Search

Neutral-point-clamped (NPC) inverters are the most widely used topology of multilevel inverters in high-power applications (several megawatts). This paper presents in a very simple way the basic operation and the most used modulation and control techniques developed to date. Special attention is paid to the loss distribution in semiconductors, and an active NPC inverter is presented to overcome this problem. This paper discusses the main fields of application and presents some technological problems such as capacitor balance and losses. [ABSTRACT FROM AUTHOR]

In this paper, the authors propose a mathematical model for a new topology called "stacked multicell converter" (SMC). Each phase of the SMC n x m multilevel inverter is formed by a stack of m flying-capacitor multilevel inverters, and each stack or stage is realized by connecting in series n controllable commutation cells. An original multicarrier subharmonic pulsewidth modulation (PWM), called disposition band carrier and phase-shifted carrier PWM (DBC-PSC-PWM), method is developed to produce (n x m + 1) output voltage levels and to improve the output voltage harmonic spectrum with a wide output frequency range. A diagram state machine is then used to decode the DBC-PSC-PWM modulator and distribute the commutations evenly to each inverter cell in a cyclical fashion. To carry out, in practice, the SMC n x m modulation technique, the implementation of the modulation control strategy has been done in a field-programmable gate array circuit XC4010E+ of XILINX to control a three-phase SMC 3 x 2 seven-level inverter, and the experimental results are carried out to confirm the high performance of this inverter. [ABSTRACT FROM AUTHOR]

The time integral of the output voltage vector of a three-phase inverter is often termed the inverter flux vector. This paper addresses the control of a three-phase four-leg sine-wave output inverter having an LC filter at its output, by controlling the flux vector in three dimensions. Flux vector control has the property that an output filter resonance is actively damped by an output voltage control loop alone. Furthermore, an inverter switching action inherently regulates the output voltage rapidly against dc-bus voltage variations. The flux vector control of sine-wave output inverters finds several applications in three-phase four-wire systems. This paper presents a flux modulation method for threephase four-leg inverters feeding unbalanced and nonlinear loads. All the necessary steps for the digital implementation of the flux modulator are presented. The switching behavior of the modulator has been evaluated, which is useful for the variable fundamental frequency applications of the inverters. To provide experimental validation, the modulator is implemented as a part of the control system for a stand-alone three-phase four-leg inverter with an LC filter at its output. Control system details are also provided. Experimental results indicate the effectiveness of the modulator and control system in providing balanced voltages at the output of the LC filter even under highly unbalanced conditions with nonlinear loads. The resonance damping and voltage regulation properties of the modulator are also apparent from the experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a passivity-based control (PBC) strategy for single-phase pulsewidth-modulated current-source inverters feeding a resistive load. In this strategy, it is required to estimate the load resistance and the inductor-current reference. However, the estimation accuracy depends on the inductance value when the load resistance and the inductor-current reference are estimated using an existing adaptive algorithm. In this paper, it is shown that the inductor-current reference estimation is possible at the expense of a constant reference function for the inductor current. In addition, the design of inductor and the sensitivity of the output voltage to the value of output capacitor used in the controller are studied extensively through the steady-state analysis of the proposed PBC. It is also shown that even very large values of the estimation error do not give rise to significant change in the output-voltage amplitude. Computer simulations and experiments are presented to verify the theoretical considerations. [ABSTRACT FROM AUTHOR]

This paper presents a technique for predicting phase currents in vector-controlled pulsewidth-modulated (PWM) inverters using single dc-link current sensor. There is a limitation for current measurements in the inverters. The measurements are executed in the effective voltages, not in the zero voltage. The current measured in the dc-link and then reconstructed to the phase value is used as a feedback value for controlling an ac machine. Here, there is a difference between the reconstructed cur- rent and measured currents at the top or bottom of PWM carrier such as in the conventional inverters. It changes according to the reference voltages, load conditions, noises, and disturbances. Next, it can cause deterioration of the performance of current control by increasing ripples in currents and torque. This paper analyzes the difference in the drives of a permanent magnet synchronous motor and proposes a method for reducing it. The proposed method predicts the currents measured at the fixed sampling points from those measured in the dc-link and the PWM patterns of voltage outputs. It can reduce current ripples and the experimental results verify it. It is applicable to the vapor compression refrigeration process with the low-cost and high-performance feature. [ABSTRACT FROM AUTHOR]

In double-stage grid-connected photovoltaic (PV) in. verters, the dynamic interactions among the dc/dc and dc/ac stages and the maximum power point tracking (MPPT) controller may reduce the system performances. In this paper, the detrimental effects, particularly in terms of system efficiency and MPPT performances, of the oscillations of the PV array voltage, taking place at the second harmonic of the grid frequency are evidenced. The use of a proper compensation network acting on the error signal between a reference signal provided by the MPPT controller and a signal that is proportional to the PV array voltage is proposed. The guidelines for the proper joint design of the compensation network (which is able to cancel out the PV voltage oscillations) and of the main MPPT parameters are provided in this paper. Simulation results and experimental measurements confirm the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

The boost inverter is a device that is able to generate a sinusoidal voltage with an amplitude larger than the input voltage. Based on the idea of indirectly controlling the output voltage through the inductor current, a dynamical sliding-mode controller for the boost inverter is proposed in this paper. Unlike the usual approach of generating a sinusoidal voltage in both capacitors of the boost inverter, the strategy proposed in this paper focuses on generating a sinusoidal voltage on the load despite the voltage form of both capacitors. A consequence of doing so is that only the desired output voltage is required as reference to implement the controller. Furthermore, it has a fast response, is robust under load and input voltage variations, and yet, is remarkably simple to implement. Although it is strongly nonlinear, it can be implemented using standard electronics circuitry and only needs voltage measurements. [ABSTRACT FROM AUTHOR]

This paper presents a novel soft-switching half-bridge dc-dc converter with high-frequency link. The newly proposed soft-switching dc-dc converter consists of a single-ended half-bridge inverter controlled by an asymmetrical pulsewidth-modulation scheme and a center-tapped diode rectifier. In order to attain the wide range of soft commutation under constant switching frequency, the single active edge-resonant snubber cell composed of a lossless inductor and a switched capacitor is employed for the half-bridge inverter leg, providing and assisting zero-current-switching operations in the switching power devices. The practical effectiveness of the proposed soft-switching dc-dc converter is demonstrated by the experimental results from an 800 W-55 kHz prototype. In addition, the feasibility of the dc-dc converter topology is proved from the viewpoints of the high efficiency and high power density. [ABSTRACT FROM AUTHOR]

This paper presents the results of an investigation of the multilevel multicell converters with a passive RLC balancing circuit applied for the maintenance of the voltage sharing on the capacitive sources of the converters. The topologies of dc/dc, ac/ac, dc/ac, and ac/dc multicell converters were analyzed. For the purpose of multilevel modulation, a multicell converter employs several capacitors that are connected to every cell--the flying capacitors, charged to a given level of voltage. An inadequate relation between the voltages across the flying capacitors (the unbalance state) results in the increase of the voltages above their rated values across the switches. An RLC series circuit (a balancing circuit) with the properly selected resonance frequency is connected in parallel to the load in order to eliminate the unbalance. The balancing process with the use of passive RLC depends on the configuration and parameters of the balancing circuit, the parameters of the converter, as well as on the operating conditions. This paper presents the mathematical description of both the converter and the balancing process, the balancing circuit approach in the different topologies of the multicell converters, a selection of the balancing circuit parameters, and the analysis of the improper control conditions. [ABSTRACT FROM AUTHOR]

This paper focuses on direct torque control (DTC) for three-phase ac electric drives. A novel model predictive control scheme is proposed that keeps the motor torque, the stator flux, and (if present) the inverter's neutral point potential within given hysteresis bounds while minimizing the switching frequency of the inverter. Based on an internal model of the drive, the controller predicts several future switch transitions, extrapolates the output trajectories, and chooses the sequence of inverter switch positions (voltage vectors) that minimizes the switching frequency. The advantages of the proposed controller are twofold. First, as underlined by the experimental results in the second part of this paper, it yields a superior performance with respect to the industrial state of the art. Specifically, the switching frequency is reduced by up to 50% while the torque and flux are kept more accurately within their bounds. Moreover, the fast dynamic torque response is inherited from standard DTC. Second, the scheme is applicable to a large class of (three-phase) ac electric machines driven by inverters. [ABSTRACT FROM AUTHOR]

This paper proposes a multilevel inverter configuration which produces a hexagonal voltage space vector structure in the lower modulation region and a 12-sided polygonal space vector structure in the overmodulation region. A conventional multilevel inverter produces 6n ± 1 (n = odd) harmonics in the phase voltage during overmodulation and in the extreme square-wave mode of operation. However, this inverter produces a 12-sided polygonal space vector location, leading to the elimination of 6n ± 1 (n = odd) harmonics in the overmodulation region extending to a final 12-step mode of operation with a smooth transition. the benefits of this arrangement are lower losses and reduced torque pulsation in an induction motor drive fed from this converter at higher modulation indexes. The inverter is fabricated by using three conventional cascaded two-level inverters with asymmetric dc-bus voltages. A comparative simulation study of the harmonic distortion in the phase voltage and associated losses in conventional multilevel inverters and that of the proposed inverter is presented in this paper. Experimental validation on a prototype shows that the proposed converter is suitable for high-power applications because of low harmonic distortion and low losses. [ABSTRACT FROM AUTHOR]

This paper proposes a generalized method for the generation of space vector pulsewidth modulation (SVPWM) signals for multilevel inverters. In the proposed method, the actual sector containing the tip of the reference space vector need not be identified. A method is presented to identify the center of a subhexagon containing the reference space vector. Using the center of the subhexagon, the reference space vector is mapped to the innermost subhexagon, and the switching sequence corresponding to a two-level inverter is determined. A new technique is proposed in this paper, by which these two-level vectors are translated to the switching vectors of the multilevel inverter by adding the center of the subhexagon to the two-level vectors. The proposed method can be extended to any n-level inverter, and a generalized algorithm is proposed. The scheme is explained for a five-level inverter, and experimental results are presented for a three-level inverter. [ABSTRACT FROM AUTHOR]

An improved division–summation (D–Σ) digital control strategy without high-speed communication is proposed in this paper. Unlike the traditional zero-sequence circulating current control method, the interphase circulating current suppression method is discussed and implemented on the basis of the single-phase equivalent circuit in this strategy. The generation mechanism of the interphase circulating current is emphatically analyzed. Output voltage compensation and virtual impedance compensation are studied and integrated with the D–Σ digital control method to suppress the circulating current. Experimental platform with three paralleled three-level T-type grid-connected inverters has been set up. The experiments are carried out under the conditions of the impedance match or mismatch among the inverters, the unbalanced grid voltages, and the dynamic process such as start-up and step current change. The experimental results verify that the improved D–Σ digital control strategy may effectively restrain the circulating current under various conditions. [ABSTRACT FROM AUTHOR]

This paper presents a new four-quadrant (4Q) boost-type converter with the use of active virtual ground (AVG) technology. The presented topology can step up the ac grid voltage to a regulated dc voltage under a stable bidirectional current flow and support the power transmission in either real power or reactive power delivery. With the use of the proposed modulation method, only two high-frequency switches are required through the 4Q operation. Under a full-bridge converter structure, an efficient system can be guaranteed. Also, benefiting from the AVG technology, an LCL filter is formed at the system input over the 4Q operation. Both magnitude of leakage current and grid current ripple are also minimized to a small value. Thus, a high-efficiency and low-noise 4Q converter is guaranteed. The presented topology is successfully implemented on a 750 VA prototype and the performance is experimentally verified on it, which shows good agreement with the theoretical knowledge. [ABSTRACT FROM AUTHOR]

In $LCL$ -type grid-tied inverters, the lumped disturbance, including parameter uncertainties, unmodeled dynamics, and grid harmonics, will deteriorate the current tracking performance and lead to high total harmonic distortion (THD). The uncertainty and disturbance estimator (UDE) scheme provides an effective way to attenuate the lumped disturbance. However, UDE for the current control in $LCL$ -type grid-tied inverters is far from perfect. In this paper, a discrete-time modified UDE (MUDE) scheme is proposed to improve the current tracking performance and robustness. In MUDE, a reduced-order model of the $LCL$ filter, combined with an active damping scheme, is proposed. The stability performance is discussed in detail, and a rigorous stability condition is proposed for the first time. Moreover, the analysis of the controller sampling frequency and grid impedance, and their influences on the system stability is presented. A design case based on the stability condition is also given. Comparative experiments are conducted to verify the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Input-series-output-parallel (ISOP) inverter system is very suitable for high dc input voltage and large ac output current power conversion applications. If this topology is applied to the grid-connected occasion, a bulky-capacity grid-connected inverter can be built with multiple small-capacity standardized inverter modules, which can significantly improve the reliability of the system operation and facilitate system design. In this paper, we investigate an ISOP LCL-type grid-connected inverter system, and put forward an optimized way for the topology of combined LCL filters first. Then, the inverter-side inductor current is carefully chosen as the controlled variable and thus a concise distributed control method is proposed to achieve multiple control objectives. On the basis of the presented distributed control, the redundant control technique is also provided to heighten the reliability of the grid-tied ISOP inverter indeed. Finally, a three-module prototype is built and the experimental results validate the effectiveness of the raised strategy. [ABSTRACT FROM AUTHOR]

Failure of the commutation process is a serious malfunction in line-commutated high-voltage direct current (HVdc) converters, which mainly occurs due to inverter ac faults and may lead to outage of the HVdc system. In this paper, an improved strategy is developed that functions based on the SIEMENS HVdc control system under normal conditions and switches to a designed commutation failure inhibition module (CFIM) during an inverter ac fault. From the response speed point of view, since the designed CFIM does not require any proportional-integral controller, the inverter control system has a quick performance in prevention of the commutation failure. This is achieved by direct measurement of the overlap area using the waveforms of the valves anode–cathode and commuting voltages. In addition, from the accuracy aspect, the proposed method has a superior performance in comparison with the existing strategies. It is because of the fact that by direct measurement of the overlap area, variations of both direct current and the commutation inductance are considered, and hence, the unnecessary increase of the inverter reactive power consumption during the fault and the repetitive commutation failures are prevented. The practical performance and feasibility of the proposed strategy is validated through the laboratory testing, using the real-time Opal-RT hardware prototyping platform. The experimental results demonstrate that the proposed strategy can effectively inhibit the commutation failure or repetitive commutation failures under different fault types by considering the lowest possible reactive power consumption. [ABSTRACT FROM AUTHOR]

The accuracy of a power-hardware-in-loop (PHIL) simulation is influenced by factors such as dynamics of the power amplifier (PA) and discretization of the real-time simulated part of the system. An open-loop voltage-source inverter (VSI) without an output filter is demonstrated to be a good choice for power amplification in terms of cost, size, design effort, bandwidth, and accuracy when the load on the PA is significantly inductive. An open-loop-VSI-based PA is shown to be accurate in emulating a synchronous generator, including the fast transients in the excitation control system. The discretization effects of the real-time simulator are captured effectively by the discrete-time (DT) modeling approach proposed in this paper. The DT model is shown to replicate fast transients in the PHIL simulation better than the existing continuous-time-based model. Stability of the PHIL simulation of a benchmark circuit is analyzed using the proposed DT modeling approach. The stability analysis is validated through simulations and experiments. The stability limits derived based on the proposed analysis are capable of suggesting maximum and minimum values of certain circuit parameters, as required for stability. [ABSTRACT FROM AUTHOR]

This paper introduces a new controller for a three-phase inverter, with a control goal of generating desired three-phase output voltages. The proposed controller employs a circuit-level decoupling method, and it is implemented by logic circuitry in combination with a control core and a feedback signal processor. Almost all dc-dc controllers can be adapted as the control core, whereas the feedback signal processor can be implemented by either voltage compensator and/or current compensator. The controller's implementation is simple and flexible with logic and analog circuitry. Yet, it demonstrates experimentally excellent load handling, source voltage noise rejection, and reference tracking ability. The prototype is tested under resistive load, highly nonlinear load, and extreme load transients; aside from that, it is also tested under noisy source voltage and sudden reference change. In all cases, the prototype demonstrated high- quality output voltages. [ABSTRACT FROM AUTHOR]

An open-end winding induction-motor drive with two two-level inverters achieves three-level inversion. The draw- back of this configuration is the presence of a high zero-sequence current, stressing the semiconductor switching devices and the motor. To avoid this, two isolated dc power supplies are needed to feed individual inverters. In this paper, it is shown that it is possible to operate this drive with a single dc power supply, with reasonable engineering compromises. The new decoupled space-vector-based pulsewidth modulation (PWM) strategy proposed in this paper achieves this objective. This PWM strategy exploits the dependence of the zero-sequence voltage on the placement of the zero-vector of individual inverters. It is shown that the zero-sequence voltage of the dual-inverter system is suppressed by forcing the zero-sequence voltage of the individual inverters to a value of zero, in the average sense, in each sampling-time interval. This strategy, therefore, achieves a dynamic balancing of the zero-sequence current. It is also shown that this PWM scheme achieves the center spacing of effective-time period for the dual-inverter drive and its associated advantages. In addition, the effect of the placement of the zero-vector for individual inverters on the dual-inverter drive is investigated, and the experimental results are presented. [ABSTRACT FROM AUTHOR]

A performance analysis of three-phase and dual three-phase (DTP) induction pulsewidth modulation (PWM) inverter-fed motor drives is conducted in this paper. The focus is on the efficiency performance of high-frequency DTP machines compared to their three-phase counterparts in low/medium power applications. For this purpose, a DTP machine, having two sets of stator three-phase windings spatially shifted by 30 electrical degrees (asymmetrical six-phase winding configuration), was tested for both six-phase and three-phase winding configurations under the same magnetic conditions. Simulation and experimental results are presented to evaluate the efficiency performance of three-phase and dual-three induction motor drives employing PWM voltage source inverters. [ABSTRACT FROM AUTHOR]

In the last few years, interest in multiphase converter technology has increased due to the benefits of using more than three phases in drive applications. Besides, multilevel converter technology permits the achievement of high power ratings with voltage limited devices. Multilevel multiphase technology combines the benefits of both technologies, but new modulation techniques must be developed in order to take advantage of multilevel multiphase converters. In this paper, a novel space vector pulsewidth modulation (SVPWM) algorithm for multilevel multiphase voltage source converters is presented. This algorithm is the result of the two main contributions of this paper: the demonstration that a multilevel multiphase modulator can be realized from a two-level multiphase modulator, and the development of a new two-level multiphase SVPWM algorithm. The multiphase SVPWM algorithm presented in this paper can be applied to most multilevel topologies; it has low computational complexity and it is suitable for hardware implementations. Finally, the algorithm was implemented in a low-cost field-programmable gate array and it was tested in a laboratory with a real prototype using a five-level five-phase inverter. [ABSTRACT FROM AUTHOR]

This paper presents power-control strategies of a grid-connected hybrid generation system with versatile power transfer. The hybrid system is the combination of photovoltaic (PV) array, wind turbine, and battery storage via a common dc bus. Versatile power transfer was defined as multimodes of operation, including normal operation without use of battery, power dispatching, and power averaging, which enables grid- or user-friendly operation. A supervisory control regulates power generation of the individual components so as to enable the hybrid system to operate in the proposed modes of operation. The concept and principle of the hybrid system and its control were described. A simple technique using a low-pass filter was introduced for power averaging. A modified hysteresis-control strategy was applied in the battery converter. Modeling and simulations were based on an electromagnetic-transient-analysis program. A 30-kW hybrid inverter and its control system were developed. The simulation and experimental results were presented to evaluate the dynamic performance of the hybrid system under the proposed modes of operation. [ABSTRACT FROM AUTHOR]

This paper investigates certain novel switching sequences involving division of active vector time for space vector-based pulsewidth modulation (PWM) generation for a voltage source inverter. This paper proposes two new sequences, and identifies all possible sequences, which result in the same average switching frequency as conventional space vector PWM (CSVPWM) at a given sampling frequency. This paper brings out a method for designing hybrid PWM techniques involving multiple sequences to reduce line current ripple. The three proposed hybrid PWM techniques (three-zone PWM, five-zone PWM and seven-zone PWM) employ three, five and seven different sequences, respectively, in every sector. Each sequence is employed in a spatial region within the sector where it results in the lowest rms current ripple over the given sampling period. The proposed techniques lead to a significant reduction in THD over CSVPWM at high line voltages. The five-zone technique results in the lowest THD among real-time techniques with uniform sampling, while the seven-zone technique is the best among real-time techniques with twin sampling rates. The superior harmonic performance of the proposed techniques over CSVPWM and existing bus-clamping PWM techniques is established theoretically as well as experimentally. [ABSTRACT FROM AUTHOR]

This paper presents a new low-loss modulation technique for the hybrid three-level four-leg converter. The total losses of the converter are reduced by about 18% on average compared to the standard three-leg neutral-point-clamped converter. Furthermore, the low-frequency voltage oscillation in the neutral point is completely cancelled, and the maximum benefit of the dc-link voltage is obtained. All these facts, together with the fault-tolerant ability due to the fourth leg, make this topology very interesting for applications such as wind generation, in which it is important to maximize efficiency and reliability. Some experimental results confirm the good performance of the proposed modulation technique. [ABSTRACT FROM AUTHOR]

Selective harmonic elimination pulsewidth modulation techniques are some of the control methods used in voltage/current source converters. However, challenges such as the task of finding all the multiple sets of solutions of the switching angles for a given problem may be difficult to deal with. In this paper, a direct minimization of the nonlinear transcendental trigonometric Fourier functions in combination with a random search is discussed. The unipolar (three-level) waveform is used to illustrate the proposed method confirming its ability to find multiple sets of solutions, including a case where 51 angles are sought for single- and three-phase applications. A simple harmonic distortion factor is studied for each set of solutions to assess their performance against the noneliminated harmonics. The results presented both at theoretical and experimental level are in close agreement and confirm the robustness of the proposed approach. [ABSTRACT FROM AUTHOR]

Multilevel converters are a very interesting alternative for medium and high power drives. One of the more flexible topologies of this type is the cascaded multicell converter. This paper proposes the use of a single-phase reduced cell suitable for cascaded multilevel converters. This cell uses a reduced single-phase active rectifier at the input and an H-bridge inverter at the output side. This topology presents a very good performance, effectively controlling the waveform of the input current and of the output voltage and allowing operation in the motoring and re- generative mode. The results presented in this paper confirm that this medium voltage inverter effectively eliminates low frequency input current harmonics at the primary side of the transformer and operates without problems in regenerative mode. [ABSTRACT FROM AUTHOR]

This paper presents a continuous control set model-predictive control with a receding horizon for a three-phase voltage-source inverter with an LCL filter. In this proposal, a reduced model of the converter with an embedded integrator and a Kalman filter are used to obtain the inverter-side currents without oscillation. Then, a cost function makes use of these currents to generate the optimum duty cycles for the space vector modulator. With the proposed method, active damping performances, a zero state error, and a reduction of the computational burden are achieved. Compared to the finite control set model-predictive control, the proposed method operates with a fixed switching frequency. Simulations and experimental results show that this proposal works correctly even in the case of grid harmonics and voltage sags. [ABSTRACT FROM AUTHOR]

Two-stage single-phase inverter system has found many applications in distributed generators and grid-connected systems. However, the existence of double-line frequency ripple on the dc-bus voltage due to single-phase inverter's operation has given rise to some design and reliability issues. Some of these are the degradation in the quality of inverter's output voltage and shortening of dc-bus capacitor's lifetime. The problem is rooted in the controller design of the front-end dc–dc converter, which is often designed based on a simplified mathematical model of the dc–dc converter that fails to account for some complex phenomena, such as system's nonlinearities and circuit parameter variations due to temperature and aging effects. In this paper, a disturbance-observer-based control is proposed where these unmodeled effects are treated in the form of lumped disturbance and compensated for in a feed-forward manner. The major advantage of this control solution is that only minimal plant information is needed for controller design and that a decoupled composite controller can be realized, where the feedback compensator is designed to meet certain stability criteria while the disturbance-observer-based feed-forward compensator is designed to handle fast transient events. A prototype of a dual-active-bridge (DAB) dc–dc converter implemented with disturbance-observer-based control is constructed to verify its effectiveness. Experimental results show that the dynamic response of the DAB converter is significantly improved and the double-line frequency ripple on the dc-bus voltage is effectively suppressed by the proposed control solution. [ABSTRACT FROM AUTHOR]

With different structures and control strategies, inverter-interfaced renewable energy generators (IIREGs) have different fault characteristics from synchronous generators. This makes the conventional distance protection used in networks with synchronous generators not applicable for transmission lines emanating from IIREGs. Therefore, supported by the fault current analysis, operating performances of distance relays on both sides of the transmission line are unveiled. It reveals the problem that the conventional distance relay on the IIREG side has a high risk of malfunction or refusing to operate. To cope with this adaptability problem, an improved scheme based on time delay and zero-sequence impedance is proposed in this paper. To validate the operating performances of the scheme, a detailed IIREG model is built in a real-time digital simulator, and simulation tests are carried out. Apart from these, a field short-circuit test is performed in a real wind power plant to examine the practical feasibility of the proposed scheme. Both the simulation results and the field test confirm the problem of the conventional distance protection and verify the reliability of the improved scheme. [ABSTRACT FROM AUTHOR]

The main idea developed in this paper is a novel biconverter structure to supply a doubly fed induction machine (DFIM). Two voltage source inverters (VSIs) feed the stator and rotor windings. The outputs of the two VSIs are combined electro-mechanically in the machine, and as a result, novel features can be obtained. For example, for high power drive applications, this configuration uses two inverters dimensioned for a half of the DFIM power. A new dual direct torque control is developed with flux model of DFIM. Two switching tables linked to VSI are defined for stator and rotor flux vector control. The satisfactory experimental and simulation results are shown, and they confirm good dynamic behavior in four quadrants of the speed-torque plane. Moreover, experimental results show the correct flux vector control behavior and speed tracking performances. [ABSTRACT FROM AUTHOR]

A pulsewidth-modulated (PWM) voltage applied to a squirrel-cage induction motor (SQIM) can cause high bearing currents, heating of rotor shaft, voltage spike across the motor terminals, etc. Filtering of this PWM voltage to obtain a sinusoidal output voltage can be a solution to this problem. However, a passive L-C filter makes the dynamic performance of the drive poor for high-performance control application. In this paper, a feed-forward control strategy for the L-C filter is proposed to have a good bandwidth for the filter output voltage. This filter control strategy is introduced along with a sensorless vector control strategy for the, SQIM drive. This complete strategy retains the high dynamic performance of the drive even with the L-C filter. In this paper, a three-level converter is used as a voltage source inverter for the drive to have a less filter-size requirement. The control strategy is verified on a 7.5-hp SQIM drive with a three-level insulated-gate bipolar-transistor inverter and L-C filter. Experimental results validate the high dynamic performance of the drive with filter. [ABSTRACT FROM AUTHOR]

This paper proposes a novel synthesis methodology to generate integrated zero-voltage-transition (ZVT) topologies, as well as to systematize the topologies that are already known. An approach is presented to classify single-pole ZVT converters. This approach can be expanded to multipole systems, such as polyphase inverters and rectifiers, uninterruptible power supplies, and variable-speed drives. The connections among several presented topologies are easily seen by means of the general diagrams that are shown in this paper. Analysis and understanding of the advantages, disadvantages, merits, and limitations of each converter are made easier since these converters are obtained from a common approach that is presented in this paper. [ABSTRACT FROM AUTHOR]

This paper analyzes a recently introduced two-motor five-phase drive system with series connection of stator windings. It has been shown, using physical reasoning, that the introduction of an appropriate phase transposition in the series connection of two machines leads to a complete decoupling of the flux/torque-producing currents of one machine from the flux/torque-producing currents of the second machine. Consequently, independent vector control of the two machines becomes possible while using a single current-controlled five-phase voltage source inverter as the supply. The drive system modeling and control are first elaborated in this paper by taking both machines as induction motors. It is shown, using rigorous mathematical derivations, that the independent control of the two machines results due to the placement of machines in two different subspaces of the five-dimensional space. This is enabled by phase transposition in series connection. The models of the complete drive in the stationary common reference frame and in the rotor-flux-oriented reference frames of the two machines are developed. An associated vector control scheme for the two-motor drive is presented next. The second part of this paper describes an experimental setup used further on to evaluate the dynamic behavior of the two-motor drive. Performance is investigated by extensive experimentation for various transients (acceleration, deceleration, reversing, and disturbance rejection). Excellent decoupling of control of the two machines is achieved. Both the concept of the drive and the approach to modeling and control are thus fully verified experimentally. Finally, the advantages and shortcomings of the series- connected five-phase two-motor drive are discussed, and potential application areas are highlighted. [ABSTRACT FROM AUTHOR]

This paper presents a numerical analysis for the Class DE inverter with eliminated driving pulses. The harmonics, low-frequency components, and power conversion efficiency are calculated and shown. The Class DE inverter is one of the high-frequency high-efficiency resonant power sources that has the hybrid features of Class D and Class E circuits. To regulate the output power while maintaining high efficiency, various methods have been proposed. Thinned-out method is one of them, which regulates the output power by eliminating the driving pulses. The principle is quite simple and is seen suitable for digital control. However, the harmonics, low-frequency components, and discontinuous regulation have been pointed out. An aim of this paper is to clarify the characteristic of the Class DE inverter with thinned-out driving patterns. Basic models with three different Q-factors of 5, 10, and 20 are designed, built, tested, and analyzed. In a series of 16 cycles of switching pulses, 1-15 cycles are eliminated. The calculated waveforms show good agreement with circuit experiments. Various waveforms, harmonics, total harmonic distortion, low-frequency components, their total, and harmonics of the lowest frequency component are calculated and discussed. [ABSTRACT FROM AUTHOR]

This paper investigates the new Class EF2 inverter and the already known Class E and E/F3 inverters. The purpose of this paper is to find the answer to the question: what should be added to a classic Class E inverter to improve its performance with minimum expenditure? Class EF2 and E/F3 inverters meet this requirement through operating under zero-voltage switching and zero-voltage slope switching conditions (as in Class E) with improved transistor waveforms (as in Class F and 1/F). Selected results of a numerical analysis of Class E, EF2, and E/F3 inverters are shown and discussed. The analysis is aimed at maximizing the power output capabilities of these inverters for soft-switching conditions and fixed voltage and current stresses on their transistors as constraints. Measurement results of the Class EF2 and E inverters are given to verify the validity of the presented theory. The measured output power of the inverters was 526 and 366 W with the total efficiency of 97.1% and 96.6% at 1 MHz, respectively. [ABSTRACT FROM AUTHOR]

Fuel cells probably represent the power-production technology receiving the most development attention. The objective of this paper is to regulate the output voltage of inverters for fuel-cell power plants. This lettter describes an effective fuzzy-logic (FL) flux control. The paper demonstrates that the proposed method provides an improved response compared to the conventional flux-control method. [ABSTRACT FROM AUTHOR]

New and stronger power quality requirements are issued due to the increased amount of photovoltaic (PV) installations. In this paper different methods are used for continuous grid monitoring in PV inverters. By injecting a noncharacteristic harmonic current and measuring the grid voltage response it is possible to evaluate the grid impedance directly by the PV inverter, providing a fast and low-cost implementation. This principle theoretically provides an accurate result of the grid impedance but when using it in the context of PV integration, different implementation issues strongly affect the quality of the results. This paper also presents a new impedance estimation method including typical implementation problems encountered, and it also presents adopted solutions for online grid impedance measurement. Practical tests on an existing PV inverter validate the chosen solution. [ABSTRACT FROM AUTHOR]

This paper presents a flyback inverter controlled by sensorless current maximum power point tracking (MPPT) for a small photovoltaic (PV) power system. Although the proposed system has small output power such as 300 W, a few sets of small PV power systems can he easily connected in parallel to yield higher output power. When a PV power system is constructs with a number of small power systems, the total system cost will in- crease and will be a matter of concern. To overcome this difficulty, this paper proposes a PV system that uses no expensive dc current sensor but utilizes the method of estimating the PV current from the PV voltage. The paper shows that the application of this novel sensorless current flyback inverter to an MPPT-operated PV system exhibits satisfactory MPPT performance similar to the one exhibited by the system with a dc current sensor as well. This paper also deals with the design method and the operation of the unique flyback inverter with center-tapped secondary winding. [ABSTRACT FROM AUTHOR]

A novel single-stage series-resonant buck-boost inverter (SRBBI) is proposed in this paper. The main attribute of the novel inverter topology is the fact that it generates an ac output voltage larger or lower than the dc input one, depending on the instantaneous duty cycle. This property is not found in the classical voltage-source inverter, which produces an ac output instantaneous voltage which always lower than the dc input voltage. The state-space averaging approach is used to estimate and examine the steady-state and dynamic character of the new single-stage SRBBI. The principle of operation, theoretical analysis, and experimental results of the proposed inverter, rated 500 W and operating at 40 kHz, are provided in this paper to verify the performance of the SRBBI. [ABSTRACT FROM AUTHOR]

This paper presents analytical techniques for the determination of the expressions for the modulation signals used in the carrier-based sinusoidal and generalized discontinuous pulse-width modulation schemes for two-level, three-phase voltage source inverters. The proposed modulation schemes are applicable to inverters generating balanced or unbalanced phase voltages. Some results presented in this paper analytically generalize the several expressions for the modulation signals already reported in the literature and new ones are set forth for generating unbalanced three-phase voltages. Confirmatory experimental and simulation results are provided to illustrate the analyses. [ABSTRACT FROM AUTHOR]

This paper proposes a snubber circuit for a flying Capacitor multilevel inverter and converter. It also explains the concept of constructing a snubber circuit for a multilevel inverter and converter. The proposed snubber circuit makes use of an Undeland snubber as a basic snubber unit, and thus can be regarded as a generalized Undeland snubber for a flying capacitor multilevel inverter and converter. It has such an advantage of Undeland snubber used in the two-level inverter. compared with a conventional RLD/RCD snubber for multilevel inverter and converter, the proposed snubber keeps such good features as fewer number of components, reduction of voltage stress of main switching devices due to low overvoltage, and improved efficiency of system due to low snubber loss. In this paper, the proposed snubber is applied to a three-level flying capacitor inverter, and its features are in detail demonstrated by computer simulation and experimental result. [ABSTRACT FROM AUTHOR]

The second harmonic pulsating power flows through the dc side of a single-phase quasi-Z-source inverter (qZSI), which requires bulky capacitor banks and inductors to suppress low-frequency ripple of dc-link voltage and inductor currents in the passive ripple reduction way. However, the resultant huge qZS network seriously deteriorates the system reliability, efficiency, volume, weight, and cost. This paper proposes an active-filter-integrated single-phase qZSI to transfer low-frequency power ripple directly from ac load to active filter's ac capacitor, so that low-frequency power ripple does not present in dc side anymore and constant inductor currents and constant capacitor voltages are ensured. Thus, much small qZS impedance is employed to only smooth high-frequency ripple and active filter's capacitor supports ac voltage (large ripple allowed) with small capacitance. The operation principle, parameter design method, and modeling and control strategy of the proposed topology are investigated. Comparative evaluation, simulation, and experimental results verify the proposed new topology system. [ABSTRACT FROM PUBLISHER]

For cascaded multilevel inverter topologies with a single dc supply, closed-loop capacitor voltage control is necessary for proper operation. This paper presents zero and reduced common-mode voltage (CMV) operation of a hybrid cascaded multilevel inverter with predictive capacitor voltage control. Each phase of the inverter is realized by cascading two- three-level flying capacitor inverters with a half-bridge module in between. For the presented inverter topology, there are redundant switching states for each inverter voltage levels. By using these switching state redundancies, for every sampling instant, a cost function is evaluated based on the predicted capacitor voltages for each phase. The switching state that minimizes cost function is treated as the best and is switched for that sampling instant. The inverter operates with zero CMV for a modulation index upto 86%. For modulation indices from 86% to 96%, the inverter can operate with reduced CMV magnitude (Vdc/18) and reduced CMV switching frequency using the new space-vector pulsewidth modulation (SVPWM) presented herein. As a result, the linear modulation range is increased to 96% as compared to 86% for zero CMV operation. Simulation and experimental results are presented for the inverter topology for various steady state and transient operating conditions by running an induction motor drive with open loop $V/f$ control scheme. [ABSTRACT FROM PUBLISHER]

In this paper, a direct torque control (DTC) using vector evaluation factor table (VEF-Table) is proposed for voltage source inverter (VSI)-permanent magnet synchronous motor (PMSM) drive system. In this approach, the candidate vectors can expand from six active vectors to 3\times N\text {d} vectors with duty cycle, and the change trends of torque and flux generated by all candidate vectors can be quantified and stored in the VEF-Table offline. In order to choose the output vector from the VEF-Table, a “Two-Step” selection algorithm is established based on the cost function, which can select two vectors with duty cycle sequentially. By synthesizing two vectors, the proposed approach is capable of adjusting the magnitude and angle of the output vector, so as to reduce the undesirable ripples of torque and flux. The feasibility and effectiveness of the proposed approach is verified by both numerical simulations and experimental results. [ABSTRACT FROM PUBLISHER]

This paper presents a model predictive control (MPC) scheme for quasi-Z-source (qZS) three-phase four-leg inverter. In order to cope with the drawbacks of traditional voltage source inverters (VSIs), a qZS three-phase four-leg inverter topology is proposed. This topology features a wide range of voltage gain which is suitable for applications in renewable energy-based power systems, where the output of the renewable energy sources varies widely with operating conditions such as wind speed, temperature, and solar irradiation. To improve the capability of the controller, an MPC scheme is used which implements a discrete-time model of the system. The controller handles each phase current independently, which adds flexibility to the system. Simulation and experimental studies verify the performances of the proposed control strategy under balanced and unbalanced load conditions as well as single-phase open-circuit fault condition. [ABSTRACT FROM PUBLISHER]

In this paper, a new predictive control for grid-connected inverters is presented, which provides the best performance in terms of transient response and stability margins when analog filters in current sensing circuits are used. These filters are necessary to avoid aliasing in the A/D conversion process, which causes an important ripple on injected current into the grid and increases the total harmonic distortion. However, predictive controls are very sensitive to delays on the acquisitions, so when such anti-aliasing filters are used a reduction of stability margins is produced and the transient response is affected. The proposed predictive control allows to compensate the effect of the filter, for any order and cutoff frequency, providing the best transient response in the reference tracking and the highest stability margins. Furthermore, a modification on the adaptive strategy of current error is proposed, which allows to work with single-phase grid-connected inverters. The proposed predictive control has been tested on a 3.3-kVA single-phase grid-connected inverter with LCL filter, using different anti-aliasing filters, obtaining in all cases, better results than those offered by other predictive controls recently published. [ABSTRACT FROM PUBLISHER]

In this paper, a new topology for half-bridge switched boost inverter (HB-SBI) is proposed. The proposed half-bridge inverter uses more active elements rather than capacitors and inductors in comparison with the conventional half-bridge Z-source inverter with two Z-networks, which results in reduction of weight, size, and cost. Moreover, the proposed inverter has the capability of eliminating inverter leg short-circuit issues and is able to further increase the output voltage level in comparison with its conventional types. Generating zero-level output voltage is also another advantage of this topology. Based on theoretical calculations, the steady-state and small-signal analysis of the proposed topology in different operating modes are performed and the ripple of inductors’ current and capacitors’ voltage are also calculated. A comprehensive comparison is also made between the proposed inverter and the conventional types. Finally, the experimental results are provided to reconfirm the performance of the proposed inverter. [ABSTRACT FROM PUBLISHER]

Modular multilevel converter (MMC) is a promising new topology for high-voltage applications. The MMC is made of several identical submodules. For proper operation, each submodule can be considered as a controlled voltage source where capacitor’s voltage should be maintained at a certain level. Besides, the minimization of the circulating current, which does not flow to the load, is crucial for achieving stable and efficient operation of the MMC. The interrelations among the load current, circulating current, and capacitor voltages complicate the MMC control. This paper aims to achieve stable and balanced voltage and current control with reduced circulating current in various operating conditions. The proposed control uses weighted model predictive control based on a normalized cost function to select the inverter switching patterns, which control the load current, while minimizing voltage fluctuation and circulating current. The weighting factors were selected based on minimizing the load-current total harmonic distortion (THD) and circulating current. The analysis is conducted on a low-power case study of single-phase four-cells MMC with possible extension to higher number of cells. The low-power three-level prototype is designed and built to validate this proposed method. Theoretical analysis, simulation, and experimental results are presented and compared. Parameter sensitivity analysis was also conducted. They all confirm the effectiveness of the proposed control method. [ABSTRACT FROM PUBLISHER]

The cost reduction of power-electronic devices, the increase in their reliability, efficiency, and power capability, and lower development times, together with more demanding application requirements, has driven the development of several new inverter topologies recently introduced in the industry, particularly medium-voltage converters. New more complex inverter topologies and new application fields come along with additional control challenges, such as voltage imbalances, power-quality issues, higher efficiency needs, and fault-tolerant operation, which necessarily requires the parallel development of modulation schemes. Therefore, recently, there have been significant advances in the field of modulation of dc/ac converters, which conceptually has been dominated during the last several decades almost exclusively by classic pulse-width modulation (PWM) methods. This paper aims to concentrate and discuss the latest developments on this exciting technology, to provide insight on where the state-of-the-art stands today, and analyze the trends and challenges driving its future. [ABSTRACT FROM PUBLISHER]