Search

A novel single-phase transformerless dual-mode interleaved multilevel inverter (DMIMI) is proposed in this paper, which can inject a highly sinusoidal ac current to the grid even with the input dc voltage less than the peak grid voltage. Furthermore, dead-beat controllers are developed to directly calculate the optimal duty cycles in a digital control platform. As compared to the traditional dual-mode multilevel approaches, the DMIMI offers high efficiency, less number of components conducting simultaneously in each operation mode, and the capability of reactive power exchange. This paper presents the theoretical analysis and the experimental results obtained from a 1-kW prototype. The experimental results reveal that the proposed DMIMI can achieve high conversion efficiency and low leakage current as well as high quality grid current. [ABSTRACT FROM AUTHOR]

One promising solution for future large wind turbine power conversion is to use medium-voltage power conversion systems for high efficiency and high power-density, e.g., by using a cascaded structure presented in this paper. However, a medium-frequency (MF) oscillation issue was observed while testing the experimental prototype of the above system. Given the complex multivariable coupling models of the multiwinding transformer and multiple inverters, conventional single-variable theories are not suitable. Therefore, this paper presents a new decoupling analysis method. Using a proposed linear transformation matrix, the output currents of multiple inverters can be transformed to a summation (grid) current and difference currents, where the former is the summation of inverter currents and the latter are the differences between currents of any two inverters. This transformation has enabled the use of single-variable theories, e.g., Bode diagram to analyze the summation current and difference current separately. It was found that the MF oscillation issue is caused by the equivalent impedance of the summation current being much larger than that of the difference current. Specifically, using small controller parameters cannot ensure the grid current quality and using large parameters may cause oscillation in the difference current. The proposed method can provide guidelines to design the parameters of current regulators and filters in multiple-inverter systems. Experimental results of a downscaled wind energy conversion system are presented to verify the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes an integrated method of three-phase ac excitation and high-frequency voltage signal injection (HFVSI) for sensorless controlled starting of brushless synchronous machines (BSM) used as starter/generator in variable frequency ac power systems of civil aircraft. Fixed 400 Hz of the three-phase ac power is adopted both for the ac excitation and HFVSI in the initial starting process to eliminate the bulky rotor position sensor for BSM. The resulting 6th sequence harmonic voltage determined by the rotating rectifier is utilized as the HFVSI into the field-winding of main generator without any extra high-frequency signal injection. The rotor position is estimated by the high-frequency response signals extracted from the armature windings of main generator. Due to the nonlinear rotating rectifier linked in the HFSI chain, a novel frequency-insensitive asynchronous demodulation strategy is proposed in this paper for rotor position estimation. Furthermore, the initial rotor position detection is calibrated by polarity decision of the induced currents of the armature windings within the establishment procedure of field current of the main generator at standstill. The effectiveness of the ac excitation and feasibility of rotor position estimation for sensorless starting control of BSM are validated by the simulation and experimental results. [ABSTRACT FROM AUTHOR]

The extra-LC auxiliary-resonant-snubber-based converter (ELCARSC), as a soft-switching converter, has the advantages of low switching loss, low electromagnetic interference noise, and low-output distortion caused by the dead-time effect. However, the dead-time effect is still the main problem in a high-precision application. Besides, the traditional control and compensation methods rely on the accuracy of the circuit parameters, especially the junction capacitance, thus resulting in compensation error. Therefore, this paper proposed a novel high-precision control method through modulating the initial resonant current to eliminate the dead-time effect of ELCARSC. The method does not need complicated online calculation and reduces the dependence on the accuracy of the circuit parameters, especially the influence of the junction capacitances of the switches. In this paper, the dead-time effect of the ELCARSC is analyzed. The output voltage error with the conventional variable-timing control method is analyzed. Subsequently, the proposed modulated initial resonant current control is introduced. Finally, a prototype was developed that verified the effectiveness of the control method. The quality of the output current and voltage is improved by utilizing the proposed control method. However, the peak current of the auxiliary switch is increased. [ABSTRACT FROM AUTHOR]

Motors and inverters are increasingly being used in vehicles. As more vehicle components are being adopted, requirement for safety and installation space are increasing. Growing attention is being paid to the use of six-phase permanent magnet synchronous motors (PMSMs) and their parallel control by two inverters, because of their reduced size and enhanced safety. This paper proposes a pulsewidth modulation (PWM) control algorithm for a six-phase PMSM. In low-voltage vehicle components such as electric power steering, the current is usually acquired with resistors placed under the low-side switching devices. In the proposed control algorithm, the neutral voltage of the two inverters is shifted to either a lower or higher voltage within a certain range to allow for the signal conditioning time delay after the switching device turns on or off, and the time taken for the phase current to rise or settle. The paper proposes a novel PWM control algorithm that ensures that the on-time of the low-side switching devices satisfies the restriction on current sensing with resistors and reduces the current in the capacitor at the same time. The work contributes to reducing the current in a practical configuration involving low-voltage vehicle components. [ABSTRACT FROM AUTHOR]

In grounded photovoltaic (PV) systems, dc leakage to earth is formed by the contribution of the PV modules, inverters, and dc circuitry. Ground faults in such systems occur when there is an unintentional connection between any dc-carrying conductor with a grounded surface or earth. On the occurrence of ground faults, dc leakage will be intensified and will flow through the earth or other conductive paths, before returning back to the source. Thus, it is highly probable that some portion of the dc leakage will attempt to return to the source through any low-resistance paths that exist in the nearby vicinity, for example, through nearby buried pipelines. Therefore, unattended dc leakage may induce stray current corrosion concerns on these pipelines. To this extent, this paper presents a detailed dc interference modeling for assessing the dc leakage activity in PV systems—particularly under undetected fault conditions. First, the modeling carried out embraces the topologically accurate consideration of all sources of dc leakage that are associated with the grounded PV systems. A second feature presented in the paper is the modeling of blind-spot dc faults and their impact on third-party infrastructure. [ABSTRACT FROM AUTHOR]

Induction motor drives operated with a low ratio of the switching frequency to the fundamental frequency (i.e., pulse number) are prone to have high magnitudes of low-order torque harmonics. This paper proposes a method to analytically evaluate the harmonic torque spectrum based on the pulsewidth-modulated voltage waveforms for low pulse number operation. The predicted torque harmonic spectra for different pulsewidth-modulated waveforms are validated through simulations and experiments. This paper also reports optimal switching angles to minimize the total rms value of torque harmonics of order 6, 12, 18 $,\dots,$ $6(N-1)$ , considering the number of switching angles per quarter ($N$) to be 4 and 5. Compared to sine triangle pulsewidth modulation and selective harmonic elimination pulsewidth modulation with the same pulse number, the proposed optimal switching angles significantly reduce the total rms value of all harmonic torques of order lower than $6N$. Extensive simulation and experimental results on a 3.7-kW induction motor drive are presented to demonstrate the superior performance of the proposed optimal pulsewidth modulation. [ABSTRACT FROM AUTHOR]

This paper proposes a new dc–ac converter with a reduced number of semiconductor switches and gate drive and control circuit components. A salient feature of this inverter is its pure sinusoidal line-to-line voltages with no need for output filter. Also, the proposed inverter improves the voltage utilization factor of the input dc supply compared to the four-switch three-phase inverter (first best topology in the literature from the number of switches point of view) and standard six-switch inverter with sine pulsewidth modulation. The proposed inverter is capable of operating with a wide range of output voltages from zero to the full value of the dc input voltage by appropriately altering the instantaneous duty cycle. A three-phase universal inverter with only three power switch has been proposed in this paper for the first time ever, to our knowledge. For the purpose of providing a good compromise between fast transient response and stability, a model predictive controller is proposed. All the design expressions have been derived. A comparison with other dc–ac converters is given to show the merits of the proposed converter. Finally, satisfactory circuit operation is confirmed by experimental results from a laboratory prototype. [ABSTRACT FROM AUTHOR]

In a sensorless vector control method, at low speeds and self-commissioning for AC motors, it is necessary to model dead time, rise/fall time, parasitic capacitors, and semiconductors’ voltage drop accurately. In this paper, voltage drop in each phase due to the nonlinear behavior of the inverter from the control system's viewpoint is precisely modeled by using two lookup tables. These lookup tables make the proposed model robust against the dc-link voltage and carrier frequency variations in the entire current range. In conventional methods at standstill, the measured voltage in the dc current test contains the nonlinearity of two phases. Therefore, to obtain the nonlinear voltage of each inverter phase, it is assumed that the values of inverter nonlinearity voltage at both high current and low current are equal. This simplification leads to distortion in measured voltage in the low currents. However, in this paper, the voltage drop equation of each phase is solved by numerical method that is realizable in conventional processors. The results obtained in actual voltage measurement show the superiority of the proposed method over traditional methods in the low currents. The proposed method is verified by using a voltage source inverter feeding a 2.2-kW three-phase induction motor. [ABSTRACT FROM AUTHOR]

Impedance source inverters have experienced a fast evolution since the first release of the Z-source inverter in 2003. This evolution is not limited to improving the structure of the different topologies but also includes their modulation and control schemes in order to reinforce their performance from many perspectives. Accordingly, many modulation schemes have been proposed in different research activities, but the full differences among these modulation schemes, including their effect on the switching losses, are not clear yet to the researchers. Hence, this research activity is aimed at introducing a comprehensive review of these different modulation schemes employed for the three-phase impedance source inverters, which is enhanced with a comparative assessment. In this paper, different modulation schemes are classified and reviewed, introducing an important benchmark in order to identify the basic differences between these modulation schemes. In other words, from this paper, the concept of classifying the different modulation schemes and the mandatory equations to implement each scheme can easily be drawn. Moreover, this review is essential for the next part, i.e., Part II, in which a comparative assessment is introduced. [ABSTRACT FROM PUBLISHER]

In this paper, two structures are proposed for the switched-inductor quasi-Z-source inverter. In the proposed structures to extend the voltage gain of inverter, two new switched-inductor structures are used. In the proposed structures, voltage across the capacitors in the Z-source network and in the switched-inductor is symmetric and small. The other advantage of proposed inverters is known as achieving higher voltage gain per smaller duty cycle ratio (of shoot-through). Compared with a conventional continuous input current switched-inductor quasi-Z-source inverter at the same gain voltage, there is less of a ripple of input current in the proposed inverters. In this paper, at first the both the conventional and the proposed switched-inductor Z-source inventers are thoroughly investigated to find out their governing relations. Then, the proposed Z-source inverters are compared with the conventional ones by simulating them to assess their performance efficiency. Next, applying the simple boost pulse width modulation the simulation is carried out with PSCAD/EMTDC software. After that, to evaluate the obtained results, the prototypes of the proposed inverters with 48 Vdc input voltage are used. [ABSTRACT FROM PUBLISHER]

This paper analyzes the combination in a cascaded H-bridge multilevel inverter (CHBI) based on transformers with the different turn ratios for increasing the voltage level and proposes the switching method for achieving the output voltage distribution among H-bridge cells (HBCs). The transformers used in this paper are connected to the output of the respective HBCs, and the secondary sides of all the transformers are connected in series for generating the final output voltage. Only one of the transformers, in particular, has a different turn ratio for increasing the output voltage level. In this paper, the possible turn ratio of the special transformer with a different turn ratio is discussed in detail, and a switching method based on the level-shifted switching method for the topology used in this paper is proposed. To verify the effectiveness of the proposed method, a three-phase 21-level CHBI is experimentally tested. [ABSTRACT FROM PUBLISHER]

Microinverters require high voltage gain capability for interfacing the low dc voltage output of photovoltaic (PV) module to single-phase ac grid. A two-stage nonisolated inverter is proposed in this paper, with a first boost stage and a second traditional pulse width modulated grid-tied inverter stage. The proposed boost stage consists of a coupled inductor added with a voltage multiplier to achieve the required high voltage gain at high efficiency and to operate over a wide input voltage range. Among the coupled inductor topologies, adding a clamp circuit is a common solution to limit the voltage spike caused by leakage inductance. In the proposed converter, the resonance between the coupled inductor and voltage multiplier is used to address the issue of voltage spike, thereby reducing both component count and device voltage stress. Detailed analysis and design procedure of the proposed converter is presented in this paper. Experimental results of a 250-W microinverter are presented to validate the proposed converter. [ABSTRACT FROM PUBLISHER]

In this paper, new half-bridge and full-bridge topologies for a switched-boost inverter are proposed. These topologies have a high boost factor in comparison with conventional types. In addition, the proposed topologies have continuous input current and can generate zero-voltage level in output. In this paper, the operation of the proposed inverters in different operating modes is analyzed. Then, the values of inductors and capacitors are designed. Moreover, the used switching pattern and its logical diagram are introduced. A comprehensive comparison between the proposed and conventional topologies in terms of the boost factor, the number of elements, and the efficiency shows the good features of the proposed inverters. Finally, to verify correct operation of the proposed topologies and derived equations, simulation and experimental results are presented. [ABSTRACT FROM AUTHOR]

Digital current controllers have the key impact on the performance of grid-side converters and ac drives. The voltage disturbances are commonly suppressed by enhancing the controller with an inner active resistance feedback. In cases where the switching noise and parasitic oscillations introduce sampling errors, conventional sampling is replaced by the oversampling-based error-free feedback acquisition which derives the average of the measured currents over the past switching period. The time delay introduced into the feedback path creates difficulties in designing the current controller with the active resistance. In this paper, we introduce a novel structure of the current controller, which includes the error-free sampling and the active resistance feedback. Devised structure improves the disturbance rejection by extending the range of permissible values of the active resistance. Controller structure is based on the internal model principles, and it maintains the input step response unaffected. This paper comprises analytical design, the gain setting procedure, computer simulation, and experimental results obtained from an experimental setup with a three-phase inverter, digital controller, and a permanent-magnet synchronous motor. [ABSTRACT FROM PUBLISHER]

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 novel hybrid pulse width modulation (PWM) technique is proposed in this paper for two parallel interleaved, two-level, three-phase voltage source inverters (VSIs). The two parallel interleaved VSIs are analyzed as a three-level (3L) inverter. A 3L inverter provides minimum line current ripple when nearest three voltage vectors are applied. In this paper, a novel carrier-based PWM technique is developed that ensures the application of nearest voltage vectors, keeping average circulating current to zero over a switching period. For carrier-based implementation of the proposed PWM, generalized common-mode (CM) offsets are derived in terms of maximum and minimum values of reference signals. Each 3L space-vector sector is divided into seven subsectors. For each subsector, a CM offset is calculated, addition of which to reference signals ensures the minimum line current ripple and zero average circulating current. Simulation and experimental results are provided to validate the proposed technique. [ABSTRACT FROM PUBLISHER]

The traditional evaluation method of vehicle noise can no longer meet the requirements of electric vehicle as the sound character of electric powertrain (E-PT) is totally different from that of internal combustion engine vehicles. This paper presents a complete investigation of the sound quality of an E-PT. An acoustic test is conducted to analyze the sound character of E-PT, based on which the acoustic origins of E-PT including electromagnetic noise, gear whine, and inverter switching noise are studied. After that the sound quality of E-PT is investigated by objective and subjective evaluations. Subsequently, a sound quality model of E-PT based on support vector machine is established. The relative error testifies the effectiveness of the prediction model. The influence of the main noise origins on the sound quality of E-PT is determined. Finally, the structure of gearbox housing is redesigned based on topology optimization to improve the sound quality. The main original contributions of this paper are not only the sound quality method developed for E-PT but also improvement feasibility of sound quality. [ABSTRACT FROM PUBLISHER]

For power converters with inductive loads, a freewheeling path is needed for the current due to reactive power. The MOSFET synchronous rectification (SR) is widely used to reduce the conduction loss during the freewheeling period. Due to the wide band gap of silicon carbide (SiC), the intrinsic body diode of SiC MOSFET exhibits a high voltage drop. Hence, an antiparallel SiC Schottky diode is normally implemented to eliminate its conduction. However, the external SiC Schottky diode is not fully utilized as it only works during the dead time. In this paper, the hard-switching SR is investigated in an SiC three-phase inverter and compared with a conventional inverter using freewheeling diode (FWD). An improved power loss model for the two inverters has been developed. It is found that the inverter using SR has higher efficiency due to the smaller switching loss. A 7-kW prototype of SiC three-phase inverter is built, which achieves a peak efficiency of 98.8% ($\pm$ 0.15%) and 98.5% ($\pm$ 0.15%) at 40 kHz using SR and FWD, respectively. This paper confirms that the SiC MOSFET is an ideal candidate for the SR. [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]

This paper proposes and validates model predictive control as an alternative control strategy for H-bridgeneutral-point-clamped (H-NPC) converters for single-phase grid-tied string photovoltaic (PV) systems. The presented control scheme achieves good quality current waveforms with unity power factor, dc-link voltage control, and neutral-point voltage minimization. Furthermore, the predictive controller has been further enhanced by including an average device switching frequency restriction and a $dv/dt$ mitigation. The main contribution of this paper is the avoidance of the potential leakage current due to parasitic capacitance of the PV modules by using a predictive model based control technique instead of modulated schemes and eliminating high-frequency common-mode voltage components. Experimental results during steady state and dynamic operation are presented to illustrate the behavior of the H-NPC converter commanded by the proposed control scheme. [ABSTRACT FROM AUTHOR]

This paper proposes an effective sliding mode controller (SMC) for a grid-connected 7-level packed U-cell (PUC7) inverter. The aim is to design a simple controller that deals effectively with the complex control problem of the PUC7 inverter (multiobjective control problem). The selection of the control actions is achieved according to the system state error at every sampling time, regardless of the previous values, which makes the control technique model-independent. The control algorithm evaluates online two cost functions (one for each state error), which are derived on the basis of sliding mode theory, and it selects the optimal control input in order to satisfy the reaching conditions of the two cost functions. Compared with the existing solutions, the proposed SMC technique ensures lower average switching frequency by tuning the hysteresis bandwidth of the capacitor-voltage error. The fast implementation, needless of gains tuning, and simple design procedure are the main features of the proposed algorithm. Simulation and experimental results are presented to prove the effectiveness of the proposed technique in controlling the PUC7 inverter with high dynamic performance and robustness against disturbances and parameters mismatch. [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]

This paper investigates the use of resistor–inductor (RL) filters for motor overvoltage issues caused by cables connecting inverters to electric machines. Using existing filter design formulae and fixing the desired peak motor voltage, an RL filter is designed for a 15-kW industrial drive. This filter is used with a Silicon IGBT inverter and a matching between the design targets and the experimental results are found. Using the same filter with an SiC MOSFET inverter, a detrimental effect in overvoltage mitigation task is found. From simulations, it is noticed that combination of filter resistor parasitic inductance and high $dv/dt$ causes unpredicted additional high-frequency motor overvoltages. These phenomena require higher filter inductance values with respect to the standard one, thus previous filter design formulae cannot be used. New design formulae, valid when inverters with high $dv/dt$ are used, are proposed and experimentally validated. Interestingly, using the proposed formulae, it is discovered that the best filter resistance is not equal to the characteristic impedance of the cable. A design guideline that permits to optimally design the RL filter without simulation tool, considering the resistor parasitic inductance and the inverter $dv/dt$ is finally proposed. [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]

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]

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]

A fast and accurate analytical inverter-fed induction machine iron loss calculation model is proposed in this paper. The proposed model takes account of the influence of the output voltage harmonics from the inverter on the iron loss of the motor based on the piecewise variable coefficients method. Additionally, the proposed method incorporates slot harmonic component's influence on the iron loss. The validity of the proposed model is verified by comparing its calculated core loss values with measured ones of a 5.5- and a 55-kW inverter-fed induction motors under different speeds and load conditions. Compared with the classical iron loss model and piecewise variable coefficient iron loss model based on the finite-element method, the proposed model can reduce the computational burden significantly with desirable accuracy. [ABSTRACT FROM AUTHOR]

Passivity-based analysis and controller design offer a promising approach to guarantee the stability of power systems. If all grid-connected components act strictly passive, critical oscillations cannot arise. Recent research establishes design guidelines for current-controlled voltage-source converters that allow obtaining a nonnegative real part of the converter input admittance for a wide frequency range. In this paper, the findings are extended and generalized from a control engineering point of view. Utilizing passivity indices to quantify the degree of the input admittance's passivity, the current controller design is reviewed and assessed. Further, generic and necessary passivation design criteria are proposed and carried out exemplarily for passive as well as for active damping using voltage feed-forward filters. Finally, the theoretical findings are validated by experiments. [ABSTRACT FROM AUTHOR]

Resonant controller (RSC) is one of the most popular approaches for ac current/voltage control due to the high performance of zero steady-state tracking error. However, it suffers from heavy computational burden issue when multiple RSCs are demanded to control multiple harmonics. To alleviate this issue, a down-sampled multirate RSCs (MRRSCs) scheme is proposed for controlling power inverters, e.g., grid-connected inverters and active power filters. The proposed control scheme is composed of an inner control loop with a fast sampling rate, which is identical to the switching frequency, and an array of paralleled MRRSCs-based external control loop with a reduced sampling rate. With the reduced sample rate, the MRRSCs can be executed alternatively in the dq-frame, such that the computational burden can be significantly reduced per cycle. Moreover, to reinforce the wide frequency adaptability of the MRRSC, its central resonant frequency is online updated according to the output of the phase-locked loop. In the paper, the equivalent single-rate closed-loop model of the overall system is developed, based on which the controller parameters are designed employing the Nyquist diagram and root locus. Finally, experiments are performed on a grid-connected inverter to validate the superiority of the proposed scheme. [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]

In this paper, a new Lyapunov-based model predictive controller is proposed for a 7-level packed U-cell (PUC7) grid-connected inverter. A cost function of the proposed model predictive controller is designed from the system stability point of view, inspired by the Lyapunov control theory. The proposed controller eliminates the need for gains that are associated with cost function coefficients, as seen in classical model predictive control based controllers. Therefore, the control design problem is significantly simplified. In addition, the average switching frequency is also reduced, as shown in both simulation and experimental results, leading to a reduction in switching losses. Simulation and experimental tests, on a PUC7 lab prototype, demonstrate the excellent performance of the proposed control system, in terms of high disturbance rejection, robustness to parameter mismatches, and fast dynamic response. Such features qualify the proposed control strategy as a good candidate for grid-tied applications. [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 magnetic resonant wireless power transfer (WPT) system with a class E inverter has wide application in mid-range power transmission, especially for several megahertz scenario. However, it is cumbersome for the existing modeling and analysis methods to obtain the analytical solution to the system variables, which limits the evaluation of system characteristic. This paper proposes a nonlinear mathematical model for the WPT system with a class E inverter. The equivalent small parameter method (ESPM) is first adopted to solve the proposed six-order nonlinear model, and symbolic-form steady-state periodic solutions are yielded. During the whole solving process, only three iteration steps are needed for the proposed method. Compared with other existing modeling and analysis methods, the symbolic periodic results acquired by the ESPM exhibit sufficient accuracy by virtue of the combination of more higher harmonics. Meanwhile, the calculation complexity is simplified remarkably. Simulation and available experiments validate the effectiveness and accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

The high penetration of renewable energy generation (REG) systems influences the dynamics of power systems. Accurate transient simulations and dynamic models are indispensable for the research on the dynamic behavior of REG systems. A large-scale REG system consisting of numerous grid-connected inverters consumes a large amount of computation time, so a dynamic aggregated model is needed for efficient simulation. The aggregated model established by coherent equivalence fits the detailed model well, and thus it is widely used in the aggregation of large-scale dynamic systems. Presently, the key challenge of the application of the coherent-equivalence method is to identify inverters that have similar dynamic characteristics. The Hamilton principle expressed by Hamilton's action has a simple form and indicates the comprehensive dynamic characteristics of inverters. Thus, this paper proposes a coherent-equivalence method of inverters based on Hamilton's action. The proposed method is applied to a wind farm of 16 permanent magnet synchronous generator-based wind turbines for demonstration. Compared with the existing equivalence method without considering coherency, the proposed method shows advantages in effectiveness and accuracy. [ABSTRACT FROM AUTHOR]

In this paper, a generalized switching modification method is proposed to reduce the root-mean-square (rms) current flowing into a dc-link capacitor in a dc–dc–ac structure consisting of a boost converter and a three-phase inverter. To analyze the current flowing into the capacitor, we analyze the currents at the respective converter and inverter sides. Based on this analysis, we derive an analytical method to find the modification time between the converter and inverter pulsewidth modulation (PWM) carriers through a Fourier series and approximation to facilitate real-time implementation. Real-time implementation of the modification time calculated by the derived method is described. To verify the validity of the proposed method, experimental results are presented based on a 1-kVA dc–dc–ac prototype. The proposed method has a better capacitor rms current reduction performance than the conventional method in various inverter power factors and PWM strategies. Especially, it has an excellent performance in mid-power factor level (0.95–0.1). [ABSTRACT FROM AUTHOR]

The instability issues of the three-phase grid-tied inverters are major concerns in the distributed generated systems. However, due to the coupling effects in dq frame, the instability mechanisms of the three-phase inverters have not yet been clearly revealed. This paper presents a divided dq small-signal model of the grid-tied inverter in dq frame. The specific expressions of a small-signal output admittance of the inverter are investigated based on the proposed model, which can reveal the influencing mechanism of coupling effects and phase-locked loop (PLL) on the stability of the inverter. Then, a stability analysis method based on the divided dq small-signal model is proposed, by which the procedure of checking the stability of the grid-tied inverters with strong/weak grid is executed step by step, and it could be helpful for the system parameters design. Furthermore, the stability of the multiparalleled inverter system is also studied, and it indicates that as the number of the paralleled inverters increases, the parameters of PLL could be adjusted for a stable operation. The theoretical stability analysis results are verified by simulations and experiments. [ABSTRACT FROM AUTHOR]

In this paper, an improved direct torque control (DTC) technique of a two-level inverter-fed five-phase induction motor (FPIM) drive for low-speed operation is presented. The operation of the FPIM with distributed winding requires the elimination of third-harmonic voltage, which, if not taken care of, would generate distorted stator current. For this, the concept of virtual vectors (VVs) is utilized. Before selecting the VVs for the DTC operation, a theoretical analysis is carried out to find the effects of these VVs on the torque and flux response of the FPIM under different speed and loading conditions. It is found that the rotor speed plays a crucial role in the selection of VVs. A particular VV has different effects on the torque change as speed varies. This analysis helps in the selection of VVs, hysteresis torque bandwidth, flux bandwidth, and efficient formation of sectors. Furthermore, in this paper, the performance of the FPIM during low-speed operation is improved by avoiding the demagnetization of the stator flux. To achieve this, a new modified lookup table is proposed. Superiority of the proposed method over the reported approaches is illustrated by experimental results. [ABSTRACT FROM PUBLISHER]

A grid-forming inverter (GFI) is an important component for the operation of an islanded microgrid. Its purpose, similar to a conventional slack bus generator, is to build up a reference voltage for other distributed generating units in the microgrid. Usually, a nested-loop proportional plus integral (PI) control structure is employed to control a GFI in a dq reference frame. However, conventional PI-based nested-loop control method has a deteriorative performance under parameter variations. In this paper, a novel nested-loop control strategy is proposed for control of a GFI system containing an {LC} output filter and loads. The proposed method does not require a precise model for the inverter system and can better deal with uncertainties and {LC}$ filter resonance without using any passive or active damping mechanisms. It utilizes a sliding-mode control in the inner current loop and a mixed {H_2}/{H_\infty } optimal control in the outer voltage loop, which provides the advantages of constant switching frequency, low total harmonic distortion, robustness against parameters variations, and fast transient response. The simulation and hardware experiments presented in this paper demonstrate the proposed controller's improved transient and steady-state performance in various key criteria, over conventional PI-based nested-loop control strategy. [ABSTRACT FROM AUTHOR]

In this paper, a new topology for half-bridge Z-source inverter is proposed. The proposed topology has only one impedance network. Unlike to the conventional half-bridge inverter, the proposed topology can provide zero voltage level at the output. It also increases output voltage level and stabilizes it in the desired value. Capacitor voltage stress in the proposed topology is low, and, therefore, nominal voltage of capacitor and cost decreases. In this paper, the steady-state analysis of the proposed inverter in two new operations which are named synchronous operation of diodes and asynchronous operation of diodes is conducted based on mathematics calculations. A method to obtain high-voltage gains by cascading the Z-network and combining middle inductors is presented that leads to cost, size, and weight reduction. Comparison among the proposed converter with conventional ones shows its excellent performance. The experimental results have good agreement with analytical analysis for the proposed topology. [ABSTRACT FROM PUBLISHER]

Harmonic current sharing is an important aspect of islanded microgrid due to the presence of nonlinear loads. The existing harmonic current sharing strategies suggest a tradeoff between voltage quality and harmonic sharing effectiveness. In this paper, a novel control strategy is proposed to achieve current harmonic sharing by directly controlling the inherent impedance of the inverter. The proposed technique achieves harmonic sharing, in a distributed generation (DG)-based islanded microgrid, without using any communication infrastructure. This paper also proposes a droop technique to distribute the harmonics generated by the nonlinear load as per the kVA ratings of different DG units in the system. The proposed concept of variable inverter impedance and its application in an islanded microgrid is validated using experimental studies. [ABSTRACT FROM PUBLISHER]

Repetitive control strategies have been commonly applied in pulse-width-modulated (PWM) voltage source inverters (VSIs) for many industrial applications. This paper presents a repetitive controller for voltage harmonic mitigation of VSI-based islanded microgrids. The phase delay in the overall control system, e.g., the delay caused by the digital duty cycle calculation, PWM generation, and repetitive controller, has to be compensated by elaborately designed phase lead filters in order to prevent control performance deterioration and system instability. Nevertheless, quantificational analysis and practical design of the time advance unit in such filters are hardly found in existing literature works. In view of this, this paper proposes an explicit analysis of the phase lead filters and a novel design method of the time advance unit in repetitive controllers to ensure system stability. Moreover, with the help of the proposed method, the overall system stability margin is predictable and improved controller performance is achieved as well. The proposed method is implemented experimentally to show the accurate stability margin calculation as well as the excellent steady state and dynamic performances of the repetitive control scheme. [ABSTRACT FROM PUBLISHER]

Zero-common-mode-voltage (CMV) operation of multilevel inverters results in reduced dc-bus utilization and reduced linear modulation range. In this paper, a method to increase the linear modulation range for zero-CMV operation without increasing the dc-bus voltage using a cascaded multilevel inverter with a single dc supply is presented. Using this method, the peak fundamental output voltage can be increased from 0.499 to 0.637  \textV\text{dc} with zero CMV inside the linear modulation range. Also, various pulse width modulation (PWM) switching sequences are analyzed in this paper, and the PWM sequence that gives minimum current ripple is used for the zero-CMV operation of the inverter. The inverter topology with single dc supply is realized by cascading a two-level inverter with two floating-capacitor-fed full-bridge modules. Simulation and experimental results for steady-state and dynamic operating conditions are presented to validate the proposed method. [ABSTRACT FROM AUTHOR]

Model predictive control (MPC) method has been developed as a simple and effective current control technique for voltage-source inverters (VSI). The conventional MPC method applied to VSI adopts all the seven voltage vectors (VVs), including one zero VV and six nonzero VVs, to implement the predictive current control in order to improve the current control performance. However, the common-mode voltage (CMV) is large due to the use of zero VV. Although the MPC strategy only using six nonzero VVs can reduce the CMV, its switching frequency is relatively high due to too many switches between nonadjacent VVs. In this paper, a new MPC-based CMV suppression strategy for VSI is proposed and applied to a permanent-magnet synchronous generator (PMSG)-based control system. Only four nonzero VVs, including three adjacent nonzero VVs and one nonadjacent nonzero VV, are utilized to perform the optimization in every sampling period, resulting in switching frequency and calculation effort reduction with no adverse effect on the current response. Moreover, the influence of the MPC-based CMV reduction algorithm on the speed control of PMSG is tested in this paper. The experimental results confirm the effectiveness of the proposed strategy. [ABSTRACT FROM PUBLISHER]

This paper presents a model predictive control technique applied to a dual-active bridge inverter where one of the bridges is floating. The proposed floating bridge topology eliminates the need for isolation transformer in a dual inverter system and therefore reduces the size, weight, and losses in the system. To achieve multilevel output voltage waveforms, the floating inverter dc-link capacitor is charged to the half of the main dc-link voltage. A finite-set model predictive control technique is used to control the load current of the converter as well as the floating capacitor voltage. Model predictive control does not require any switching sequence design or complex switching time calculations as used for space vector modulation; thus, the technique has some advantages in this application. A detailed analysis of the converter as well as the predictive control strategy is given in this paper. Simulation and experimental results to validate the approach are also presented. [ABSTRACT FROM PUBLISHER]

Typical value of rated phase voltage of pole phase modulated multiphase induction motor (PPMMIM) drives with wider speed range is in the order of few hundreds of kilovolts. This high value of phase voltage for high power density applications results in higher dc-link voltage requirement and switch voltage rating of two-level multiphase inverter. Further, using multiphase space vector pulse width modulation (SVPWM) yields less dc-link utilization. Methods to increase dc-link utilization using SVPWM with offset value of third harmonic order introduces dominant lower order harmonic currents into phase windings. One more major problem in high pole mode of PPMMIMs is higher torque pulsation due to decrease in phase number. To address these problems this paper proposes a dual-inverter-based multilevel voltage excitation scheme for nine-phase PPMMIM with 1:3 speed ratio. In four-pole mode a simple phase grouping technique to eliminate lower order harmonic currents in the phase windings is proposed. In addition each inverter feeding these phase groups is modulated using carrier-based three-phase SVPWM to achieve higher dc-link utilization. This paper also proposes a multilevel voltage generation scheme for 12-pole mode of operation using carrier phase shifted pulse width modulation (PWM) for inherently available equal voltage profile coils with the same dual inverter structure. The torque ripple using phase shifted carriers PWM and single carrier PWM are compared. Finite element method model of nine-phase PPMMIM is developed in Ansys Maxwell two-dimension and is cosimulated with three three-phase dual inverters in Simplorer environment. Experimental validation is done for linear and over modulation case on 9ФIM fed from three three-phase dual inverters controlled using Spartan six field programmable gate array board programmed in VHDL. [ABSTRACT FROM PUBLISHER]

This paper presents a three-phase open-end winding induction motor drive. The drive consists of a three-phase induction machine with open stator phase windings and dual-bridge inverter supplied from a single dc voltage source. To achieve multilevel output voltage waveforms, a floating capacitor bank is used for the second of the dual bridges. The capacitor voltage is regulated using redundant switching states at half of the main dc-link voltage. This particular voltage ratio (2:1) is used to create a multilevel output voltage waveform with three levels. A modified modulation scheme is used to improve the waveform quality of this dual inverter. This paper also compares the losses in the dual-inverter system in contrast with a single-sided three-level neutral point clamped converter. Finally, detailed simulation and experimental results are presented for the motor drive operating as an open-loop v /f controlled motor drive and as a closed-loop field-oriented motor controller. [ABSTRACT FROM PUBLISHER]