Showing papers on "Partial discharge published in 2009"

RF-Based Partial Discharge Early Warning System for Air-Insulated Substations

Abstract: Partial discharges (PDs) generate wideband radio-frequency interference which can be used for noninvasive monitoring of discharges. This paper presents a novel method based on this principle for PD monitoring of substations. The significant advantage of this method lies in the ability to detect PD sources in energized equipment anywhere within a substation compound during normal operating conditions. The results obtained from the prototypes installed in the U.K. and U.S. substations are reported. Results include correlation with apparent charge and daily recordings obtained before, during, and after the failure of a 132-kV current transformer and 69-kV voltage transformer.

Electrical treeing characteristics in XLPE power cable insulation in frequency range between 20 and 500 Hz

Abstract: Electrical treeing is one of the main reasons for long term degradation of polymeric materials used in high voltage AC applications. In this paper we report on an investigation of electrical tree growth characteristics in XLPE samples from a commercial XLPE power cable. Electrical trees have been grown over a frequency range from 20 Hz to 500 Hz and images of trees were taken using CCD camera without interrupting the application of voltage. The fractal dimension of electric tree is obtained using a simple box-counting technique. Contrary to our expectation it has been found that the fractal dimension prior to the breakdown shows no significant change when frequency of the applied voltage increases. Instead, the frequency accelerates tree growth rate and reduces the time to breakdown. A new approach for investigating the frequency effect on trees has been devised. In addition to looking into the fractal analysis of tree as a whole, regions of growth are being sectioned to reveal differences in terms of growth rate, accumulated damage and fractal dimension.

Application possibilities of artificial neural networks for recognizing partial discharges measured by the acoustic emission method

Abstract: The genesis of the research work presented in this paper constitutes the issue of the effective and efficient recognition of single-source one-time partial discharge forms that can occur in insulation systems of power transformers. The paper presents research results referring to the use of single-direction artificial neural networks for recognizing basic partial discharge forms that can occur in paper-oil insulation impaired by aging processes. The research work results presented show the recognition effectiveness of basic partial discharge forms depending on the descriptor of the analysis of the acoustic emission signal analysis. The detailed cognitive aim was selection of input parameters and an artificial neural network which would be the best, considering recognition effectiveness and processing time, and which could be used as a classifier in an expert diagnostic system making identification of partial discharges measured by using the acoustic method possible.

Enhancement of the Electromechanical Performances of a Single Dielectric Barrier Discharge Actuator

Abstract: In this paper, a parametric study of a single dielectric barrier discharge (SDBD) plasma actuator has been performed in order to enhance its electromechanical performances. Three input parameters have been investigated: the dielectric thickness, the waveform of the applied high voltage and the voltage slew rate. The goal is to increase the velocity of the discharge-induced electric wind and/or to reduce the electrical power consumption of the discharge. First, electrical measurements with high voltages ranging from 5 to 30 kV have been conducted, with a frequency set here to 1 kHz. This consisted of current recordings and electrical power consumption computations. Secondly, the velocity of the produced electric wind has been measured with a Pitot tube probe. From these velocity profiles, the maximum velocity, the produced mass flow rate and the electromechanical efficiency of the discharge have been determined. It highlights that the way to enhance the electromechanical performances of such a surface discharge consists in using a thick dielectric (at least a few millimeters) and a sine high voltage (several tens of kV) at a moderate frequency value.

Dielectric heating in insulating materials subjected to voltage waveforms with high harmonic content

Abstract: Dielectric heating is one potential aging mechanism active below partial discharge inception voltage in materials used as high voltage insulation. When exposed to voltage waveforms containing high amount of harmonics, the heat generation will be larger due to increased power losses as compared with power frequency excitation. This may result in a decreased life or even failure of insulation due to the increased operating temperature or to thermal runaway. An analysis of the power developed due to dielectric heating in two different materials subjected to voltage waveforms with high harmonic content is presented in this paper. By expressing the non-sinusoidal loss as an enhancement factor to the sinusoidal one, a geometry-independent formalism is derived. From dielectric response measurements at low voltage and at several temperatures the dielectric power loss in the material can be calculated for different voltage levels and waveforms. Two important material parameters can be extracted from the calculated dielectric power loss: (i) non-sinusoidal loss compared with sinusoidal loss with the same fundamental frequency (pfact) and (ii) change of loss with changing temperature (dpfact/dT). These two parameters could potentially be used to indicate the suitability of materials for use in applications where voltage waveforms contain high harmonic content.

Motor drive system

Abstract: The invention provides a motor drive system for suppressing the deterioration of insulation between turns of a motor winding caused by a surge voltage generated in an inverter driven motor For solving the problem, an optimum rising time is determined for suppressing partial discharge generated between the turns of the winding of the motor driven by an inverter from a relation between a rising time of the surge voltage and a surge-resistant voltage, and an optimum reactor capacity is determined from a relation between the rising time of the surge voltage and the inductance of an inserted reactor The reactor having the optimum capacity is arranged between the inverter and the motor winding The reactor may be the one only wound with wiring, an air-core reactor or the like

Partial discharge characteristics over differently aged oil/pressboard interfaces

Abstract: The choice of the materials used as the dielectric media in large transformers continues to be a combination of pressboard and mineral oil. However, a number of failures in the field have been attributed to the surface discharge or creeping discharge over the oil/pressboard interface. This paper reports on an experiment to study surface discharge over oil/pressboard. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD) from inception to flashover. The max PD charge, pattern, frequency and current pulse were measured during the flashover process. Four oil/pressboard specimens with different aging degrees were used and their influence on the flashover process over oil/paper interface was studied in this experiment. The results indicate that the max PD charge and PD frequency increase with the increase of applied voltage. The oil aging has great influence while the pressboard aging has little influence on PD characteristic over oil/pressboard. The PD pattern and current pulse shape show great difference for various applied voltages. The results therefore suggest that the PD detection on the oil/pressboard can be used effectively to implement the early monitor of surface discharge thereby to avoid the formation of surface flashover.

Detecting characteristics of various type antennas on partial discharge electromagnetic wave radiating through insulating spacer in gas insulated switchgear

Abstract: A widely used, effective method for diagnosing the insulation of a gas-insulated switchgear (GIS) is to detect partial discharges using signals in the ultrahigh frequency (UHF) band. To detect partial discharge signals that propagate inside a GIS, it is necessary to install a sensor inside or set on the outside of a GIS tank. For GISs that have already been constructed, it is effective to use external diagnosis using electromagnetic wave (EM-wave) signals that radiate (leak) from the insulation spacer of the GIS tank, and there is strong demand for such a diagnosis technology. Externally leaking EM-waves are often detected using an antenna, and multiple antennas supporting different frequency bands are commercially available. The accuracy of external diagnosis could be improved by comparing and evaluating the characteristics of various antennas. The insulation spacer of a GIS tank is usually installed on the flange and fastened with metal bolts. GIS tanks are made electrically continuous by these metal bolts and have the same grounding potential. EM-waves leak from the part of the solid insulator surrounded by these metal bolts and the GIS tanks. This leaking part forms a slot antenna and has a resonant frequency that depends on the spacing between the bolts in the circumferential direction of the flange. At this resonant frequency, the output of externally leaking EM-waves is higher and enables more sensitive measurement. In this study, the detection characteristics with different antenna positions and directions were compared in consideration of the number and positions of the bolts on the spacer, and the frequency characteristics of various antennas were obtained. To improve the detection sensitivity, a prototype of an improved dipole antenna was fabricated and its characteristics were evaluated. Based on the experimental results, an ideal diagnostic system based on the externally leaking EM-waves is proposed.

Acoustic measuring of partial discharge in power transformers

Abstract: Power transformers' reliability can be seriously affected by partial discharges. For this reason, it is necessary to implement technical methods to identify endangered equipment before catastrophic failures occur. A well-known method that can be applied either in the laboratory or in the field is the detection and localization of partial discharges, by means of the analysis of the acoustic signals they produced. An innovative partial discharge detector was developed based on the analysis of an acoustic or electrical PD signal envelope. This paper describes the architecture of the developed acoustic detector, which is composed of a set of ultrasonic sensors, signal conditioning and control modules, a graphical interface and the required software for the location of the affected area within the transformer. The conditioning and control modules perform analog to digital conversion, arrival time measurement, communication and control operations. Finally, some power transformer diagnostic testing is presented and discussed.

Surface charge accumulation and partial discharge activity for small gaps of electrode/epoxy interface in sf 6 gas

Abstract: The electrical insulation reliability of solid spacers in gas insulated switchgears (GISs) is an important issue to achieve a safe operation of such equipment. Among different phenomena, charge accumulation represents the most important matter that can degrade the overall performance of these insulation systems. For this respect, this paper discusses the contribution of partial discharge (PD) activity by ac voltage application to charge accumulation in the small gap at the electrode/epoxy interface as one of the weakest points in GIS solid spacers. The partial discharge inception voltages for non-accumulated charge case (PDIV0) and after exposing to PD activity (PDIVn) are measured among different gap lengths, simulating delamination at the electrode/epoxy interface. The PD activity is generated using applied voltage with 1.2×PDIV0 for all gap lengths examined in this study (50~500 ?m). In these measurements, PDIV increased with increasing the number of PD pulses as a result of accumulated charges. The accumulated surface charge density is estimated using the boundary equations and is compared for the different gap lengths. The accumulated charge density was larger for the smaller gap lengths. Comparing PD parameters with accumulated charge density enabled us to identify that the number of negative and positive PD pulses is the main parameter that corresponds to charge accumulation process.

Condition monitoring of power transformers using DGA and Fuzzy logic

Abstract: Dissolved Gas Analysis (DGA) is a widely used technique to estimate the condition of oil-immersed transformers. The measurement of the level and the change of combustible gases in the insulating oil is a trustworthy diagnostic tool which can be used as indicator of undesirable events occurring inside the transformer, such as hot spots, electrical arcing or partial discharge.

Atmospheric pressure glow discharge plasma in air at frequency 50?Hz

Abstract: Homogeneous atmospheric pressure discharge using a dielectric barrier discharge in air at 50 Hz was investigated. Porous alumina ceramic sheets were used as a dielectric barrier. In this paper we compare the filamentary discharge initiated by using glass sheets as a dielectric and the glow discharge obtained by the porous alumina sheets as a dielectric.Generally, the discharge in a dielectric barrier arrangement consists of a large number of filaments. Using porous alumina sheets Al2O3, an atmospheric pressure glow discharge (APGD) plasma has been obtained. This APGD discharge is characterized by a homogeneous profile of long lifetime (~5 ms) and the current is one pulse per half cycle of the power supply. The electrical measurements and the emission spectrum confirm that the discharge has been formed in the glow mode.

Rogowski air coil sensor technique for on-line partial discharge measurement of power cables

Abstract: For monitoring the insulation status of medium- and high-voltage power cables, on-line partial discharge (PD) diagnosis tests such as detection, location and identification are used to find the defects or faults and to assess the aging degree of the cable insulation, especially for XLPE cable systems. However, owing to signal propagation attenuation and noise pollution, the PD signals are weak, which impose challenges for PD detection. Fine detection methods require a sensor with high sensitivity and wide bandwidth to detect the PD pulses. Rogowski air coil sensor (RACS) technique has been used for on-line PD signal detection in recent years. It requires reasonable methods to raise the upper frequency under the condition of high sensitivity. Traditional RACS has narrow bandwidth because of the limitation of the discrete resonance frequencies. A novel physical design for on-line RACS is proposed. It is aimed at applying transmission line theory and adjustable balance impedance to restrain or eliminate the first resonance notch frequency. Based on this design, the bandwidth of RACS increases effectively, whereas the sensitivity remains essentially unchanged. A real RACS on the basis of coaxial cable windings is designed and developed. In comparison with the traditional capacitance sensor and RACS, experimental results verified that the RACS based on the proposed novel design technique can not only increase bandwidth but also improve the sensitivity of PD detection.

Pattern formation and boundary effect in dielectric barrier glow discharge.

Abstract: In this paper, we report the experimental results on the characteristics of plasma patterns with different planar electrode shapes in dielectric barrier glow discharge. The formation and the evolution of the discharge patterns at different voltages were investigated. The results show that the plasma patterns in this glow-barrier system form at the beginning of the discharge pulse. The limited size of planar electrodes and the electric field distribution are important factors for the pattern formation.

Electrical Model of an Atmospheric Pressure Dielectric Barrier Discharge Cell

Abstract: This paper presents a model of the typical dielectric barrier plasma discharge at atmospheric pressure, structured as an equivalent electric circuit whose elements are identified with, and deducted from, the main influential variables of the process, namely, the applied gas, the geometry of the reactor, the breakdown parameters, as well as the power supply associated to the dielectric barrier discharge cell. Considering a parallel-plate reactor and a high-voltage sinusoidal power supply, an electrical comprehensive Simulink/MATLAB model has been developed in order to reveal the interaction between these two elements. The main components of this discharge model are as follows: (1) a double dielectric capacitance; (2) a voltage-controlled current source; and (3) a gas capacitance associated to the ionized gas. A sinusoidal voltage of up to 15 kV peak to peak at frequencies of 12.5 and 47 kHz has been applied to the discharge electrodes. The electrical model is based on the power law proposed by Roth, which defines the V-I behavior during the discharge startup. A series of simulations has been carried out in order to estimate the total current and voltage consumed during each discharge and to identify those parameters which are not measurable during the process. Finally, both the experimental and simulated voltage and current results in helium, argon, and nitrogen, as well as their Q-V graphics, are shown, and a comparison between them is discussed.

Measurement and simulation of partial discharge in oil impregnated pressboard with an electrical aging process

Abstract: The continuous test on oil impregnated pressboard insulation with internal void defect was developed and the phase resolved partial discharge (PRPD) pattern of partial discharge (PD) signals during the electrical aging process was measured. Two different void structures which have different void volume were used in this experiment. It shows that the PD pattern could be classified into five stages and a great diversity in the first four stages is observed. The larger void volume leads to larger PD magnitude. The computer numerical simulation model which is based on a physical discharge process was used and the causes of PD pattern change were interpreted by comparison with computer numerical simulation results. The initial values and change tendency of gas pressure and surface conductivity were determined through experiment. The model parameters in different stages have been studied as well as the insight into the physical changes in the void during electrical aging. The results provide rules for the identification of the electrical aging stage through the partial discharge measurements.

A versatile method to study electrical treeing in polymeric materials

Abstract: Electrical treeing in high voltage cable insulation has been studied widely as it is one of the major breakdown mechanisms for solid dielectrics subjected to high electrical stresses. The most frequently experimental set-up used to study electrical treeing is utilizing needles to achieve a high divergent field to promote the initiation of electrical trees. In this paper a new type of electrode configuration is described, which will be beneficial when examining electrical treeing in terms of both the capability to follow the course of events in real time and the small amount of insulation material is needed. This new electrode configuration makes use of a thin tungsten wire as electrode to obtain the high divergent electrical stress needed to initiate electrical trees. Multiple electrical trees are at times formed at different points along the wire during the testing, this indicates that the field are homogenous along the wire. This allows for determining the tree initiation field and the rate of tree growth. It is also shown that this electrode configuration is able to distinguish the increased electrical tree inception obtained by the addition of voltage stabilizers.

Remote electrical safety diagnosis system and apparatus

Abstract: The present invention relates to an electrical safety diagnosis system and apparatus, which are configured to enable sensed data on user surroundings in a U-city environment, such as electrical safety data for switchboards and distribution boards, and water leakages, faulty outlet connections, etc., to be managed remotely by a server at a U-city integrated information center. In particular, the present invention relates to an electrical safety diagnosis system and apparatus which use a U-city integrated information network to preemptively perform checking and analyses for potential electrically-induced disasters that can occur in public facilities, stores, buildings, and homes within a U-city, by means of a U-city integrated information center, and prevent electrical fires or shocks at the electrical equipment of switchboards or distribution boards, and dangerous conditions (such as water leakages, abnormal temperatures, gas, and faulty outlet connections) in facilities used by users. The remote electrical safety diagnosis system of the present invention comprises: a digital switchboard including the functions of checking electrical equipment in a switchboard room using a contactless arc sensor and a partial discharge sensor on a power line, to diagnose the safety of the electrical equipment, diagnosing an arc or partial discharge in a detected waveform and storing the diagnosed data, and transmitting the diagnosed data to a higher level server; and a digital distribution board including a main circuit breaker and a partial circuit breaker, for checking an AC voltage or current input through the power line, storing the checked data, and transmitting the diagnosed data to the higher level server.

Frequency domain correlation technique for PD location in transformer winding

Abstract: Partial discharge (PD) is one of the sources of insulation failures in Power transformers. Knowledge of the PD source locations are important for transformer maintenance and repair. Due to the complex structure of the transformer, it is very difficult to locate the PD accurately in transformer winding. Correlation methods are applied to locate the PD source across the winding. The PD across parts of the winding are simulated by applying voltage pulse across the sections. The winding response for a reference PD impulse across transformer sections are taken as base for correlation and correlated with winding response of PD impulse across any section with different PD pulse widths. In this paper the limitation of time domain correlation method in locating PD in transformer windings is found out. To overcome the limitation of time domain correlation method, frequency domain correlation with Gram Schmidt orthogonalization is proposed. Simulation and experiments are performed on layer, continuous disc and interleaved windings to prove the feasibility of the proposed method.

Inductive Sensor for Measuring High Frequency Partial Discharges Within Electrical Insulation

Abstract: Partial discharges are not only a consequence but a symptom of insulation degradation in electrical equipment. Partial discharges are revealed outside the insulation as very narrow high-frequency pulses that were superimposed to the grid frequency high-level voltages or currents and thus are fairly hard to detect. This paper presents an application of a new inductively coupled sensor that allows these signals to be measured in an accurate inexpensive manner. The sensor model and its frequency and geometry-dependent behavior are studied to increase the probe sensitivity as much as possible. Its measuring capability in real insulation systems that were subjected to high partial discharge activity will be tested.

Simulation of Propagating Electromagnetic Wave Due to Partial Discharge in GIS Using FDTD

Abstract: It takes a lot of trouble to measure and calibrate the sensitivity of ultra-high-frequency couplers which are set in various gas-insulated switchgears (GIS). This paper is related to the approach of calculating the attenuation of the electromagnetic (EM) wave due to the partial discharge propagating through insulators constituting a GIS. The use of a simulator based on the finite-difference time-domain method for analyzing the three-dimensional transient EM fields shows the degree of precision of the modeling descriptions employed and their ability to confirm experimental results.

Propagation characteristics of electromagnetic waves in three-phase-type tank from viewpoint of partial discharge diagnosis on gas insulated switchgear

Abstract: The authors have been examining the diagnosis method for partial discharge (PD) occurring inside gas insulated switchgear (GIS) using the UHF method with a focus on electromagnetic (EM) wave mode conversions at L-shaped, T-shaped pipes, and components existing in actual GIS. Based on these conversion characteristics, an on-site, high-accuracy PD diagnosis algorithm was constructed for use on high-order mode EM-waves. All the researches so far were targeted at an isolated-phase-type (singlephase- type) tank. The present study was targeted at a straight-pipe three-phase structure, the most basic form, in order to apply the evaluation method to a three-phase-common-type tank (three-phase-type tank). Through the implementation of EM-wave propagation experiments and the EM-field analysis, the presence of TEM, TE11, and TE21 modes was verified also in the three-phase-type tank. Next, the EM-field analysis was conducted on models equivalent to actual apparatuses of 72 to 550 kV classes. Then, formulas for simplified calculation of cutoff frequencies in TE11 and TE21 in a three-phase-type tank were proposed. Furthermore, the basic concept of the PD diagnosis method was examined and the work flow was presented.

Off-line and on-line pd measurements on induction motors fed by power electronic impulses

Abstract: Partial Discharge (PD) off- line and on-line measurements are required to prevent early failures on low voltage induction motors fed by inverter drives. Conventional methods use commercial Digital Scopes to visualize simultaneously the waveform of the applied impulsive voltage and the acquired PD pulses, as well as appropriate solutions to attenuate power electronic voltage impulses. A new approach is proposed in this paper, relying upon a Phase-Resolved PD (PRPD) pattern, which constitutes a simple and efficient tool for PD activity detection in adjustable speed drives. Off-line PD measurements were done on low voltage induction motors fed by a voltage impulse generator and detecting PD Inception Voltage (PDIV) on each phase. Then, PD on line measurements were carried out using a commercial PWM inverter and showing interesting correlation between off-line and on-line tests. All the measurements were performed using antenna sensors, achieving excellent sensitivity.

Electromagnetic wave radiated from an insulating spacer in gas insulated switchgear with partial discharge detection

Abstract: As a means of diagnosing partial discharge (PD) signals propagate inside a gas insulated switchgear (GIS), a study for the leakage of electromagnetic waves (EM-waves) emitted from the insulating spacer was implemented. The EM-waves leaking out from the solid insulator have the resonance frequencies depend on the spacing between adjacent bolts in the direction of the flange circumference, because the leakage portion is the equivalent of a slot antenna. In this paper, using an electromagnetic analysis model which has a simulated spacer on a concentrically-shaped GIS tank, the output characteristics of the EM-waves that leaked out from the slit were analyzed under various conditions such as the spacing between adjacent bolts, the width of the spacer, the dielectric constant of the spacer and the form of the flange. Also the actual measurement by the experimental equipment used to simulate the model was implemented for comparison with the analytical results. Consequently, the optimal specifications of the sensor and the measurement method used to achieve highly-sensitive detection for practical use were summarized and proposed as well as evaluating the effectiveness of the electromagnetic analysis model adopted in this paper.

Dielectric properties of natural esters and their influence on transformer insulation system design and performance — An update

Abstract: There is considerable knowledge of Transformer Insulation System design based on the use of cellulose insulation used in conjunction with mineral oil. This knowledge is based on over 100 years of transformer design and manufacture. Insulation design is based on the stress distribution between the solid insulation, in this case Kraft, and the fluid. In power and distribution transformers the stress is distributed in accordance with the permittivity of the various insulation components. The insulation designer must determine the stress in the fluid, in the solid insulation and along the interface. Design curves have been established for mineral oil, which give limits to the allowable stress at each of these critical areas. This paper presents the results of a test program to develop these criteria for Kraft insulation in Natural Ester fluids. This paper is an update to a paper presented at the IEEE T&D Conference in Dallas Texas in 2006. [1] At that time we had not completed all of the dielectric tests. These have now been completed and are presented in this paper.

Structure-variable gas insulation composite apparatus partial discharge experiment device and method thereof

Abstract: The invention discloses a partial discharging experiment device of a structure-variable combined electrical gas-insulated apparatus and a method thereof, and belongs to the technical field of insulation online detection of electrical equipment. The partial discharging experiment device mainly comprises a GIS simulating element, a digital storage oscillograph and a signal generator, etc. By utilizing the device, the partial discharging experiment method carries out a pressurized experiment so as to collect ultrahigh frequency PD signals, carries out a signal injecting experiment, collects and corrects the ultrahigh frequency PD signals through programs, thus obtaining real waveform of the ultrahigh frequency PD signals. The device can simulate the T-shaped, L-shaped and bar-shaped structures of real GIS equipment. The method can carry out the experiment to the T-shaped, L-shaped or bar-shaped GIS structures by one to four insulated defect models, and the experiment is real, accurate and wide in range. The device and the method can be widely applied to the experimental research of the insulated state online monitoring of GIS equipments in teaching practices, scientific research institutions and equipment manufacturing plants, and the detected PD signals can be used for theoretical analysis and applied research.

Experience with optical partial discharge detection

Abstract: Partial discharge (PD) diagnostics was used to evaluate electrical insulations of high voltage equipment as a quality control and to detect insulation deterioration. Improvements, new developments as well as lower costs of sensors, electronics and processing units are some reasons for the increasing usage of PD measurements. The paper presents investigations in the area of optical partial discharge measurement technique. In an overview, PD measurement systems were presented, advantages and disadvantages of the optical method were discussed. An unconventional optical partial discharge detection system was developed and comparative measurements to a conventional electrical PD measurement system were accomplished. The system was tested by using a PD source in air or alternatively in oil to evaluate the sensitivity and the impulse behaviour in correlation to the conventional measurement technique.

On the regime transitions during the formation of an atmospheric pressure dielectric barrier glow discharge

Abstract: The atmospheric pressure dielectric barrier discharge in helium is a pulsed discharge in nature If during the electrical current pulse a glow discharge is reached, then this pulse will last only a few microseconds in operating periods of sinusoidal voltage with lengths of about 10 to 100 µs In this paper we demonstrate that right before a glow discharge is reached, the discharge very closely resembles the commonly assumed Townsend discharge structure, but actually contains some significant differing features and hence should not be considered as a Townsend discharge In order to clarify this, we present calculation results of high time and space resolution of the pulse formation The results indicate that indeed a maximum of ionization is formed at the anode, but that the level of ionization remains high and that the electric field at that time is significantly disturbed Our results also show where this intermediate structure comes from (Some figures in this article are in colour only in the electronic version)