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Two-stage single-phase inverters have been widely used as they can achieve voltage matching and galvanic isolation between the input and output. Due to the pulsating output power of the downstream inverter, an ac current at twice the output frequency, which is called second harmonic current (SHC), arises in the input side of the downstream inverter. This SHC will penetrate to the front-end dc-dc converter, leading to reduced conversion efficiency. This paper first analyzes the propagation mechanism of the SHC and load transient response of two-stage single-phase inverters from the viewpoint of output impedance. Then, based on the current mode control and load current feed forward, two control methods to achieve low SHC in the front-end dc-dc converter and fast dynamic performance during load transient are proposed in this paper. Finally, a 2-kW two-stage single-phase inverter prototype has been constructed and tested, and the experimental results are provided to verify the effectiveness of the proposed control methods. [ABSTRACT FROM AUTHOR]

This paper describes a variable-speed motion-sensorless control system for permanent-magnet synchronous generator (PMSG) connected to grid via back-to-back inverters for wind energy generation. The grid-side inverter control system employs proportional-integral (PI) current controllers with cross-coupling decoupling and line-voltage feedforward disturbance compensation. Also, a D-module filter is used to robustly extract the line voltage positive sequence followed by a phase-locked-loop (PLL) based observer to estimate the positive-sequence angle for control, including the case of asymmetric voltages and automatic seamless transfer method from grid connected to stand alone and vice versa. In stand-alone mode, a voltage control scheme with selective harmonic compensation is employed. The PMSG motion-sensorless control system uses an active power controller and a PLL-based observer to estimate the rotor position and speed without using the electromotive force (EMF) integration and initial rotor position. The paper investigates and validates the ride-through performance of this proposed system during asymmetric power grid-voltage sags, transition from grid connected to stand alone and vice versa and voltage harmonics compensation. While some of the aforementioned issues have been treated rather individually in previous conference publications of the authors, the present paper integrates them into a comprehensive control system of PMSG. [ABSTRACT FROM AUTHOR]

Microinverters operating into the single-phase grid from solar photovoltaic (PV) panels or other low-voltage sources must buffer the twice-line-frequency variations between the energy sourced by the PV panel and that required for the grid. Moreover, in addition to operating over wide average power ranges, they inherently operate over a wide range of voltage conversion ratios as the line voltage traverses a cycle. These factors make the design of microinverters challenging. This paper presents a multilevel energy buffer and voltage modulator (MEB) that significantly reduces the range of voltage conversion ratios that the dc-ac converter portion of the microinverter must operate over by stepping its effective input voltage in pace with the line voltage. The MEB partially replaces the original bulk input capacitor, and functions as an active energy buffer to reduce the total size of the twice-line-frequency energy buffering capacitance. The small additional loss of the MEB can be compensated by the improved efficiency of the dc-ac converter stage, leading to a higher overall system efficiency. The MEB architecture can be implemented in a variety of manners, allowing different design tradeoffs to be made. A prototype microinverter incorporating an MEB, designed for 27 to 38 V dc input voltage, 230-V rms ac output voltage, and rated for a line cycle average power of 70 W, has been built and tested in a grid-connected mode. It is shown that the MEB can successfully enhance the performance of a single-phase grid-interfaced microinverter by increasing its efficiency and reducing the total size of the twice-line-frequency energy buffering capacitance. [ABSTRACT FROM AUTHOR]

In order to improve the performance of sensorless field-oriented controlled motor drives at low speed, the inverter nonlinearity should be compensated for by an accurate voltage error model. This paper proposes a novel method to automatically obtain the total output voltage delay curve, which is a significant part of the inverter voltage error model. Unlike manual measurements of the output delays in earlier literature works, the proposed automatic measurement method achieves low cost in time and labor. Meanwhile, the compensation accuracy based on the automeasured output voltage delay curve is approaching to the performance of compensation by the manual measurement method, far better than the accuracy of compensation by the standard square-wave model at low speed. Moreover, in contrast to other inverter nonlinearity automeasurement methods, the measured results of the proposed method can be applied to compensate for inverter nonlinearities at different switching frequencies. The effectiveness of this method is validated in a voltage source inverter fed induction motor drive system experimentally. [ABSTRACT FROM AUTHOR]

In this paper, a stationary-frame, single-loop controller for three-phase standalone inverter supplying nonlinear loads has been derived from synchronous reference frame proportional-integral multiloop controller. In this method, load voltage harmonics are eliminated by a single loop and only one control variable. The proposed method has several advantages such as lack of Park transformation and requires tuning only one control parameter. It can also be implemented with less computational burden, less complexity, and easier tuning, compared to conventional proportional-integral multiloop controller. Experimental results with laboratory prototype under linear and nonlinear load conditions validate the proposed method. [ABSTRACT FROM AUTHOR]

The parallel operation of inverters in microgrids is mainly based on the droop method. The conventional voltage droop method consists of adjusting the output voltage frequency and amplitude to achieve autonomous power sharing without control wire interconnections. Nevertheless, the conventional voltage droop method shows several drawbacks, such as complicated inner multiloop feedback control, and most importantly, frequency and voltage deviations. This paper proposes a new control strategy in microgrid applications by drooping the virtual flux instead of the inverter output voltage. First, the relationship between the inverter virtual flux and the active and reactive powers is mathematically obtained. This is used to develop a new flux droop method. In addition, a small-signal model is developed in order to design the main control parameters and study the system dynamics and stability. Furthermore, a direct flux control algorithm is employed to regulate the virtual flux according to the droop controller, which avoids the use of proportional-integral controllers and pulse-width modulation modulators. Both the simulation and experimental results show that the proposed flux droop strategy can achieve active and reactive power sharing with much lower frequency deviation than the conventional voltage droop method, thus highlighting the potential use in microgrid applications. [ABSTRACT FROM AUTHOR]

In this paper, a model-predictive control of medium voltage, grid-tied four-level diode-clamped inverter for use in high-power wind energy conversion systems is presented. The control objectives such as regulation of net dc-link voltage, reactive power generation to meet the grid operator request, dc-link capacitor voltages balancing, and switching frequency minimization are included in the cost function. The latter objective is an important requirement for high-power converters, and this has been achieved without any modifications to the software or hardware configuration. The future behavior of the grid currents and dc-link voltages is predicted for all the possible switching states using the discrete-time model of the inverter, dc-link, inductive filter, and grid. The switching state that minimizes the cost function has been chosen and applied to the inverter directly. During different operating conditions, the switching frequency is regulated between 750 and 850 Hz with the help of a lookup-table-based weighting factor, and thus, the proposed methodology appears as a promising tool to control the high-power grid-tied inverters. The results obtained through the MATLAB simulations on 4 MVA/4000 V system and dSPACE DS1103-based experiments on 5 kW/208 V prototype are in a close relationship and thus validate the proposed control strategy. [ABSTRACT FROM AUTHOR]

This paper proposes a new family of three-switch three-state single-phase Z-source inverters (TSTS-ZSIs) that can be classified into two groups, boost-based TSTS-ZSI and buck-boost-based TSTS-ZSI, according to the step-up circuit. All of the topologies have the merits of buck-boost capability and low voltage stress, and some of them have the feature of dual grounding. In addition, the step-up in dc side and inversion in ac side is completely decoupled in the proposed topologies. In terms of a linear voltage gain, the conventional linear control methods can be used, which makes the control system very simple. The operating principles of the two types of topologies are described, respectively, and the comprehensive comparison between them is provided. Finally, the theoretical analysis and comparison of the proposed topologies are verified by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper presents an improved variable-band hysteresis current controller for a two-level three-phase voltage source inverter (VSI). The controller takes the average voltages of the phase-leg switched outputs as an approximation to the load back-EMF voltages, and uses these results to vary the hysteresis bands so as to maintain constant phase-leg switching frequencies. The switching frequency control process is then further refined by fine tuning the hysteresis band variations to synchronize the zero crossings of the phase-leg current errors with a fixed reference clock so as to achieve a nearest space vector switching sequence, which further ensures that the switched output spectrum has been optimized. Finally, a technique is proposed to replace the third phase-leg current regulator with a fixed-frequency open-loop pulse-width modulator, where its commanded reference is generated from the average switched output voltages of the other two phase legs. This avoids the hazard of the three independent hysteresis current regulators adversely interacting with each other in a conventional system, resulting from an overconstrained control problem with only two degrees of freedom. Additionally, this approach allows the linear modulation range to be increased by adding a common-mode third-harmonic component to the third phase-leg reference command signal. [ABSTRACT FROM AUTHOR]

Nowadays, the majority of the photovoltaic (PV) power sources are connected to the public grid. One of the main connection problems occurs when voltage sags appear in the grid due to short circuits, lightning, etc. International standards regulate the grid connection of PV systems, forcing the source to remain connected during short-time grid-voltage faults. As a consequence, during the voltage sag, the source should operate with increasing converter currents to maintain the injection of the generated power. This abnormal operation may result in nondesired system disconnections due to overcurrents. This paper proposes a controller for a PV three-phase inverter that ensures minimum peak values in the grid-injected currents, as compared with conventional controllers. From the system analysis, a design method is presented in order to set the parameters of the control scheme. Selected experimental results are reported in order to validate the effectiveness of the proposed control. [ABSTRACT FROM PUBLISHER]

Conventional hysteresis current controllers have the disadvantage of a variable switching frequency. This paper presents a novel hysteresis current control method which uses the adjacent current error zero-crossing time to calculate the upper/lower hysteresis bandwidth after the measured half current error period. This strategy makes the algorithm independent of load parameters and thus it can achieve a stable switching frequency. An improved switching scheme has been successfully applied to a single-phase three-level H-bridge voltage source inverter driving a vibration test bench based on the novel hysteresis current. It can select appropriate switching states to overcome dead-time effects without dead-time compensation according to the relation of the current switching state, the current error, and the upper/lower hysteresis bandwidth. Field-programmable gate array has enough computational power to allow multiple sampling within a current error period and logic control power to implement state machine transitions. The simulation and experimental results are given to demonstrate the validity and features of the novel hysteresis current controller with the improved switching scheme. [ABSTRACT FROM AUTHOR]

Inverter-fed motor drive systems are widely used in industrial applications because of their energy efficiency and flexible control of machinery using maintenance-free induction motors. As switching inverters generate fast-switching voltage pulses, the transmission-line effects of the motor cable and motor stator windings become significant, and may lead to a doubling of the supply voltage to the motor and cable, overvoltage inside the motor windings, and an increase in the common-mode current. Such phenomena cause premature failure of the motor and cable insulation. A typical protective measure is the use of a passive filter to reduce the voltage surges and alter the rise time of the voltage pulses at the motor terminals. However, most passive filters have the drawbacks of bulky size and high power loss. This paper presents an active terminal filter that can perform the same functions as the passive filters with low power dissipation. An experimental filter has been built and evaluated on a 1-hp three-phase motor drive system with different motor cable lengths. A comparative study into the performance among the commonly used RLC filter and RC filter, and the proposed filter is provided. [ABSTRACT FROM AUTHOR]

An energy system powered by a low-voltage fuel cell (FC) is conditioned to generate higher--voltage regulated ac voltage. The power conditioning system typically requires two power stages: a boost stage and an inversion stage. In this paper, the buck--boost inverter topology that achieves both boosting and inversion functions in a single stage is used as a building block to develop a single-phase FC-based energy system that offers high conversion efficiency, low-cost and compactness. The proposed system incorporates a backup energy storage unit to support the slow dynamics of the FC. The single-phase buck--boost inverter is voltage-mode controlled and the backup unit is current-mode controlled. The low-frequency current ripple is supplied by the backup unit that minimizes the detrimental effects of such ripple being drawn directly from the FC itself. Experimental results from a 1-kW prototype operating at 20 kHz are presented to validate the analysis and performance of the proposed system. [ABSTRACT FROM AUTHOR]

Renewable power systems as distributed generation units often experience big changes in the inverter input voltage due to fluctuations of energy resources. Often, a front-end boost converter is added to step up the dc voltage when the energy resources are at a weak point. However, when a very high boost gain is demanded, the duty cycle may come to its extreme, and large duty cycles causes serious reverse-recovery problems. This paper proposes a novel single-stage boost-type inverter with coupled inductor. By introducing impedance network, including coupled inductor, into the three-phase bridge inverter, and adjusting the previously forbidden shoot-through zero state, the converter can realize a high boost gain and output a stable ac voltage. The single-stage operation of the converter may lead to improved reliability and higher efficiency. Theoretical analysis, simulation, and experimental results are presented to verify good performance. [ABSTRACT FROM AUTHOR]

The main drawback of line-interactive and passive standby uninterruptible power supplies (UPS) is the transfer time when changing from normal mode of operation to battery mode of operation. During this time, the critical loads protected by the UPS are under the influence of the grid disturbance that forced the mode change. On the other hand, equipment protected by a double-conversion UPS is never affected by disturbance in the mains. Unfortunately, the efficiency of this topology is lower and its size and cost are considerably higher. Hence, the elimination of the transfer process between modes in line-interactive and passive standby UPSs would make them more competitive than double-conversion UPSs in many situations. In this paper, a method for eliminating the transfer time is proposed. It is based on a half-bridge inverter with a hysteretic controller whose output voltage is only applied to the load during the transfer time. Considering the random nature of the grid failures and the short duration of the transfer time, the design and selection of the components are nothing but standard. This also leads to a nonstandard hysteretic controller that needs to be fully analyzed. Experimental results are provided for a 750-W line-interactive UPS whose transfer time of 4 ms was eliminated by the proposed system. [ABSTRACT FROM AUTHOR]

This paper proposes an indirect current control algorithm for seamless transfer of three-phase utility-interactive voltage source inverters. With the proposed method the inverter is able to provide critical loads with a stable and seamless voltage during the whole transition period including both clearing time and control mode change. The LCL filter design which is suitable for the indirect current control is also proposed to meet the harmonic limits. The proposed control method is validated through simulation and experiment. [ABSTRACT FROM AUTHOR]

Grid-interfacing converter systems with enhanced voltage quality are proposed for microgrid applications in this paper. By adapting the conventional series-parallel structure, a group of grid-interfacing system topologies are proposed for the purpose of interfacing local generation/microgrid to the grid, or interconnecting microgrids. The functionality of the proposed systems is also reconfigured in order to ease the control design and to improve overall system performance, differing from existing series-parallel structure-based systems. Experiments with a concrete laboratory system are given to detail the proposed concepts and to demonstrate the practical implementations. Two three-phase four-leg inverters, together with dc microsources and nonlinear loads, are employed to construct a general series-parallel grid-interfacing system. Through the introduction of multilevel control objectives, it is illustrated that the proposed system could ride through voltage disturbances and continue the power transfer between the local generation and the grid, while a high-quality voltage is maintained for the local loads. The system also shows the possibility to achieve auxiliary functions such as voltage unbalance correction and harmonic current compensation. The main design aspects of the controllers are specified, and the entire system is effectively validated on a laboratory setup. [ABSTRACT FROM AUTHOR]