Second-order Harmonic Component Research Articles

CFD Study for Wave Run-up Characteristics Around a Truncated Cylinder with Damper

In this study, numerical simulations for a single fixed truncated circular cylinder in regular waves were conducted to investigate the nonlinear wave run-up under various dampers and wave period conditions. The present study used the volume of fluid (VOF) technique to capture the air-water interface. The unsteady Reynolds-averaged Navier–Stokes (URANS) equation with the k– ϵ turbulence model was solved using the commercial computational fluid dynamics (CFD) software STAR-CCM+. First, a systematic spatial convergence study was conducted to assess the performance and precision of the present numerical wave tank. The numerical scheme was validated by comparing the numerical results of wave run-up on a bare truncated cylinder with the experimental results, and a good agreement was achieved. Then, a series of parametric studies were carried out to examine the wave run-up time series around the truncated cylinder with single and dual dampers in terms of the first- and second-order harmonic and mean set-up components. Additionally, the local wave field and the flow velocity vectors adjacent to the cylinder were evaluated. It was confirmed that under short wave conditions, the high position of the damper led to a noticeable increase in the wave run-ups with significant changes in the first- and second-order harmonic components.

Circulating Current Control for a Three-Phase Modified Modular Multilevel Converter With a Reduced Number of Current Sensors

The high capacitance requirement becomes a significant problem when modular multilevel converters (MMCs) are applied to medium or low-voltage systems. This paper proposes a capacitor voltage control scheme for a three-phase modular multilevel converter (MMC) with optimized capacitor sizing. Unlike the existing circulating current control, the proposed controller uses the middle capacitor voltage to control the second-order harmonic component of the circulating current. Therefore, it is unnecessary to measure the two arm currents; only one voltage sensor is required, which reduces the number of current sensors. The proposed controller can effectively control the circulating current to be constant or inject second-order harmonics to reduce the capacitor sizing. The detailed analysis, controller design, and performance of the proposed scheme are presented in this paper. The effectiveness of this study is confirmed by simulation and experimental results.

Coordinated Second-Order Voltage Harmonic Mitigation in Hybrid Microgrids

This paper introduces a compensation methodology that enables the coordination of multiple DC/DC converters to mitigate the second-order voltage harmonic component in hybrid microgrids. The proposed methodology utilizes metaheuristic algorithms to prohibit circulating currents and share harmonic currents among the compensating circuits. The proposed method can be easily implemented by any DC/DC converter while its efficacy is independent of the feeders' impedances and the physical location of the compensating equipment. The elimination of circulating currents, and the capability to share the total compensation current between the compensating circuits are not affected by the line impedances. In addition, the implementation of the proposed scheme does not require any expensive real-time communication. The mathematical analysis of hybrid microgrids seen from the perspective of harmonic compensation by coordinating multiple DC/DC converters is also presented, being a useful tool to design compensation methods. In addition, a qualitative and a quantitative comparison of the proposed methodology with prior-art methods in the literature are presented to further demonstrate the performance of the proposed solution. Finally, simulations and experimental tests verify the effectiveness of the proposed methodology.

Comparisons of second- and third-order ultrasonic nonlinearity parameters measured using through-transmission and pulse-echo methods

It is well known that ultrasonic nonlinearity parameters are effective for assessing material degradation, so many applied studies have been actively conducted. However, a method of measuring the second-order nonlinearity parameter by the through-transmission method (TT) has been used in most cases. In these cases, the second-order harmonic component generated during the transmission of ultrasonic waves in the material is measured. In this study, we demonstrate the utility of the method for measuring the third-order nonlinearity parameter by the pulse-echo method (PE), which is advantageous for field applications because it measures only one side. In addition, the second harmonic component is difficult to measure by PE due to the phase inversion effect, but the third harmonic is not affected by phase inversion. To experimentally verify such differences in different measurement methods, second- and third-order nonlinearity parameters were measured using both TT and PE for the same specimen. For this, two Al6061-T6 alloy specimen groups, high-temperature heat-treated and tensile fatigued, were prepared, of which trends in the second-order nonlinearity parameters measured by TT were well-known. In both specimen groups, second- and third-order nonlinearity parameters measured by TT exhibited the same trend according to heat treatment time or fatigue. On the other hand, in the results measured by PE, only the third-order nonlinearity parameter showed the same tendency as the results of TT, whereas almost no change in the second-order nonlinearity parameter was observed. This result confirms that the third-order nonlinearity parameter measurement by PE can be used equally with the second-order nonlinearity parameter measurement by TT and is expected to remarkably expand the field applicability of nonlinear ultrasonic techniques.

PV/Battery Grid Integration Using a Modular Multilevel Isolated SEPIC-Based Converter

Photovoltaic (PV) plants can be built rapidly when compared with other conventional electrical plants; hence, they are a competent candidate for supplying the electricity grid. The output power of the PV modules can be used in plug-in electric vehicles (PEVs) DC charging stations to reduce the burden on the electricity grid, particularly during peak load hours. To integrate PV modules and electric vehicles (EVs) with the electricity grid, the modular multilevel converters (MMCs) topologies producing staircase voltage waveforms are preferred as they are able to deliver less total harmonic distortion (THD) and higher efficiency in addition to lower voltage stress on semiconductor switches. In conventional centralized MMC topologies, a direct connection to a high-DC-link input voltage is required which is not appropriate for PV plants. A new MMC topology for PV/EV/grid integration is proposed in this paper, where the individual PV arrays are directly connected to each phase of the AC grid to harvest the maximum available power point. A current-source converter (CSC) based on a single-stage isolated SEPIC converter is adopted as the submodule (SM) for the proposed MMC topology given its outstanding features, such as low input ripple current, high efficiency, high power factor, and flexible output voltage higher or lower than the input voltage. The single-stage SMs can operate in both DC/DC and DC/AC operating modes. Proper controllers for each mode of operation are designed and applied to supply constant current from either the PV modules or the battery cells by eliminating the second-order harmonic component. The performance of the proposed converter is verified by simulations and a downscaled prototype controlled by TMSF28335 DSP.

Modular Single-Stage Three-Phase Flyback Differential Inverter for Medium/High-Power Grid Integrated Applications.

This paper proposes a single-stage three-phase modular flyback differential inverter (MFBDI) for medium/high power solar PV grid-integrated applications. The proposed inverter structure consists of parallel modules of flyback DC-DC converters based on the required power level. The MFBDI offers many features for renewable energy applications, such as reduced components, single-stage power processing, high-power density, voltage-boosting property, improved footprint, flexibility with modular extension capability, and galvanic isolation. The proposed inverter has been modelled, designed, and scaled up to the required application rating. A new mathematical model of the proposed MFBDI is presented and analyzed with a time-varying duty-cycle, wide-range of frequency variation, and power balancing in order to display its grid current harmonic orders for grid-tied applications. In addition, an LPF-based harmonic compensation strategy is used for second-order harmonic component (SOHC) compensation. With the help of the compensation technique, the grid current THD is reduced from 36% to 4.6% by diminishing the SOHC from 51% to 0.8%. Moreover, the SOHC compensation technique eliminates third-order harmonic components from the DC input current. In addition, a 15% parameters mismatch has been applied between the flyback parallel modules to confirm the modular operation of the proposed MFBDI under modules divergence. In addition, SiC MOSFETs are used for inverter switches implementation, which decrease the inverter switching losses at high-switching frequency. The proposed MFBDI is verified by using three flyback parallel modules/phase using PSIM/Simulink software, with a rating of 5 kW, 200 V, and 50 kHz switching frequency, as well as experimental environments.

A novel inductive angular displacement sensor based on time-grating

The paper proposes a novel inductive angular displacement sensor to measure large-diameter hollow rotating mechanical components in large science facilities. The sensor design includes a stator, a rotor with two layers of windings. The stator is fabricated according to the dimension of the rotor, which is the large-diameter hollow rotary mechanical components. Based on the principle of electromagnetic induction, the theoretical model of the proposed sensor is acquired by mathematical equations in detail. The proposed sensor design ensures integration of the large-diameter hollow rotating mechanical components with the stator, and solves the problem that the sensors cannot be coaxially installed in the closed-loop control process of large mechanical rotating parts. The performance of the sensor is evaluated by tests of the prototype sensor fabricated by printed circuit board technology, which manifests the effectiveness of the proposed sensor. The results demonstrate the value of original dynamic relative error is ±97.6″ over 0°–360° range, and the measurement error mainly includes first-order and second-order harmonic components. Therefore, the original errors are reduced by optimizing the angular displacement algorithm of the sensor, optimized results demonstrate the final relative accuracy of the sensor is ±3.5″ over the 0°–360° measurement range.

Fluid resonance in the narrow gap of a box-wall system under cnoidal wave action

Fluid resonance within a narrow gap formed by a box in close proximity to a plane wall under cnoidal wave action is investigated using the Computational Fluid Dynamics (CFD) package OpenFOAM®. Fluid resonance behavior excited by the second-order harmonic component of the incident cnoidal waves is concerned. Numerical simulations show the large second-order induced wave height in the gap which increases as the incident wave height gets larger or the water depth gets shallower. The large double-frequency component of the cnodial waves is the major reason for the phenomenon. Harmonic analysis indicates that the CFD results can be represented well by a second-order approximation, and the free surface nonlinearity is an indispensable factor in the action of the cnoidal incident wave. Similar phenomenon is also found in the horizontal wave force. However, the second-order induced resonance within the gap has insignificant effect on the vertical wave force.

Three-Phase PWM Inverter for Isolated Grid-Connected Renewable Energy Applications

This paper proposes a three-phase isolated flyback inverter (IFBI) for single-stage grid-tied solar PV applications, considering a simple sinusoidal pulse-width modulation (SPWM) scheme. The proposed single-stage inverter employs a reduced passive elements count by considering three input-parallel output-differential (IPOD) flyback converter modules. Additionally, a single small size LC-input low-pass filter is utilized at the input paralleling point for ripple-free input current operation, which is essential in grid-connected renewable energy applications. In addition, a mathematical model of the IFBI is presented to confirm the existence of its low-order harmonic components. A simple PI controller-based control scheme, considering only two loops and five sensors, is used to control the proposed grid-tied IFBI. Continuous modulation scheme (CMS) combined with SPWM is used to diminish the low-frequency harmonic components. Moreover, a simple selective harmonic elimination (SHE) loop is used for second-order harmonic components (SOHC) elimination from grid-injected currents. The SHE has decreased the SOHC from 43% to 0.96%, which improves the grid current THD from 39% to 3.65%, to follow the IEEE harmonic standard limits. Additionally, the harmonic elimination technique decreases the circulating power between the inverter paralleled modules, which enhances the grid currents power factor. The proposed inverter is verified through a grid-connected 200 V, 1.6 kW, 60 Hz experimental prototype, and the switching frequency is 50 kHz. TMS-based DSP controller is used to control the grid-injected power to follow the reference power set-point.

A 17.8–34.8 GHz (64.6%) Locking Range Current-Reuse Injection-Locked Frequency Multiplier with Dual Injection Technique

A 17.8–34.8 GHz (64.6%) locking range current-reuse injection-locked frequency multiplier (CR-ILFM) with dual injection technique is presented in this paper. A dual injection technique is applied to generate differential signal and increase the power of the second-order harmonic component. The CR core is proposed to reduce the power consumption and compatibility with NMOS and PMOS injectors. The inductor-capacitor (LC) tank of the proposed CR-ILFM is designed with a fourth-order resonator using a transformer with distributed inductor to extend the locking range. The self-oscillated frequency of the proposed CR-ILFM is 23.82 GHz. The output frequency locking range is 17.8–34.8 GHz (64.6%) at a 0-dBm injection power without any additional control including supply voltage, varactor, and capacitor bank. The power consumption of the proposed CR-ILFM is 7.48 mW from a 1-V supply voltage and the die size is 0.75 mm × 0.45 mm. The CR-ILFM is implemented in a 65-nm CMOS technology.

Measurement of Absolute Acoustic Nonlinearity Parameter Using Laser-Ultrasonic Detection

The absolute acoustic nonlinearity parameter β is defined by the displacement amplitudes of the fundamental and second-order harmonic frequency components of the ultrasonic wave propagating through the material. As β is a sensitive index for the micro-damage interior of industrial components at early stages, its measurement methods have been actively investigated. This study proposes a laser-ultrasonic detection method to measure β. This method provides (1) the β measurement in a noncontact and nondestructive manner, (2) inspection ability of different materials without complex calibration owing to direct ultrasonic displacement detection, and (3) applicability for the general milling machined surfaces of components owing to the use of a laser interferometer based on two-wave mixing in the photorefractive crystal. The performance of the proposed method is validated using copper and 6061 aluminum alloy specimens with sub-micrometer surface roughness. The experimental results demonstrated that the β values measured by the proposed method for the two specimens were consistent with those obtained by the conventional piezoelectric detection method and the range of previously published values.

Protecting Distribution Systems With Inverter-Interfaced PV Plants Using Q-Axis Components

Satisfying fault ride-through requirements, solar photovoltaic (PV) plant remains connected to the grid even during a fault. With current limiting feature, the plant current during a fault becomes comparable to its rated current. This imposes challenges to conventional local data-based protection methods employed in the relay on PV-side. In such a situation, the selectivity of a protection scheme is crucial for maintaining an uninterrupted supply to customers. In this article, a q-axis component-based protection scheme is proposed for the distribution system with a PV plant. The method extracts the second-order harmonic components of q-axis voltage and current at relaying point for fault detection and direction identification. Using this direction information at both ends of a line segment to be protected, internal and external faults are discriminated. The performance of the proposed method is tested on 14-bus distribution system for various fault cases. A comparative analysis with existing techniques shows the strength of the proposed method. Hardware-in-loop testing using OPAL-RT simulator as system and Arduino NANO microcontroller as relay validates the performance of the proposed method in real-time.

Nonlinear wave generation using a heaving wedge

This paper investigates the optimization of second-order control signals required to produce stable non-linear, deep-water waves using a wedge-shaped, plunger-type wave generator. Both numerical and experimental methods are utilized. A fully non-linear and dispersive potential flow (FNPF) solver developed at DTU is used for the numerical work, following improvements that reduce reflection to 1%. The numerical solver is validated against theoretical and experimental data. A defect correction optimization scheme is employed, resulting in optimized control signals for non-dimensional wave numbers of 2.04-8.17 and steepness of 3-7%. These control signals produce waves that are within 2% of stream function theory solutions for the fundamental harmonic component, and within 10% for the second-order harmonic component. The results demonstrate both the applicability of this optimization procedure and the suitability of a heaving wedge for generating stable non-linear deep-water waves.

Analysis of the Influence of Surface Roughness on Measurement of Ultrasonic Nonlinearity Parameter Using Contact-Type Transducer

The ultrasonic nonlinearity parameter is used to evaluate the nonlinear elasticity of a material, which is determined from the displacement amplitude of the fundamental and second-order frequencies components in an ultrasonic wave propagating through a material. However, the displacement amplitude of the second-order harmonic component generated during propagation through a material is very weak because it is easily affected by measurement conditions such as surface roughness. In this study, we analyzed the influence of surface roughness on the measurement of the ultrasonic nonlinearity parameter. For this purpose, Al6061-T6 and SUS304 specimens were prepared with different surface roughness ranging from 0.5 to 2.9 μm. Then, the absolute and relative ultrasonic nonlinearity parameter measurements were conducted using a through-transmission technique involving two cases: both surfaces being rough, and one being a rough surface and the other being a smooth surface. The experimental results showed that the surface roughness had a lesser influence on the absolute measurement than on the relative measurement and that the transmission surface was less affected by the reception surface. These results were similar regardless of the types of specimens. Therefore, to perform accurate measurements, it is desirable to measure the nonlinearity parameter after polishing the material surface.

Harmonic Suppression RF Rectifier Design With a New Experimental Analysis Based on Voltage and Efficiency Level Curves

This letter describes the design, simulation, and measurement process of a series radio frequency (RF) rectifier at the 2.4 GHz ISM band, composed of a distributed filter with radial stub elements. The proposal is to reduce the first- and second-order harmonic components that are generated by the diode nonlinearities at the rectifier output. The efficiency and output voltage are presented according to load values ranging from 1 to 10 kQ. These figures of merits are obtained with a new methodology, using an automatized measuring setup, which describes in detail the behavior and nuances of the circuit according to the frequency and input power through experimental level curves. The proposed RF rectifier prototype reaches over 20% efficiency for input power levels over -20 dBm for the lower load values, showing a satisfactory performance in comparison with other proposals for low-power applications.

Characteristics Analysis of Capacitor Voltage Ripples and Dimensioning of Full-Bridge MMC With Zero Sequence Voltage Injection

Compared to the conventional half-bridge submodule-based modular multilevel converter (MMC), the full-bridge submodule (FBSM) capacitance of FB-MMC can be reduced significantly with consideration of negative voltage state of the FBSM, which has benefits to cost and size reduction. This paper analyzes the characteristics of the FBSM voltage fundamental frequency and second-order harmonic fluctuations of the FB-MMC without and with zero sequence voltage injection (ZSVI) comparatively. The analyses show that ZSVI has almost no impacts on the FBSM voltage fundamental frequency fluctuations, while the second-order harmonic components become 5/6 times of the one without. Based on these analyses, with the same fluctuation constraint as the case without ZSVI, the FBSM capacitance dimensioning method is also presented. The correctness of the presented analyses is verified by comparative simulation results on a ±200 kV FB-MMC system and experimental results on a laboratory platform. The simulation and experimental results indicate that the FBSM capacitance of the FB-MMC can achieve a reduction about 10% with ZSVI.

Active Cross-Correlation Anti-Islanding Scheme for PV Module-Integrated Converters in the Prospect of High Penetration Levels and Weak Grid Conditions

This paper introduces a new cross-correlation anti-islanding detection scheme for module-integrated converters (MICs) with pseudo dc-link. The proposed scheme periodically injects a second-order harmonic current component of low magnitude (in open-loop mode) and evaluates grid response by means of correlation. The proposed scheme is highly reliable, providing nondetection-zone free operation, fast detection, and compliance with the relevant power quality and anti-islanding standards. Additionally, it excels in lower computational cost, reduced fault detection zone, and higher output power quality, compared to the referenced anti-islanding schemes that are compatible with MICs. Last but not least, it can be implemented in MIC platforms without requiring an output current sensor, being an easily upgradable solution in software level. A firm theoretical framework that considers both high penetration levels of PV systems and weak grid conditions is also included in this paper. The theoretical framework, as well as the effectiveness of the proposed scheme, is verified through extensive experimental results for various electrical-network parameters and nonlinear loading conditions. Finally, a comprehensive comparison between the proposed method and the most common anti-islanding techniques (compatible with MICs) is performed, leading to some valuable conclusions from MICs perspective.

A Pulse Inversion-Based Nonlinear Ultrasonic Technique using a Single-Cycle Longitudinal Wave for Evaluating Localized Material Degradation in Plates

General nonlinear ultrasonic techniques (NUTs) use tone-burst ultrasonic waves with a narrow bandwidth so that the fundamental frequency component and the second-order harmonic component can be clearly separated in the frequency domain. Meanwhile, when using a longitudinal wave propagating in the thickness direction, the number of cycles of tone-burst signals can be limited by the object thickness. In some cases, only a pulsed signal or a single-cycle signal, which has broad bandwidth, may be applicable to avoid superposition of the first transmitted wave and the multi-reflected waves. Such cases complicate the use of general NUTs, and it is necessary to apply a method to precisely extract the second-order harmonic component even in broadband signals to the NUT. In this study, the pulse inversion (PI) method, which can extract only even harmonics or odd harmonics by superposing or subtracting two wave signals obtained from 180° out-of-phase inputs, is applied to an NUT using broadband ultrasound. The performance of the PI-based NUT with respect to material degradation is verified using a series of heat-treated aluminum alloy specimens with various levels of precipitates. The experimental results show that the nonlinearity parameters measured with single-cycle signals agree well with the previous well-validated experimental results obtained using narrowband signals. Next, the PI-based NUT is used to assess the localized material degradation of a stainless steel plate subjected to high-cycle fatigue. The experimental results show that the profile of the measured nonlinearity parameters as a function of scan position is consistent with the intended distribution of localized fatigue damage, which demonstrates the potential feasibility of the proposed technique for evaluation of localized material degradation in plates.

Synthetic aperture imaging of contact acoustic nonlinearity to visualize the closing interfaces using tone-burst ultrasonic waves

Abstract Contact acoustic nonlinearity (CAN) effect was visualized by synthetic aperture focusing technique (SAFT) via a signal-processing method for acoustic nonlinearity imaging. The proposed signal-processing technique, named Synthetic Aperture Imaging of Acoustic Nonlinearity (SAIAN), allows the SAFT to visualize CAN that is difficult to be visualized by linear ultrasonic imaging. Using the SAIAN algorithm, the fundamental and second-order harmonic frequency components are extracted from the tone-burst wave signal and then these component signals are transformed into pulse-like signals. From the pulse-like signals, the CAN parameter signals can be obtained in the form of pulses, which allows CAN to be visualized by SAFT. In order to verify the effectiveness of the SAIAN, the nonlinear ultrasonic measurements using tone-burst waves were conducted to visualize closing interfaces. As a result, CAN effect appeared at partially closed part of a contacting interface in the images of the CAN signals converted by the SAIAN; on the contrary, the CAN feature was not shown in linear ultrasonic images based on the fundamental frequency component. These results support that the SAIAN is a useful signal processing for acoustic nonlinearity visualization that improves the probability of detection of closing interfaces and the accuracy in crack sizing.

Influence of switch angles on second‐order current harmonic and resonance in switched reluctance motors

This study investigates the origin of resonance in switched reluctance motor (SRM). It has been found that the second-order harmonic component of phase current is one of the major origins, which may cause resonance when its frequency multiplied by three caused by three phases coincides with natural frequency. It has also been demonstrated that there is a natural-frequency-centred band, saying that as long as the excitation frequency or its harmonics locates in the band, resonance will be excited. Accordingly, influence of critical parameters such as turn-on and turn-off angles on second-order current harmonic and corresponding vibration caused by resonance have been fully studied. Based on the study, optimal switch angles can be used to reduce second-order current harmonic, and thus to reduce vibration and noise caused by resonance. Both simulation and experimental results show that resonance can be effectively reduced by selection of optimal switch angles.