Showing papers on "Harmonics published in 2017"

Damping Methods for Resonances Caused by LCL-Filter-Based Current-Controlled Grid-Tied Power Inverters: An Overview

Abstract: Grid-tied voltage source inverters using LCL filter have been widely adopted in distributed power generation systems (DPGSs). As high-order LCL filters contain multiple resonant frequencies, switching harmonics generated by the inverter and current harmonics generated by the active/passive loads would cause the system resonance, and thus the output current distortion and oscillation. Such phenomenon is particularly critical when the power grid is weak with the unknown grid impedance. In order to stabilize the operation of the DPGS and improve the waveform of the injected currents, many innovative damping methods have been proposed. A comprehensive overview on those contributions and their classification on the inverter- and grid-side damping measures are presented. Based on the concept of the impedance-based stability analysis, all damping methods can ensure the system stability by modifying the effective output impedance of the inverter or the effective grid impedance. Classical damping methods for industrial applications will be analyzed and compared. Finally, the future trends of the impedance-based stability analysis, as well as some promising damping methods, will be discussed.

Plasmon-enhanced high-harmonic generation from silicon

Abstract: High-harmonic emission from crystalline silicon can be made ten times brighter by exploiting local plasmonic fields in arrays of nano-antennas. Plasmonic antennas can enhance the intensity of a nanojoule laser pulse by localizing the electric field in their proximity1. It has been proposed that the field can become strong enough to convert the fundamental laser frequency into high-order harmonics through an extremely nonlinear interaction with gas atoms that occupy the nanoscopic volume surrounding the antennas2,3,4. However, the small number of gas atoms that can occupy this volume limits the generation of high harmonics5,6,7. Here we use an array of monopole nano-antennas to demonstrate plasmon-assisted high-harmonic generation directly from the supporting crystalline silicon substrate. The high density of the substrate compared with a gas allows macroscopic buildup of harmonic emission. Despite the sparse coverage of antennas on the surface, harmonic emission is ten times brighter than without antennas. Imaging the high-harmonic radiation will allow nanometre and attosecond measurement of the plasmonic field8 thereby enabling more sensitive plasmon sensors9 while opening a new path to extreme-ultraviolet-frequency combs10.

Sequence Impedance Modeling of Modular Multilevel Converters

Abstract: This paper presents the development of sequence impedance models for modular multilevel converters (MMC). The intended applications of the models include stability and resonance analysis of high-voltage dc transmission, static synchronous compensation, and other systems that use MMC. The basis of the modeling method is harmonic linearization that has been applied to other types of converters, but a new formulation is presented to allow the inclusion of harmonic effects. This generalization, called multiharmonic linearization, is necessary for MMC because of the significant second harmonics present in the arm currents, capacitor voltages, and control signals. To accommodate multiple harmonics in the linearization process, a matrix formulation is introduced and used to model both the converter power stage and its control. The developed sequence impedance models are validated by point-by-point simulation of detailed converter circuit models, and used to understand the effects of harmonics and control on MMC frequency-domain characteristics.

Harmonic Instability Assessment Using State-Space Modeling and Participation Analysis in Inverter-Fed Power Systems

Abstract: This paper presents a harmonic instability analysis method using state-space modeling and participation analysis in the inverter-fed ac power systems. A full-order state-space model for the droop-controlled distributed generation (DG) inverter is built first, including the time delay of the digital control system, inner current and voltage control loops, and outer droop-based power control loop. Based on the DG inverter model, an overall state-space model of a two-inverter-fed system is established. The eigenvalue-based stability analysis is then presented to assess the influence of controller parameters on the harmonic instability of the power system. Moreover, the harmonic-frequency oscillation modes are identified, where participation analysis is presented to evaluate the contributions of different states to these modes and to further reveal how the system gives rise to harmonic instability. Based on the participation analysis, a reduced-order model for harmonic instability analysis is also proposed. The experimental results are presented for validating the theoretical analyses.

Review of harmonic analysis, modeling and mitigation techniques

Abstract: Power quality problems are manifested in voltage, current or frequency deviations causing malfunction of sensitive equipment. Integration of inverter connected PV and wind power plants, and rampant rise in nonlinear loads have led to harmonic problem in power system. Nonlinear loads and switched devices energized by sinusoidal sources or linear loads and switched devices with non-sinusoidal sources, produce harmonics in distribution system. Academic harmonic analysis study consists of modeling nonlinear loads to develop Norton and Thevenin equivalent circuits of devices for integration into harmonic analysis software. Experimental researchers often use harmonic analyzers to measure the harmonics in real systems to evaluate suitable mitigation alternatives. The distortive power losses force utilities to increase apparent power to maintain reliable and uniform power supply. Harmonic analyzers use data acquisition hardware and inbuilt software algorithms to perform onsite measurements. Harmonic analyzers help find true power factor, total harmonic distortions, reactive and distortive power losses. Use of shunt capacitance at unity power factor worsens the situation instead of supplying distortive power compensation. Active power factor correction techniques, using smart algorithm to cancel the distortive power, have been reviewed for further research. Nonlinear physics of harmonic phenomenon is described to explore its applications. Harmonic mitigation technologies have been compared, current state of the art technology reviewed and demonstrated by designing a harmonic filter. Measurement of harmonics, waveform distortions, and true power factor (TPF) of single and three phase electronic loads is carried out to test their compliance to harmonic standard limits. Energy conservation concept requires reduction of harmonics in distribution networks. This study found 60±10% reduction in power factor and more than 2% increase in line losses due to widespread use of nonlinear loads. Utility apparent power demand increases due to consumers’ inadvertent violation of IEC Standard 61000-3-2 and IEEE Standard 519–1992.

Nonreciprocal electromagnetic scattering from a periodically space-time modulated slab and application to a quasisonic isolator

Abstract: Scattering of obliquely incident electromagnetic waves from periodically space-time modulated slabs is investigated. It is shown that such structures operate as nonreciprocal harmonic generators and spatial-frequency filters. For oblique incidences, low-frequency harmonics are filtered out in the form of surface waves, while high-frequency harmonics are transmitted as space waves. In the quasisonic regime, where the velocity of the space-time modulation is close to the velocity of the electromagnetic waves in the background medium, the incident wave is strongly coupled to space-time harmonics in the forward direction, while in the backward direction it exhibits low coupling to other harmonics. This nonreciprocity is leveraged for the realization of an electromagnetic isolator in the quasisonic regime and is experimentally demonstrated at microwave frequencies.

Higher-order power harmonics of pulsed electrical stimulation modulates corticospinal contribution of peripheral nerve stimulation

Abstract: It is well established that electrical-stimulation frequency is crucial to determining the scale of induced neuromodulation, particularly when attempting to modulate corticospinal excitability. However, the modulatory effects of stimulation frequency are not only determined by its absolute value but also by other parameters such as power at harmonics. The stimulus pulse shape further influences parameters such as excitation threshold and fiber selectivity. The explicit role of the power in these harmonics in determining the outcome of stimulation has not previously been analyzed. In this study, we adopted an animal model of peripheral electrical stimulation that includes an amplitude-adapted pulse train which induces force enhancements with a corticospinal contribution. We report that the electrical-stimulation-induced force enhancements were correlated with the amplitude of stimulation power harmonics during the amplitude-adapted pulse train. In an exploratory analysis, different levels of correlation were observed between force enhancement and power harmonics of 20–80 Hz (r = 0.4247, p = 0.0243), 100–180 Hz (r = 0.5894, p = 0.0001), 200–280 Hz (r = 0.7002, p < 0.0001), 300–380 Hz (r = 0.7449, p < 0.0001), 400–480 Hz (r = 0.7906, p < 0.0001), 500–600 Hz (r = 0.7717, p < 0.0001), indicating a trend of increasing correlation, specifically at higher order frequency power harmonics. This is a pilot, but important first demonstration that power at high order harmonics in the frequency spectrum of electrical stimulation pulses may contribute to neuromodulation, thus warrant explicit attention in therapy design and analysis.

Advanced High Torque Density PM Vernier Machine With Multiple Working Harmonics

Abstract: In recent years, permanent magnet (PM) vernier machines have gained more and more attention due to their high torque density and simple mechanical structure. However, PM vernier (PMV) machines with lap windings always suffer from long end winding length, and regular nonoverlapping winding may result in torque reduction for PMV machines. In this paper, an advanced PMV machine topology with multiple working harmonics is proposed. With specially designed stator auxiliary teeth, this topology could achieve ∼20% higher torque density than that of a regular nonoverlapping winding PMV machine, with the same magnet usage. Through finite element algorithm and theoretical analysis, the production of additional flux density harmonics and their contributions to back-electromotive force (EMF) are verified. Moreover, the electromagnetic performances of this novel machine topology, such as back-EMF and output torque, are quantitatively investigated with the geometric parameters’ effect considered. Finally, analysis results are verified by experimental test on a 21 Nm prototype, which is designed to have similar volume and weight with a 14 Nm regular commercial PM machine.

Mapping the electron band structure by intraband high-harmonic generation in solids

Abstract: High-harmonic generation (HHG) has recently been extended to solids, enabling all-optical reconstruction of electron band structure. However, material absorption of above-the-bandgap interband harmonics used for this purpose in earlier work limits the applicability of this promising technique. Here, sub-100-fs mid-infrared pulses tunable within the range of wavelengths from 5.0 to 6.7 μm are used to study HHG in ZnSe. Below-the-bandgap high-order harmonics generated by such driver pulses fall within the transparency range of a solid material, thus removing absorption-related limitations on the depth of HHG. Such harmonics are shown to be ideally suited to probe the nonlinearities of electron bands, enabling an all-optical mapping of the electron band structure in bulk solids.

Mitigation of Harmonics in Grid-Connected and Islanded Microgrids Via Virtual Admittances and Impedances

Abstract: Optimization of the islanded and grid-connected operation of microgrids is important to achieve a high degree of reliability. In this paper, the authors consider the effect of current harmonics in single phase microgrids during both modes of operation. A detailed analysis of the effect of the output impedance of the considered primary control loops on the harmonic output of the considered voltage source inverters is initially carried out. A virtual admittance loop is proposed to attenuate the current harmonic output in grid-connected operation that is generated due to the grid voltage distortion present at the point of common coupling (PCC) and due to local non-linear loads. This paper also considers the harmonic current sharing and resulting voltage harmonics at the PCC during islanded operation of the microgrid. A capacitive virtual impedance loop was implemented to improve the harmonic current sharing and attenuate the voltage harmonics at the PCC. Experimental results are given to validate the operation of the proposed algorithms.

A second-order sliding mode and fuzzy logic control to optimal energy management in wind turbine with battery storage

Abstract: The optimal control of large-scale wind turbine has become a critical issue for the development of renewable energy systems and their integration into the power grid to provide reliable, secure and efficient electricity, despite any possible constraints such as sudden changes in wind speed. This paper deals with the modeling and control of a hybrid system integrating a permanent magnet synchronous generator (PMSG) in variable speed wind turbine (VSWT) and batteries as energy storage system (BESS). Moreover a new supervisory control system for the optimal management and robust operation of a VSWT and a BESS is described and evaluated by simulation under wind speed variation and grid demand changes. In this way, the proposed coordinated controller has three subsystems (generator side, BESS side and grid side converters). The main function of the first one is to extract the maximum wind power through controlling the rotational speed of the PMSG, for this a maximum power point tracking algorithm based on fuzzy logic control and a second-order sliding mode control (SOSMC) theory is designed. The task of the second one is to maintain the required direct current (DC) link voltage level of the PMSG through a bidirectional DC/DC converter, whereas in the last, a (SOSMC) is investigated to achieve smooth regulation of grid active and reactive powers quantities, which provides better results in terms of attenuation of the harmonics present in the grid courant compared with the conventional first-order sliding controller. Extensive simulation studies under different conditions are carried out in MATLAB/Simulink, and the results confirm the effectiveness of the new supervisory control system.

Laser waveform control of extreme ultraviolet high harmonics from solids.

Abstract: Solid-state high-harmonic sources offer the possibility of compact, high-repetition-rate attosecond light emitters. However, the time structure of high harmonics must be characterized at the sub-cycle level. We use strong two-cycle laser pulses to directly control the time-dependent nonlinear current in single-crystal MgO, leading to the generation of extreme ultraviolet harmonics. We find that harmonics are delayed with respect to each other, yielding an atto-chirp, the value of which depends on the laser field strength. Our results provide the foundation for attosecond pulse metrology based on solid-state harmonics and a new approach to studying sub-cycle dynamics in solids.

Square-Wave Voltage Injection Algorithm for PMSM Position Sensorless Control With High Robustness to Voltage Errors

Abstract: Rotor position estimated with high-frequency (HF) voltage injection methods can be distorted by voltage errors due to inverter nonlinearities, motor resistance, and rotational voltage drops, etc. This paper proposes an improved HF square-wave voltage injection algorithm, which is robust to voltage errors without any compensations meanwhile has less fluctuation in the position estimation error. The average position estimation error is investigated based on the analysis of phase harmonic inductances, and deduced in the form of the phase shift of the second-order harmonic inductances to derive its relationship with the magnetic field distortion. Position estimation errors caused by higher order harmonic inductances and voltage harmonics generated by the SVPWM are also discussed. Both simulations and experiments are carried out based on a commercial PMSM to verify the superiority of the proposed method and the investigations in this paper.

Control Algorithms of Shunt Active Power Filter for Harmonics Mitigation: A Review

Abstract: Current harmonics is one of the most significant power quality issues which has attracted tremendous research interest. Shunt active power filter (SAPF) is the best solution to minimize harmonic contamination, but its effectiveness is strictly dependent on how quickly and accurately its control algorithms can perform. This manuscript reviews various types of existing control algorithms which have been employed for controlling operation of SAPF. Harmonic extraction, DC-link capacitor voltage regulation, current control and synchronizer algorithms are examined and discussed. The most relevant techniques which have been applied for each control algorithm are described and contrasted in an organized manner to identify their respective strengths and weaknesses. It is found that the applied control algorithms differ in two conditions: (1) the condition where harmonic current distortion is treated by the SAPF in the presence of non-ideal source voltage; and (2) the condition where multilevel inverter is employed as the circuit topology of SAPF.

Reduction of EMI with Chaotic Space Vector Modulation in Direct Torque Control

Abstract: In this paper a study is presented about the reduction of electromagnetic interference (EMI) in Direct Torque Control (DTC) method by the means of chaotic space vector modulation. Space Vector Modulated Direct Torque Control (SV-DTC) is a method developed to reduce current and moment fluctuations which stem from the hysteresis controllers used in classical DTC method. The switching frequency amplitude was changed chaotically in a Space Vector Modulated Direct Torque Control (FFSV-DTC) having a Fixed switching frequency, and Chaotic Space Vector Modulated Direct Torque Control (CAFSV-DTC) method having chaotic amplitude modulated switching frequency was obtained. CAFSV-DTC method was suggested to reduce EMI, acoustic noise and current harmonics which occur in FFSV-DTC method. For this purpose, the proposed CAFSV-DTC method, using a Permanent Magnet Synchronous Motor (PMSM) drive, is compared to Random Amplitude Frequency method (RSV-DTC) and FFSV-DTC. Simulation results indicate that the proposed method yields better results. DOI: http://dx.doi.org/10.5755/j01.eee.22.1.14097

On Resonances and Harmonics in HVDC-MMC Station Connected to AC Grid

Abstract: Resonance and harmonic phenomena can occur when HVDC and Flexible Alternating Current Transmission System devices are inserted into ac grids. Frequency response analysis and electromagnetic transient (EMT) type simulations of converter stations are essential for analyzing and preventing negative effects. This paper presents two approaches for deriving the high frequency response of a converter station: using simplified analytical representation and accurate EMT-type model. Validations with control parameter impacts are presented. This paper also presents a test case, based on a recent HVdc project, where potential resonances can occur between the converter station and its ac network. It is demonstrated analytically that these resonances are mainly due to control system parameters and ac network configurations.

Suppression of Dead-Time Distortion Through Revised Repetitive Controller in PMSM Drives

Abstract: This paper proposes a new dead-time compensation method for voltage-source inverters (VSIs) used in permanent-magnet synchronous motor (PMSM) drives. The proposed technique is developed based on the revised repetitive controller (RRC) to reduce current harmonics and distortion, which is essential to improve efficiency, rotor position observer performance at very low speed, and self-commissioning accuracy. This method significantly suppresses the sixth-order harmonics and its multiples in synchronous reference frame and reduces the current total harmonic distortion (THD) without depending on the precise current sampling especially in the zero-crossing region. Unlike in most average value theory or pulse based compensations, it does not require additional hardware. It is also shown that RRC-based compensation has better robustness against motor parameter variations than disturbance observer based methods and requires very low computational effort. Compared to other repetitive controllers (RC), the proposed RRC is dedicated for dead-time compensation and can easily be integrated because of its simpler structure. In addition, it can be conveniently activated or deactivated on the fly, which adds more flexibility to the control overall algorithm. The effectiveness of the proposed method is validated by theoretical analysis, spectrum analysis, as well as simulation and experimental results.

Helicity-Selective Enhancement and Polarization Control of Attosecond High Harmonic Waveforms Driven by Bichromatic Circularly Polarized Laser Fields

Abstract: High harmonics driven by two-color counterrotating circularly polarized laser fields are a unique source of bright, circularly polarized, extreme ultraviolet, and soft x-ray beams, where the individual harmonics themselves are completely circularly polarized. Here, we demonstrate the ability to preferentially select either the right or left circularly polarized harmonics simply by adjusting the relative intensity ratio of the bichromatic circularly polarized driving laser field. In the frequency domain, this significantly enhances the harmonic orders that rotate in the same direction as the higher-intensity driving laser. In the time domain, this helicity-dependent enhancement corresponds to control over the polarization of the resulting attosecond waveforms. This helicity control enables the generation of circularly polarized high harmonics with a user-defined polarization of the underlying attosecond bursts. In the future, this technique should allow for the production of bright highly elliptical harmonic supercontinua as well as the generation of isolated elliptically polarized attosecond pulses.

Comprehensive Analysis and Reduction of Torque Ripples in Three-Phase Four-Switch Inverter-Fed PMSM Drives Using Space Vector Pulse-Width Modulation

Abstract: As a result of their reduced number of switches, three-phase four-switch (TPFS) inverters are generally applied as cost-reduction topologies for permanent magnet synchronous motor (PMSM) drives. However, the torque ripples of PMSM severely deteriorate the performance and reliability of the entire system. Hence, comprehensive considerations for torque ripple reduction, including high- and low-frequency torque ripples, are elaborated considering TPFS inverter-fed PMSM drives. The second-order torque harmonics produced by dc-capacitor voltage fluctuations are first demonstrated, and a very simple compensation method is presented by introducing a novel nonorthogonal coordinate transformation. Then, to evaluate the effects on the high-frequency torque ripples of space vector modulation (SVM) schemes, three SVM schemes for TPFS inverter-fed PMSM drives are assessed based on the torque ripple root-mean-square value. Consequently, the preferred SVM scheme is obtained for high-frequency torque ripple minimization. Moreover, the linear modulation range of the TPFS inverter-fed PMSM drive is derived considering capacitor voltage fluctuations, therein avoiding the low-frequency torque ripples caused by overmodulation. Meanwhile, an adaptive capacitor voltage offset suppression method is proposed to fully exploit the dc-link voltage. The experimental results demonstrate the validation and effectiveness of the proposed analysis and methods for torque ripple reduction.

Harmonic Interaction Analysis in a Grid-Connected Converter Using Harmonic State-Space (HSS) Modeling

Abstract: An increasing number of power-electronic-based distributed generation systems and loads generate not only characteristic harmonics but also unexpected harmonics. Several methods, such as impedance-based analysis, which are derived from the conventional average model, are introduced to perform research about the harmonic interaction. However, it is found that the linear-time-invariant-based model analysis makes it difficult to analyze these phenomena because of the time-varying properties of the power-electronic-based systems. This paper investigates a grid-connected converter by using the harmonic state-space (HSS) small-signal model, which is based on a linear time-varying periodically theory. The proposed model can include the switching behavior of the model, where it makes the model possible to analyze how harmonics are transferred into both the ac-side and dc-side circuits. Furthermore, a harmonic matrix of the grid-connected converter is developed to analyze the harmonic interaction at the steady-state behavior. Besides, the frequency-domain results are compared with time-domain simulation results by using the HSS modeling to verify the theoretical analysis. Experimental results are finally discussed to verify the proposed model and study.

Variable-Angle Phase-Shifted PWM for Multilevel Three-Cell Cascaded H-Bridge Converters

Abstract: Multilevel cascaded H-bridge converters have become a mature technology for applications where high-power medium ac voltages are required. Normal operation of multilevel cascaded H-bridge converters assumes that all power cells have the same dc voltage, and each power cell generates the same voltage averaged over a sampling period using a conventional phase-shifted pulse width modulation (PWM) technique. However, this modulation method does not achieve good results under unbalanced operation per H-bridge in the power converter, which may happen in grid-connected applications such as photovoltaic or battery energy storage systems. In the paper, a simplified mathematical analysis of the phase-shifted PWM technique is presented. In addition, a modification of this conventional modulation method using variable shift angles between the power cells is introduced. This modification leads to the elimination of harmonic distortion of low-order harmonics due to the switching (triangular carrier frequency and its multiples) even under unbalanced operational conditions. The analysis is particularized for a three-cell cascaded H-bridge converter, and experimental results are presented to demonstrate the good performance of the proposed modulation method.

Direct Control of the Inverter Impedance to Achieve Controllable Harmonic Sharing in the Islanded Microgrid

Abstract: 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.

Effect of transition dipole phase on high-order-harmonic generation in solid materials

Abstract: High-order harmonic spectra from solid materials driven by single-color multicycle laser fields sometimes contain even harmonics. In this work we attribute the appearance of even harmonics to the nonzero transition dipole phase (TDP) when the solid system has broken symmetry. By calculating the harmonic efficiency from graphene and gapped graphene by using the semiconductor Bloch equations under the tight-binding approximation, we demonstrate the role of the TDP, which has been ignored for a long time. When the crystal has inversion symmetry, or reflection symmetry with the symmetry plane perpendicular to the laser polarization direction, the TDP can be neglected. Without such symmetry, however, the TDP will lead to the appearance of even harmonics. We further show that the TDP is sensitive to the crystal geometry. To extract the structure information from the harmonic spectra of a solid the TDP cannot be ignored.

Lyapunov Function and Sliding Mode Control Approach for the Solar-PV Grid Interface System

Abstract: This paper deals with control of a solar-photovoltaic (PV) power-generating system interfaced with the grid. A sliding mode control approach is used for achieving maximum power tracking control of a solar-PV array. The Lyapunov function-based control approach is designed and modeled for the dc–ac inverter to serve the functions of an active power injection to the grid, balanced grid currents at unity power factor and load currents harmonics compensation. The proposed approaches eliminate the need of adjustment of system parameters under changing loads and generation scenario. The effectiveness of the proposed control strategies is established using its stability analyses. The performance of the solar-PV power-generating system with the proposed control algorithms is demonstrated using simulation and experimental studies under various operating conditions.

A Fast and Fixed Switching Frequency Model Predictive Control With Delay Compensation for Three-Phase Inverters

Abstract: Finite control set-model predictive control (FCS-MPC) has been used in power converters due to its advantages, such as fast dynamics, multi-objective control, and easy implement. However, due to variable switching frequency, the harmonics of inverter output current spread in a wide range of frequency. Furthermore, a large amount of computation is required for the implementation of the traditional FCS-MPC method. Here, an improved FCS-MPC algorithm with fast computation and fixed switching frequency is proposed in this paper for two-level three-phase inverters. First, according to the principle of deadbeat control, the inverter voltage vector reference can be constructed. Then, the operation durations and sequences of different voltage vectors are determined according to the location of the inverter voltage vector reference and the cost functions of different voltage vectors. In this algorithm, the operation durations of different voltage vectors are arranged inversely proportional to their cost functions. Compared with the conventional fixed switching frequency FCS-MPC control, the number of sectors involved in the FCS-MPC calculation can be reduced from 6 to 1, which greatly improves the computation efficiency. Moreover, the delay due to digital implementation is effectively compensated in the proposed algorithm. Finally, experimental tests are carried out to verify the advantages of the proposed method in terms of both steady-state and dynamic performance.

Design Optimization and Comparative Study of Novel Dual-PM Excited Machines

Abstract: This paper systematically studies a new kind of PM machines with both stator and rotor PM excitations, namely, dual-PM excited machines. The key is to rely on the PM-iron structure in the machine to provide both PM excitation and flux modulation. Besides the fundamental field component in the air-gap, some other predominant harmonics introduced by the flux modulating effect can also contribute to the electromagnetic torque production. Therefore, this kind of machines can be designed with high torque density. Four dual-PM excited machine concepts with the same rotor configuration but different stator structures are comparatively studied, which include double-stator PM machine, stator multitooth-PM machine, stator slot-PM machine, and stator tooth-PM machine (STPM). Based on the flux modulating effect, the general design principle of the dual-PM machines is proposed in this paper. Through analytically investigating the air-gap field harmonics, the physical insight of the dual-PM machines is brought forward. All the four machines are optimized using an improved Tabu search coupled with finite element method, and their electromagnetic performances are comprehensively studied and compared. A prototype of STPM is manufactured. Experimental tests are conducted and the results well verify the electromagnetic design.

Harmonic Emissions of Three-Phase Diode Rectifiers in Distribution Networks

Abstract: Harmonic emissions have been changed in distribution networks, with respect to frequency range and magnitude, due to the penetration of modern power electronics systems. Two new frequency ranges 2–9 and 9–150 kHz have been identified as new disturbing frequency ranges affecting distribution networks. This paper presents the effects of grid-connected three-phase systems with different front-end topologies: conventional, small dc-link capacitor, and electronic inductor. A power converter with a small dc-link capacitor can create a resonant frequency with the line impedance below and above 1 kHz depending on the grid configurations. The resonant effects depend on many factors, such as load power levels, filter types, and the number of parallel drives. These issues can affect the grid current harmonics and power quality of the distribution networks. Analyses and simulations have been carried out for three different topologies and the results have been verified by experimental test at system level. Current harmonic emissions have been considered for 0–2, 2–9, and 9–150 kHz frequency ranges.

Adaptive Feedforward Algorithm Without Grid Impedance Estimation for Inverters to Suppress Grid Current Instabilities and Harmonics Due to Grid Impedance and Grid Voltage Distortion

Abstract: The performance of the grid-connected inverter was affected by the uncertainty of the grid conditions including the background distortion and the grid impedance. Typically, the feedforward of the grid voltage at the point of common coupling (PCC) highly suppressed the grid current harmonics caused by the grid voltage distortion; however, the PCC grid usually had a nonnegligible grid impedance, and the PCC voltage feedforward aroused serious grid current harmonics or instability. This study proposes a novel adaptive algorithm for the PCC voltage feedforward to work well with the varied grid impedance. In the proposal, the band-pass filters at the harmonic frequencies are used to detect the variation of the grid impedance as well as to facilitate the adaptive PCC voltage feedforward. It is not necessary to inject an additional harmonic to estimate the grid impedance. The basic principles as well as the realization and logic of the proposed algorithm are detailed, and some selected waveforms are provided to verify the superior performance. Compared with the typical robust design or adaptive control, the proposed algorithm does not have to sacrifice the dynamic or the harmonics rejection performance, or to use the on or offline grid impedance estimation.