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The neutral grounding method is one of the crucial technologies used to ensure the safe and stable operation of distribution networks. With the development of power electronics-based devices, the application of highly controllable inverters to grounding systems has become a new research direction. In this review paper, based on the summary and analysis for the research status, the flexible grounding system is systematically classified according to its control strategy and structure. And then, the following core technologies are summarized: faulted line selection, current component detection, high-resistance ground identification, ground parameter measurement, neutral point voltage control, and fault location. Subsequently, deficiencies of flexible grounding systems are summarized from the following four perspectives: reliability, accuracy, speed, and stability. Finally, some discussions and recommendations for future development and research direction are presented. Especially, the reliability-enhanced hybrid grounding system that combines active inverters and low resistance for high-resistance ground faults is recommended. This paper provides a systematic literature review of the crucial technologies for flexible grounding systems and can be used as a technical reference for future research. [ABSTRACT FROM AUTHOR]

In this paper, on the basis of a conventional 12-stator-coils/17-rotor-teeth (12/17) flux-switching permanent magnet (PM) (FSPM) machine and a 6/17 V-shaped FSPM machine, a novel 6/17 multi-tooth FSPM machine with a pair of magnets located in the upper apex of stator tooth is proposed. Then, four multi-tooth topologies sharing the same 6-stator-coils but with different rotor teeth combinations, namely, 6/17, 6/10, 6/11, and 6/13, are compared by the 2-D finite element analysis (2-D-FEA). The FEA-predicted results reveal the optimal combinations of stator coils and rotor teeth should be 6/17 for high-performance multi-tooth machines. Thereafter, an optimal design criterion of multi-tooth FSPM machines for the combinations of stator coils and rotor teeth is proposed. Afterward, electromagnetic performance evaluation between the 6/17 V-shaped and 6/17 multi-tooth machines is conducted, in terms of back electromotive force (back EMF), electromagnetic torque, cogging torque, phase inductance, unbalanced magnetic force (UMF), torque/magnet ratio, and efficiency. The comparative results indicate that the proposed 6/17 multi-tooth machine is superior to the V-shaped one, since it exhibits much higher torque/magnet ratio, larger torque, lower inductance, and much lower UMF, though the cogging torque and total harmonic distribution of phase back EMF are unfavorably larger. [ABSTRACT FROM AUTHOR]

For modular permanent magnet (PM) machines with overlapping (OLP) windings widely employed in wind power generation, the large torque ripple and long end winding are major issues. In order to solve these problems, PM machines with non-overlapping (NOLP) windings and redundant teeth for easy modularity are proposed in this paper. The comparative study between modular machines with these two kinds of windings is necessary and is the major focus of this paper. For the sake of clarity, two modular dual 3-phase machines with 42-slots/32-poles (42S/32P) and 192S/32P combinations are chosen as examples to show the differences in terms of machine performance. The proposed 42S/32P modular machine adopts NOLP winding, while the conventional 192S/32P one uses the OLP type. Based on the results, it is found that the modular machine with NOLP winding has comparable average torque and efficiency. In the meantime, much lower torque ripple exists for the proposed modular machine regardless of the current value. The shorter and simpler end windings are beneficial to manufacturability. Moreover, the proposed modular machine with NOLP winding will be more fault tolerant due to smaller mutual inductances between phases and larger d-axis inductance. Finally, the proposed 42S/32P modular machine is prototyped and the experiments validate the correctness of the analyses in this paper. Despite two specific examples being used, the conclusion should be generic and can be employed to modular machines with other slot and pole number combinations. [ABSTRACT FROM AUTHOR]

This paper investigates the source and implication of efficiency minima that is typically observed during the second-harmonic load–pull of transistor amplifiers. The study starts with the theoretical derivation of time-domain voltage and current waveforms as a function of conduction angle ($\alpha$) at the second-harmonic efficiency minima, where the output power and drain efficiency (DE) are at minimum. Thereafter, this paper unfolds a systematic re-engineering approach that is developed to recover the performance degradation and to exploit the region of efficiency minima in favor of design and implementation of high-efficiency inverse class-F power amplifiers (PAs). Interestingly, the inferences drawn from the in-depth analysis are shown to provide a simplified first-pass design approach that guarantees inverse class-F PA operation without an a priori knowledge of device parasitic elements. Theoretical postulations and simulation results are experimentally validated using an on-wafer active harmonic load–pull and a prototype design using 1.95-mm NXP gallium nitride die at a frequency of 2.6 GHz. The designed PA delivers an output power of 40 dBm with DE of 76% and gain of 12 dB at 3-dB gain compression. The measurement results confirm the theoretical framework reported in this paper. [ABSTRACT FROM AUTHOR]

In this paper, we propose a loss calculation method considering minor hysteresis loops for permanent magnet (PM) motors driven by pulsewidth modulated (PWM) inverters. In the governing equation of the proposed method, both the effects of the major and minor hysteresis phenomena are expressed by a reaction magnetic field. This field is determined by iterative calculations of finite-element analysis (FEA) with a simple hysteresis modeling that considers the minor loops. The proposed method is applied to a PM motor, and the experimental verification is carried out. It is clarified that the minor hysteresis loops affect not only the hysteresis loss but also the eddy current loss because the skin depth varies with the differential permeability of the minor loops. [ABSTRACT FROM AUTHOR]

Distributed grid and microgrid are becoming increasingly important in power system nowadays. In these grids, reactive compensations and harmonic current suppression are commonly employed to improve power quality. For the doubly fed induction generator (DFIG), high frequency resonance (HFR) will occur when it is connected to parallel compensation grids. When DFIG system is connected to the harmonic grid with parallel compensation, frequency ranges of HFR and harmonics may have intersections. In this paper, some cases for DFIG system with the HFR damping and harmonics suppressor are given to reveal that the coupling between HFR damping control and harmonics suppression control may lead DFIG system into the unstable situation. Furthermore, a frequency detection is necessary for the existed HFR damping control when compensation capacitor changes, which will deteriorate the HFR damping performance due to the additional detection time on HFR frequency. This paper proposes an impedance reshaping control strategy based on a variable frequency resistance and chebyshev filter, the HFR damping control and harmonic current suppression can be achieved simultaneously to improve power quality for DFIG system connected to the harmonic grid with parallel compensation. Theoretical analysis, simulations, and experiments verify availability of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Ferroelectric and negative-capacitance field-effect transistors (FeFETs and NCFETs) have recently garnered great attention as devices for applications in memory and low-power logic, respectively. As these technologies are pursued, it is critical to have a variety of measurement approaches, including methods familiar to the electron-device and microwave communities that can aid in fully understanding the behavior of ferroelectrics in FeFETs and NCFETs. In this paper, we propose and show the viability of using frequency-domain electrical measurement techniques employing the well-known microwave S-parameters, and their large-signal generalization and X-parameters. Our methods provide the means to trace the intrinsic polarization versus electric-field curve of the ferroelectric, i.e., with the parasitics de-embedded, thereby showing the innate ferroelectric response, which cannot be done using conventional techniques. These methods also enable extraction of all the parameters of the Landau–Khalatnikov equation, which is commonly used to model ferroelectric behavior in FeFETs and NCFETs. This paper hence takes a useful step toward methods familiar to the electron-device community that can help to better understand and optimize FeFET and NCFET technologies. [ABSTRACT FROM AUTHOR]

This paper investigates the critical parameter design technique for a cascaded H-Bridge (CHB) with selective harmonic elimination (SHE)/compensation (SHC) for single-phase systems. The critical parameters include the filter inductance, the number of H-bridge cells, the dc-bus voltage and the switching frequency. This paper derives the relationship of electrical parameters, topologies, and modulation techniques to the voltage/current harmonic envelopes, and compares the filter inductor design for an H-Bridge with sinusoidal pulsewidth modulation (SPWM), an H-Bridge (HB) with SHE/SHC and a CHB with SHE/SHC. Furthermore, a harmonic-envelope-based approach is developed to determine the optimum design parameters. The developed approach designs the harmonic envelope with two proposed criteria: the first one is the capacity criterion, which guarantees that the fundamental and controllable components are within the applicable solution range to avoid overmodulation; the other one is the attenuation criterion, which ensures that the uncontrollable harmonic complies with the grid limit. Simulations and experiments verified that a CHB designed with the proposed technique can simultaneously fulfill the compensation objectives and meet the harmonic limits. [ABSTRACT FROM AUTHOR]

A parallel hybrid modular multilevel converter (PHMMC) belongs to the class of modular multilevel converters, which have become potential candidates for high-voltage direct-current (HVDC) transmission systems. Due to the circuit topology of a PHMMC, the dc bus voltage contains low-order harmonics and cannot be fully regulated at a constant dc voltage. The dc bus voltage, if not properly controlled, leads to improper power transfer and increases the magnitude of dc current ripple on the dc transmission line. This paper proposes a zero-sequence voltage injection (ZSVI)-based model predictive control (MPC) strategy to control the dc current/power flow and simultaneously minimize the dc current ripple. The proposed strategy takes advantage of a cost function minimization technique to determine and inject the optimal zero-sequence voltage components into the dc- bus voltage of a PHMMC system. This paper derives a discrete-time dynamic model of the dc transmission-line current and, correspondingly, develops a predictive model. The predictive model is used to inject the appropriate amount of zero-sequence voltage components to the dc bus reference voltage waveform. Compared with the existing triplen harmonics injection method, the proposed ZSVI-MPC strategy improves the performance of a PHMMC system in terms of minimization of the dc current/voltage ripple. Performance of the proposed strategy for a 21-level PHMMC-based HVDC station system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported results demonstrate superior performance of the PHMMC-HVDC station operating based on the proposed ZSVI-MPC strategy, under various operating conditions, as opposed to the existing triplen harmonics injection method. [ABSTRACT FROM AUTHOR]

Harmonic loads and unbalanced loading in a distribution transformer lead to higher losses, an increase in temperature that leads to premature failure, and a shorter lifetime. To prevent these difficulties, the rated power of a transformer supplying nonlinear loads or unbalanced load is reduced; commonly known as derating. In this paper, an alternative to traditional loss based derating is proposed. It is shown that derating might actually not be required when contrasted with the traditional approach. A perspective is presented on how to make the kVA rating “adaptive” and utilize the same kVA rating with or without harmonics. Actual field data are presented to support this viewpoint. [ABSTRACT FROM AUTHOR]

This paper proposes a measurement strategy to remove the random phase contributions of the sweeping local oscillator (LO) from modulated radio frequency phase measurements based on downconversion. To obtain a stable phase spectrum for each run of tone-by-tone sweeping measurement, the LO used for downconversion is coupled out to an extra path and modulated to be a wideband multisine signal. By monitoring the phase change of multisine signal at a specific and fixed frequency point, random LO phase offsets during the frequency sweep can be measured and compensated. In this paper, two experimental test benches with the same core measurement strategy but different architectures are established for method validation. According to the experimental results using test bench one, the phase stability is ±(0.2°–0.4°) at 1.8 GHz for fundamental mixing-based downconversion and ±(0.6°–1°) for harmonic mixing at 6 GHz. Based on test bench two, relative phase deviations of ±(0.8°–1.2°) at 22 GHz for fundamental mixing and ±(2°–3°) at 66 GHz for harmonic mixing are reachable. [ABSTRACT FROM AUTHOR]

The need for lighter and smaller railway traction drives and recent advances in power semiconductor technology have resulted in a new energy conversion paradigm, one without the classical heavy and bulky line-frequency transformer. Several multilevel topologies have been introduced over the past years, all having in common the use of medium-frequency (MF) energy conversion to reduce the weight and volume of the magnetic components. This paper is concerned with the design of the input filter for an MF traction converter, focusing on a solution that minimizes the filter's overall mass. Two filter topologies are investigated, namely a single-pole filter (inductor) and a passively damped $LCL$ filter. The paper first describes the requirements to be fulfilled, establishes the spectrum to consider for the design, and proposes a method to determine the initial parameters of the filter's components. The results show that an $LCL$ -type filter achieves considerable weight savings in comparison to the single-pole solution. [ABSTRACT FROM AUTHOR]

This paper studies the output performance improvement of the spherical multi-degree-of-freedom (multi-DOF) motor. The multi-DOF motor is a motor that is designed to drive in multiple axes. Therefore, it is important that the shape of the motor should be fabricated to enable multi-axis driving when designing the multi-DOF motor. In addition, it is essential to consider the output density characteristics and stability characteristic analysis to reduce the volume of the multi-DOF motor and to operate of the motor in the stable range, respectively. In this paper, the structure and operating characteristics of the spherical multi-DOF motor with dual air gaps are described. In order to improve the output characteristics of the multi-DOF motor, characteristics analysis was performed according to the change of the number of the phases and pole pitch angles, and stability characteristic analysis for stable operation of the multi-DOF motor was performed. [ABSTRACT FROM AUTHOR]

High-speed electric machines with permanent magnets are sometimes equipped with a shielding cylinder; a conductive sleeve that is wrapped around the magnets in order to reduce the rotor losses and/or retain the magnets. Despite the every increasing popularity of such high-speed permanent-magnet synchronous machines (PMSMs), there are still a lot of question regarding the effect of the shielding cylinder. Earlier work in literature evaluates how the shielding cylinder’s design affects the machine’s performance. However, that work assumes a sinusoidal current, hence neglects the stator current’s higher time-harmonic orders. Therefore, this paper aims at evaluating how different stator sources affect the way in which the shielding cylinder influences the torque production in high-speed PMSMs. The study is performed using a 2-D analytical subdomain model that accounts for slotting and the eddy current reaction field. Both current and voltage sources are considered. The torque is divided in two components, the classical torque due to interaction between the magnets and the stator currents and the torque due to interaction with the eddy currents in the shielding cylinder. This approach results in a better insight in the machine’s physics. [ABSTRACT FROM AUTHOR]

This paper investigates the performance of switch-mode power supplies used in desktop PCs (PC-SMPS’) across the entire range of operating powers. Experimental results show that at low to medium operating powers, the input ac current might lose its periodicity with supply voltage frequency, when PCS-MPS’ can be an unexpected source of interharmonics and dc component. In such cases, there is a significant increase of PC-SMPS’ current waveform distortion and a substantial decrease of efficiency and operational power factors, which requires application of appropriate measurement and calculation procedures during the analysis. To correctly assess these power-dependent changes in performance and power quality indicators, this paper introduces a novel testing and evaluation methodology, based on the known or assumed PC-SMPS operating cycle. [ABSTRACT FROM AUTHOR]

Cogging torque is due to the interaction between a magnet’s magnetomotive force and a slotted stator or rotor structure. In this paper, a compact equation for cogging torque is presented for surface-mounted permanent magnet motors. This equation has four terms of field interactions involving tangential and radial components of airgap flux density. These four terms are considered as sources of cogging. Variations of these sources with pole embrace are examined. In earlier publications, closed-form expressions for pole embrace corresponding to the minimum cogging were presented considering only radial fields. In this paper, an attempt is made to recognize the influence of tangential fields in cogging. The compact form given here distinctly reflects the significance of both radial and tangential field components. Furthermore, to study the trends of cogging sources with pole number and airgap radius, simplified expressions for harmonic content of field components are derived using the first-order binomial approximation. The analysis on cogging presented in this paper helps in determining search domains in which optimum embrace for minimum cogging can be found when both the radial and tangential field components are considered. [ABSTRACT FROM AUTHOR]

This paper presents a new asymmetric Class-F−1/F Doherty power amplifier (DPA) consisting of a Class-F−1 carrier amplifier and a Class-F peaking amplifier. The asymmetric current waveform due to different harmonic loading conditions offers an asymmetric fundamental current in load modulation region, ensuring an improvement in the performance at the peak output power. In this paper, improvements in the large signal performances, such as the output power and efficiency, are analyzed using the derived fundamental current models. For verification, a 2.4-GHz GaN Class-F−1/F DPA was designed and fabricated. The designed Class-F−1/F DPA achieves a peak output power of 44 dBm with a peak drain efficiency (DE) of 86.7%. Using a local thermal equilibrium 10-MHz signal with a 6.6-dB peak-to-average power ratio, the Class-F−1/F DPA achieves an average output power of 37.3 dBm with a DE of 68% at a supply voltage of 28 V. The measured adjacent channel leakage power ratio with a digital predistortion is below −50 dBc. [ABSTRACT FROM AUTHOR]

In the rapidly growing field of electrified transportation technology, hybrid electric and electric vehicle (HEV/EV) manufacturers have been placing increasing demands on the designers to improve the efficiency and reduce the costs of vehicle components to sustain a niche in the EV market. To understand the significance and challenges in an efficient and economical drivetrain design, this paper presents a part of an ongoing project that aims at developing a comprehensive study on dc/ac drive motor technology in EV applications. In this paper, a new three-level three-phase matrix converter topology, along with two conventional frequently used three-level three-phase dc/ac converter topologies, namely, diode-clamped and H-bridge converters, are implemented. Furthermore, comprehensive analysis and comparative studies are conducted to examine the performance and efficiency of these converter topologies according to EV design criteria. In addition, experimental investigations are carried out on the aforementioned converter configurations to help EV designers choose the most suitable converter topology. [ABSTRACT FROM PUBLISHER]

In this paper, an analytical torque ripple model for interior permanent magnet synchronous machine (IPMSM) is introduced at first, which can be used to estimate the torque ripple. Then, the inductance variation due to magnetic saturation is analyzed through experimental investigations, which demonstrates that magnetic saturation has a great influence on the inductances, and hence the torque ripple minimization performance. Thus, this paper proposes an online parameter identification approach to estimate the inductances and incorporates them into the torque ripple minimization. The proposed parameter identification approach can estimate the dq-axis inductances accurately such that it brings significant torque ripple minimization improvement for IPMSMs. Numerical and experimental investigations have been conducted to validate the proposed approach based on a laboratory IPMSM. [ABSTRACT FROM AUTHOR]

This paper proposes a novel hybrid excitation vernier permanent magnet machine (HE-VPM). The characteristics of the proposed machine are: 1) the PMs are embedded in the stator teeth, which is helpful for the heat dissipation; 2) the rotor is salient structure without PMs and windings, which ensures the reliability of high-speed operation; 3) the nonoverlapping concentrated winding shortens the end-connection, and improves the torque density, 4) only half of the stator teeth possess PMs, and the total amount of PMs are decreased; and 5) the dc current are injected to the armature coils, therefore, the exciting field can be adjusted flexibly. In this paper, the electromagnetic performance, including back electromotive force, torque, etc. are analyzed with finite element analysis. The results show that the proposed machine exhibits flexible field adjustment capability, which may be suitable for applications requiring wide speed range operation. Besides, it is found that, compared with the existing pure sinusoidal phase current, higher torque density can be obtained with dc-biased phase current under constant stator copper loss. [ABSTRACT FROM AUTHOR]

Coreless axial flux machines are of interest because of the absence of stator core losses and cogging torque. These machines generally employ concentrated windings. One of the challenges with such a winding is that the torque producing MMF component that corresponds to the fundamental of the magnet excitation is accompanied by substantial asynchronous components. These harmonics cause losses in the rotor core and magnets, which can become significant at high speeds. This paper proposes a new multilayer winding arrangement to eliminate the non-torque producing MMF components. This winding is applied to a 12-coil 16-pole coreless axial flux machine. The efficacy of the winding is established by 3-D finite-element analysis. [ABSTRACT FROM AUTHOR]

This paper proposes a flexible neutral grounding method based on electromagnetic hybrid Petersen coil (EHPC), which consists of a magnetically controlled reactor and an active power compensator (APC). Under normal conditions, the system's neutral point grounds through a reactor of high reactance. When a single line-to-earth fault occurs, the EHPC can be controlled flexibly to suppress the recovery voltage of faulty phase and to reduce the ground-fault current to almost zero. This paper expounds the two operating modes and corresponding characteristics, the measuring method of insulation parameters of the system (mainly including capacitance to earth and leakage resistance), and the full compensation principle for arc suppression based on the equivalent negative impedance of EHPC. This paper also proposes a compound control method of neutral-to-earth impedance based on neutral-point displacement voltage and a compensating method for the harmonic component of the ground-fault current. Compared to traditional grounding methods, this method can be fast and reliably make the residual current approach almost zero without arc reignition while greatly reducing the output capacity of APC and, thus, decreasing the cost of this device. Simulations and experiments verify the correctness of this measuring method and feasibility of this flexible grounding control strategy. [ABSTRACT FROM PUBLISHER]

The use of power electronics has led to a growing importance of higher time-harmonic content in electrical machines. To gain insight in phenomena related to these higher harmonics, such as noise and losses, a good understanding of the magnetic field’s harmonic content is mandatory. Moreover, the development of fast and accurate, harmonic-based, analytical models requires a qualitative knowledge of the machine’s time- and spatial-harmonic content. Although the harmonic content of electric machines is an extensively studied topic, previous publications tended to focus on one type of synchronous machines and often did not consider higher time-harmonic orders. This paper complements the existing theory by providing a more general approach, thereby covering machines and operating points that were not covered until now. It considers both three-phase and multi-phase machines with an odd number of phases. The winding distribution can either have an integer or a fractional number of slots per pole and per phase, and higher time-harmonic content is regarded as well. Note that the saturation is neglected. Despite its general validity, this paper succeeds at providing one simple equation to determine the machine’s time-and spatial-harmonic content. Moreover, this paper also extensively discusses the physical causes of the harmonic content. The combination of this general validity, the simple result, and the insight in the physics makes that this paper is a strong tool to both study harmonic-related phenomena in electric machines and to develop harmonic-based analytical models. [ABSTRACT FROM AUTHOR]

This paper presents analytic and experimental results for high-efficiency GaN HEMT class-F and inverse class-F (class-F−1) synchronous rectifiers. The rectifier designs are based on time-reversed duals of corresponding class-F amplifiers. A detailed analysis of the power efficiency of a class-F rectifier is presented. The analysis shows that, unlike the amplifier dual, the rectifier configuration does not have significant overlap loss because of device constraints in terms of operation in quadrant III. Consequently, the power efficiency of the class-F rectifier is slightly higher than the amplifier circuit under equal source power conditions. Experimental measurements are shown, which compare the rectification efficiency of a class-F rectifier with class-F−1 rectifiers. It is shown that operation of the rectifiers under back-off conditions mimics the power efficiency of an amplifier with envelope tracking. Based on this paper, it is concluded that the class-F−1 mode has better rectification efficiency compared with that of class-F, both at peak power and under back-off conditions. [ABSTRACT FROM PUBLISHER]

This paper presents novel nonlinear vector network analyzer (NVNA)-based techniques for characterizing the harmonic and intermodulation nonlinearity of dual-band radio frequency (RF) power amplifiers (PAs). On the one hand, a wideband Schroeder-phase multisine is used to drive a step recovery diode comb generator, so that a novel multiharmonic-band NVNA phase reference on a dense spectral grid is obtained. On the other hand, a novel phase calibration approach is developed to synchronize the NVNA multiband measurements. To validate the NVNA testbed with novel techniques, an RF PA under two typical kinds of dual-band multisine stimulus is tested, where the NVNA measurement is compared with that of an oscilloscope. This paper can be easily adapted for various multiband conditions. [ABSTRACT FROM PUBLISHER]

Harmonic resonance has become an important concern for the application of shunt capacitors in recent years. A potential solution to address this challenge is to convert a shunt capacitor into a passive filter. This paper presents design methods to configure a shunt capacitor as a C-type filter or a third-order high-pass filter with guaranteed resonance-free performance. The concept of resonance-free condition is first introduced in this paper. It is then used to develop filter design methods that always meet the resonance-free condition. The two filter configurations are also compared. It was found that the third-order high-pass filter has more advantages than the C-type filter. Another useful finding of this work is that the filter parameters, as determined using the proposed design methods, are independent of the system conditions. As a result, a lookup table for the filter parameters has been created to facilitate immediate use by the industry. [ABSTRACT FROM PUBLISHER]

This paper presents a novel realization of nonlinear vector network analyzer (NVNA) test-bed with a new phase calibration technique, in order to characterize the multiharmonic intermodulation nonlinearity of a radio frequency (RF) power amplifier (PA) under a large-signal modulated input. The NVNA test-bed uses a step recovery diode comb generator as the phase reference, which is driven by a pulsed-RF signal with a high duty cycle of over 99%. Hence all the multiharmonic intermodulation tones of interest can be measured at once. A multisine phase standard provided by a vector signal analyzer is used to calibrate the intermodulation tones at each harmonic on a dense spectral grid. Two harmonic phase standards with different fundamental frequencies are used to synchronize the phase calibration results at different harmonics. The validity of this paper is verified by comparing NVNA measurements with oscilloscope measurements of the same PA under test. This paper can be used to inspect the nonlinearity of RF amplifiers under large-signal modulated excitations in both the frequency and time domains. [ABSTRACT FROM AUTHOR]

This paper studies quasi-synchronization in networked heterogeneous harmonic oscillators. By introducing a leader, two distributed synchronization protocols are first proposed for heterogeneous networks by utilizing continuous real-time information and aperiodic sampled-data information. Then, the sufficient conditions on quasi-synchronization are established for heterogeneous networks coupled with nonidentical harmonic oscillators. It is found that each follower oscillator can converge to a bounded region of the leader by adopting either a continuous-time protocol or sampled-data protocol. The upper bound of the region is solved for networked heterogeneous harmonic oscillators. Finally, an electrical network is provided to illustrate the applicability of the theoretical results, and two examples are provided to illustrate the effectiveness of the sufficient criteria. [ABSTRACT FROM AUTHOR]

Due to their good rotational invariance and stability, image continuous orthogonal moments are intensively applied in rotationally invariant recognition and image processing. However, most moments produce numerical instability, which impacts the image reconstruction and recognition performance. In this paper, a new set of invariant continuous orthogonal moments, polar harmonic Fourier moments (PHFMs), free of numerical instability is designed. The radial basis functions (RBFs) of the PHFMs are much simpler than those of the Chebyshev-Fourier moments (CHFMs), orthogonal Fourier-Mellin moments (OFMMs), Zernike moments (ZMs), and pseudo-Zernike moments (PZMs). For the same degree, the RBFs of the PHFMs have more zeros and are more evenly distributed than those of the ZMs and PZMs. Therefore, PHFMs do not suffer from information suppression problem; hence, the image description ability of the PHFMs is superior to that of the ZMs and PZMs. Moreover, the RBFs of the PHFMs are always less than or equal to 1.0 near the unit disk center, whereas those of the OFMMs, PZMs, CHFMs, and radial harmonic Fourier moments (RHFMs) are infinite (implying numerical instability). This indicates that PHFMs can outperform these moments in image reconstruction tasks. We theoretically and experimentally demonstrate that PHFMs outperform the above moments in reconstructing images and recognizing rotationally invariant objects considering noise and various attacks. This paper also details the significance of the PHFM phase in image reconstruction, angle estimation using PHFMs, and the accurate moment selection of the PHFMs. [ABSTRACT FROM AUTHOR]

This paper reports a practical challenge in Distributed Energy Resources (DER) operation: open phase detection. The paper presents a wide variety of naturally occurred open phase events recorded by power quality meters in DER commercial operations. The events demonstrate the complexity of open phase detection due to electrical and magnetic interphase couplings through transformer winding and core configurations. The coupling effect could recreate the missing phase in various patterns so that the DER control and protection systems or control room operators may not detect the anomaly for prolonged period of time. This paper shares experiences of timely DER open phase detection by using power quality data and suggests practical detection guidelines based on waveform signature analytics. The original field records provide credible benchmarking references for algorithm development to meet the new DER open phase detection requirement in the IEEE Stdandard 1547–2018 revision. [ABSTRACT FROM AUTHOR]

The inter‐turn short‐circuit fault of a power transformer has a large current in the winding, which will cause serious heating and deformation of the winding and may threaten the safety of the power transformer. Under this background, this paper proposes a scheme based on open‐delta voltage monitoring and identification of inter‐turn short circuits in power transformers. First, the mathematical modeling and analysis of the principle of inter‐turn short‐circuit monitoring of power transformers are carried out, and the mathematical expression to characterize power transformers with different short‐circuit turns are given. Second, the magnetic field distribution of the power transformer under different short‐circuit conditions, the relationship between the eigenvalue and the number of short‐circuit turns etc. are given. Finally, the experimental study verifies the correctness of the proposed scheme. [ABSTRACT FROM AUTHOR]

In this paper, firstly, optimal sizing and placement of photovoltaic-based distributed generation units (PVDGUs) and passive harmonic filters (PFs) are proposed to improve both the PVDGU hosting capacity and bus voltage profile of the distorted radial distribution systems. The objective of the proposed approach (PA) is defined as minimizing the rms deviation and total harmonic distortion of the bus voltages while maximizing the hosting capacity. Its constraints are considered as bus voltage total and individual harmonic distortion limits placed in IEEE standard 519, permissible bus voltage rms intervals, and the numbers and total powers of the PVDGUs and PFs. Secondly, it is aimed to investigate the performance of the PA on the improvement of the hosting capacity and voltage profile. Thus, for the modified IEEE 33 bus test system containing nonlinear loads, it is comparatively evaluated with a traditional PF planning approach (TA) based on minimization of the total voltage harmonic distortion. Finally, the effects of the harmonic analysis algorithm and PVDGU harmonic model preferences on the solutions of both approaches are also analysed. One of the major conclusions obtained from the analysis is that PA achieves significantly higher PVDGU hosting capacity while providing total voltage harmonic distortion mitigation performance very close to TA. In addition, PA achieves almost the same hosting capacity for all considered harmonic models and harmonic analysis algorithms. However, this is not the case for TA. [ABSTRACT FROM AUTHOR]

Tidal-level prediction is crucial for ensuring the safety and efficiency of offshore marine activities, port and channel management, water transportation resource development, and life-saving operations. Although tidal harmonic analysis is among the most prevalent methods for predicting tidal water level fluctuations, it relies on extensive data, and its long-term prediction accuracy can be limited. To enhance prediction performance, this paper proposes a model that combines the variational mode decomposition (VMD) algorithm with the long short-term memory (LSTM) neural network. The initial step involves decomposing the original data using the VMD algorithm, followed by applying the LSTM to each decomposition component. Finally, all prediction results are superimposed and summed. The model is tested using the 2018 tidal time series data from the Lvsi station in Zhoushan City and the 2020 tidal time series data from the Ganpu station. The results are compared with those from the classical harmonic analysis model, the traditional machine learning model, and the decomposition-based machine learning method. The experimental outcomes demonstrate the superior predictive capabilities of the proposed model. [ABSTRACT FROM AUTHOR]

Some research progress has been made on fractional-order continuous orthogonal moments (FrCOMs) in the past two years. Compared with integer-order continuous orthogonal moments (InCOMs), FrCOMs increase the number of affine invariants and effectively improve numerical stability. However, the existing types of FrCOMs are still very limited, of which all are planar image oriented. No report on stereoscopic images is available yet. To this end, in this paper, FrCOMs corresponding to various types of InCOMs are first deduced, and then, they are combined with trinion theory to construct trinion FrCOMs (TFrCOMs) applicable to stereoscopic images. Furthermore, the reconstruction performance and geometric invariance of TFrCOMs are analyzed theoretically and experimentally. Finally, an application in the stereoscopic image zero-watermarking algorithm is investigated to verify the superior performance of TFrCOMs. [ABSTRACT FROM AUTHOR]

A phase-locked loop (PLL) is commonly used in grid-connected power converter for synchronization. A single input single output (SISO) linear model of PLL with the phase angle of the point of common coupling (PCC) voltage as input and the estimated phase angle as the output is generally used in the PLL analysis and design. However, an undesirable coupling between the magnitude and phase of the input voltage is present when a filtering stage is incorporated before the control loop, which a SISO model cannot capture, and could result in a significant disturbance in the PLL estimated phase during grid faults. This paper proposes a generalized multi-input multi-output (MIMO) linear model that captures the complete PLL dynamics during symmetrical faults for a PLL equipped with any prefilter. Furthermore, a compensation method that decouples the magnitude dynamics of the input voltage from the phase dynamics is proposed in this paper. The proposed compensation improves the PLL's phase tracking performance by ensuring that the prefiltered PLL acts only on the PCC voltage phase changes. A vital application of the proposed compensation method for PLL is during a grid fault case, wherein the converters are expected to contribute to the fault current, which requires an accurate measurement of the PCC voltage phase. The compensation method's effectiveness is demonstrated using power hardware in the loop simulation of a hardware voltage source converter interfaced to a distribution system simulated in real-time. [ABSTRACT FROM AUTHOR]

The harmonic impedance of the grid needs to be calculated for the harmonic assessment of new connections in the high voltage (HV) network. For these studies, one of the elements is the modeling of the downstream distribution networks. In the Netherlands, these medium voltage (MV) networks are cabled with a length between 20 to several hundreds km. These distribution networks could have a significant impact on the network impedance. For the HV network operator, it is almost impossible to model these networks in detail as not all information of the distribution network operators is readily available. The aim of this paper is to present a simplified representation for the distribution networks and develop a method to determine the parameters to be used in the model via a clustering method. To verify the method of clustering and compare the various equivalents, a comparison is made with an exact representation of a distribution grid. A sensitivity analysis has been performed on the most important parameters. It has been found that the Type-2 load model gives the best result compared the conventional ones. If the network information is known by the transmission system operators (TSOs), the Type-2 load model can be constructed directly. Otherwise the approach based on the clustering process as described in this paper can be used as an alternative. [ABSTRACT FROM AUTHOR]

Harmonic resonances in DFIG-based wind parks can be investigated by using extensive electromagnetic transient simulations, which is a time-consuming process. This paper shows that not all wind parks need such a detailed analysis. In response, the paper proposes a practical, easy-to-use method for a first-cut assessment of the risk of harmonic resonance of DFIG-based wind parks. First, simple equations are obtained to estimate resonance frequency and amplification in terms of wind park parameters readily available in practice. These equations reveal combinations of wind park parameters that cause problematic resonance. Such combinations are used to trace regions of concern in a chart. If the wind park is out of all regions of concern of the chart, one can directly declare this park is resonance-free, without running any simulation study. In addition to the analytic (model-based) method, a measurement-based method is provided for obtaining the resonance chart without running any simulation. Examples show how the proposed approach can be applied to facilitate resonance assessment during the wind park planning and operation. [ABSTRACT FROM AUTHOR]

The use of power converters is very important in maximizing the power transfer from renewable energy sources such as wind, solar, or even a hydrogen-based fuel cell to the utility grid. An LCL filter is often used to interconnect an inverter to the utility grid in order to filter the harmonics produced by the inverter. Although there is an extensive amount of literature available describing LCL filters, there has been a gap in providing a systematic design methodology. Furthermore, there has been a lack of a state-space mathematical modeling approach that considers practical cases of delta- and wye-connected capacitors showing their effects on possible grounding alternatives. This paper describes a design methodology of an LCL filter for grid-interconnected inverters along with a comprehensive study of how to mitigate harmonics. The procedures and techniques described in this paper may be used in small-scale renewable energy conversion systems and may be also retrofitted for medium- and large-scale grid-connected systems. [ABSTRACT FROM PUBLISHER]

This paper presents a technique to mitigate zero-sequence harmonics in power distribution systems. The method is based on the concept of passive zero-sequence harmonic filters. However, its basic configuration has been expanded to create a double-tuned filtering feature. This feature makes it possible to trap two harmonics with one filter and is especially attractive in solving harmonic-caused telephone interference problems. Furthermore, this paper has shown that common utility service transformers can be used to construct the filter. As a result, a practical and low-cost solution to mitigating zero-sequence harmonics has been found. A method for sizing and loading assessment of filters has also been developed. As an example application, the proposed filter package has been applied to mitigate a telephone interference problem. Issues, such as filter location, the number of filters required, and the effectiveness of filtering harmonics produced by distributed residential loads have been investigated. The results show that the proposed filter is a very promising technique to reduce zero-sequence harmonics in primary power distribution systems. [ABSTRACT FROM PUBLISHER]

This paper investigates a feasible nondestructive method for micron-sized kissing defects in the composite insulators, which are commonly utilized in ultra-high voltage (UHV) power grid. In this paper, cylindrical adhesive contact model was established to describe the obvious distortion of ultrasound propagation caused by kissing defects on the polymeric interface of composite insulators. The ultrasonic nonlinearity parameter was measured for each of the kissing defective and intact specimens. Moreover, the theoretical analysis has been proved by the test results on the kissing defective samples, which indicates that the 0.7 μm kissing defects existed in the composite insulators could be successfully recognized by the nonlinear ultrasonic methods and the nonlinearity is reduced by interface roughness. [ABSTRACT FROM AUTHOR]

Due to different solar radiation, temperature, and other reasons of modules in the three-phase cascaded H-bridge (CHB) photovoltaic (PV) inverter, the output power among PV modules will be unequal and lead to unbalanced grid currents, which cannot meet the requirements of grid codes. On the other hand, the second-order voltage ripple is aroused in the dc-link capacitor since each phase-leg of the CHB inverter is made up of a single-phase inverter. Concerning these two issues, this paper proposes an optimized third harmonic injection method based on fundamental frequency zero sequence injection (FFZSI), which is a conventional method to handle the slight power imbalance problem. In this paper, the FFZSI is adopted to redistribute the power among three phases, and then the third harmonic is injected into the three-phase combined reference waveforms according to the two requirements: all modules are free from overmodulation and the dc-link voltage fluctuation is mostly reduced. Therefore, the proposed method can reduce the dc-link voltage fluctuation and extend the power balance range of FFZSI at the same time. The validity and effectiveness of the proposed method are verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Cascaded H-bridge converter (CHB) is a multilevel topology that is a well-suited solution for multiple applications such as flexible ac transmission systems or motor drives. This paper is focused on a CHB where the cells present an aging mismatch. This can be caused by the maintenance operation which forces the replacement of some damaged cells of the converter with new or repaired ones. In this paper, a new improved approach of the active thermal control (ATC) of the CHB using discontinuous pulsewidth modulation (PWM) (D-PWM) is presented. The D-PWM technique is used to reduce the power losses of one cell reducing its average temperature in order to increase its remaining lifetime. However, the combination of D-PWM with traditional phase-shifted PWM (PS-PWM) introduces high harmonic distortion in the output voltage of the CHB converter at twice the carrier frequency. A detailed harmonic distortion analysis of the CHB output voltage when the D-PWM based ATC is active is presented. From this analysis, a modification of the traditional PS-PWM is derived to eliminate the harmonic distortion at twice the carrier frequency. Experimental results show how the ATC using D-PWM is achieved whereas the harmonic distortion around twice the carrier frequency is eliminated. [ABSTRACT FROM AUTHOR]

This paper proposes a novel polygonal voltage space vector structure (SVS) having 30 sides, for a star-connected induction motor drive. The SVS eliminates the presence of harmonics up to 25th order from motor phase voltage throughout the entire modulation range, providing a torque profile devoid of lower order pulsations. Linear modulation is extended till 99.63% of base speed without exceeding the motor phase voltage rating. Topology consists of a dc-link fed primary inverter and two equal low voltage modular capacitor fed secondary inverters. Here the harmonics generated by the primary inverter is canceled by the secondary inverter which acts as a switched capacitive filter. Detailed description of the SVS generation and timing calculations are provided in this paper. Effectiveness of the proposed scheme is validated using experimental results, inverter loss calculations, and harmonic analysis. [ABSTRACT FROM AUTHOR]

This paper proposes a new control scheme to eliminate the 3rd harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical 3rd harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a 3rd harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup. [ABSTRACT FROM AUTHOR]

This paper proposes a sensorless vibration harmonic frequency estimation for a servo system driven by a permanent magnet (PM) synchronous machine. Traditional vibration harmonic estimation relies on the measurement using external sensors. An accelerometer is typically installed in the servo system to measure the vibration harmonic signal. Because the accelerometer must be attached on the visibly vibrational location inside the system, this vibration detection results in both issues about installation and reliability. To overcome this drawback, this paper proposes a disturbance observer to estimate the vibration harmonic by using current and position signal. A real-time signal process is developed to identify the vibration frequency from estimated harmonic. A servo system with a 400-W PM machine is built to verify the proposed vibration harmonic and frequency estimation. This paper includes the comparative evaluation between the accelerometer-based vibration harmonic measurement and the sensorless estimation. [ABSTRACT FROM AUTHOR]

Modern power systems face several problems at frequencies above the range of traditional stability studies due to the increasing penetration of power electronic devices. This phenomenon has been recently referred to as electromagnetic transient stability and is a growing issue in systems with power electronic converters at any power and voltage level, such as FACTS, high-voltage DC, interfaces for grid connection of renewable energies, custom power devices for power quality enhancement, among others. The study of power systems stability is usually carried out by simulation tools that represent the power system through phasor models. However, due to the faster dynamics of modern systems, the harmonic interaction among the controllers, converters, and electric power components, recent research results suggest that some stability analyses should be conducted with models suitable for wider frequency ranges, e.g., electromagnetic transient models. This paper reviews the use of frequency domain methods to analyze the influence of the modeling approach of passive power components on the results of electromagnetic transient stability studies. [ABSTRACT FROM AUTHOR]

Harmonic blocking is usually applied by converter transformer differential protection to prevent protection mal-operation during an inrush condition, but it may incorrectly block the protection during an internal fault. This paper first makes an in-depth analysis on factors that lead to differential protection failure to trip for converter transformer internal faults. The main factor responsible for the protection failure to trip is half-cycle saturation of the converter transformer core, which results from the dc component in the fault current. This suggests that harmonic blocking may not be suitable for differential protection of converter transformers to identify inrush conditions. In this regard, this paper proposes a novel method using mathematical morphology to effectively discriminate between the conditions caused by inrush and internal faults. This method implements the waveform symmetry criterion and improved morphological gradient criterion to distinguish between these two conditions. The effectiveness of the proposed method is evaluated based on extensive simulation, and the presented results show that the proposed method can ensure operation for all types of internal faults. Additionally, this method improves the accuracy of inrush identification, even in extreme cases, such as sympathetic inrush and inrush with current transformer saturation. [ABSTRACT FROM AUTHOR]

This paper presents an optimization method for selective harmonic elimination in a cascaded multilevel inverter using teaching–learning-based optimization (TLBO). The main objective in selective harmonic elimination (SHE) strategy is to eliminate low-order harmonics by solving nonlinear equations and reaching optimal solution, while the fundamental component is satisfied. In this paper, in one side, the influence of optimal dc sources is investigated to reach the SHE goal, and in another side, comparing optimization methods is considered. In this paper, the TLBO as a recently emerged nature-inspired algorithm is presented to provide better results for the SHE in comparison with genetic algorithm, artificial bee colony, imperialistic competitive algorithm, harmony search, ant colony optimization, particle swarm optimization, and differential evolution. For better comparison of those methods and influence of optimal dc sources, 5-, 9-, and 15-level inverters are chosen and MATLAB software is used for optimization. Simulation results show the superiority of TLBO, higher precision and probability of convergence than other mentioned algorithms. Finally, to validate the influence of optimal dc sources and the accuracy of TLBO results, the experimental setup is conducted for a 5-level cascaded H-bridge inverter with optimal dc sources. [ABSTRACT FROM AUTHOR]

The main technical challenge of the medium-voltage motor drives with modular multilevel converter (MMC) is the serious low-frequency fluctuation on the capacitor voltages, and the existing suppression methods inevitably inject high-frequency circulating currents in MMC. However, the output currents are evidently distorted by the large injected components, while little research has been done to study and solve this problem. This paper analyzes and proves the essential interactions between the output current distortion and the injected components, and the specific dominant output harmonic groups are derived and summarized, revealing the nature of the problem, and it can be taken as a favorable reference for the controller designing. Based on the derivations, this paper proposes a novel suppression method for the greatly improved harmonic characteristics of the output currents during the low-frequency operation. The output harmonics can be accurately measured by the proposed control and effectively suppressed for much lower current total harmonic distortion, benefiting for the superior output control performances with improved efficiency. The simulation results on PSCAD/EMTDC and experimental verifications on a 2.2 kW MMC-based motor drive prototype prove the validity of the presented harmonic analysis and suppression method. [ABSTRACT FROM AUTHOR]