Hot-spot temperature of transformer windings is a crucial indicator of internal defects.However,current methods for measuring the hot-spot temperature of transformers do not apply to those already in operation and suf...Hot-spot temperature of transformer windings is a crucial indicator of internal defects.However,current methods for measuring the hot-spot temperature of transformers do not apply to those already in operation and suffer from data lag.This study introduces a novel inversion method that combines ultrasonic sensing technology,multiphysics simulation,and the K-nearest neighbors algorithm.Leveraging the penetrative ability and temperature sensitivity of ultrasonic sensing,a detailed physical field simulation model was established.This study extensively investigates the characteristics of ultrasonic wave signals inside transformers.The investigation includes different temperature fields,ranging from 40℃ to 110℃ at 10℃ intervals,and various ultrasonic wave emitter conditions.By extracting the key features of the acoustic signals,such as the peak time,propagation time,and peak amplitude,an accurate inversion of the winding hot-spot temperature is successfully achieved.The results demonstrate that this method achieves a high accuracy rate(98.57%)in inverting the internal winding hot-spot temperatures of transformers,offering an efficient and reliable new approach for measuring winding hot-spot temperatures.展开更多
Basin-type insulator often has small cracks due to stress concentration.The current method cannot accurately reflect the stress condition of the insulator to find the stress concentration areas.To solve these problems...Basin-type insulator often has small cracks due to stress concentration.The current method cannot accurately reflect the stress condition of the insulator to find the stress concentration areas.To solve these problems,a method for detecting two-dimensional plane stress(δ_(1) andδ_(2))within different depth ranges in a basin-type insulator is proposed based on critically refracted longitudinal(LCR)wave.First,the acoustoelastic equation characterising the relationship between the variation of LCR wave propagation time and the plane stress was derived.Next,the propagation characteristics of LCR wave in epoxy resin samples were investigated.Then,the stress distribution within different depth ranges of the insulator subjected to hydraulic load was measured using the proposed method,including direction(θ),δ_(1) andδ_(2).The results show that the magnitude of the stress alone cannot accurately characterise the stress state.Points with equal distances to the centre have similar stress magnitudes,but their directions are not the same.With increasing depth,θremains essentially unchanged at the same location,whileδ1 andδ2 decrease,and the rate of decrease varies at different locations.Comparing the measured and simulated data,the results showed that they were in good agreement,and the maximum errors of stress value andθwere 0.69 MPa and 2.97°,respectively,which confirmed the feasibility and accuracy of the stress detection in the proposed method.展开更多
High‐voltage electronic equipment is widely used in power systems,which normally operate under the pulse voltage condition.However,compared to DC or AC stress,the electrical insulation suffers a more severe ageing pr...High‐voltage electronic equipment is widely used in power systems,which normally operate under the pulse voltage condition.However,compared to DC or AC stress,the electrical insulation suffers a more severe ageing process under a pulse voltage with steep edges and so may cause electrical equipments to fail early.The charge behaviour in polyimide under the pulse voltage with different edge times was investigated in this study.The experimental results show that charges vibrate at the pulse edge,while the amplitude of charge vibration increases with decreasing edge times.Furthermore,the detected vibration signal is strong within the nanosecond edge times,while being negligible in the microsecond range.Notably,the amplitude of charge vibration at the falling edge is larger than that at the rising edge in the presence of the column‐plate electrode.It is believed that the resultant force acting on the charge increases drasti-cally at the pulse edges,leading to the charge movement and the generation of ultra-sonic waves,with the resultant force becoming larger when the edge times decrease.Charge vibration behaviour is harmful to the microstructure of insulation materials,and so it may be concluded that the rise and fall times of the pulse voltage play an important role in insulation failure.展开更多
The oil‐pressboard insulation on the valve side of the converter transformer withstands pulsating square wave voltage.The voltage contains lots of DC components and high‐order harmonics,which is likely to cause insu...The oil‐pressboard insulation on the valve side of the converter transformer withstands pulsating square wave voltage.The voltage contains lots of DC components and high‐order harmonics,which is likely to cause insulation failures.The partial discharge and space charge experiments of oil‐impregnated pressboard under DC superimposed har-monic voltage are conducted to study the effects of harmonic frequency and DC com-ponents on the discharge and charge accumulation and to explore the correlation mechanism of charge dynamic behaviour and discharge.Firstly,it is found that high‐order harmonic voltage promotes the partial discharge generation,but DC component sup-presses the discharge.Moreover,the charge inside the pressboard is mainly injected in the same polarity and accumulates over time.The space charge density near the electrode increases with the increase of harmonic frequency,and the large amount of charge in-jection leads to electric field distortion.Meanwhile,the electric field direction changes faster,and the frequent charge injection and extraction behaviours make the charge recombination intensify,which promotes the discharge.Through the above analysis,the influence law of harmonic voltage on the failure of oil‐pressboard insulation is obtained,providing a theoretical basis for the structural optimisation and condition assessment of the converter transformer insulation.展开更多
Silicone elastomers are widely used to encapsulate power electronic devices.However,such devices may be subjected to square‐wave pulsed voltages with a high rate of change,which can create significant challenges for ...Silicone elastomers are widely used to encapsulate power electronic devices.However,such devices may be subjected to square‐wave pulsed voltages with a high rate of change,which can create significant challenges for encapsulation insulation.In this article,the molecular vibration of silicon elastomer at the edge of pulsed electric field is studied.Firstly,the relationship between the intensity of molecular vibration and the parameters of pulsed electric field is explored.The experimental results show that the amplitude of the vibrations decreases as the pulse‐edge time increases,and it increases linearly as the pulseedge slope increases.Furthermore,the amplitude of the vibrations is proportional to the square of the amplitude of the pulsed electric field,and it increases as the space charge density increases.Then,the force analysis of charged molecule at the pulse edges is calculated,and the theoretical change law of molecular vibration intensity with pulse edge slope is deduced.Comparing the theoretical results with the experimental results,it is found that they are highly consistent.Finally,electrically induced mechanical stress caused by molecular vibration was shown to be an important factor in insulation failure.展开更多
When high‐voltage direct current(HVDC)cables are subjected to a direct current(DC)‐superimposed pulsed electric stress,the pulsed voltage applied facilitates electrical tree generation and breakdown;however,the mech...When high‐voltage direct current(HVDC)cables are subjected to a direct current(DC)‐superimposed pulsed electric stress,the pulsed voltage applied facilitates electrical tree generation and breakdown;however,the mechanism involved remains unclear.To study the deterioration mechanism of cable insulation under a pulsed electric field,an experi-ment on the dynamic characteristics of space charge under a DC‐superimposed pulsed electric field was conducted.For this experiment,a pulsed time trigger control circuit was developed to accurately measure the change law of the space charge in cross‐linked polyethylene samples at the rising and falling edges of the pulsed electric field.The experimental results showed an unusual change law:the space charge density increases with falling voltage amplitude and decreases with rising voltage amplitude.The sudden change in the electric field breaks the balance of the forces acting on the space charge,leading to injection,extraction and migration.The energy released during these dynamic processes may destroy the microstructure of the insulation material and contributes to the growth of the electrical tree.This study provides an insight on the initiation mechanism of electrical tree in HVDC cables and proposes a strategy to suppress electrical tree initiation.展开更多
Converter transformers are the core device in ultrahigh voltage direct current(UHVDC)transmission systems,and the oil-paper insulation at its rectifying valve side deals with the composite AC-DC voltage.A detailed inv...Converter transformers are the core device in ultrahigh voltage direct current(UHVDC)transmission systems,and the oil-paper insulation at its rectifying valve side deals with the composite AC-DC voltage.A detailed investigation of the partial discharge(PD)mechanism in the oil-paper insulation under the composite AC-DC voltage is,therefore,very important.The oilpaper insulated needle plate electrode simulation model for the electric field and charge distribution can be established using hydrodynamic drift-diffusion theory and bipolar charge carrier theory,and the simulation results can reflect the morphological changes of PD.The development characteristics of PD under composite AC-DC voltage are researched.In addition,the effects of simulation time,the AC-DC component proportion,and model parameters on PD are also explored.Results indicate that differences in applied voltage,interface charge,and geometry structure can cause changes in the electric field strength,which consequently influences the PD process in the simulation model.Oil-immersed paperboard is a type of liquid-immersed dielectric(LID).When the LID is arranged in perpendicular,only the positive streamers have narrow channels oriented toward the LID.When the LID is arranged in parallel,the PDs are in the form of creeping discharges and the negative streamers are more dispersed.The development characteristics of PD and its influencing factors reported here may provide theoretical and simulation support to explain the physical mechanism of PD at the rectifying valve side of the converter transformer.展开更多
Hybrid reactive power compensation(HRPC)combines step-controlled shunt reactors and series compensation,and will be employed in ultra-high-voltage(UHV)power systems.The single-phase auto-reclosure characteristics of s...Hybrid reactive power compensation(HRPC)combines step-controlled shunt reactors and series compensation,and will be employed in ultra-high-voltage(UHV)power systems.The single-phase auto-reclosure characteristics of secondary arcs in systems with HRPC require further investigation.In this paper,both the arc-recalling voltage and subsidiary variations in arc current are investigated with and without HRPC.The frequency components of the secondary arc current and variations in arcing time are analyzed for various influential factors,such as the neutral reactor,arc resistance,fault location,degrees of compensation of HRPC,and the length of the transmission line.The non-dominated sorting genetic algorithm II(NSGA-II)and support vector machine regression are combined to create a multi-variable dependent forecasting algorithm to predict the characteristics of the secondary arc in UHV systems with HRPC.This paper provides a theoretical reference for optimizing the parameters of HRPC,and for developing adaptive auto-reclosure schemes and protection equipment.展开更多
基金Supported by National Natural Science Foundation of China(U1966209,52277155 and 2021CXGC010210).
文摘Hot-spot temperature of transformer windings is a crucial indicator of internal defects.However,current methods for measuring the hot-spot temperature of transformers do not apply to those already in operation and suffer from data lag.This study introduces a novel inversion method that combines ultrasonic sensing technology,multiphysics simulation,and the K-nearest neighbors algorithm.Leveraging the penetrative ability and temperature sensitivity of ultrasonic sensing,a detailed physical field simulation model was established.This study extensively investigates the characteristics of ultrasonic wave signals inside transformers.The investigation includes different temperature fields,ranging from 40℃ to 110℃ at 10℃ intervals,and various ultrasonic wave emitter conditions.By extracting the key features of the acoustic signals,such as the peak time,propagation time,and peak amplitude,an accurate inversion of the winding hot-spot temperature is successfully achieved.The results demonstrate that this method achieves a high accuracy rate(98.57%)in inverting the internal winding hot-spot temperatures of transformers,offering an efficient and reliable new approach for measuring winding hot-spot temperatures.
基金Major Scientific and Technological Innovation Project of Shandong Province,Grant/Award Number:2021CXGC010211。
文摘Basin-type insulator often has small cracks due to stress concentration.The current method cannot accurately reflect the stress condition of the insulator to find the stress concentration areas.To solve these problems,a method for detecting two-dimensional plane stress(δ_(1) andδ_(2))within different depth ranges in a basin-type insulator is proposed based on critically refracted longitudinal(LCR)wave.First,the acoustoelastic equation characterising the relationship between the variation of LCR wave propagation time and the plane stress was derived.Next,the propagation characteristics of LCR wave in epoxy resin samples were investigated.Then,the stress distribution within different depth ranges of the insulator subjected to hydraulic load was measured using the proposed method,including direction(θ),δ_(1) andδ_(2).The results show that the magnitude of the stress alone cannot accurately characterise the stress state.Points with equal distances to the centre have similar stress magnitudes,but their directions are not the same.With increasing depth,θremains essentially unchanged at the same location,whileδ1 andδ2 decrease,and the rate of decrease varies at different locations.Comparing the measured and simulated data,the results showed that they were in good agreement,and the maximum errors of stress value andθwere 0.69 MPa and 2.97°,respectively,which confirmed the feasibility and accuracy of the stress detection in the proposed method.
基金financed by the National Natural Science Foundation of China(Grant Nos.51907105 and U1966209)the Shandong Provincial Natural Science Foundation(Grant No.ZR2019QEE013).
文摘High‐voltage electronic equipment is widely used in power systems,which normally operate under the pulse voltage condition.However,compared to DC or AC stress,the electrical insulation suffers a more severe ageing process under a pulse voltage with steep edges and so may cause electrical equipments to fail early.The charge behaviour in polyimide under the pulse voltage with different edge times was investigated in this study.The experimental results show that charges vibrate at the pulse edge,while the amplitude of charge vibration increases with decreasing edge times.Furthermore,the detected vibration signal is strong within the nanosecond edge times,while being negligible in the microsecond range.Notably,the amplitude of charge vibration at the falling edge is larger than that at the rising edge in the presence of the column‐plate electrode.It is believed that the resultant force acting on the charge increases drasti-cally at the pulse edges,leading to the charge movement and the generation of ultra-sonic waves,with the resultant force becoming larger when the edge times decrease.Charge vibration behaviour is harmful to the microstructure of insulation materials,and so it may be concluded that the rise and fall times of the pulse voltage play an important role in insulation failure.
基金supported by the National Natural Science Foundation of China(U1966209)Natural Science Foundation of Shandong Province(ZR2021QE134).
文摘The oil‐pressboard insulation on the valve side of the converter transformer withstands pulsating square wave voltage.The voltage contains lots of DC components and high‐order harmonics,which is likely to cause insulation failures.The partial discharge and space charge experiments of oil‐impregnated pressboard under DC superimposed har-monic voltage are conducted to study the effects of harmonic frequency and DC com-ponents on the discharge and charge accumulation and to explore the correlation mechanism of charge dynamic behaviour and discharge.Firstly,it is found that high‐order harmonic voltage promotes the partial discharge generation,but DC component sup-presses the discharge.Moreover,the charge inside the pressboard is mainly injected in the same polarity and accumulates over time.The space charge density near the electrode increases with the increase of harmonic frequency,and the large amount of charge in-jection leads to electric field distortion.Meanwhile,the electric field direction changes faster,and the frequent charge injection and extraction behaviours make the charge recombination intensify,which promotes the discharge.Through the above analysis,the influence law of harmonic voltage on the failure of oil‐pressboard insulation is obtained,providing a theoretical basis for the structural optimisation and condition assessment of the converter transformer insulation.
基金National Natural Science Foundation of China,Grant/Award Numbers:51907105,52277155。
文摘Silicone elastomers are widely used to encapsulate power electronic devices.However,such devices may be subjected to square‐wave pulsed voltages with a high rate of change,which can create significant challenges for encapsulation insulation.In this article,the molecular vibration of silicon elastomer at the edge of pulsed electric field is studied.Firstly,the relationship between the intensity of molecular vibration and the parameters of pulsed electric field is explored.The experimental results show that the amplitude of the vibrations decreases as the pulse‐edge time increases,and it increases linearly as the pulseedge slope increases.Furthermore,the amplitude of the vibrations is proportional to the square of the amplitude of the pulsed electric field,and it increases as the space charge density increases.Then,the force analysis of charged molecule at the pulse edges is calculated,and the theoretical change law of molecular vibration intensity with pulse edge slope is deduced.Comparing the theoretical results with the experimental results,it is found that they are highly consistent.Finally,electrically induced mechanical stress caused by molecular vibration was shown to be an important factor in insulation failure.
基金National Natural Science Foundation of China,Grant/Award Number:51907105Natural Science Foundation of Shandong Province,Grant/Award Number:ZR2019QEE013National Natural Science Foundation of China,Grant/Award Number:U1966209。
文摘When high‐voltage direct current(HVDC)cables are subjected to a direct current(DC)‐superimposed pulsed electric stress,the pulsed voltage applied facilitates electrical tree generation and breakdown;however,the mechanism involved remains unclear.To study the deterioration mechanism of cable insulation under a pulsed electric field,an experi-ment on the dynamic characteristics of space charge under a DC‐superimposed pulsed electric field was conducted.For this experiment,a pulsed time trigger control circuit was developed to accurately measure the change law of the space charge in cross‐linked polyethylene samples at the rising and falling edges of the pulsed electric field.The experimental results showed an unusual change law:the space charge density increases with falling voltage amplitude and decreases with rising voltage amplitude.The sudden change in the electric field breaks the balance of the forces acting on the space charge,leading to injection,extraction and migration.The energy released during these dynamic processes may destroy the microstructure of the insulation material and contributes to the growth of the electrical tree.This study provides an insight on the initiation mechanism of electrical tree in HVDC cables and proposes a strategy to suppress electrical tree initiation.
基金This work was supported in part by the Science and Technology Project of State Grid Corporation of China(No.52060020002P).
文摘Converter transformers are the core device in ultrahigh voltage direct current(UHVDC)transmission systems,and the oil-paper insulation at its rectifying valve side deals with the composite AC-DC voltage.A detailed investigation of the partial discharge(PD)mechanism in the oil-paper insulation under the composite AC-DC voltage is,therefore,very important.The oilpaper insulated needle plate electrode simulation model for the electric field and charge distribution can be established using hydrodynamic drift-diffusion theory and bipolar charge carrier theory,and the simulation results can reflect the morphological changes of PD.The development characteristics of PD under composite AC-DC voltage are researched.In addition,the effects of simulation time,the AC-DC component proportion,and model parameters on PD are also explored.Results indicate that differences in applied voltage,interface charge,and geometry structure can cause changes in the electric field strength,which consequently influences the PD process in the simulation model.Oil-immersed paperboard is a type of liquid-immersed dielectric(LID).When the LID is arranged in perpendicular,only the positive streamers have narrow channels oriented toward the LID.When the LID is arranged in parallel,the PDs are in the form of creeping discharges and the negative streamers are more dispersed.The development characteristics of PD and its influencing factors reported here may provide theoretical and simulation support to explain the physical mechanism of PD at the rectifying valve side of the converter transformer.
文摘Hybrid reactive power compensation(HRPC)combines step-controlled shunt reactors and series compensation,and will be employed in ultra-high-voltage(UHV)power systems.The single-phase auto-reclosure characteristics of secondary arcs in systems with HRPC require further investigation.In this paper,both the arc-recalling voltage and subsidiary variations in arc current are investigated with and without HRPC.The frequency components of the secondary arc current and variations in arcing time are analyzed for various influential factors,such as the neutral reactor,arc resistance,fault location,degrees of compensation of HRPC,and the length of the transmission line.The non-dominated sorting genetic algorithm II(NSGA-II)and support vector machine regression are combined to create a multi-variable dependent forecasting algorithm to predict the characteristics of the secondary arc in UHV systems with HRPC.This paper provides a theoretical reference for optimizing the parameters of HRPC,and for developing adaptive auto-reclosure schemes and protection equipment.
