A time and frequency system is a critical component of Very Long Baseline Interferometry(VLBI)stations,providing stable and reliable standards that directly impact data processing quality.At the Tianma 65 m radio tele...A time and frequency system is a critical component of Very Long Baseline Interferometry(VLBI)stations,providing stable and reliable standards that directly impact data processing quality.At the Tianma 65 m radio telescope(TMRT),this system has been meticulously designed to ensure long-term reliability and high performance.It incorporates high-performance hydrogen atomic clocks,high-precision time standards,automatic signal switching,and robust system software.This comprehensive approach has enabled the system to achieve long-term reliable operation,successfully supporting both major national engineering tasks and daily scientific observations.The effectiveness of the system is evidenced by its consistent delivery of the precision and stability required for radio astronomy.This article provides an in-depth exploration of the design and operation of the time and frequency system at the Tianma 65 m telescope,examining various aspects of its architecture,implementation,and performance.By sharing these insights,we aim to contribute knowledge that could benefit similar systems at other VLBI stations,greatly advancing radio astronomy infrastructure.展开更多
Large-scale new energy grid connection leads to the weakening of the system frequency regulation capability,and the system frequency stability is facing unprecedented challenges.In order to solve rapid frequency fluct...Large-scale new energy grid connection leads to the weakening of the system frequency regulation capability,and the system frequency stability is facing unprecedented challenges.In order to solve rapid frequency fluctuation caused by new energy units,this paper proposes a new energy power system frequency regulation strategy with multiple units including the doubly-fed pumped storage unit(DFPSU).Firstly,based on the model predictive control(MPC)theory,the state space equations are established by considering the operating characteristics of the units and the dynamic behavior of the system;secondly,the proportional-differential control link is introduced to minimize the frequency deviation to further optimize the frequency modulation(FM)output of the DFPSU and inhibit the rapid fluctuation of the frequency;lastly,it is verified on theMatlab/Simulink simulation platform,and the results show that the model predictive control with proportional-differential control link can further release the FM potential of the DFPSU,increase the depth of its FM,effectively reduce the frequency deviation of the system and its rate of change,realize the optimization of the active output of the DFPSU and that of other units,and improve the frequency response capability of the system.展开更多
The utilization of hybrid energy systems has necessitated to address the various Power Quality(PQ)concerns in Distributed Generation(DG)networks.Owing to the emergence of DG networks in recent times,it is envisaged fo...The utilization of hybrid energy systems has necessitated to address the various Power Quality(PQ)concerns in Distributed Generation(DG)networks.Owing to the emergence of DG networks in recent times,it is envisaged for every utility⁃grid⁃tied system to generate and utilize harmonic⁃less electric power.Therefore,the present research critically evaluates the operation of a utility⁃grid coordinated DG system and studies its islanding operation under faulted conditions.To achieve this,an Anti⁃Islanding Protection(AIP)scheme is developed which is capable of controlling the frequency and voltage variations.This scheme is operated by a coordinated operation of multivibrators.Their operation continuously traces the pre⁃defined limits of voltage,reactive,and real power,and matches with their reference values to avoid mismatch.It is revealed that,if the mismatched values of real and reactive power exceeded its threshold value of 0.1 p.u.,then the islanding condition is detected.Especially,the proposed system is assessed in two modes:utility⁃grid and islanding modes.In utility⁃grid mode,reactive power compensation is obtained by the control of voltage and frequency signals.However,in islanding mode,the real power requirement of the connected load is obtained with reduced harmonics under unsymmetrical faulted conditions.Incremental Conductance(IC)based Maximum Power Point Tracking(MPPT)technique ensures the extraction of maximum power under varying and stochastically atmospheric conditions.Simulation results reveal that the AIP scheme promptly disconnects the utility grid from the DG network in the minimum time during dynamic variations in frequency and voltage to prevent islanding.It is justified that there is violation of the considered threshold limits even under the faulted condition.The strategy of the switchgear scheme ensures the minimum detection time of the islanding operation.Total Harmonic Distortion(THD)is 0.26%for grid voltage.It validates according to the IEEE⁃1547 standard which stipulates that the THD of grid voltage must be less than 5%.Overall,satisfactory and accurate results are obtained,which are compared with the IEEE⁃1547 standard for validation.展开更多
Highly accurate international height reference frames with long-term stability,global consistency,and homogeneity are crucial for monitoring sea level variations,understanding climate change,managing disasters,and sup...Highly accurate international height reference frames with long-term stability,global consistency,and homogeneity are crucial for monitoring sea level variations,understanding climate change,managing disasters,and supporting other applications that benefit scientific research and societal well-being.Currently,there are over 100 local height reference systems worldwide.Unifying these systems is a pivotal step toward constructing international height reference frames.The method introduced in this study-the gravity frequency shift via Satellite Frequency Signal Transfer(SFST)-represents a groundbreaking relativistic geodetic approach,demonstrating its potential to surpass the constraints of conventional techniques.The advent of high-precision optical atomic clocks(OAC)with an accuracy level of 1×10^(-18) has facilitated this method's implementation.The International Association of Geodesy(IAG)has established the International Height Reference System(IHRS)and its practical realization,the International Height Reference Frame(IHRF).Our study focuses on two neighbouring height systems:the China Height System(CHS)and the Nepal Height System(NHS),separated by the Himalayas and the Xizang plateau.We aim to unify these two systems by determining the geopotential and orthometric height differences between their respective height datum stations:the Qingdao Height Datum Station(QHDS)and the Madar Height Datum Station(MHDS)using a simulation experiment with the method mentioned above.Using an OAC with an accuracy of 1×10^(-18),we identified a geopotential difference of-8.348±0.464 m^(2)s^(-2) and an orthometric height difference of 0.786±0.047 m between QHDS and MHDS.These results suggest that the introduced method could unify any two height systems with fewcentimeter-level precision,emphasizing its significance in contributing to the construction of the IHRS/IHRF with today's required precision.In summary,the SFST technique is a novel geodetic method that offers an alternative for height system unification,delivering centimeter-level precision,surpassing traditional methods,and supporting the development of the IHRF.展开更多
Grid-forming(GFM)control is a key technology for ensuring the safe and stable operation of renewable power systems dominated by converter-interfaced generation(CIG),including wind power,photovoltaic,and battery energy...Grid-forming(GFM)control is a key technology for ensuring the safe and stable operation of renewable power systems dominated by converter-interfaced generation(CIG),including wind power,photovoltaic,and battery energy storage.In this paper,we challenge the traditional approach of emulating a synchronous generator by proposing a frequency-fixed GFM control strategy.The CIG endeavors to regulate itself as a constant voltage source without control dynamics due to its capability limitation,denoted as the frequency-fixed zone.With the proposed strategy,the system frequency is almost always fixed at its rated value,achieving system active power balance independent of frequency,and intentional power flow adjustments are implemented through direct phase angle control.This approach significantly reduces the frequency dynamics and safety issues associated with frequency variations.Furthermore,synchronization dynamics are significantly diminished,and synchronization stability is enhanced.The proposed strategy has the potential to realize a renewable power system with a fixed frequency and robust stability.展开更多
This study develops a GWO-optimized cascaded fuzzy-PID controller with triangular membership functions for load frequency control in interconnected power systems.The controller’s effectiveness is demonstrated on ther...This study develops a GWO-optimized cascaded fuzzy-PID controller with triangular membership functions for load frequency control in interconnected power systems.The controller’s effectiveness is demonstrated on thermal–thermal and hybrid thermal–hydro–gas power systems.The controller parameters were tuned using the Integral Time Absolute Error(ITAE)objective function,which was also evaluated alongside other objective functions(IAE,ISE,and ITSE)to ensure high precision in frequency stabilization.To validate the effectiveness of the triangular membership function,comparisons were made with fuzzy-PID controllers employing trapezoidal and Gaussian membership functions.Performance metrics,including ITAE,settling time,overshoot,and undershoot of frequency deviation,as well as tie-line power deviation,were evaluated.Robustness was established through a comprehensive sensitivity analysis with T_(G),T_(T),andT_(R) parameter variations(±50%),a non-linearity analysis incorporating Generation Rate Constraint(GRC)and Governor Deadband(GDB),a random Step Load Perturbation(SLP)over 0–100 s,and also Stability analysis of the proposed scheme is conducted using multiple approaches,including frequency-domain analysis,Lyapunov stability theory,and eigenvalue analysis.Additionally,the system incorporating thermal,hydro,and gas turbines,along with advanced components like CES and HVDC links,was analysed.Comparisons were conducted against controllers optimized using Modified Grasshopper Optimization Algorithm(MGOA),Honey Badger Algorithm(HBA),Particle Swarm Optimization(PSO),Artificial Bee Colony(ABC),and Spider Monkey Optimization(SMO)algorithms.Results demonstrate that the GWO-based fuzzy-PID controller outperforms the alternatives,exhibiting superior performance across all evaluated metrics.This highlights the potential of the proposed approach as a robust solution for load frequency control in complex and dynamic power systems.展开更多
The variation of the nonlinear contact stiffness induced by the elastic wheel-holding effect between the aircraft and tractor has an important effect on the vibration characteristics of an airfield towbarless traction...The variation of the nonlinear contact stiffness induced by the elastic wheel-holding effect between the aircraft and tractor has an important effect on the vibration characteristics of an airfield towbarless traction system,making it essential for ensuring the safety of this emerging towing-out mode.In this paper,the frequency evolutionary tendency of the traction system is studied and obtained considering the variation of nonlinear contact stiffness for the first time.A novel modal analysis method,based on a derived nonlinear contact relationship,is proposed to investigate the vibration characteristics for mechanical system.Frequency veering and mode exchange phenomena in the low-order modes are observed due to the variation of nonlinear contact stiffness.These findings are further validated by the experimental results of a scaled-down model.In addition,it is also found that the veering critical point will be shifted with the external loads.The study provides valuable insights into the vibration characteristics and frequency veering behavior of similar mechanism-based systems,such as towbarless traction system,and has important implications for improving their design and operational performance.展开更多
With the increasing penetration of renewable energy resources in power systems,conventional timescale separated load frequency control(LFC)and economic dispatch may degrade frequency performance and reduce economic ef...With the increasing penetration of renewable energy resources in power systems,conventional timescale separated load frequency control(LFC)and economic dispatch may degrade frequency performance and reduce economic efficiency.This paper proposes a novel data-driven adaptive distributed optimal disturbance rejection control(DODRC)method for real-time economic LFC problem in nonlinear power systems.Firstly,a basic DODRC method is proposed by integrating the active disturbance rejection control method and the partial primal–dual algorithm.Then,to deal with the tie-line power flow constraints,the logarithmic barrier function is employed to reconstruct the Lagrange function to obtain the constrained DODRC method.By analyzing the sensitivity of the uncertain parameters of power systems,a data-driven adaptive DODRC method is finally proposed with a neural network.The effectiveness of the proposed method is demonstrated by experimental results using real-time equipment.展开更多
Background Interconnection of different power systems has a major effect on system stability.This study aims to design an optimal load frequency control(LFC)system based on a proportional-integral(PI)controller for a ...Background Interconnection of different power systems has a major effect on system stability.This study aims to design an optimal load frequency control(LFC)system based on a proportional-integral(PI)controller for a two-area power system.Methods Two areas were connected through an AC tie line in parallel with a DC link to stabilize the frequency of oscillations in both areas.The PI parameters were tuned using the cuckoo search algorithm(CSA)to minimize the integral absolute error(IAE).A state matrix was provided,and the stability of the system was verified by calculating the eigenvalues.The frequency response was investigated for load variation,changes in the generator rate constraint,the turbine time constant,and the governor time constant.Results The CSA was compared with particle swarm optimization algorithm(PSO)under identical conditions.The system was modeled based on a state-space mathematical representation and simulated using MATLAB.The results demonstrated the effectiveness of the proposed controller based on both algorithms and,it is clear that CSA is superior to PSO.Conclusion The CSA algorithm smoothens the system response,reduces ripples,decreases overshooting and settling time,and improves the overall system performance under different disturbances.展开更多
To improve design accuracy and reliability of structures,this study solves the uncertain natural frequencies with consideration for geometric nonlinearity and structural uncertainty.Frequencies of the laminated plate ...To improve design accuracy and reliability of structures,this study solves the uncertain natural frequencies with consideration for geometric nonlinearity and structural uncertainty.Frequencies of the laminated plate with all four edges clamped(CCCC)are derived based on Navier's method and Galerkin's method.The novelty of the current work is that the number of unknowns in the displacement field model of a CCCC plate with free midsurface(CCCC-2 plate)is only three compared with four or five in cases of other exposed methods.The present analytical method is proved to be accurate and reliable by comparing linear natural frequencies and nonlinear natural frequencies with other models available in the open literature.Furthermore,a novel method for analyzing effects of mean values and tolerance zones of uncertain structural parameters on random frequencies is proposed based on a self-developed Multiscale Feature Extraction and Fusion Network(MFEFN)system.Compared with a direct Monte Carlo Simulation(MCS),the MFEFNbased procedure significantly reduces the calculation burden with a guarantee of accuracy.Our research provides a method to calculate nonlinear natural frequencies under two boundary conditions and presentes a surrogate model to predict frequencies for accuracy analysis and optimization design.展开更多
This paper presents an innovative and effective control strategy tailored for a deregulated,diversified energy system involving multiple interconnected area.Each area integrates a unique mix of power generation techno...This paper presents an innovative and effective control strategy tailored for a deregulated,diversified energy system involving multiple interconnected area.Each area integrates a unique mix of power generation technologies:Area 1 combines thermal,hydro,and distributed generation;Area 2 utilizes a blend of thermal units,distributed solar technologies(DST),and hydro power;andThird control area hosts geothermal power station alongside thermal power generation unit and hydropower units.The suggested control system employs a multi-layered approach,featuring a blended methodology utilizing the Tilted Integral Derivative controller(TID)and the Fractional-Order Integral method to enhance performance and stability.The parameters of this hybrid TID-FOI controller are finely tuned using an advanced optimization method known as the Walrus Optimization Algorithm(WaOA).Performance analysis reveals that the combined TID-FOI controller significantly outperforms the TID and PID controllers when comparing their dynamic response across various system configurations.The study also incorporates investigation of redox flow batteries within the broader scope of energy storage applications to assess their impact on system performance.In addition,the research explores the controller’s effectiveness under different power exchange scenarios in a deregulated market,accounting for restrictions on generation ramp rates and governor hysteresis effects in dynamic control.To ensure the reliability and resilience of the presented methodology,the system transitions and develops across a broad range of varying parameters and stochastic load fluctuation.To wrap up,the study offers a pioneering control approach-a hybrid TID-FOI controller optimized via the Walrus Optimization Algorithm(WaOA)-designed for enhanced stability and performance in a complex,three-region hybrid energy system functioning within a deregulated framework.展开更多
In order to solve the multiple power extreme value point problem caused by system frequency splitting during wireless energy transmission at short distances a transmission model of the system is established.With the c...In order to solve the multiple power extreme value point problem caused by system frequency splitting during wireless energy transmission at short distances a transmission model of the system is established.With the comprehensive consideration of the resonance frequency load parameters and the coupling between coils the internal factors of frequency splitting and boundary conditions are discussed.The results show that under the condition of the fixed load the higher the natural resonance frequency the easier the frequency splitting. As the frequency splitting occurs the frequency of the maximum power transfer is no longer with the natural resonance frequency which can make the system unstable and the transfer power more difficult to control. Therefore a decreasing-frequency method is proposed to avoid the system frequency splitting. And decreasing the system resonance frequency can make the system successfully withdraw the frequency splitting area at a short-distance range.Under the fixed load condition the transmission power of the system can be increased by 400% and the transmission efficiency is reduced by only 14% which greatly improves the transmission performance of the system.展开更多
On the basis of high precision requirement for input signals in the power system protection and control system,this paper,only for the influence of power system frequency deviation on extracting fundamental harmonic,s...On the basis of high precision requirement for input signals in the power system protection and control system,this paper,only for the influence of power system frequency deviation on extracting fundamental harmonic,studies the amplitude error of Fourier algorithm,presents a method of correcting frequency deviation,and further derives the formulas of improved Fourier algorithm.The simulation results verified the effectiveness of the algorithm,it not only can greatly weaken the influence of frequency deviation,but also increase the precision of the power system protection and control.As a result the study in this paper has practical application value.展开更多
Diffraction effects will bring about more difficulties in actuating resonators,which are electrostatically actuated ones with sub-micrometer or nanometer dimensions,and in detecting the frequency of the resonator by o...Diffraction effects will bring about more difficulties in actuating resonators,which are electrostatically actuated ones with sub-micrometer or nanometer dimensions,and in detecting the frequency of the resonator by optical detection.To avoid the effects of diffraction,a new type of nanoelectromechanical systems（NEMS） resonators is fabricated and actuated to oscillate.As a comparison,a doubly clamped silicon beam is also fabricated and studied.The smallest width and thickness of the resonators are 180 and 200 nm,respectively.The mechanical oscillation responses of these two kinds of resonators are studied experimentally.Results show that the resonant frequencies are from 6.8 to 20 MHz,much lower than the theoretical values.Based on the simulation,it is found that over-etching is one of the important factors which results in lower frequencies than the theoretical values.It is also found that the difference between resonance frequencies of two types of resonators decreases with the increase in beam length.The quality factor is improved greatly by lowering the pressure in the sample chamber at room temperature.展开更多
Based on the frequency domain training sequences, the polynomial-based carrier frequency offset （CFO） estimation in multiple-input multiple-output （ MIMO ） orthogonal frequency division multiplexing （ OFDM ） sys...Based on the frequency domain training sequences, the polynomial-based carrier frequency offset （CFO） estimation in multiple-input multiple-output （ MIMO ） orthogonal frequency division multiplexing （ OFDM ） systems is extensively investigated. By designing the training sequences to meet certain conditions and exploiting the Hermitian and real symmetric properties of the corresponding matrices, it is found that the roots of the polynomials corresponding to the cost functions are pairwise and that both meger CFO and fractional CFO can be estimated by the direct polynomial rooting approach. By analyzing the polynomials corresponding to the cost functions and their derivatives, it is shown that they have a common polynomial factor and the former can be expressed in a quadratic form of the common polynomial factor. Analytical results further reveal that the derivative polynomial rooting approach is equivalent to the direct one in estimation at the same signal-to-noise ratio（SNR） value and that the latter is superior to the former in complexity. Simulation results agree well with analytical results.展开更多
Sum frequency generation vibrational spectroscopy （SFG-VS） is a robust technique for inter- facial investigation at molecular level. The performance of SFG-VS mostly depends on the spectral resolution of the SFG sys...Sum frequency generation vibrational spectroscopy （SFG-VS） is a robust technique for inter- facial investigation at molecular level. The performance of SFG-VS mostly depends on the spectral resolution of the SFG system. In this research, a simplified function was deduced to calculate the spectral resolution of picosecond SFG system and the lineshape of SFG spectra based on the Guassian shaped functions of IR beam and visible beam. The function indicates that the lineshpe of SFG spectra from nonresonant samples can be calculated by the Guassian widths of both IR beam and visible beam. And the Voigt lineshape of SFG spectra from vibrational resonant samples can be calculated by the Homogeneous broadening （Lorentzian width） and Inhomogeneous broadening （Guassian width） of vibrational modes, as well as the Guassian widths of both IR beam and visible beam. Such functions were also applied to verify the spectral resolution of the polarization-resolved and frequency-resolved picosecond SFG-VS system which was developed by our group recently. It is shown that the linewidths of IR beams that generated from current laser system are about 1.5 cm-1. The calculated spectral resolution of current picosecond IR scanning SFG-VS system is about 4.6 cm-1, which is consist with he spctral resolution shown in the spectra of cholesterol monolayer （3.5-5 cm-1）.展开更多
Low-fi＇equency vertical vibration isolation systems play important roles in precision measurements to reduce seismic and environmental vibration noise. Several types of active vibration isolation systems have been de...Low-fi＇equency vertical vibration isolation systems play important roles in precision measurements to reduce seismic and environmental vibration noise. Several types of active vibration isolation systems have been developed. However, few researches focus on how to optimize the test mass install position in order to improve the vibration transmissibility. An active low-frequency vertical vibration isolation system based on an earlier instrument, the Super Spring, is designed and implemented. The system, which is simple and compact, consists of two stages： a parallelogram-shaped linkage to ensure vertical motion, and a simple spring-mass system. The theoretical analysis of the vibration isolation system is presented, including terms erroneously ignored before. By carefully choosing the mechanical parameters according to the above analysis and using feedback control, the resonance frequency of the system is reduced from 2.3 to 0.03 Hz, a reduction by a factor of more than 75. The vibration isolation system is installed as an inertial reference in an absolute gravimeter, where it improved the scatter of the absolute gravity values by a factor of 5. The experimental results verifies the improved performance of the isolation system, making it particularly suitable for precision experiments. The improved vertical vibration isolation system can be used as a prototype for designing high-performance active vertical isolation systems. An improved theoretical model of this active vibration isolation system with beam-pivot configuration is proposed, providing fundamental guidelines for vibration isolator design and assembling.展开更多
Ideally, diesel hybrid autonomous power systems would operate with high penetration of renewable energy sources such as wind and photovoltaic to minimize fuel consumption. However, since these are inherently intermitt...Ideally, diesel hybrid autonomous power systems would operate with high penetration of renewable energy sources such as wind and photovoltaic to minimize fuel consumption. However, since these are inherently intermittent and fluctuating, the grid-forming diesel engine generator sets are usually required to operate with larger amounts of spinning reserve, often at low loading conditions what tends to increases operating and maintenance costs. Frequency stability is of great concern in “small” systems, such as mini-grids, where any individual generator in-feed represents a substantial portion of the total demand. There, the initial rate of change of frequency is typically larger and a lower value of frequency can be reached in a shorter time than in conventional systems with all generation supplied by rotating machines, possibly resulting in under-frequency load shedding and tripping of renewable energy generators. The first part of this paper, discusses some general concepts regarding frequency stability in a diesel hybrid mini-grid and how energy storage systems can be used to enhance system performance. Then, a particular technique based on a virtual synchronous generator is presented and its effectiveness is demonstrated with simulation results.展开更多
This paper proposes an autopilot system that can be used to control the small scale rotorcraft during the flight test for linear-frequency-domain system identification. The input frequency-sweep is generated automatic...This paper proposes an autopilot system that can be used to control the small scale rotorcraft during the flight test for linear-frequency-domain system identification. The input frequency-sweep is generated automatically as part of the autopilot control command. Therefore the bandwidth coverage and consistency of the frequency-sweep are guaranteed to produce high quality data for system identification. Beside that, we can set the safety parameters during the flight test （maximum roll/pitch value, minimum altitude, etc.） so the safety of the whole flight test is guaranteed. This autopilot system is validated using hardware in the loop simulator for hover flight condition.展开更多
基金supported by the National Natural Sci-ence Foundation of China(12273098).
文摘A time and frequency system is a critical component of Very Long Baseline Interferometry(VLBI)stations,providing stable and reliable standards that directly impact data processing quality.At the Tianma 65 m radio telescope(TMRT),this system has been meticulously designed to ensure long-term reliability and high performance.It incorporates high-performance hydrogen atomic clocks,high-precision time standards,automatic signal switching,and robust system software.This comprehensive approach has enabled the system to achieve long-term reliable operation,successfully supporting both major national engineering tasks and daily scientific observations.The effectiveness of the system is evidenced by its consistent delivery of the precision and stability required for radio astronomy.This article provides an in-depth exploration of the design and operation of the time and frequency system at the Tianma 65 m telescope,examining various aspects of its architecture,implementation,and performance.By sharing these insights,we aim to contribute knowledge that could benefit similar systems at other VLBI stations,greatly advancing radio astronomy infrastructure.
基金supported by the National Natural Science Foundation of China(Project No.52377082)the Scientific Research Program of Jilin Provincial Department of Education(Project No.JJKH20230123KJ).
文摘Large-scale new energy grid connection leads to the weakening of the system frequency regulation capability,and the system frequency stability is facing unprecedented challenges.In order to solve rapid frequency fluctuation caused by new energy units,this paper proposes a new energy power system frequency regulation strategy with multiple units including the doubly-fed pumped storage unit(DFPSU).Firstly,based on the model predictive control(MPC)theory,the state space equations are established by considering the operating characteristics of the units and the dynamic behavior of the system;secondly,the proportional-differential control link is introduced to minimize the frequency deviation to further optimize the frequency modulation(FM)output of the DFPSU and inhibit the rapid fluctuation of the frequency;lastly,it is verified on theMatlab/Simulink simulation platform,and the results show that the model predictive control with proportional-differential control link can further release the FM potential of the DFPSU,increase the depth of its FM,effectively reduce the frequency deviation of the system and its rate of change,realize the optimization of the active output of the DFPSU and that of other units,and improve the frequency response capability of the system.
文摘The utilization of hybrid energy systems has necessitated to address the various Power Quality(PQ)concerns in Distributed Generation(DG)networks.Owing to the emergence of DG networks in recent times,it is envisaged for every utility⁃grid⁃tied system to generate and utilize harmonic⁃less electric power.Therefore,the present research critically evaluates the operation of a utility⁃grid coordinated DG system and studies its islanding operation under faulted conditions.To achieve this,an Anti⁃Islanding Protection(AIP)scheme is developed which is capable of controlling the frequency and voltage variations.This scheme is operated by a coordinated operation of multivibrators.Their operation continuously traces the pre⁃defined limits of voltage,reactive,and real power,and matches with their reference values to avoid mismatch.It is revealed that,if the mismatched values of real and reactive power exceeded its threshold value of 0.1 p.u.,then the islanding condition is detected.Especially,the proposed system is assessed in two modes:utility⁃grid and islanding modes.In utility⁃grid mode,reactive power compensation is obtained by the control of voltage and frequency signals.However,in islanding mode,the real power requirement of the connected load is obtained with reduced harmonics under unsymmetrical faulted conditions.Incremental Conductance(IC)based Maximum Power Point Tracking(MPPT)technique ensures the extraction of maximum power under varying and stochastically atmospheric conditions.Simulation results reveal that the AIP scheme promptly disconnects the utility grid from the DG network in the minimum time during dynamic variations in frequency and voltage to prevent islanding.It is justified that there is violation of the considered threshold limits even under the faulted condition.The strategy of the switchgear scheme ensures the minimum detection time of the islanding operation.Total Harmonic Distortion(THD)is 0.26%for grid voltage.It validates according to the IEEE⁃1547 standard which stipulates that the THD of grid voltage must be less than 5%.Overall,satisfactory and accurate results are obtained,which are compared with the IEEE⁃1547 standard for validation.
基金the National Natural Science Foundation of China(NSFC)(Grant Nos.42030105,42274011,42074019,41974034,42204006)。
文摘Highly accurate international height reference frames with long-term stability,global consistency,and homogeneity are crucial for monitoring sea level variations,understanding climate change,managing disasters,and supporting other applications that benefit scientific research and societal well-being.Currently,there are over 100 local height reference systems worldwide.Unifying these systems is a pivotal step toward constructing international height reference frames.The method introduced in this study-the gravity frequency shift via Satellite Frequency Signal Transfer(SFST)-represents a groundbreaking relativistic geodetic approach,demonstrating its potential to surpass the constraints of conventional techniques.The advent of high-precision optical atomic clocks(OAC)with an accuracy level of 1×10^(-18) has facilitated this method's implementation.The International Association of Geodesy(IAG)has established the International Height Reference System(IHRS)and its practical realization,the International Height Reference Frame(IHRF).Our study focuses on two neighbouring height systems:the China Height System(CHS)and the Nepal Height System(NHS),separated by the Himalayas and the Xizang plateau.We aim to unify these two systems by determining the geopotential and orthometric height differences between their respective height datum stations:the Qingdao Height Datum Station(QHDS)and the Madar Height Datum Station(MHDS)using a simulation experiment with the method mentioned above.Using an OAC with an accuracy of 1×10^(-18),we identified a geopotential difference of-8.348±0.464 m^(2)s^(-2) and an orthometric height difference of 0.786±0.047 m between QHDS and MHDS.These results suggest that the introduced method could unify any two height systems with fewcentimeter-level precision,emphasizing its significance in contributing to the construction of the IHRS/IHRF with today's required precision.In summary,the SFST technique is a novel geodetic method that offers an alternative for height system unification,delivering centimeter-level precision,surpassing traditional methods,and supporting the development of the IHRF.
基金supported by the National Key Research&Development Program of China under Grant 2024YFB2408900.
文摘Grid-forming(GFM)control is a key technology for ensuring the safe and stable operation of renewable power systems dominated by converter-interfaced generation(CIG),including wind power,photovoltaic,and battery energy storage.In this paper,we challenge the traditional approach of emulating a synchronous generator by proposing a frequency-fixed GFM control strategy.The CIG endeavors to regulate itself as a constant voltage source without control dynamics due to its capability limitation,denoted as the frequency-fixed zone.With the proposed strategy,the system frequency is almost always fixed at its rated value,achieving system active power balance independent of frequency,and intentional power flow adjustments are implemented through direct phase angle control.This approach significantly reduces the frequency dynamics and safety issues associated with frequency variations.Furthermore,synchronization dynamics are significantly diminished,and synchronization stability is enhanced.The proposed strategy has the potential to realize a renewable power system with a fixed frequency and robust stability.
文摘This study develops a GWO-optimized cascaded fuzzy-PID controller with triangular membership functions for load frequency control in interconnected power systems.The controller’s effectiveness is demonstrated on thermal–thermal and hybrid thermal–hydro–gas power systems.The controller parameters were tuned using the Integral Time Absolute Error(ITAE)objective function,which was also evaluated alongside other objective functions(IAE,ISE,and ITSE)to ensure high precision in frequency stabilization.To validate the effectiveness of the triangular membership function,comparisons were made with fuzzy-PID controllers employing trapezoidal and Gaussian membership functions.Performance metrics,including ITAE,settling time,overshoot,and undershoot of frequency deviation,as well as tie-line power deviation,were evaluated.Robustness was established through a comprehensive sensitivity analysis with T_(G),T_(T),andT_(R) parameter variations(±50%),a non-linearity analysis incorporating Generation Rate Constraint(GRC)and Governor Deadband(GDB),a random Step Load Perturbation(SLP)over 0–100 s,and also Stability analysis of the proposed scheme is conducted using multiple approaches,including frequency-domain analysis,Lyapunov stability theory,and eigenvalue analysis.Additionally,the system incorporating thermal,hydro,and gas turbines,along with advanced components like CES and HVDC links,was analysed.Comparisons were conducted against controllers optimized using Modified Grasshopper Optimization Algorithm(MGOA),Honey Badger Algorithm(HBA),Particle Swarm Optimization(PSO),Artificial Bee Colony(ABC),and Spider Monkey Optimization(SMO)algorithms.Results demonstrate that the GWO-based fuzzy-PID controller outperforms the alternatives,exhibiting superior performance across all evaluated metrics.This highlights the potential of the proposed approach as a robust solution for load frequency control in complex and dynamic power systems.
基金co-supported by the Key Projects of the Civil Aviation Joint Fund of the National Natural Science Foundation of China(No.U2033208)。
文摘The variation of the nonlinear contact stiffness induced by the elastic wheel-holding effect between the aircraft and tractor has an important effect on the vibration characteristics of an airfield towbarless traction system,making it essential for ensuring the safety of this emerging towing-out mode.In this paper,the frequency evolutionary tendency of the traction system is studied and obtained considering the variation of nonlinear contact stiffness for the first time.A novel modal analysis method,based on a derived nonlinear contact relationship,is proposed to investigate the vibration characteristics for mechanical system.Frequency veering and mode exchange phenomena in the low-order modes are observed due to the variation of nonlinear contact stiffness.These findings are further validated by the experimental results of a scaled-down model.In addition,it is also found that the veering critical point will be shifted with the external loads.The study provides valuable insights into the vibration characteristics and frequency veering behavior of similar mechanism-based systems,such as towbarless traction system,and has important implications for improving their design and operational performance.
基金supported in part by the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources under Grant LAPS24009in part by the Guangdong Basic and Applied Basic Research Foundation under Grant 2021A1515110016in part by the National Natural Science Foundation of China under Grant 52206009.
文摘With the increasing penetration of renewable energy resources in power systems,conventional timescale separated load frequency control(LFC)and economic dispatch may degrade frequency performance and reduce economic efficiency.This paper proposes a novel data-driven adaptive distributed optimal disturbance rejection control(DODRC)method for real-time economic LFC problem in nonlinear power systems.Firstly,a basic DODRC method is proposed by integrating the active disturbance rejection control method and the partial primal–dual algorithm.Then,to deal with the tie-line power flow constraints,the logarithmic barrier function is employed to reconstruct the Lagrange function to obtain the constrained DODRC method.By analyzing the sensitivity of the uncertain parameters of power systems,a data-driven adaptive DODRC method is finally proposed with a neural network.The effectiveness of the proposed method is demonstrated by experimental results using real-time equipment.
基金Supported by the Russian Science Foundation(Agreement 23-41-10001,https://rscf.ru/project/23-41-10001/).
文摘Background Interconnection of different power systems has a major effect on system stability.This study aims to design an optimal load frequency control(LFC)system based on a proportional-integral(PI)controller for a two-area power system.Methods Two areas were connected through an AC tie line in parallel with a DC link to stabilize the frequency of oscillations in both areas.The PI parameters were tuned using the cuckoo search algorithm(CSA)to minimize the integral absolute error(IAE).A state matrix was provided,and the stability of the system was verified by calculating the eigenvalues.The frequency response was investigated for load variation,changes in the generator rate constraint,the turbine time constant,and the governor time constant.Results The CSA was compared with particle swarm optimization algorithm(PSO)under identical conditions.The system was modeled based on a state-space mathematical representation and simulated using MATLAB.The results demonstrated the effectiveness of the proposed controller based on both algorithms and,it is clear that CSA is superior to PSO.Conclusion The CSA algorithm smoothens the system response,reduces ripples,decreases overshooting and settling time,and improves the overall system performance under different disturbances.
基金the research project funded by the Fundamental Research Funds for the Central Universities(No.HIT.OCEP.2024038)the National Natural Science Foundation of China(No.52372351)the State Key Laboratory of Micro-Spacecraft Rapid Design and Intelligent Cluster,China(No.MS02240107)。
文摘To improve design accuracy and reliability of structures,this study solves the uncertain natural frequencies with consideration for geometric nonlinearity and structural uncertainty.Frequencies of the laminated plate with all four edges clamped(CCCC)are derived based on Navier's method and Galerkin's method.The novelty of the current work is that the number of unknowns in the displacement field model of a CCCC plate with free midsurface(CCCC-2 plate)is only three compared with four or five in cases of other exposed methods.The present analytical method is proved to be accurate and reliable by comparing linear natural frequencies and nonlinear natural frequencies with other models available in the open literature.Furthermore,a novel method for analyzing effects of mean values and tolerance zones of uncertain structural parameters on random frequencies is proposed based on a self-developed Multiscale Feature Extraction and Fusion Network(MFEFN)system.Compared with a direct Monte Carlo Simulation(MCS),the MFEFNbased procedure significantly reduces the calculation burden with a guarantee of accuracy.Our research provides a method to calculate nonlinear natural frequencies under two boundary conditions and presentes a surrogate model to predict frequencies for accuracy analysis and optimization design.
文摘This paper presents an innovative and effective control strategy tailored for a deregulated,diversified energy system involving multiple interconnected area.Each area integrates a unique mix of power generation technologies:Area 1 combines thermal,hydro,and distributed generation;Area 2 utilizes a blend of thermal units,distributed solar technologies(DST),and hydro power;andThird control area hosts geothermal power station alongside thermal power generation unit and hydropower units.The suggested control system employs a multi-layered approach,featuring a blended methodology utilizing the Tilted Integral Derivative controller(TID)and the Fractional-Order Integral method to enhance performance and stability.The parameters of this hybrid TID-FOI controller are finely tuned using an advanced optimization method known as the Walrus Optimization Algorithm(WaOA).Performance analysis reveals that the combined TID-FOI controller significantly outperforms the TID and PID controllers when comparing their dynamic response across various system configurations.The study also incorporates investigation of redox flow batteries within the broader scope of energy storage applications to assess their impact on system performance.In addition,the research explores the controller’s effectiveness under different power exchange scenarios in a deregulated market,accounting for restrictions on generation ramp rates and governor hysteresis effects in dynamic control.To ensure the reliability and resilience of the presented methodology,the system transitions and develops across a broad range of varying parameters and stochastic load fluctuation.To wrap up,the study offers a pioneering control approach-a hybrid TID-FOI controller optimized via the Walrus Optimization Algorithm(WaOA)-designed for enhanced stability and performance in a complex,three-region hybrid energy system functioning within a deregulated framework.
基金Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of Chinathe Scientific Innovation Research of College Graduates in Jiangsu Province(No.CXZZ11-0150)+1 种基金the National Natural Science Foundation of China(No.51177011)the National High Technology Research and Development Program of China(863 Program)(No.2012AA050210)
文摘In order to solve the multiple power extreme value point problem caused by system frequency splitting during wireless energy transmission at short distances a transmission model of the system is established.With the comprehensive consideration of the resonance frequency load parameters and the coupling between coils the internal factors of frequency splitting and boundary conditions are discussed.The results show that under the condition of the fixed load the higher the natural resonance frequency the easier the frequency splitting. As the frequency splitting occurs the frequency of the maximum power transfer is no longer with the natural resonance frequency which can make the system unstable and the transfer power more difficult to control. Therefore a decreasing-frequency method is proposed to avoid the system frequency splitting. And decreasing the system resonance frequency can make the system successfully withdraw the frequency splitting area at a short-distance range.Under the fixed load condition the transmission power of the system can be increased by 400% and the transmission efficiency is reduced by only 14% which greatly improves the transmission performance of the system.
文摘On the basis of high precision requirement for input signals in the power system protection and control system,this paper,only for the influence of power system frequency deviation on extracting fundamental harmonic,studies the amplitude error of Fourier algorithm,presents a method of correcting frequency deviation,and further derives the formulas of improved Fourier algorithm.The simulation results verified the effectiveness of the algorithm,it not only can greatly weaken the influence of frequency deviation,but also increase the precision of the power system protection and control.As a result the study in this paper has practical application value.
基金The National High Technology Research and Development Program of China(863 Program)(No.2007AA04Z301)
文摘Diffraction effects will bring about more difficulties in actuating resonators,which are electrostatically actuated ones with sub-micrometer or nanometer dimensions,and in detecting the frequency of the resonator by optical detection.To avoid the effects of diffraction,a new type of nanoelectromechanical systems（NEMS） resonators is fabricated and actuated to oscillate.As a comparison,a doubly clamped silicon beam is also fabricated and studied.The smallest width and thickness of the resonators are 180 and 200 nm,respectively.The mechanical oscillation responses of these two kinds of resonators are studied experimentally.Results show that the resonant frequencies are from 6.8 to 20 MHz,much lower than the theoretical values.Based on the simulation,it is found that over-etching is one of the important factors which results in lower frequencies than the theoretical values.It is also found that the difference between resonance frequencies of two types of resonators decreases with the increase in beam length.The quality factor is improved greatly by lowering the pressure in the sample chamber at room temperature.
基金The National Natural Science Foundation of China(No.60702028)the National High Technology Research and Development Program of China(863Program)(No.2007AA01Z268)
文摘Based on the frequency domain training sequences, the polynomial-based carrier frequency offset （CFO） estimation in multiple-input multiple-output （ MIMO ） orthogonal frequency division multiplexing （ OFDM ） systems is extensively investigated. By designing the training sequences to meet certain conditions and exploiting the Hermitian and real symmetric properties of the corresponding matrices, it is found that the roots of the polynomials corresponding to the cost functions are pairwise and that both meger CFO and fractional CFO can be estimated by the direct polynomial rooting approach. By analyzing the polynomials corresponding to the cost functions and their derivatives, it is shown that they have a common polynomial factor and the former can be expressed in a quadratic form of the common polynomial factor. Analytical results further reveal that the derivative polynomial rooting approach is equivalent to the direct one in estimation at the same signal-to-noise ratio（SNR） value and that the latter is superior to the former in complexity. Simulation results agree well with analytical results.
文摘Sum frequency generation vibrational spectroscopy （SFG-VS） is a robust technique for inter- facial investigation at molecular level. The performance of SFG-VS mostly depends on the spectral resolution of the SFG system. In this research, a simplified function was deduced to calculate the spectral resolution of picosecond SFG system and the lineshape of SFG spectra based on the Guassian shaped functions of IR beam and visible beam. The function indicates that the lineshpe of SFG spectra from nonresonant samples can be calculated by the Guassian widths of both IR beam and visible beam. And the Voigt lineshape of SFG spectra from vibrational resonant samples can be calculated by the Homogeneous broadening （Lorentzian width） and Inhomogeneous broadening （Guassian width） of vibrational modes, as well as the Guassian widths of both IR beam and visible beam. Such functions were also applied to verify the spectral resolution of the polarization-resolved and frequency-resolved picosecond SFG-VS system which was developed by our group recently. It is shown that the linewidths of IR beams that generated from current laser system are about 1.5 cm-1. The calculated spectral resolution of current picosecond IR scanning SFG-VS system is about 4.6 cm-1, which is consist with he spctral resolution shown in the spectra of cholesterol monolayer （3.5-5 cm-1）.
基金supported by National Natural Science Foundation of China(61533013,61273144)Scientific Technology Research and Development Plan Project of Tangshan(13130298B)Scientific Technology Research and Development Plan Project of Hebei(z2014070)
基金Supported by Tsinghua University’s Scientific Research Initiative Program,China(Grant No.2010THZ05)
文摘Low-fi＇equency vertical vibration isolation systems play important roles in precision measurements to reduce seismic and environmental vibration noise. Several types of active vibration isolation systems have been developed. However, few researches focus on how to optimize the test mass install position in order to improve the vibration transmissibility. An active low-frequency vertical vibration isolation system based on an earlier instrument, the Super Spring, is designed and implemented. The system, which is simple and compact, consists of two stages： a parallelogram-shaped linkage to ensure vertical motion, and a simple spring-mass system. The theoretical analysis of the vibration isolation system is presented, including terms erroneously ignored before. By carefully choosing the mechanical parameters according to the above analysis and using feedback control, the resonance frequency of the system is reduced from 2.3 to 0.03 Hz, a reduction by a factor of more than 75. The vibration isolation system is installed as an inertial reference in an absolute gravimeter, where it improved the scatter of the absolute gravity values by a factor of 5. The experimental results verifies the improved performance of the isolation system, making it particularly suitable for precision experiments. The improved vertical vibration isolation system can be used as a prototype for designing high-performance active vertical isolation systems. An improved theoretical model of this active vibration isolation system with beam-pivot configuration is proposed, providing fundamental guidelines for vibration isolator design and assembling.
文摘Ideally, diesel hybrid autonomous power systems would operate with high penetration of renewable energy sources such as wind and photovoltaic to minimize fuel consumption. However, since these are inherently intermittent and fluctuating, the grid-forming diesel engine generator sets are usually required to operate with larger amounts of spinning reserve, often at low loading conditions what tends to increases operating and maintenance costs. Frequency stability is of great concern in “small” systems, such as mini-grids, where any individual generator in-feed represents a substantial portion of the total demand. There, the initial rate of change of frequency is typically larger and a lower value of frequency can be reached in a shorter time than in conventional systems with all generation supplied by rotating machines, possibly resulting in under-frequency load shedding and tripping of renewable energy generators. The first part of this paper, discusses some general concepts regarding frequency stability in a diesel hybrid mini-grid and how energy storage systems can be used to enhance system performance. Then, a particular technique based on a virtual synchronous generator is presented and its effectiveness is demonstrated with simulation results.
文摘This paper proposes an autopilot system that can be used to control the small scale rotorcraft during the flight test for linear-frequency-domain system identification. The input frequency-sweep is generated automatically as part of the autopilot control command. Therefore the bandwidth coverage and consistency of the frequency-sweep are guaranteed to produce high quality data for system identification. Beside that, we can set the safety parameters during the flight test （maximum roll/pitch value, minimum altitude, etc.） so the safety of the whole flight test is guaranteed. This autopilot system is validated using hardware in the loop simulator for hover flight condition.
