Design of a high-frequency transformer based on amorphous cut cores for insulation breakdown testing

Innovative test methods are required to keep up with the increased demand for insulation materials which can withstand the high-frequency square-wave voltages generated by power-electronic equipment.Test systems using high-voltage,high-frequency transformers have proven versatile and easy to realise.The authors investigate the application of amorphous cut cores in such transformers.Analytical and numerical models for the transformer and its frequency response are developed,aiding the design process.An amorphous core-based transformer is designed for 8 kVpk output at 100 kHz and compared to two previous designs based on ferrite cores.The frequency and pulse response,as well as the high-voltage and thermal performance,are evaluated.The comparison shows that while the low parasitics of the amorphous-based transformer allow for superior frequency response,they are unsuitable for long-duration tests with high pulse repetition frequencies(25-100 kHz)due to increased core losses.The partial discharge inception and flashover voltage are comparable to the ferrite-based transformers.

GIS用超高频传感器的校准:不同校准方法的比较及一种采用锥形天线的校准系统测试(英文)

Different methods of calibrating ultra high frequency(UHF) sensors for gas-insulated substations(GIS) were investigated in the past.The first approach was to use strip lines,triplates and TEM calibration cells.These cells had already been in use for years for example to test the electromagnetic compatibility of electronic devices.The smaller the size of the cell,the higher its bandwidth-but the cell should be large enough to not disturb the electric field with the installed sensor under test.To overcome this problem,a calibration procedure using a gigahertz transverse electromagnetic (GTEM) test cell and a pulsed signal source were introduced in 1997.Although this procedure has many advantages and is easy to understand,measurements show several shortcomings of this calibration method.To overcome the disadvantages of the known systems,a calibration cell using a monopole cone antenna and a metallic ground plane were developed and tested.The UHF sensor was placed in a region with minimum distortion of the electric field due to its installation.Experience shows that the new method for calibrating UHF sensors is necessary in order to overcome the limits in the calibration of large sensors and to suppress the propagation of higher order modes and reflections.Due to its surprisingly simple structure,its low price and low overall measurement uncertainty,it is the preferred method for calibrating UHF sensors for GIS applications.

Investigation of a CO2 Switching Arc Using Spectroscopic Diagnostics and Computational Fluid Dynamics Simulations

A deep understanding of the physical processes coming along with the current interruption in high voltage circuit breakers is essential for the optimization of today’s switching technologies.Therefore a switching arc in a model circuit breaker is studied by means of computational fluid dynamics(CFD)simulations and optical emission spectroscopy(OES)in this contribution.Experimental investigations are performed in carbon dioxide(CO2)at absolute filling pressures of 0.1 and 0.5 MPa.CFD simulations are carried out based on a model of the arcing zone including a consistent treatment of the radiation transport and the wall ablation.Carbon ion line radiation is analysed in the experiment using an optical path in the heating channel between the electrodes inside the nozzle system.The pressure value in the arc is estimated based on the line width-intensity dependence.Obtained values correspond to the measured pressure outside the arc.For the temperature profiles,a good agreement within the accuracy of the approaches is observed between the CFD simulations and the results of OES.

超声波与人工智能在现场绝缘诊断中的应用(英文)

This work studies the feasibility of implementing a system for diagnosis of electrical insulation in the field based on ultrasonic noise and artificial neural networks.Such system,proved functional under laboratory conditions,extracts spectral information from the ultrasonic noise emitted by the corona discharges in electric equipments,and correlates it with previously defined degrees of pollution.To achieve this classification,artificial neural networks are employed.The results show the viability of the method in the site;nevertheless they also show that its reliability is proportional to the size and diversity of the available database.

Study of Formation and Propagation of Streamers in SF_6 and Its Gas Mixtures with Low Content of SF_6 Using a One-Dimensional Fluid Model

Using a hybrid Monte Carlo Collision/Fluid model, the formation and propagation of streamers in SF6 and its gas mixtures are simulated. The simulation is based on an accurate numerical solution of Poisson＇s equation in conjunction with the continuity fluid equation for electrons, negative ions, and positive ions. The factors that influence the formation and propagation of streamers are investigated. The electron density, positive and negative ion density, and electric field in the discharge channel are also presented, which are very important in understanding the phenomena of streamers and in assessing the insulation strength of the gas mixture.

Formation of RF Plasma Transport Channel in External B-Field

The paper describes experiments on formation of a plasma channel with imbedded B-field for transporting high power ion beam. The plasma was generated with a 5-turn loop inductive antenna driven by an H-bridge type RF generator. The azimuthal B-field in the channel of 0.5-1.5 kG was formed by a pulsed current from external capacitor bank. Control of the hydrogen gas pressure was provided by an electromagnetic puffvalve. The paper describes experimental devices and results on the generated plasma parameters as function of RF frequency, antenna voltage, pulse duration and puffgas pressure. When operating at-1 kG B-field, ambient gas pressure in the range of few -10 mTorr, and 5 kV antenna voltage at resonant frequency of 150 kHz, the plasma density range was （3-7）×10^12 cm3 with a temperature of a few eV.

Pressure Rise in Electrical Installations due to Internal Arcing in CO2 as Insulating Gas

Internal arcs cause a rapid increase in pressure in electrical installations. The type of insulation gas has influence on pressure development. Typically SF6 is used incompact metal-clad switchgear, however, it has a high global warming potential. Because of this, the replacement of SF6 by alternative gases such as CO2 is under discussion. The pressure developments in a closed vessel filled with air, SF6 and CO2 are measured and compared. During internal arcing in gas-insulated switchgear, overpressure causes a rupture of a burst plate and hot gas escapes into the surrounding room mixing with air. In order to predict the pressure development in electrical installations reliably, the portion of energy causing pressure rise, arc voltage as well as reliable gas data i.e., thermodynamic and transport properties, must be known in a wide range of pressure and temperature. These data are up to now not available for CO2/air mixtures. The thermodynamic properties are directly calculated from the number densities, internal partition functions and enthalpies of formation. The transport coefficients are deduced using the Chapman-Enskog method. Comparing measured and calculated pressure developments in a test arrangement demonstrates the quality of the calculation approach.

Investigation of impact of DC component on breakdown characteristics for different electric fields under composite AC&DC voltage

The valve side of the converter in the high‐voltage direct current is subjected to mixed voltages such as composite AC&DC voltage.In this study,the effects of the homo-geneity of electric field on breakdown voltage were investigated for different�DC component amplitudes of the composite voltage.The field efficiency factor was calcu-lated using mean and maximum field strengths for all of them.Variation of breakdown voltage of air was examined under the composite AC&DC voltage for different ratios�DC.As one result of the study,the breakdown occurs at the positive half‐wave of the AC voltage despite−DC voltage being applied due to positive corona discharge pulses.This breakdown point is named as the polarity change point.The breakdown voltage increases with the decrease of DC voltage component up to polarity change point in non‐uniform electric field.In less uniform electric field,the AC breakdown voltage was measured slightly higher than the DC breakdown voltage.In less uniform electric field,as the ratio of the applied AC voltage to DC voltage increases,the breakdown voltage gradually approaches the AC breakdown voltage.This result is similar to the result ob-tained for the+DC component in non‐uniform electric field experiments.

Time-domain Dynamic State Estimation for Unbalanced Three-phase Power Systems

In this paper,we present a time-domain dynamic state estimation for unbalanced three-phase power systems.The dynamic nature of the estimator stems from an explicit consideration of the electromagnetic dynamics of the network,i.e.,the dynamics of the electrical lines.This enables our approach to release the assumption of the network being in quasi-steady state.Initially,based on the line dynamics,we derive a graphbased dynamic system model.To handle the large number of interacting variables,we propose a port-Hamiltonian modeling approach.Based on the port-Hamiltonian model,we then follow an observer-based approach to develop a dynamic estimator.The estimator uses synchronized sampled value measurements to calculate asymptotic convergent estimates for the unknown bus voltages and currents.The design and implementation of the estimator are illustrated through the IEEE 33-bus system.Numerical simulations verify the estimator to produce asymptotic exact estimates,which are able to detect harmonic distortion and sub-second transients as arising from converterbased resources.

Study on the influence mechanism of ambient humidity on the temperature rise of decay-like composite insulators

At present,observing the temperature rise of composite insulators is an effective method for the detection of decay-like defects.However,ambient humidity will affect the temperature rise of composite insulators,which will cause problems in screening the decaylike composite insulators.Therefore,it is urgent to study the temperature rise principle of decay-like insulators and find out the influence mechanism of humidity on temperature rise.In this study,a four-electrode system is designed to measure the surface current,volume current and interface current of the decay-like short sample and the intact short sample under different humidity.Then the equivalent circuit models of short samples and the 500 kV full-size decay-like composite insulator are built to analyse the contribution rates of leakage conductance loss and polarisation loss to temperature rise.The results show that the contribution rates of the polarisation loss to the temperature rise of two samples are 35.8%and 23.6%in low humidity,while they are 98.3%and 64.4%in high humidity.For the full-size decay-like composite insulator,the contribution rates of polarisation loss to its temperature rise in high or low humidity are more than 99%.In addition,the proportion of polarisation loss increases at first and then decreases with the increase of the length of the decay-like section.The conclusions of this study provide a theoretical basis for further understanding the corresponding relationship between decaylike defects and temperature rise and improve the detection rate of decay-like insulators.

Proposed methodology for online frequency response analysis based on magnetic coupling to detect winding deformations in transformers

Frequency response analysis is widely used as a method for the offline diagnosis of winding deformations in power transformers.To apply it to a working transformer,people need to determine how to inject the excitation signal and measure the response signal for windings that bear a rated voltage and current.In this study,a method to obtain the frequency response curve online is proposed.It uses the principle of magnetic field coupling to inject a frequency sweep signal into the windings through a Rogowski coil.Another Rogowski coil sensor placed at the root of a high-voltage bushing is used to measure the response current signal.Experiments on a 72.5 kV bushing show that metal accessories of the bushing have no influence on the injection or the measurement.The feasibility of this method was verified by experiments on charged 110/35/10 kV transformers at a factory and a working 35 kV transformer in a power station.The results show that it is safe to install the Rogowski coil at the root of the high-voltage bushing.The excitation signal can be injected into live windings and the response signal is measurable.Strong electromagnetic power frequency noise can be reduced and the frequency response curve can be measured online.