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Harmonic distortions at a point of common coupling (PCC) are caused by the harmonic sources in both supply system and customer facilities. In order to verify compliance with harmonic limits for an operating facility, it is essential to determine the contribution of a facility to the harmonic distortions measured at the PCC. Over the past many years, a significant amount of research work has been conducted in developing different harmonic analysis methods and especially harmonic contribution assessment methods. A common need of this line of research is consistent test systems that can be used to verify/validate a proposed method or to compare different approaches. To address this gap, a benchmark test system representing a typical medium-voltage/low-voltage network supplying industrial loads is proposed in this paper. This paper also includes examples on how to use the system to assess the performance of harmonic contribution determination methods. [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]

Many research works report the use of LCL filters for grid-interfaced PV systems. Indiscreet selection of passive LCL filter parameters may result in poor filtering and increased power losses. LCL filter parameter selection is critical to get a good performance and at the same time ensure that losses and costs stay low. This paper proposes a novel approach to design the LCL filter for a grid-interfaced PV system. A cost-based minimization approach on a cost per joule basis for each of the filter elements is explored to obtain the optimal filter design. Simulation results for a 10-kVA grid-connected inverter with an LCL filter are presented in order to verify the efficacy of the proposed design. The impact of designed filter parameters on the ripple current is also discussed in this paper. The total inductance of the LCL filter is varied and filter parameter values which give lowest cost is identified while meeting harmonic standard requirements and the fundamental voltage drop across the filter. Finally, the proposed design is also experimentally validated via a hardware prototype. Results validate the design method in terms of the harmonic attenuation and total harmonic distortion. [ABSTRACT FROM AUTHOR]

In this paper, the theoretical analysis of the position estimation error in position-sensorless operation using pulsating square wave signal injection is presented. The purpose of this paper is to analyze the phenomenon that the position estimation performance varies depending on the injection frequency. Mathematical derivation supported by simulation and experimental results shows that, in no-load condition, the voltage distortion induces position estimation error with 3rd-order harmonics if the frequency of the injected square wave is the same to the switching frequency. While, if the frequency is a half of the switching frequency or heavier load condition in either injection frequency, the effect of nonideal characteristics of inverter disappears and there is no 3rd harmonics in the position estimation error. Therefore, only the effect of cross-coupling inductance is reflected in the estimation error. As the result, the position estimation error appears in the form of 6th-order harmonics due to the spatial harmonics of inductances, and the error is relatively small compared with the switching frequency injection case. The theoretical analysis was verified by simulation and experimental results. [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]

Z-source inverter has recently been introduced to overcome the limitations of conventional voltage source inverter. This paper deals with a novel topology of single-phase Z-source inverter (ZSI). This topology reduced the number of passive elements and active switches in order to make the inverter cheaper and smaller in size compared to traditional ZSI. Detailed analysis of the proposed topology is presented in this paper which includes calculations of boost factor, total harmonic disorder, magnitude of output voltage etc. Modulation technique used to control the switching of the proposed inverter is explained in detail. This paper also compares the proposed topology with conventional topologies of Z-source inverter. Simulation results of MATLAB and real-time simulators are shown to explain the working of the proposed topology. [ABSTRACT FROM AUTHOR]

This paper proposes a split-capacitor four-wire current source inverter, which is the dual of the split-capacitor four-wire voltage source inverter. Since the midpoint of the dc link is tied to the neutral point of ac filter capacitors, the common-mode voltage (CMV) is reduced significantly. Consequently, the leakage current issue is effectively addressed. The proposed circuit is cost-effective as no extra switch is added. This paper, first, establishes the equivalent common-mode circuit of the proposed inverter. The impact of the neutral line inductance on CMV is also analyzed. Then, a specific modulation is introduced to balance the dc-link voltages/currents. To achieve good input/output performance, a nonlinear control method is developed based on time-domain models. Finally, all the proposed methods and related theoretical analysis are verified by simulations and experimental results. [ABSTRACT FROM AUTHOR]

Under the aspect of decreasing costs for power electronic elements due to mass production, machines with feeding from multiphase inverters can be considered where each slot is activated as a separate phase. The number of IGBT switches is increased, but the current rating of the IGBTs becomes very small, thus reaching the region of cheap mass production elements. Additionally, this allows a smaller capacitor in the intermediate circuit and a mechanical structure of the inverter that can be easily integrated into the machine housing. By this, the often critical du/dt problems can be reduced significantly. In the machine, new features such as an ideal sinusoidal or even rectangular distribution of electric loading and flux density become possible. Moreover, if induction machines with squirrel cage are considered, a rather free adjustment of the pole pitch is possible. In the paper, this approach and its consequences on the performance and the construction of the machine are discussed for an exemplary induction machine. [ABSTRACT FROM AUTHOR]

Foil-winding transformers with high currents in low voltage winding normally have copper/aluminum bars as leads of the coils. A practical solution for locating bar leads without increasing the size of the active part is to flatten the coil in the leads zone causing a deformation. Abnormalities in the leakage reactance have been identified in this type of transformers. In order to take this effect into account, this paper proposes a correction to the mathematical formulation for calculating the leakage reactance in transformers with deformations in the low voltage winding. Three methods for calculating the leakage reactance have been compared for four real case studies: the proposed formulation, the commonly used formulation, and detailed simulations with the finite element method in three-dimension (FEM3D). The leakage reactance calculated with the proposed method has been found to have an improved accuracy compared with the one calculated using the formulation without correction in all case studies. The main contribution of this paper is that the proposed correction in the formulation makes it possible to increase the accuracy in the calculation of leakage reactance, avoiding the need to resort to detailed simulations with FEM3D. [ABSTRACT FROM AUTHOR]

Four novel architectures of wideband five-port single-ended-to-balanced power dividers with broadband common-mode suppression are proposed in this paper. The bandpass filtering capability with/without close-to-passband transmission zeros are also incorporated in some of them to obtain multifunctional single-ended-to-differential-mode filtering/power division operation. Two ideal 180° phase inverters are used to replace the conventional half-wavelength transmission line-based inverters in the basic power divider structure so that wideband common-mode rejection and a smaller circuit size are achieved. Quarter-wavelength transmission lines and coupled-line sections are exploited to simultaneously realize the bandpass filtering and wideband power division functionalities. Each five-port power divider exhibits broadband 3-dB single-ended-to-balanced power division, high in-band input–output power-matching levels, high in-band power isolation levels between the two balanced outputs and wideband common-mode rejection. The theoretical analysis, parametric study, and design procedure for the conceived single-ended-to-differential-mode power dividers are provided. Furthermore, for experimental and demonstration purposes, four microstrip prototypes designed at 1 GHz are manufactured and characterized. [ABSTRACT FROM AUTHOR]

This paper proposes a new high-efficiency photovoltaic (PV) converter for grid connection through a high-leg delta transformer. The converter is composed of a symmetrically connected boost converter and three half-bridge inverters. One of the three half-bridge inverters is connected to the boost converter, and the others are directly connected to the PV terminals. As a result, this circuit configuration enables to reduce the power losses in both boost converter and inverters. This paper also proposes a new cooperative control method between the symmetrically connected boost converter and inverter. The control method can reduce the average switching frequency to 75% of that in a conventional one, resulting in a great reduction in the switching power loss. Experimental results confirm that the proposed circuit configuration makes it possible to improve its European efficiency from 91.6% to 94.5%. Moreover, system performance is evaluated on the assumption of maximum power point tracking operation. [ABSTRACT FROM AUTHOR]

A multifunctional grid-connected inverter (MFGCI) has been a promising solution to mitigate power quality problems in distributed generation system and microgrid applications. However, the capability of a conventional MFGCI is limited when compensating a voltage-based power quality problem. In this paper, a new configuration of MFGCI is proposed to improve both voltage-based and current-based power quality issues and is named shunt-series-switched MFGCI (SSS-MFGCI). The SSS-MFGCI can be connected in series or parallel to the grid by implementing bidirectional switches to provide different modes of operation depending on various grid disturbances. The required compensating current and voltage are analytically derived considering the grid and load conditions for voltage regulation. This paper also presents the compensation strategies, control scheme, and transition scheme between different modes of the proposed SSS-MFGCI. Simulation results to prove the concept are also included for unbalanced sag and swell cases. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the variation in performance of variable speed induction motors fed by inverters with grades of core materials used for stators and rotors from both results of measurement and finite element analysis. Identical shaped stator/rotor cores are manufactured by using several kinds of electrical steel sheets from low to high grades. Then, the characteristics of these motors are measured. Finite element analysis is applied to these motors to understand the measured results by separating losses and torques according to origins. The measured motor characteristics are found to be in good agreement with the results of the analysis. The analysis reveals the variation in each loss component with electrical steel sheets. Finally, the strong and weak points of the application of high-grade electrical steel sheets to the variable speed induction motors are discussed. [ABSTRACT FROM AUTHOR]

This paper proposes a zero-voltage ride-through (ZVRT) method and an LCL filter design method to meet the fault ride-through (FRT) requirements for a single-phase grid-connected inverter with a minimized LCL filter. A high inverter output current overshoot occurs at a grid voltage sag and a grid voltage recovery, when a low-inductance LCL filter design is employed in order to minimize the filter volume. In order to suppress the current overshoot, a high-speed gate-block operation and the design method of the minimized LCL filter are proposed. In particular, the filter design considers the delay time in the gate-block operation in order to clarify the reduction limitation of the inductance in the LCL filter. As a result, the maximum inverter output current overshoot is suppressed to 146% by the high-speed gate-block operation and the designed LCL filter, where the impedances of an interconnected inductor and a filter inductor are 1.0% and 0.78% of the inverter normalized impedance, respectively. Therefore, it is possible to meet the FRT requirements with the proposed ZVRT method even with the minimized LCL filter. [ABSTRACT FROM AUTHOR]

A programmable electronic load (PEL) solution is usually needed for comprehensive testing of power supplies. In this paper, a novel PEL based on a hybrid five-level voltage source inverter (VSI) is introduced to emulate a variety of static load behaviors, such as R, L, C loads, high-frequency harmonic loads, and unbalanced loads. Dynamic load behavior, for example, modulation loads, can also be achieved in this paper. A dedicated control structure is proposed for the emulation of different load behaviors and the detailed compensator design is presented to evaluate the system dynamic performance. The proposed PEL system has isolated active dc links to support multilevel converter topology, and it can either dump the energy to a passive load or recycle the energy back to the utility grid through another VSI. Simulation and experimental results are provided to validate the effectiveness of proposed control methods in terms of successfully achieving a variety of load profile functions. [ABSTRACT FROM AUTHOR]

A circuit configuration, a circuit topological family, a buck-mode active clamped circuit, and an instantaneous output voltage feedback control strategy of combined bidirectional buck–boost dc–dc chopper-mode inverter with high-frequency (HF) link (HFL) were proposed and fully investigated in this paper. The steady principle characteristic and the design criteria for the key circuit parameters were given in this paper. The circuit configuration is composed of two identical isolated bidirectional buck–boost dc–dc choppers with the same input and output filters. These two choppers in parallel at input end and in series at output end generate unipolarity sinusoidal pulsewidth modulation current waveforms with positive and negative half low-frequency cycles separately. The circuit topological family includes four circuit topologies, such as one-transistor mode. Taking the one-transistor mode circuit topology as an example, the 750VA48VDC/220V50HzAC prototype is designed and implemented. The theoretical analysis and principle test show the inverters have excellent performance. [ABSTRACT FROM PUBLISHER]

This paper presents theoretical and experimental discussions on a practical speed-sensorless start-up method for an induction motor driven by a modular multilevel cascade inverter based on double-star chopper cells (MMCI-DSCC) from standstill to middle speed. This motor drive is suitable, particularly for a large-capacity fan- or blower-like load. The load torque is proportional to a square of the motor mechanical speed. The start-up method is characterized by combining capacitor-voltage control with motor-speed control. The motor-speed control with the minimal stator current plays a crucial role in eliminating a speed sensor from the drive system and in reducing an ac-voltage fluctuation occurring across each dc capacitor. Experimental results obtained from the 400-V 15-kW downscaled system with no speed sensor verify that the motor-speed control proposed for the DSCC-based drive system can enhance the start-up torque by a factor of three under the same ac-voltage fluctuation. Several start-up waveforms show stable performance from standstill to middle speed with different load torques. [ABSTRACT FROM AUTHOR]

This paper presents a single-phase ac power supply circuit that based on modified voltage-fed quasi-Z-source inverter topology; it suits to the light or the heavy load and is fit for the resistive, inductive, or capacitive load. The topology, operating principle, voltage gain, control method, and its realization by DSP are described in this paper. The feasibility and rationality of the system is verified by the simulation and experimental results. [ABSTRACT FROM PUBLISHER]

This paper presents an algorithm for the economical design of a utility-scale photovoltaic (PV) power plant via compromising between the cost of energy and the availability of the plant. The algorithm inputs are the plant peak power and the price of inverters with respect to their power ratings. The outputs are the optimum inverter ratings and the interconnection topology of PV panels. This paper introduces the effective levelized cost of energy (LCOE) (ELCOE) index as the core of the proposed design algorithm. ELCOE is an improved index based on the conventional LCOE that includes the availability of a power plant in economical assessments. The conventional LCOE index determines centralized topology (e.g., 1-MW inverter for a 1-MW PV power plant) for minimizing the energy generation cost, whereas based on ELCOE, a multistring topology (e.g., a 1-MW PV plant consists of fifty 20-kW inverters) despite of higher investment cost becomes the economically winning topology. Given the price of commercially available PV inverters at present, the case studies in this paper show that, for 0.1–100-MW PV power plants, the economical ratings of inverters range from 8 to 100 kW. The recently installed PV power plants confirm the feasibility of the calculations based on the suggested algorithm. [ABSTRACT FROM PUBLISHER]

There are several ways to model three-phase gridconnected voltage-source inverters (VSIs) with an $LCL$ filter. Such models often use simplifications like constant dc-link voltage or averaging over a switching period. These simplifications remove transistor switching and dc-link capacitor dynamics from the model. In this paper, a comprehensive modeling of a three-phase VSI, including an $LCL$ filter, is described and compared to models using one or more of the aforementioned simplifications. Advantages and disadvantages of different models are discussed. All simulations are verified by comparing with experimental results. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a methodology to evaluate the potential of submicrometer organic thin-film transistors (OTFTs) with nanoimprinted gate and self-aligned source and drain contacts. In the first step, the dedicated static model we previously reported is updated to account for the subthreshold regime and is used to simulate a zero- $V_{{\rm GS}}$ inverter (one of the most basic unipolar logic gate). Based on the extracted noise margins, two methodologies were studied to assess the potential of this technology in terms of p-logic digital circuits. The first one is De Vusser's V_{T} -based method that we adapted to our OTFT model. The second one relies on statistical modeling and takes into account the actual worst case scenario for an inverter in terms of noise margins. It better represents the experimental distribution because of specific corner models. The different analysis studied in this paper shows that these OTFTs, in the current state of the technology, are still not ready for complex digital circuits as the throughput is expected to be quite low. In addition, the proposed methodology and related interpretation are technology-independent therefore this analysis may serve as a basis to characterize unipolar-logic printed electronics and can be further extended to complementary-logic circuits. [ABSTRACT FROM PUBLISHER]

Three-level voltage space phasor can be obtained using a dual two-level voltage source inverter (VSI). Two zero sequence voltage elimination pulse width modulation (PWM) switching variants are proposed in this paper for complete elimination of zero sequence voltage (ZSV) in the dual-VSI scheme. Both the PWM variants are designed in such a way that only one inverter is switched at any instant of time; to ensure that switching power losses in the dual-VSI is limited to one inverter only at any instant of time. Analytical expressions for the dwell times of the dual-VSI are developed and presented that successfully forces the ZSV in the dual-VSI to zero; at all instants of time. As a result, both the PWMs facilitate the dual-VSI to be driven from a single DC power source only. The implementation of the proposed PWM algorithms is greatly simplified and involves just the use of instantaneous magnitudes of the reference space vector. The implementation of PWMs totally avoids the time consuming task of sector identification and do not use look up tables. The PWMs proposed in this paper are first analysed, simulated using MATLAB/SIMULINK and then verified experimentally to validate the proposed PWMs. [ABSTRACT FROM AUTHOR]

This paper presents analytical expressions for steady-state waveforms, output power, and power conversion efficiency for the class-E inverter outside the class-E zero-voltage switching and zero-derivative switching conditions. The analytical expressions in this paper include the MOSFET-body-diode effect. By carrying out PSpice simulations and circuit experiments, it is shown that the analytical predictions agree with the simulated and experimental results quantitatively, which indicates the validity of the analytical expressions. Additionally, the switching-pattern distribution maps are also given by using the analytical waveform equations. [ABSTRACT FROM PUBLISHER]

A dual two-level voltage source inverter (VSI) can synthesize a three-level voltage space vector employing an open-end winding induction motor. Space-vector-based pulsewidth modulation (PWM) variants for this dual VSI are proposed in this paper that offer the dual advantage of limiting the switching power loss to a single VSI at all instants and also reducing the switching commutations in the dual VSI by 50%. The influence of different error volt seconds (affected with different PWM variants) on the motor phase current in the dual VSI is critically analyzed. To this end, two analytical approaches (one using error-voltage trajectory information and the other using switching state information of the dual VSI) are also proposed in this paper to predict the current trajectory and the ripple content in the drive system. Expressions for rms ripple current are developed with different PWM variants. The efficacy of the proposed analytical approaches to predict the current trajectory and the ripple content is confirmed from the experimental results. All the PWM variants are first simulated using MATLAB and verified experimentally by conducting tests on a three-phase open-end winding induction motor drive controlled with volts per hertz control. The implementation of the PWM algorithms only requires instantaneous magnitudes of three-phase reference voltages and completely avoids the sector identification and lookup tables. [ABSTRACT FROM PUBLISHER]

Transformerless grid-connected inverters for photovoltaic (PV) applications provide several advantages such as reduced cost and volume as well as an increased efficiency. However, the removal of the transformer gives rise to several problems related to leakage currents and electromagnetic interferences (EMI). This paper presents different chaotic unipolar sine-pulse width modulation (C-USPWM) techniques for a transformerless grid-connected PV inverter based on parameter selection. The main objective behind this proposed chaotic modulation technique is to reduce the conducted EMI without worsening the power quality and the common-mode voltage (CMV). The method is validated through numerical simulations and experimental tests carried out on laboratory prototype of an optimized H5 inverter (oH5). The obtained results showed its superiority to the conventional unipolar sine-pulse width modulation (USPWM). We showed also that the appropriate parameter selection of the C-UPSWM could advantageously combine an improved EMI performance and an efficient control, making it possible to deliver a significant reduction of the peaks' amplitudes in the frequency spectrum of the common-mode (CM) voltage. [ABSTRACT FROM AUTHOR]

This paper proposes a robust continuous nonlinear control method for grid‐tied photovoltaic (PV) inverters by combining model predictive control and integral sliding mode control (ISMC). The robustness of the proposed augmented controller is demonstrated mathematically by showing that the equivalent control signal of the ISMC system cancels disturbances to the system. Further, the proposed controller can compensate for system perturbations without knowing their characteristics, thereby simplifying controller design for the PV systems. The robustness of the proposed controller against parameter uncertainties and external disturbances was demonstrated via experimental and simulation verification. [ABSTRACT FROM AUTHOR]

By developing a robust control strategy based on the differential flatness concept, this paper presents a solution for the bidirectional trajectory tracking task in the "full-bridge Buck inverter–DC motor" system. The robustness of the proposed control is achieved by taking advantage of the differential flatness property related to the mathematical model of the system. The performance of the control, designed via the flatness concept, is verified in two ways. The first is by implementing experimentally the flatness control and proposing different shapes for the desired angular velocity profiles. For this aim, a built prototype of the "full-bridge Buck inverter–DC motor" system, along with Matlab–Simulink and a DS1104 board from dSPACE are used. The second is via simulation results, i.e., by programming the system in closed-loop with the proposed control algorithm through Matlab–Simulink. The experimental and the simulation results are similar, thus demonstrating the effectiveness of the designed robust control even when abrupt electrical variations are considered in the system. [ABSTRACT FROM AUTHOR]

The current world geopolitical state has driven significant interest in expanding influence in the polar regions. In order to develop new trade routes and defense of these areas, specialized ice-capable vessels are needed. Traditional propulsion technology utilized for these vessels has featured diesel and turbo electric drives with cycloconverter based technology used for speed regulation. Cycloconverter technology has the major disadvantage of characteristic output voltage waveforms that are very non-sinusoidal which induce harmonic rich currents with their resulting negative effects on total power factor and efficiency. There is also the potential to induce harmonic currents that may increase heat production in power cables, stressing insulation and reducing overall component life. Furthermore, sensitive loads such as control system power supplies may also be affected. The output waveform of a PWM drive has harmonic content that is both less severe, and more easily analyzed with current modeling tools. There are significant technical challenges that need to be assessed when considering PWM drive deployment in ice-capable vessels. This is due to the severe load torques, and torque variation imposed on propeller blades during ice-milling operations. To the authors' knowledge, an analysis of the behavior of vector-controlled PWM drives to this type of service has not been published in the open literature. This paper presents an initial modeling and simulation study that investigates whether such a PWM drive and associated controller may be effective for ice milling and other operational evolutions for these types of ships. [ABSTRACT FROM AUTHOR]

In response to the key engineering problems of photovoltaic grid-connected inverter cluster resonance suppression affected by grid-connected inverter impedance, in this paper, a control strategy based on a disturbance observer is proposed to dynamically compensate for the damping coefficient of the controlled system and improve the robustness of the system. First, an engineering mathematical model of a 200 MW photovoltaic inverter cluster is established, and the mechanism of the active damping of the cluster inverter influenced by the disturbance is analyzed. Secondly, the capacitor current feedback is utilized to constitute the virtual damping, and the inverter output impedance is reshaped to suppress the resonance peak. Then, a Kalman filter is used to improve the traditional disturbance observer to accurately detect disturbance of the system during the dynamic process of the cluster inverter so as to better adapt to changes in grid impedance and dynamically compensate for the virtual damping of the cluster resonant system. Finally, the proposed control strategy is verified with respect to a practical PV power station. The experimental results demonstrate the feasibility of the proposed control method. [ABSTRACT FROM AUTHOR]

In the grid-connected systems, LCL filters are widely used for eliminating the generated switching current ripples due to better attenuation in comparison with L or LC filter. However, it is known that stability of digital controlled LCL filter can present a challenge due to induced delay. This paper presents conditions of system stability in dependence of filter characteristics such as resonant frequency, inductors and capacitor parameter variations. Stability analysis and system performance are observed and compared. Derived results are tested and confirmed on Hardware-in-the-loop HIL402 device. Obtained results are discussed and suggestions are provided in terms of the practical implementation of the control system. [ABSTRACT FROM AUTHOR]

Summary. The linear active disturbance rejection control (LADRC) is mainly applied for tracking a step signal, and it is inadequate to realize satisfactory performance in the control of a single-phase inverter whose reference signal is sinusoidal. In this paper, an improved voltage control strategy based on compound synchronous reference frame proportional-integral (SRFPI) control and LADRC is proposed. The SRFPI and LADRC remain relatively independent without complicating the control parameter design. Detailed theoretical analyses on the compound strategy and its performance are presented, indicating that it inherits the advantages of the two controllers. Moreover, contrast experiments are carried out to verify the feasibility and superiority of the proposed control strategy. The results show that the system achieves a small steady-state error, voltage tracking error, fast transient response, and low total harmonic distortion (THD) when feeding a highly nonlinear load. [ABSTRACT FROM AUTHOR]

This paper proposes a feedback linearization controller for a grid-following inverter (GFLI) that uses a conventional Phase-Locked Loop (PLL) as the synchronization unit. The proposed controller enhances the GFLI synchronization by expanding the PLL domain of attraction to the whole plane that is limited to a small region around the equilibrium point in a conventional PLL, provided that the grid impedance and voltage are known. Linearizing the overall closed-loop system, the proposed controller provides linear system attributes for the PLL, leading to an infinite domain of attraction and only one equilibrium point. Additionally, a state-feedback controller is integrated within the feedback linearization controller so that it enables the system to have an adjustable dynamic response. Finally, it is shown that the system is robust against parametric uncertainties. The performance of the proposed control design is validated in Matlab/Simulink and experiment. It is verified that the proposed controller expands the domain of attraction and enhances the system dynamic response. [ABSTRACT FROM AUTHOR]

The power calculation is an indispensable element in droop-controlled inverters because the bandwidth of the measured power has a direct impact on the controller performance. This paper proposes a fast and accurate power calculation algorithm based on the combined Second Order Generalized Integrator (SOGI) filters in stationary coordinates for a three-phase system, which takes into consideration the use of nonlinear loads. The power calculation scheme is formed by the two-stage SOGI filters that are employed for obtaining the active and reactive powers required to perform a droop-based inverter operation, respectively. From the two-stage structure, the first SOGI is used as a band-pass filter (BPF) for filtering harmonics and obtaining the fundamental current of the nonlinear load; The second SOGI is used as a low-pass filter (LPF) for extracting the DC-component, which corresponds with the average power. A small-signal model of a two droop-controlled inverters system is built to obtain the dynamical response and stability margin of the system. And compared it with the dynamical behaviour of a standard droop-control method. Next, the proposed power calculation system is designed in order to achieve the same ripple amplitude voltage as that obtained with the standard droop-control method by adjusting the bandwidth gains. Through simulation and hardware in the loop (HIL) validation, the proposed approach presents a faster and more accurate performance when sharing nonlinear loads, and also drives the inverters' output voltage with lower distortion. [ABSTRACT FROM AUTHOR]

This paper exhibits a sensorless speed control method based on full‐order‐sliding‐mode observer applied to a fault‐tolerant permanent‐magnet‐synchronous motor (PMSM) drive system. So, this paper proposes a rapid method of fault switch detection in the power converters aiming to make sure of the continuity of service despite the fault presence of an opening phase. In fact, the full‐order‐sliding‐mode observer (FO‐SMO) observer is used to replace the mechanical sensor and a redundant inverter leg is equally employed to replace the faulty leg. The proposed new fast fault diagnosis method has the features of a simple algorithm, independent of the transient states and being simply integrated without any additional sensors. [ABSTRACT FROM AUTHOR]

In electricity generation through photovoltaic cells, efficient inverters are required to inject the generated power into the grid. Among the inverters connected to the grid, current source inverters despite their advantages are used less than voltage source ones. Different circuits are presented for these converters. In this paper, several power circuit topologies of the current source inverters, which are an interface between solar panels and the grid, are reviewed. Also, the inverters are compared from the point of some indexes like efficiency, voltage transmission ratio, total harmonic distortion, leakage currents, and their reduction methods. The importance of these indexes is investigated too. Categorization is for full-bridge inverters and special structures groups. The first group includes the conventional inverter, 4-leg inverter, CH7 CSI, H7 CSI, three-mode, and other structures. The second group consists of inverters with special structures and is independent of the conventional CSI. The summary of the studies is presented in a table. [ABSTRACT FROM AUTHOR]

The power inverter topologies are indispensable devices to incorporate distributed generation schemes, like photovoltaic energy sources into the AC main. The nonlinear behavior of the power inverter draws a challenge when it comes to their control policy, rendering linear control methods often inadequate for the application. The control complexity can be further increased by the LCL filters, which are the preferred way to mitigate the current ripple caused by the inverter switching. This paper presents a robust variable structure control for a three-phase grid-tied inverter with an LCL filter. As well to the benefits of the sliding mode control (SMC), which is one of the control methods applied by power converters founded in literature, the proposed control scheme features a novel partial state observer based on the immersion and invariance technique, which thanks to its inherent robustness and speed of convergence is adequate for this application. This observer eliminates the need for physical current sensors, decreasing the overall cost and size, as well as probable sources of noise. The proposed controller is meant for a three-phase grid-tied inverter to inject active power to the grid while harmonics generated by nonlinear loads are compensated. The simulation results prove the effectiveness of the proposed method by compensating for current harmonics produced by the nonlinear loads and maintaining a low total harmonic distortion as recommended by the STD-IEEE519-2014, regardless of whether the system provides active power or not. [ABSTRACT FROM AUTHOR]

The instantaneous output power of the two-stage inverter pulsates at twice the output frequency, resulting in the second harmonic current (SHC) in the front-end dc-dc converter. To reduce the SHC in the inductance branch while maintaining good dynamic performance, this paper proposes a control scheme based on a new inductor current feedback path in the front-end buck converter. In the scheme, through the equivalent gain, the virtual resistor, and the bandpass filter, the inductor current is fed to the front of the voltage regulator. The feedback function simplification is studied and the implementation of the proposed scheme is illustrated. With considerations of both reducing the SHC and improving the dynamic performance, the parameter design approach for the feedback function is also presented for the proposed SHC reduction control scheme, and the performance analysis and comparison from the viewpoint of output impedance are provided. The simulation model and the prototype of a 10 kW two-stage inverter are constructed. The simulation and experimental results about the low SHC in the front-end buck converter and the fast dynamic response during the load transient verify the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

This paper investigates both the fundamental and harmonic losses of permanent magnet synchronous motor (PMSM) in conjunction with inverter losses for a wide range of switching frequencies (SFs) and different direct-axis currents (Ids) under full power range. The effects of SFs and Ids on the total losses, including fundamental loss and harmonic loss in the windings and lamination core, conduction loss, and switching loss in IGBTs, are described through analytical models. Meanwhile, the loss models of the motor are modified and validated by the 2-D finite-element analysis. Furthermore, the efficiency improvement of the overall PMSM-inverter system for different operation conditions benefits from an artificial bee colony (ABC) algorithm proposed in this paper. The optimal SF and Id, in terms of PMSM-inverter overall efficiency, are preferred for every operation point, thanks to the proposed ABC and the precise calculation of losses. Experimental results validate the proposed methodology. [ABSTRACT FROM PUBLISHER]

In this paper, a cascaded H-Bridge (CHB)-multilevel converter (MC)-based photovoltaic (PV) system with no voltage or current sensors at the dc-side is proposed. Eliminating the dc-side sensors simplifies the hardware, leading to lower cost and higher reliability of the PV system. A novel scheme estimating the capacitors’ voltages from the ac output voltage of the inverter is developed. The scheme allows replacing all dc-side voltage sensors by a single-voltage sensor at the ac-side of the converter. Furthermore, the dc current sensors, conventionally required for the maximum power point tracking (MPPT), are also eliminated. Instead, the outputs from the capacitors’ voltage control systems are utilized for the MPPT. The effectiveness of the proposed dc-side sensorless system is experimentally demonstrated on a 2-kW single-phase seven-level CHB converter-based PV system. [ABSTRACT FROM PUBLISHER]

This paper presents an application of model predictive control to a variable-speed drive system operating in the multi-megawatt range. The variable-speed drive system comprises a synchronous machine fed by a line-commutated rectifier and a load-commutated inverter. The control task is to regulate the dc-link current, and hence the machine torque, to ensure the machine speed follows a given reference. The proposed control approach is model predictive control, where both the rectifier- and inverter-firing angles are considered as control inputs. The nonlinear model predictive torque controller has been implemented on an embedded system and applied in an industrial-scale pilot plant installation. The experiments show the successful operation of model predictive control on a plant with more than 48 MW power. [ABSTRACT FROM PUBLISHER]

Load-commutated inverters (LCI's) are often used to supply dual-three-phase synchronous motors in high-power variable-speed applications. A pair of LCIs is used in this arrangement to feed the two motor three-phase winding sets. In order to cope with inter-harmonic issues, a drive configuration with an interconnection of the two LCI dc-links has been proposed. In this paper, such a drive design is shown to produce an increased voltage stress on motor windings compared with traditional configurations. The problem is investigated in the paper by proposing an algebraic algorithm capable of predicting the steady-state voltage waveform applied to the motor terminals and arising between the star points of the two winding sets. Unlike conventional dynamic simulations, the proposed approach gives practically instantaneous results, making it possible to quickly investigate a wide number of possible operating conditions. Furthermore, it requires a limited knowledge of system parameters, which are often hardly available. Its reliability and accuracy are assessed by comparison with measurements on a test drive system and examples are given of the method application to the sizing of motor insulation system. [ABSTRACT FROM PUBLISHER]

This paper extends the previous virtual synchronous generator (VSG) control for voltage-source converters to modular multilevel converters (MMCs). Compared with the ac synchronous generator (SG), the VSG-based MMC is embedded with a power-angle characteristic. Starting from this feature and with reference to the existing ac system coherency-based equivalence methods, this paper proposes a novel coherency equivalence method for MMC used in large-scale renewable energy systems connected to the ac grid. This novel method mainly focuses on the coherency criterion and parameter aggregation methods which have significant differences compared with the ac system methods. The proposed equivalence method is validated on PSCAD/EMTDC and the study results indicate that: 1) the VSG-based MMC model does show the power-angle characteristic similar to the SG model; 2) the proposed coherency criterion can accurately and efficiently classify the paralleled MMC into coherent groups; and 3) the dynamic behaviors of the external systems are preserved after the aggregations of all previous coherency MMC groups. [ABSTRACT FROM AUTHOR]

Unbalanced load currents not only give rise to unbalanced voltages but also adversely affect the performance of the conventional current-limiting mechanisms. The latter might result in overcurrent stress on the distributed energy resources (DERs) or current harmonics. In this paper, a novel decentralized control method is proposed to improve the power quality and protect DERs from overload. The proposed controller makes use of the model predictive control (MPC) technique to minimize the voltage unbalance, improve current limiting, and prevent active power overload. The MPC is combined with the V-I droop method to realize coordinated operation with fast dynamic response. The proposed method is tested on the CIGRE benchmark microgrid. Simulation results demonstrate that the proposed method improves power quality but also allows for operation close to the maximum load capacity without imposing DERs to overload. [ABSTRACT FROM PUBLISHER]

The Sandia frequency-shift method, among all frequency-based active islanding detection methods (IDMs), is one of the most effective methods for inverter based distributed generators (DGs). The nondetection zone (NDZ) of the Sandia frequency shift (SFS) highly depends on its design parameters. If the design parameters are not properly tuned, they may result in failure of the method or deterioration of the system power quality (PQ) through injection of a high amount of harmonics. The main objective of this paper is to propose a new algorithm for SFS method parameter determination, which would add precision to the previous parameter design criterion. In this paper, also a new formula for multi-DG applications is proposed which results in optimally setting of all SFS parameters to eliminate NDZ. Using the new criterion, the negative impact of the conventional criterion on PQ is reduced, and the efficiency of the SFS IDM is increased. The simulation results show that the proposed algorithm results in more accurate detection of the islanding phenomenon in the shortest possible time. All of the simulations and NDZ analysis are carried out in the MATLAB environment. [ABSTRACT FROM PUBLISHER]

This paper addresses theoretical calculation of a single-phase multilevel pulsewidth modulation (PWM) inverter current total harmonic distortion (THD). Analytical approach introduced in late 1980s for a two-level inverter is generalized for an arbitrary level count for a single-phase inverter. Although the obtained closed-form piecewise analytical solutions assume (infinitely) large ratio of switching and fundamental frequencies and pure inductive load, they are practically very accurate for the ratios of (apparent) switching and fundamental frequencies larger than 25–30 and inductance-dominated loads. Along with strict mathematical solutions, simple accurate Bessel function approximation and hyperbolic average trend one are suggested. The formulas clearly reveal a single-phase PWM inverter current THD dependence on modulation index for an arbitrary voltage level count and are easily modified to cover grid-connected cases. [ABSTRACT FROM PUBLISHER]

In this paper, a control strategy for the parallel operation of three-phase inverters in a modular online uninterruptable power supply (UPS) system is proposed. The UPS system is composed of a number of dc/acs with LC filter connected to the same ac critical bus and an ac/dc that forms the dc bus. The proposed control includes two layers: 1) individual layer and 2) recovery layer. In individual layer, virtual impedance concept is employed in order to achieve active power sharing while individual reactive power is calculated to modify output voltage phases to achieve reactive power sharing among different modules. Recovery layer is mainly responsible for guaranteeing synchronization capability with the utility and voltage recovery. With the proposed control, improved voltage-transient performance can be achieved and also dc/ac modules can be plugged in and out flexibly while controlling the ac critical bus voltage, thus allowing plug’n’play or hot-swap capability. Detailed control architecture of the individual and recovery layers is presented in this paper. An experimental setup was built to validate the proposed control approach under several case-study scenarios. [ABSTRACT FROM AUTHOR]

This paper provides a theoretical discussion and comparison in energy balancing between a modular multilevel cascade inverter based on single-star bridge cells (SSBC) and that on single-delta bridge cells (SDBC). Attention is paid to applications involving asymmetric active-power generation in utility-scale grid-tied photovoltaic systems. Both qualitative and quantitative evaluation metrics to assess the energy-balancing capability are introduced and applied to both SSBC and SDBC inverters. As for the SSBC inverter, six zero-sequence voltage waveforms with different harmonic content enabling enhanced energy-balancing capability are analyzed and compared regarding their effectiveness. This paper also emphasizes on the SDBC as an alternative to the SSBC and highlights its superior operating characteristics under asymmetric active-power generation. [ABSTRACT FROM PUBLISHER]

Paralleling is the development tendency of the auxiliary inverter. This paper analyzes the problems of the parallel process in detail. First, for inherent contraction between voltage regulation performance and current sharing performance, the paper adopts the resistive droop method. Second, the paper improves dynamic performance and reliability of the droop method by improving power calculation and introducing the whole cycle adaptive droop method. Third, in order to solve the unshared current problem caused by inconsistent parameters, the paper increases active power proportion droop coefficient, and introduces voltage compensation strategy to compensate voltage drop caused by increased proportion droop coefficient. Fourth, the auxiliary parallel system has complicated load characteristics; the paper proposes voltage compensation strategy to compensate voltage drop caused by the pump load. This paper utilizes three-phase total reactive power to droop frequency and active power of each phase to droop amplitude, unbalanced current can be shared. This paper introduces special virtual impedance technology to solve the harmonic current sharing problem caused by the nonlinear load. Finally, the effectiveness of above theory is fully verified by simulation and experiment. [ABSTRACT FROM PUBLISHER]

The power factor and total harmonic distortion (THD) are important considerations when designing an output filter. In the case of the photovoltaic (PV) ac-module, the size and weight also have to be taken into consideration. This paper proposes an output filter to reduce size and weight, and provides the optimal design method for a PV ac-module. The proposed output filter consists mainly of a conventional CL filter with a trap filter. The trap filter is used to eliminate the harmonic at the switching frequency that contains most of the harmonics. Therefore, total inductance and the size of the filter can be reduced although the output filter components are increased. This paper presents analysis of the proposed filter characteristics in detail. Also, an optimal design method reducing the size of the output filter components including the damping resistor is proposed. The proposed methods are verified on the experimental prototype rated at 320 W with an ac-module based on the interleaved flyback inverter. The total filter inductance and volume are, respectively, reduced to 9.54% and 26.62% with the same performance. [ABSTRACT FROM AUTHOR]

This paper investigates possible reduction of pulsating torque in open-loop and vector-controlled induction motor drives through deployment of certain advanced bus-clamping pulsewidth modulation (ABCPWM) method. Toward this goal, a simple and machine-independent method is proposed to analyze the torque harmonic spectrum of a voltage source inverter fed induction motor, operated with any real-time pulsewidth modulation (PWM) method. The analytically evaluated torque harmonic spectra, pertaining to conventional space vector PWM (CSVPWM), bus-clamping PWM (BCPWM), and ABCPWM, are validated through simulation and experimental results. Theoretical and experimental studies bring out the superiority of the ABCPWM in terms of torque harmonics over CSVPWM and BCPWM. The magnitude of the dominant torque harmonic with the ABCPWM scheme is shown to be significantly lower than that with CSVPWM, over a wide range of speed. The rms torque ripple (i.e., total rms value of all harmonic torques) is lower with ABCPWM than with BCPWM over the entire range of speed. [ABSTRACT FROM PUBLISHER]