater potential for fault tolerance, as compared with an equivalent single-sided four-level drive. Due to the large number of switching states, development of a suitable space vector pulsewidth-modulation (PWM) method can be challenging. Hence, this paper examines the implementation of two-level-shifted carrier-based PWM methods. The effect of dead time on the drive performance is discussed, and it is shown that simultaneous PWM switching of both inverters can lead to degraded output phase voltage waveforms. Detailed analysis of this phenomenon is presented, a solution is proposed, and the modified modulation techniques are incorporated in an experimental setup, at first in conjunction with $V/f$ control. Once the proof of concept has been provided, full field-oriented control is implemented in this OeW drive topology for the first time; detailed experimental testing is conducted, and results are reported. [ABSTRACT FROM AUTHOR]

In this paper, a new cascaded multilevel inverter is proposed. For the proposed inverter, two different algorithms to determine the magnitude of the dc voltage sources are proposed. Then, in order to generate maximum numbers of output voltage levels by using a constant number of power switches and/or dc voltage sources, several optimum structures of the proposed inverter are obtained. In comparison with the conventional cascaded multilevel inverters, the proposed inverter is able to generate a high number of output voltage levels by using a lower number of power electronic devices such as power switches, driver circuits, power diodes, and dc voltage sources. In addition, the low amount of blocked voltage by switches is another advantage of the proposed inverter. The accuracy performance of the proposed inverter in generating the positive and negative voltage levels is verified through the experimental results on a 61-level inverter. [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]

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]

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]

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]

In this paper, a simple digital hysteresis current control (DHCC) with quasi-constant switching frequency (SF) for the current control of a single-phase grid-connected inverter (SPGI) is proposed. The hysteresis band is cancelled, and the SF is fixed by online adjusting the sampling period. The principle of the DHCC with constant sampling period is analyzed, and the distribution of the switching period (SP) corresponding to the phase angle of the grid is derived. The rule of online adjusting the sampling period to fix SF is then determined according to the former theoretical analysis results. Furthermore, the effect of variation of the filter inductor on the SP is analyzed. An online estimation of the filter inductor based on the linear model of the current increment within one switching cycle is developed to eliminate this kind of effect. The proposed scheme shows the features of simple control, easy implementation in a digital way, robust to the filter inductor, and excellent steady and dynamic performance. The detailed simulation and experimental results verify its accuracy and feasibility. [ABSTRACT FROM PUBLISHER]

To address the reliability of rotating machine insulation, the International Electrotechnical Commission implemented new standards and technical specifications that are relevant to low-voltage and medium-voltage inverter-fed motors. Insulation evaluation under such power supply relies upon partial discharge (PD) measurements and accelerated life tests, which should be conducted (at least for the turn-to-turn insulation) under impulsive voltage waveform. The effect of impulsive voltage parameters on PD inception voltage and, in particular, endurance tests must be investigated to design test procedures that can indicate insulation quality and its behavior in service. This paper examines the effect of repetitive impulsive voltage waveform parameters on PD and the endurance (life) of organic insulation for rotating machine wires based on experiments performed on single-contact crossed pairs. Results show that the features of repetitive voltage impulse affect PD time and frequency characteristics, PD phase distribution, and endurance. Consequently, evaluation of insulation subjected to power electronics waveforms should take into account the appropriate choice of impulsive voltage parameter to fit to the type of PD phenomenology occurring in service conditions. [ABSTRACT FROM PUBLISHER]

Dispersed generation systems need several elements that should work together. One possible way to connect these elements is through a common dc link. This paper discusses a new control method for the doubly fed induction generator when connected to a dc link. Considerable benefits are obtained in this configuration, because a simple diode bridge can be used on the stator and only an ac/dc power electronic inverter is needed to control the rotor currents. As the ac mains do not impose the stator frequency, this should be regulated to get an appropriate operation of the induction machine. This paper proposes a method to regulate the stator frequency and control the torque based on adjustments of the $d - q$ rotor reference currents by using the field-oriented control. The analysis and synthesis of the control system based on a simplified model is discussed. Simulations and experimental results show that the proposed control exhibits good dynamics in the torque control and in frequency regulation. [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]

A novel space vector pulse width modulation (SVPWM) strategy for three-phase four-wire neutral-point-clamped (NPC) inverter is proposed in this paper. The m-mode controllability of a three-phase four-wire NPC inverter, which is modeled as a switched linear system, is investigated. Then, on the basis of analysis of the m-mode controllability, taking the three-level property and the continuity of switching sequence into account, the controllable sets with the three-level property can be a new solution for the NPC inverter. With the sets of less operating modes, new modulation strategies named 18-mode SVPWM strategy, including 18-mode SVPWM(I), 18-mode SVPWM(II), and 18-mode SVPWM(III), are put forward. Compared with the conventional SVPWM strategy, the switching frequencies of the proposed 18-mode SVPWMs modulation strategy are significantly reduced by one-third. Therefore, the overall losses of the system can be reduced and the performance of the inverter can be improved. Finally, the simulation and the experiment results verify the correctness of the proposed approach. [ABSTRACT FROM PUBLISHER]

This paper proposes a multiloop framework for current control of grid-connected voltage source inverters (VSIs) with LCL output filter. The discrete-time model is of third-order with a nonminimum phase zero when controlling the grid side current. This poses some difficulties on the design of most types of controllers. Motivated by this problem, the inner loop is implemented by a discrete-time sliding mode control law to ensure the tracking of the converter side current. This can be achieved regardless the grid impedance and voltage, making the converter to behave like a current source inverter with a capacitive+inductive (CL) filter. Thus, the problem of current control falls from a third-order system to a second-order system. Several types of controllers can be designed to implement the outer loop based on the equivalent CL circuit. In this work, a resonant controller with a virtual resistor was applied. Simulations and experimental results are presented to validate the proposal. [ABSTRACT FROM PUBLISHER]

In-circuit frequency response measurement of power converters enables verification of the control loop and periodic measurement during normal operation of the converter. This enables addition of diagnostics and monitoring functions without additional cost. The disturbance injected for frequency response measurement causes output of the converter to be disturbed, which is undesirable during normal converter operation. This paper presents an auto injection control algorithm by which the disturbance amplitude is controlled while maintaining good measurement fidelity. A voltage source inverter is selected as a case study to test the proposed algorithm, in which both current and voltage loop are verified using in-circuit frequency response analysis measurement. [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]

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]

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]

This paper presents two new implementations of finite control set model predictive control (FCS-MPC) methods applied to ac–dc converters with an inductive.capacitive.inductive (LCL) filter. The LCL filter, despite its advantages, can cause a strong resonance in the grid current and also pose a substantially more complex control problem. The new solutions improve the FCS-MPC with an active damping algorithm by eliminating low-order grid current harmonics and decreasing sensitivity to grid voltage distortion. The new methods, i.e., PCi1i2uc and PCi1i2uc-2steps propose multivariable approaches using converter-side current, line-side current, and capacitor voltage. Another improvement involves extending the prediction horizon in PCi1 i2uc-2steps. Both methods have been tested and compared in steady and transient states as well as under grid voltage disturbances. The methods were also compared with a predictive control algorithm with active damping. The simulation results and experimental measurements validate the developed control schemes and show high quality of grid current (low THDi value), high dynamic performance, and immunity under distorted grid voltage conditions. [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]

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]

This paper considers the operation of a load commutated inverter (LCI) fed synchronous machine when the stator voltage is too low to ensure commutation of the machine side thyristors. Under such low-voltage conditions, the LCI is operated in a so-called pulse mode, accompanied by higher drive torque harmonics than during standard operation. Following a theoretical analysis, an alternative pulse mode is proposed, describing how to operate the LCI during pulse mode in order to keep the excitation of the natural frequencies of the drive shaft at a minimum. [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 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]

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]

Conventional droop control methods cannot achieve accurate proportional reactive power sharing, due to the mismatch of components, system disturbances, etc. In this paper, an uncertainty and disturbance estimator (UDE)-based robust droop controller is proposed to address these problems. As a result, the model nonlinearity and uncertainty (e.g., power angle and uncertain output impedance) and system disturbances (e.g., variations of output impedance, load change, and fluctuating dc-link voltage) can be estimated and compensated. Experimental results from a single-phase system with two inverters are provided to demonstrate the effectiveness of the proposed strategy. In order to further demonstrate the advantage and flexibility of the proposed UDE-based control strategy, simulation results from a three-phase system with two inverters are presented with comparison to a robust droop control strategy reported. [ABSTRACT FROM PUBLISHER]

Large transient inrush fault current in a synchronverter during a short-circuit fault can potentially damage the inverter and cause grid instability. It is therefore important to study the characteristics of and the way to restrain the inrush fault current. This paper investigates the characteristics of the synchronverter inrush fault current and proposes a new control method based on mode switching to protect synchronverters. First, an ideal voltage source inverter instantaneous inrush current calculation method for a grid symmetrical short circuit is discussed. Then, based on the inertia of synchronverter, the approximate calculation of the synchronverter instantaneous inrush current is described. The results show that the synchronverter's inrush current mainly consists of a gradually attenuated periodic component and a dc component when the phased voltage symmetry is lost. In our proposed new control approach, the synchronverter switches to the hysteresis controller to limit the output current quickly while staying connected to the grid, and supporting the grid voltage simultaneously. Simulation and experiment results verify the validity of the theoretical analysis. [ABSTRACT FROM PUBLISHER]

An introduction is presented in which the editors discuss various reports within the issue on topics including a diode-clamped converter, modulation methods for hybrid multilevel converters, and paralleled multi-level inverters and a space-vector-based hysteresis current control.

The article discusses various reports published within the issue, including one by Khoudir Marouani and colleagues on a new space vector pulsewidth modulation (PWM) method for an asymmetrical six-phase induction motor drive with two isolated neutral points and another one by Domenico Casadei and colleagues on general PWM strategy for a seven-phase voltage source inverter.

In this paper, a new general cascaded multilevel inverter using developed H-bridges is proposed. The proposed topology requires a lesser number of dc voltage sources and power switches and consists of lower blocking voltage on switches, which results in decreased complexity and total cost of the inverter. These abilities obtained within comparing the proposed topology with the conventional topologies from aforementioned points of view. Moreover, a new algorithm to determine the magnitude of dc voltage sources is proposed. The performance and functional accuracy of the proposed topology using the new algorithm in generating all voltage levels for a 31-level inverter are confirmed by simulation and experimental results. [ABSTRACT FROM PUBLISHER]

This paper presents a comparison of two modulation methods for the seven-level switched-capacitor (SC) inverter using series/parallel conversion (SCISPC). The SCISPC is a multilevel inverter which has the less number of switching devices than the conventional multilevel inverters. The level-shifted pulsewidth modulation (PWM) (LS-PWM) was applied to the seven-level SCISPC in the previous research. In this paper, the level- and phase-shifted PWM (LPS-PWM) and the LS-PWM are applied to the seven-level SCISPC, and LPS-PWM is compared with LS-PWM for the seven-level SCISPC. The LPS-PWM has both the characteristics of the phase-shifted PWM (PS-PWM) and the LS-PWM. The voltage reduction during the discharging term of the SCs is reduced by means of the shorter discharging period compared to LS-PWM. The voltage ripple of the capacitors is also reduced, which leads to higher power conversion efficiency. The difference of the two modulation methods is confirmed by the theoretical approach, simulation with PSIM, and circuit experiment. [ABSTRACT FROM PUBLISHER]

This paper presents the mathematical and harmonic analyses of output phase voltages of three-phase voltage source inverters which employ a hybrid two-dimensional–threedimensional (2-D–3-D) space vector modulation (SVM) voltage control algorithm. The explicit time-domain representation of the harmonic components in addition to the total harmonic distortion (THD) of the output phase voltages is given in terms of system and control parameters. This paper also investigates the harmonic characteristics and THD performance against the linearity of the modulation region which helps in the harmonic performance and design studies of such inverters employing the hybrid 2-D–3-D SVM. The fundamental component of the output voltages follows the reference with lower THD in the linear modulation region rather than the overmodulation region. Experimental results are used to validate these analyses as well as the effectiveness of the hybrid 2-D–3-D SVM algorithm for generating balanced and low THD phase voltages. [ABSTRACT FROM PUBLISHER]