The functional relation between partial discharges and inducedcharge

Analytical expressions are deduced for the quantitative evaluation of the charge induced on an electrode by a partial discharge (PD) within a dielectric body. Two approaches have been employed to effect this evaluation. These are characterized by the λ function and the φ function, and the essential relationships to be fulfilled by λ and φ are derived. The principal difference between these functions is how the dielectric polarization is accounted for, either implicitly (λ) or explicitly (φ). This difference is brought clearly into focus. Thereafter the application of these functions to PD studies is considered. It is demonstrated quantitatively, that perturbations in the dielectric polarization, brought about by the PD, can contribute significantly to the magnitude of the induced charge. As a consequence, the currently accepted approach to PD energy considerations should be re-examined     

Comment on “Using the λ function to evaluate probemeasurements of charged dielectric surfaces” [and reply]

For original paper, see T.O. Rerup et al., ibid., vol. 3, pp. 770-777 (1996). Rerup et al. described the calibration of electrostatic probes used to measure the surface charge on dielectrics. This work showed that the use of the probe response theorem (the λ function), rather than the simple capacitative formula, leads to better experimental values. In this discussion, the commenter wishes to show that a more detailed interpretation can be extracted from the numerical results. In reply, Rerup et al. demonstrate that simple statistics cannot be used to extract, in a conclusive manner, a more detailed interpretation of the σ_λ,σ_D patterns. Moreover, with the limitations imposed by the axially symmetric λ function, there is little to be gained in pursuing the source of the discrepancies between σ_λ and σ_F. They agree with the commenter that a multi-point scanning procedure would be essential if the functional form of the surface charge distribution were unknown     

Nano- and meso-measurement methods in the study of dielectrics

Measurements of the physical form and morphology of dielectrics and of their chemical characteristics and physical properties have become increasingly important at micron and sub-micron length scales. Such length scales are accessible by different forms of microscopy but scanning probe methods allow both chemical and physical measurements to be made, in addition to morphological and typographical imaging, at nanoscopic and mesoscopic length scales. These measurements are described in this paper with examples drawn from many areas of dielectric research and applications, including polymer morphology, surface and subsurface charge measurements along with the dynamics of charge-decay, surface potential and work function, microelectronics, ferroelectrics, photocharging, dielectric response and nanoscopic relaxation and nanoscopic Raman spectroscopy. The future of scanning probe methods for dielectric measurements is assured but more effort is required to both develop and consolidate this form of nano-metrology.     

Time-resolved space charge and electroluminescence measurements in polyethylene under ac stress

Time-resolved space charge (SC) and electroluminescence (EL) measurements are carried out on polyethylene films stressed under ac voltage at industrial frequency to probe injection, trapping and recombination of charge. SC is measured by pulsed electro-acoustic (PEA) technique at each zero voltage cross-over point of the ac voltage, thereby getting rid of the capacitive charge. EL is detected using photon counting techniques with a time resolution down to 50 /spl mu/s. Both SC and EL exhibit a threshold response as a function of the applied voltage. Numerical values of these thresholds are in good correspondence for SC and EL measurements in every tested material. Observations are consistent with a model of bipolar injection with an unbalanced situation in positive and negative charge behavior. It is also shown that different polyethylene materials behave differently relative to the field above which the charge is detected, providing thereby a way to compare the space charge behavior of those materials under ac field.     

Numerical modeling for potential relaxation of charged oil incontainers

A numerical model that simulates the relaxation process for surface potential at the surface of a charged oil in a grounded metal vessel is developed. In the model, constant (ohmic) conductivity of the oil is assumed, and a surface conduction mechanism for the dissipation of electric surface charge is introduced. Calculations for a cylindrical vessel of diameter D_t, height H_t, and fill fraction F show that the charge relaxation is influenced by vessel geometry and fill level, as expected, and by the absolute size of the vessel as well. In fact, it is found that the effective relaxation time of the surface potential correlates quite well to a dimensionless quantity D_tκ/λ, where κ (in Siemens per meter) is the bulk conductivity of the oil, and λ (in Siemens) is the effective surface conductivity at the oil/air interface     

New approach with a Hopfield modeling framework to economicdispatch

This paper presents a new Hopfield model based approach for the economic dispatch problem of power systems. To solve the economic dispatch problem using the Hopfield model, an energy function composing power mismatch, total fuel cost and the transmission line losses is defined. The weighting factors associated with the terms of the energy function can be either appropriately selected or directly estimated in the proposed model. Which, however, are determined by trial and error in the conventional Hopfield method. To minimize the value of the energy function, the computational procedures including a series of adjusting the weighting factor associated with the transmission line losses and updating the unit generations and power losses are carried out. Because the weighting factors are governed by some relationships developed, adjustment of the weighting factor is much simpler and more effective in steadily achieving solutions than adjustment of the λ-multiplier in the lambda-iteration method for economic dispatch problems. Computational results reveal that this approach can find accurate solutions more simply and fast compared with the conventional lambda-iteration method     

A practical approach to real time economic dispatch consideringunit's prohibited operating zones

This paper develops an efficient and practical approach for determining the feasible optimal solution of the economic dispatch problem when some of the online units have prohibited operating zones. When a unit has prohibited operating zones, its operating region will be broken into isolated sub-regions, which results in multiple decision spaces for the economic dispatch problem. For each of the decision spaces, it can be either feasible or unfeasible with respect to the system demand. The feasible optimal solution for the dispatch problem can only reside in one of the feasible, spaces, and can be found by performing the conventional λ-δ iterative search based on the equal-incremental-cost criterion in each of the feasible spaces. However, for a system with a large number of decision spaces, such an exhaustive search method would not be acceptable in real time operation due to its high computational time requirement. In this paper, a novel approach is proposed. It defines a small and advantageous set of decision spaces, uses an efficient algorithm to determine the most advantageous space and then utilizes the conventional λ-δ iterative method to find the feasible optimal dispatch solution     

Measurement of surface charge on opposite sides of a planar insulator using an electrostatic probe

A digital image processing technique using the Fourier transformation in the spatial domain and the Wiener filter has been developed. This method is applied to the measurement of the accumulated charge on the opposite sides of a planar insulator using an electrostatic probe. In the spatial frequency domain, the relational equations between the probe outputs and charge distribution are given by the first order simultaneous equations of two unknowns. In the course of the estimation of the charge distribution from the probe outputs, the Wiener filter is used to reduce the influence of the experimental noise. The spatial resolution of this method is also quantitatively discussed from an analysis in the spatial frequency domain. Using this technique, the distribution of the surface charge density on both surfaces of an insulating plate is measured immediately after the occurrence of a surface discharge. When the probe scans both sides of the plate, the charge distribution is estimated with a spatial resolution of 1.6 mm. In the case that the probe scans from only one side of the plate, the two-layered charge distribution is estimated with a limited resolution.     

Novel multimode fiber for narrow-band Bragg gratings

We propose a novel multimode fiber structure with modal propagation characteristics tailored to facilitate the creation of narrow-band high-reflectivity fiber Bragg gratings. The fiber structure proposed consists of concentric cylindrical shells of higher and lower refractive index material. A full vector second-order finite-element method is used to analyze the proposed multimode fiber structure. Simulations of the modal profiles show that high-order modes are localized to particular high-refractive index shells. We present the theoretical characterization of the modal propagation constant as a function of inner shell radius, shell separation, and harmonic-mode parameter. It is shown that a fiber with a minimum inner shell radius of at least 25λ (where λ is the vacuum wavelength), and a minimum shell separation of at least 10λ provides a reasonable tradeoff between fiber size and grating performance. A simulation of the multimode fiber grating shows that a grating with a full-width at half-maximum bandwidth on the order of 10^-4λ is theoretically possible, if optical power is launched strictly into modes with angular harmonic parameter p=1     

Measurement of tribo and corona charging features of materials for assessment of risks from static electricity

Many of the risks from static electricity arise in relation to the maximum surface voltage that may arise when materials are rubbed. This paper describes a new approach for assessing risks in terms of the "capacitance loading" experienced by static charge retained on materials. This involves simultaneous fast-response measurements of the quantity of charge transferred and the initial peak voltage generated. The quantity of charge transferred divided by the initial peak voltage allows calculation of the "capacitance loading." If this is large, then only low surface voltages will occur with practical quantities of charge transfer, and static problems are unlikely to arise from charge retained on the material itself. This approach provides a necessary addition to assessment of materials by the charge decay time when charge decay times are long. Studies show that decay times need to be quite short, 0.2 s, to avoid local high peak surface voltages.     

Electrical measurements for the characterization of electret properties of cellular polymers

Cellular polymeric materials may show interesting quasi-piezoelectric properties, upon charging under specific conditions of electrical field, temperature, pressure and environment. Electrical charging is the trigger of electret properties. It has been proved that the occurrence of electrical avalanches in the cavities which are present in a material is associated with the appearance of electret behavior, but the precise nature of the phenomena involved in this transformation is still under investigation. This paper provides a contribution to the study of these materials focusing on the relation between the appearance of electret properties and the presence of charge injection and partial discharge phenomena. For this purpose, space charge, partial discharges and low-frequency polarization measurements were carried out on polypropylene (PP) cellular materials, at different fields and temperatures. The existence of a threshold field for the start up of electret behavior that is larger than the threshold for space charge injection and partial discharge inception was observed. Steady charge is accumulated in the material, thus generating electret behavior, only if partial discharge repetition rate becomes sufficiently high and space charge is injected from electrodes. Space charge measurements were performed through the pulsed electroacoustic technique, in spite of significant attenuation of acoustic waves due to the cellular nature of the materials. Partial discharge measurements were carried out by an advanced system recording each discharge pulse, thus allowing observation of single and multiple pulses, besides achieving statistics of discharge pulse amplitude and phase. By such approach, space charge and partial discharge measurements proved to constitute a valuable tool to investigate the phenomena leading to piezoelectric behavior and, also, to infer electret time stability and optimize charging methodologies.     

Optical properties of pseudomorphic Si1-xGexfor Si-based waveguides at the λ=1300-nm and 1550-nmtelecommunications wavelength bands

The index of refraction for pseudomorphic Si_1-xGe_x layers grown on Si has been measured at wavelengths λ=1310 mn and λ=1550 nm. The refractive index values were obtained from waveguide mode profile measurements on a series of Si-Si_1-xGe _x-Si waveguides with Ge_x concentrations between x=0.01 and x=0.1. The index of refraction, n, is significantly larger for light polarized parallel to the growth direction than for light polarized in the plane of the epilayer. This birefringence is consistent with the anisotropic index change predicted using photoelastic theory, given the biaxial strain present in the pseudomorphic Si_1-xGe _x layers. At all wavelengths and polarizations, n varies linearly with the Ge concentration. The pseudomorphic Si_1-xGe _x waveguides layer are stable against lattice relaxation during short anneals at 950°C, but exhibit partial relaxation after annealing at 1200°C     

Analytical method for evaluating surface charge distribution on adielectric from capacitive probe measurement-application to a cone-typespacer in ±500 kV DC-GIS

The measurement of surface charging on a spacer in a ±500-kV HVDC gas-insulated switchgear (GIS) is performed by a capacitive probe set inside the gas tank. The various factors that cause measurement errors are examined using a simple model. An analytical method to correct the measurement errors and transform the probe output into the charge distribution on the surface of the spacer is developed. The method is an extension of a numerical method for 3-D surface charge calculation. It is shown that the charge distributions from the probe measurement agree fairly well with dust figures     

Advanced measurement of surface charge distribution with high potential

A measuring technique with a multilayer insulator is proposed to enable charge distribution measurement on an insulator of high potential. By using this technique, the residual charge distribution of the positive surface discharge was successfully measured with an electrostatic probe; nevertheless its potential is 10 times higher than the measurable range of an electrostatic probe. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Polymeric HVDC Cable Design and Space Charge Accumulation. Part 1: Insulation/Semicon Interface

From theory and experiments, it can be deduced that materials for DC applications should not accumulate a large amount of space charge if accelerated degradation of the insulation system is to be avoided. Therefore, the characterization of DC insulation must take into account the evaluation of space charge accumulation. This cannot be done exhaustively without taking a system approach considering both the semiconductive material and the insulation, in particular, the properties of the semicon/insulation interface. The latter interface, in fact, plays a major role in space charge injection/accumulation in the insulation bulk. Having analyzed different semiconductive and insulating materials candidate for HVDC cable applications, the best solution to be exploited for HVDC cable design would be the combination showing a high threshold for space charge accumulation, a small rate of charge accumulation as a function of electric field and a small activation energy, i.e., a space charge amount less dependent on temperature. Therefore, space charge measurements will provide important information to cable material manufacturers with the aim of tailoring insulation and semicon specifically for HVDC application and, thus, improving the reliability of polymeric cables.     

多回输电线路对短波无线电测向台无源干扰防护距离计算分析

输电线路对小孔径短波无线电测向台无源干扰防护距离计算是工程设计中的关键问题。针对短波测向台的工作频率,分析了单基铁塔高度为λ/4和3λ/4时对短波测向台无源干扰的影响;根据测向设备总的测向误差不得超过1°的限值,推导了多回输电线路对短波测向台无源干扰防护距离的计算方法。该方法对解决输电线路与短波测向台电磁兼容问题,提出的防护措施更加合理、全面、严谨。为《直流架空输电线路对无线电台电磁干扰防护设计规范》行业新标准的制订和工程设计提供了依据。     

Measurement of streamer charge distribution on an insulator in SF6 Gas

The charge density produced by streamers on an insulator surface in SF₆ has been investigated by using a probe method with a high-speed temporal resolution. Concentric circular probes, which also act as a plane electrode, are used in this probe method. Probe signals are observed oscilloscopically and converted into the charge densities through a numerical calculation. This method reveals the charge distribution before a disturbance caused by the “back discharge.” The charge density thus obtained ranges from several nC/cm² up to about 60 nC/cm². The density depends on the pressure, voltage height and the position of the streamer. The electric field on the insulator is analyzed numerically taking into account the surface charge. The internal electric field of the streamer is found to be 40 ～ 50 kV/cm · atm when the streamer ceases its propagation. However, it partly exceeds the critical one (89 kV/cm · atm) during the propagation.     

Large signal nonlinear model of anisotropic transformers fornonsinusoidal operation. II. Magnetizing and core-loss currents

For pt.I see ibid., vol.6, no.1, p.174 (1991). No-load currents and voltages of saturated single-phase transformers are computed based on given λ-i and harmonic core-loss characteristics. A circuit analysis approach is used to satisfy the nonlinear equations involved in an iterative manner. The theoretical and experimental analyses confirm that for the calculation of the iron-core losses under nonsinusoidal operating conditions the principle of superposition cannot be applied: the iron-core losses are dependent not only on the amplitudes of the fundamental and the harmonics of the flux density, but also on the phase shifts of the harmonics with respect to the fundamental. For the computation of the ohmic losses at no-load the skin effects within the windings are taken into account. To confirm the validity of the computed results some of them are compared with measurements     

The calibration of a capacitive probe for surface charge measurements

Contents A new method to calibrate a probe for charge measurements is proposed in this paper, after reviewing the principle and a calibration method. The new method is suitable to the measurements on a curved surface. Measurements on a 380 kV spacer at atmospheric conditions show the validity of the method.

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Das Kalibrierverfahren einer kapazitiven Meßsonde für die Messungen von Oberflächenladung

Übersicht In diesem Artikel wird, ausgehend von der Analyse des Prinzipes der Meßsonde und der Kalibriermethode, ein neues Eichungsverfahren für die Oberflächenladungsmessungen vorgeschlagen. Das neue Verfahren ist insbesondere für gekrümmte Oberflächen geeignet. Durch eine Reihe von Messungen an einem 380 kV Abstandshalter in Luft wurde die Gültigkeit dieses Verfahrens bestätigt.

     

Space charge and dielectric response in polar fluids

The assumption of "low signal" response is important in relation to the interpretation of the interaction of the electrical field (dc and ac) with mobile charge carriers. The existence of such a response is assumed in linear samples under the condition of absence of space charge. Experimental evidence as well as theoretical arguments of formation of hetero-charge in liquid-like materials have been reported. The materials, of course, are of low electrical conductivity. It has been demonstrated that the existence of space charge field is not always followed by sample nonlinearity. It is the case, where space charge field acts as a macrodipole with particular time or frequency response function. In principle, dc and ac sample responses should be the same because of the lack of nonlinearities. However, there is experimental evidence that this statement is not always true. The important problem seems to be the detection of space charge effects, which is particularly difficult in the case of ac measurements. There is a proposal to use dc combined with ac or dc measurements