Harmonic Amplification Research Articles

Adaptive threshold based new active islanding protection scheme for multiple PV based microgrid application

A new adaptive active islanding protection scheme is presented for a multiple photovoltaic (PV) based microgrid environment, where islanding detection parameters are estimated in adaptive manner according to operational PV penetration level. Dynamic threshold calculation is obtained by a piecewise cubic Hermite polynomial interpolation to detect islanding for a PV based voltage source converter (VSC), where inconsistent solar irradiation profile is subjected to the system connected to local utility through point of common coupling (PCC). The VSC pulse-width modulation inputs are obtained from primary second-order phase locked loop based control path, where instability to detect islanding is achieved by active injection of error dynamics (parallel islanding protection path). The disturbance injection is decided from islanding detection parameter (i.e. harmonic amplification factor, %HAF) of PCC voltage and the VSC instability is achieved by desired unstable closed-loop poles of parallel detection path. UL 1741 standard is considered for the proposed multi-PV microgrid, for worst-case scenario study.

High Orbital Angular Momentum Harmonic Generation.

We identify and explore a high orbital angular momentum (OAM) harmonics generation and amplification mechanism that manipulates the OAM independently of any other laser property, by preserving the initial laser wavelength, through stimulated Raman backscattering in a plasma. The high OAM harmonics spectra can extend at least up to the limiting value imposed by the paraxial approximation. We show with theory and particle-in-cell simulations that the orders of the OAM harmonics can be tuned according to a selection rule that depends on the initial OAM of the interacting waves. We illustrate the high OAM harmonics generation in a plasma using several examples including the generation of prime OAM harmonics. The process can also be realized in any nonlinear optical Kerr media supporting three-wave interactions.

Dynamic Mass of a Reaction Wheel Including Gyroscopic Effects: An Experimental Approach

In recent years, driven by the increasingly stringent stability requirements imposed by some satellites’ payloads (e.g., the new generation of optical instruments), the issue of accurate onboard spacecraft microvibration modeling has attracted significant interest from engineers and scientists. This paper investigates the microvibration-induced phenomenon on a cantilever-configured reaction wheel assembly including sub- and higher harmonic amplifications due to modal resonances and broadband noise. A mathematical model of the reaction wheel assembly is developed and validated against experimental test results. The model is capable of representing each configuration in which the reaction wheel assembly will operate, whether it is hard mounted on a dynamometric platform or suspended free–free. The outcomes of this analysis are used to establish a novel methodology to retrieve the dynamic mass of the reaction wheel assembly in its operative range of speeds. An alternative measurement procedure has been developed for this purpose, showing to produce good estimates over a wide range of frequencies using a less complex test campaign compared with typical dynamic mass setups. Furthermore, the gyroscopic effect influence in the reaction wheel assembly response is thoroughly examined both analytically and experimentally. Finally, to what extent the noise affects the convergence of the novel approach is investigated.

Study of a Novel Equivalent Model and a Long-Feeder Simulator-Based Active Power Filter in a Closed-Loop Distribution Feeder

The closed-loop distribution system has become a trend, owing to increasing demands of electric power reliability in high-tech parks or in metropolitan areas. Harmonic propagation caused by power-factor correction capacitors and system inductors may lead to significant harmonic amplification throughout a closed-loop distribution feeder. This paper proposes a novel equivalent model of a closed-loop distribution feeder that a closed-loop distribution feeder can be equivalent to a radial feeder. This paper analyzes the harmonic propagation in a closed-loop distribution feeder and proposes a long-feeder simulator-based active power filter (LFS-APF) to damp out the harmonic propagation, based on the novel equivalent model. The proposed LFS-APF is installed at the middle point of a closed-loop feeder. The LFS-APF has better capability of harmonic damping than the resistive APF (RAPF), especially in response to variation of both nonlinear loads and power-factor correction capacitors or even if the installation location has to deviate from the middle point. Both simulation and experiment results are provided to validate the correctness and effectiveness of the novel equivalent model and the proposed LFS-APF.

The possibility of amplification and generation of higher harmonics in a microwave device with turbulent electron flow (Natural experiment and phenomenological model)

A generator of turbulent electron beams in an independent mode and in the mode of amplification of an external single-frequency signal is experimentally investigated. The formation of higher harmonic components in the spectrum of output radiation of the generator is shown experimentally. The phenomenological model of the turbulent electron flow is constructed on the basis of a chain of interacting “electron vortices” described by the modified nonlinear Van-der-Pol differential equations. The modes of independent dynamics and the mode of amplification of an external harmonic signal are considered. The results of numerical modeling and experimental investigations are compared. The qualitative correspondence of the behavior of the dependences obtained from the experimental investigation of the generator of turbulent electron beams and the results obtained from numerical modeling is shown.

Online phase margin compensation strategy for a grid‐tied inverter to improve its robustness to grid impedance variation

According to studies on active damping and the stability margin, grid impedance variation does not make active damping ineffective; instead, it attenuates the stability margin. The harmonic resonance mechanism of grid-tied inverters has been revealed by researching the influence of stability margin reduction on harmonic current amplification coefficients around the cut-off and phase crossover frequencies. With the stability margin decreasing, amplification to harmonics around relevant frequencies is strengthened. The harmonic resonance around the cut-off frequency is triggered as the stability margin decreases to zero. An online phase margin (PM) compensation strategy is proposed in this study to improve system robustness to grid impedance variation. The strategy is implemented by the lead network to enhance the phase and the proportional component to regulate the cut-off frequency according to the grid impedance measured online. Hence, the system can be maintained with a sufficient PM to mitigate the harmonic resonance caused by grid impedance. Simulation and experimental results validate the theoretical analysis presented in this study. Comparisons show that the proposed strategy is superior to the zero compensation strategy in suppressing the harmonic resonance.

Comprehensive numerical modelling of the performance of a second harmonic generation stage coupled with a low-gain optical parametric amplifier

We present a comprehensive model for predicting the full performance of a second harmonic generation-optical parametric amplification system that aims at enhancing the temporal contrast of laser pulses. The model simultaneously takes into account all the main parameters at play in the system such as the group velocity mismatch, the beam divergence, the spectral content, the pump depletion, and the length of the nonlinear crystals. We monitor the influence of the initial parameters of the input pulse and the interdependence of the two related non-linear processes on the performance of the system and show its optimum configuration. The influence of the initial beam divergence on the spectral and the temporal characteristics of the generated pulse is discussed. In addition, we show that using a crystal slightly longer than the optimum length and introducing small delay between the seed and the pump ensures maximum efficiency and compensates for the spectral shift in the optical parametric amplification stage in case of chirped input pulse. As an example, calculations for bandwidth transform limited and chirped pulses of sub-picosecond duration in beta barium borate crystal are presented.

An Active Filter With Resonant Current Control to Suppress Harmonic Resonance in a Distribution Power System

A shunt active filter operated as a harmonic conductance is able to suppress a harmonic resonance in the distribution power system. However, due to the inherent phase-lagging in digital signal processing, the active filter really behaves as a harmonic admittance instead of conductance. This may induce unintentional harmonic amplification at other locations in the feeder when starting the active filter, which is similar to the so-called whack-a-mole phenomenon. This paper presents an active filter with resonant current control to suppress a harmonic resonance. The current control is realized by parallel-connected bandpass filters tuned at harmonic frequencies to ensure that the active filter functions as an approximately pure conductance. The conductance at dominant harmonic frequencies can be separately and dynamically adjusted to guarantee the damping performance. In addition, in order to address the harmonic resonance, the line distributed-parameter model of a radial feeder is developed considering harmonic damping by variable conductance and admittance, respectively. Simulation results show that the active filter with the resonant control provides a better damping performance compared with other control methods. A lab-scale prototype circuit rated at 220 V/20 kVA also validates the effectiveness of the proposed method.

Parametric synthesis of passive filter-compensating devices

A method of optimal design for passive power filters based on performance optimization in the space of input impedance parameters is considered. The poles and the normalization coefficient of the filter operating impedance are variable parameters. A filter designed with the help of the proposed method satisfies requirements of reactive power compensation, harmonic filtering, and minimizing the amplification of minor harmonics. A design example illustrating the proposed approach is given.

Resonance Impact on Power Factor Correction System in Power System with Harmonic Distortion

This paper presents the resonance impact on power factor correction system (PFC system) in power system with harmonic distortion. The main purpose is to show harmonic current amplification that generated by the parallel resonance phenomenon in power system and problem solution. The power system model for analysis consist of the harmonic current source and PFC system. These devices are established by MATHLAB/Simulink. In simulation, the variable speed drive, which is DC drive, instead to the harmonic current source in power system that use capacitor to improve power factor of system. The problem solution, the series reactor, which is detune filter, is used to correct parallel resonance problem, by connecting capacitor with the series reactor, which is detune filter. The result of simulation, detune filter is able to avoid the parallel resonance phenomenon and partially reduce harmonic current that flowing to capacitor, however the series reactor, which is detune filter, is unable to reduce harmonic current of the harmonic current source.

Enhancement of high-order harmonic generation in intense laser interactions with solid density plasma by multiple reflections and harmonic amplification

High order harmonic generation (HHG) from laser solid-density plasma interactions is considered as a promising way to generate bright and ultrashort burst of x-rays. One of the key issues for HHG is the low conversion efficiency due to a power law fall off in harmonic intensity. Here, we explore a possible mechanism for HHG enhancement by reflecting an intense laser pulse multiple times between two plasma surfaces. We find that HHG can be nonlinearly enhanced by orders of magnitude through multiple reflections compared with a single reflection. We argue this enhancement is due to the non-linear combination of the relativistically oscillating plasma mirror and the harmonic-rich pulse generated in preceding reflections. This mechanism should allow moderate power laser systems to efficiently generate coherent, ultrashort bursts of soft x-rays.

Harmonic amplification investigation and calculation of electric vehicle charging stations using three-phase uncontrolled rectification chargers

In electric vehicle (EV) charging stations equipped with shunt active power filters (APFs), the operation of the shunt APF may amplify the charging current harmonics at the point of power collection (PPC), an effect called “harmonic amplification.” This paper focuses on an EV charging station with three-phase uncontrolled rectification chargers that produce the most serious harmonic pollution. By analyzing the coupling relationship between harmonic current and voltage, as well as by defining the total harmonic current amplification factor (TIF), an analytical method is proposed to quantify the amplification degree of the PPC charging current. Simulations and experiments verify the correctness of the method. Then, the variation trends that the amplification factor changes with key parameters, such as quantity of chargers, compensation rate, and compensation phase difference of APFs, are discussed. The results are beneficial to gaining further understanding on capacity correction and control design of APFs in EV charging stations.

Impact of wind farms on disturbance detection and classification in distributed generation using modified Adaline network and an adaptive neuro-fuzzy information system

A new disturbance detection and classification technique based on modified Adaline and adaptive neuro-fuzzy information system (ANFIS) is proposed for a distributed generation system comprising a wind power generating system (DFIG) and a photovoltaic array. The proposed technique is based on a fast Gauss–Newton parameter updating rule rather than the conventional Widrow–Hoff delta rule for the Adaline network. The voltage and current signals near the target distributed generation (DG), particularly the DFIG, whose speed varies from minimum to the maximum cut-off speed, are processed through the modified Adaline network to yield the features like the negative sequence power, harmonic amplification factor (HAF), total harmonic distortion (THD), etc. These features are then used as training sets for the ANFIS, which employs a gradient descent algorithm to update its parameters. The proposed technique distinguishes the islanding condition of the distributed generation system with some other disturbances, such as switching faults, capacitor bank switching, voltage swell, voltage sag, distorted grid voltage, unbalanced load switching, etc. which are referred to as non-islanding cases in this paper.

Traveling-Wave Tube Harmonic Amplifier in Terahertz and Experimental Demonstration

In this paper, a novel traveling-wave tube (TWT) harmonic terahertz (THz) amplifier is proposed for a high-power, wideband, and practical electromagnetic vacuum radiation source. The amplifier is based on the reuse of harmonic information in the spent beam of a normal TWT by harmonic generation, selection, amplification, and coupling. In order to verify this concept, a W-band folded waveguide pulsed TWT with a second-harmonic extraction system in Cascade has been developed for a G-band (170-260 GHz) TWT second-harmonic amplifier. Simulation results from CST Particle Studio show that the amplifier exhibits a good performance. Then, a prototype of the amplifier was fabricated and tested. The experimental results show that over 100-mW second-harmonic output power at most frequencies of the 11.4 GHz of bandwidth at 1% duty cycle was achieved, and the maximum second-harmonic output power reached 500 mW. This paper demonstrates the feasibility that TWT harmonic amplifiers can be used as a potential THz vacuum radiation source, and further progress in development of such novel THz devices is anticipated.

Sensitivity and open-loop control of stochastic response in a noise amplifier flow: the backward-facing step

AbstractThe two-dimensional backward-facing step flow is a canonical example of noise amplifier flow: global linear stability analysis predicts that it is stable, but perturbations can undergo large amplification in space and time as a result of non-normal effects. This amplification potential is best captured by optimal transient growth analysis, optimal harmonic forcing, or the response to sustained noise. With a view to reducing disturbance amplification in these globally stable open flows, a variational technique is proposed to evaluate the sensitivity of stochastic amplification to steady control. Existing sensitivity methods are extended in two ways to achieve a realistic representation of incoming noise: (i) perturbations are time-stochastic rather than time-harmonic, (ii) perturbations are localised at the inlet rather than distributed in space. This allows the identification of regions where small-amplitude control is the most effective, without actually computing any controlled flows. In particular, passive control by means of a small cylinder and active control by means of wall blowing/suction are analysed for Reynolds number $\mathit{Re}=500$ and step-to-outlet expansion ratio ${\it\Gamma}=0.5$. Sensitivity maps for noise amplification appear largely similar to sensitivity maps for optimal harmonic amplification at the most amplified frequency. This is observed at other values of $\mathit{Re}$ and ${\it\Gamma}$ too, and suggests that the design of steady control in this noise amplifier flow can be simplified by focusing on the most dangerous perturbation at the most dangerous frequency.

Mechanical phase matching of birefringent non-linear crystals.

Second-order nonlinear processes such as second harmonic generation or parametric amplification have found numerous applications in the scientific and industrial world, from micromachining to petawatt laser facilities. These nonlinear interactions are mostly carried out in birefringent crystals because of their low cost and the possibility to operate at high powers Phase-matching configurations in birefringent crystals are determined by their refractive indexes. Here, we show that an important mechanical stress can be used to significantly change the phase-matching properties of a birefringent crystal. As an example, we demonstrate the shift of second harmonic non-critical phase matching wavelength of LiB3O5 (LBO) crystal at room temperature from 1200 nm to 1120 nm by applying compressive forces up to 100 MPa. We believe that this mechanical phase matching can be used as an additional degree of freedom to optimize nonlinear optical frequency mixing geometries.

Capacity Calculation of Shunt Active Power Filters for Electric Vehicle Charging Stations Based on Harmonic Parameter Estimation and Analytical Modeling

The influence of electric vehicle charging stations on power grid harmonics is becoming increasingly significant as their presence continues to grow. This paper studies the operational principles of the charging current in the continuous and discontinuous modes for a three-phase uncontrolled rectification charger with a passive power factor correction link, which is affected by the charging power. A parameter estimation method is proposed for the equivalent circuit of the charger by using the measured characteristic AC (Alternating Current) voltage and current data combined with the charging circuit constraints in the conduction process, and this method is verified using an experimental platform. The sensitivity of the current harmonics to the changes in the parameters is analyzed. An analytical harmonic model of the charging station is created by separating the chargers into groups by type. Then, the harmonic current amplification caused by the shunt active power filter is researched, and the analytical formula for the overload factor is derived to further correct the capacity of the shunt active power filter. Finally, this method is validated through a field test of a charging station.

The Grid Harmonic Amplification and Transmission Characteristics Research

Harmonic transmission is influenced by many factors in the grid and the harmonic resonance phenomenon may be occurred. This paper analyzes the influencing factors, and the relevant model was established, and applied to the practical case.

Time-reversed wave mixing in nonlinear optics

Time-reversal symmetry is important to optics. Optical processes can run in a forward or backward direction through time when such symmetry is preserved. In linear optics, a time-reversed process of laser emission can enable total absorption of coherent light fields inside an optical cavity of loss by time-reversing the original gain medium. Nonlinearity, however, can often destroy such symmetry in nonlinear optics, making it difficult to study time-reversal symmetry with nonlinear optical wave mixings. Here we demonstrate time-reversed wave mixings for optical second harmonic generation (SHG) and optical parametric amplification (OPA) by exploring this well-known but underappreciated symmetry in nonlinear optics. This allows us to observe the annihilation of coherent beams. Our study offers new avenues for flexible control in nonlinear optics and has potential applications in efficient wavelength conversion, all-optical computing.

Expanding the Stability Range of a Lifted Propane Flame by Resonant Acoustic Excitation

We found that the stability of an unconfined fuel-rich lifted propane–air flame is substantially enhanced and the stability range (in the Reynolds number–equivalence ratio domain) is extended when forced by periodic perturbations at 280 ± 20 Hz and an intensity of 10%. This preferred frequency appeared to be independent of the Reynolds number over the range considered (2500 to 12000), resulting in the corresponding Strouhal number (St) variation from about 1.8 to 0.4. For the stabilization heights below 5 nozzle diameters, the enhanced stability is especially effective in shifting the blow-off limit toward leaner mixtures. From high-repetition particle image velocimetry (PIV) measurements and dynamic mode decomposition (DMD), as well as flame visualization by CH* chemiluminescence, it was found that the flame affected the large-scale ring-like vortices by increasing their convection speed and suppressing their pairing and the consequent subharmonic modes of the flame instability. The resonance frequency of 308 Hz (St = 0.82) was also found to be the natural frequency of the unforced propane flame, compared to about 127 Hz (St = 0.38) in the cold jet of the same configuration. Both the forced and unforced flames stabilized on secondary (azimuthal) instabilities associated with the streamwise vortex filaments in the braid between the roll-up vortices, possibly excited by the feedback of heat-release pulsations. This leads to amplification of the first harmonic of the fundamental frequency, which, together with thermal expansion, is believed to cause a more than two-fold increase in the preferred frequency compared with that of a nonreacting jet.