Surface recovery of silicone rubber used for HV outdoor insulation

Results of a study performed to obtain a better understanding of the material characteristics responsible for hydrophobicity recovery leading to a high wet surface resistance in silicone rubbers used for outdoor HV insulation, are reported. The samples were obtained from new and artificially aged HV insulators using HTV silicone rubbers (3 different formulations) as weathershed and RTV silicone rubbers (2 different formulations) as a protective coating. The main experimental facilities employed consist of a salt fog chamber for artificially aging the insulators, and a scanning electron microscope (SEM) for material studies. New results of practical significance that have emerged from this study are: (1) hydrophobicity recovery, predominantly due to diffusion of low molecular weight (LMW) silicone polymer chains, occurs with only a fraction (<20%) of the total LMW polymer content initially available in an unaged material surface, (2) LMW chain regeneration and hence surface hydrophobicity recovery occurs even after the initial supply of LMW polymer is depleted, and (3) hydrophobicity recovery is significantly affected by ambient temperature. The results show the same pattern for different formulations of HTV and RTV rubber materials studied. The X-ray Mapping feature in the SEM provides a visual indication of the diffusion process which is a noteworthy contribution     

Silicone rubber dielectrics modified by inorganic fillers for outdoor high voltage insulation applications

The paper discusses the mechanisms by which inorganic fillers in silicone rubber dielectrics enhance the properties of thermal conductivity, relative permittivity, and electrical conductivity making them useful in outdoor high voltage insulation applications. The addition of alumina trihydrate or silica fillers to silicone elastomers, forming binary composites with enhanced thermal conductivity, is discussed in relation to filler type, particle size, shape, and concentration, and its use as a housing material for non-ceramic insulators to minimize material erosion at dry band arcing sites by lowering hot spot temperature. Also discussed is the enhanced relative permittivity of silicone dielectrics that is obtained through the addition of barium titanate powder which can be further increased with the addition of aluminium powder forming a tertiary composite, resulting in a significant grading of the surface electric field when applied as a housing material to high voltage bushings. Controlled electrical conductivity of silicone dielectrics is discussed through the use of antimony-doped tin oxide filler binary composites and when applied as a housing material to outdoor bushings, the pollution performance is greatly enhanced.     

Modern approaches to evaluate the state of insulation of high voltage machines

The use of the total index of polarization (TPI) as an integrated characteristic of the processes of polarization development in the volume of a controllable interval has been demonstrated to result in more reliable information about the condition of power facilities and to provide the necessary precision of the received estimates.     

The role of inorganic fillers in silicone rubber for outdoor insulation alumina tri-hydrate or silica

A systematic study to understand how alumina tri-hydrate (ATH) and silica fillers improve the erosion resistance of silicone rubber during dry band arcing showed that the thermal conductivity of the resulting composite material is the main criterion governing material erosion. The thermal conductivity of the composite material is dependent on the thermal conductivity, concentration, particle size, and bonding of the filler particles to the silicone matrix. In this context, either filler can be shown to perform better than the other, depending on the formulation, in the ASTM inclined plane tracking and erosion test. Therefore, the industry perception that ATH filler imports better erosion resistance than silica in silicone rubber can be misleading. The release of water of hydration from ATH appears to have a secondary effect that may be more relevant in silicone compositions having a low concentration of a filler.     

Leakage current suppression and resistance to tracking and erosion of HTV silicone rubber with added silicone plasticizer

The present study evaluated the leakage current suppression and the resistance to tracking and erosion of high-temperature vulcanized silicone rubbers (HTV-SIRs) containing different levels of silicone plasticizer. Slab HTV-SIRs unfilled and filled with alumina trihydrate (ATH, 50 wt%) and with added linear silicone plasticizer (0-6.0 wt%), which was terminated with hydroxyl groups and of which repetition number was 13-18, were tested. Evaluation using the IEC 60587 inclined-plane method indicated that the plasticizer had no influence on the tracking and erosion behavior. Salt-fog test results indicated that a higher level of plasticizer in the ATH-filled HTV-SIR showed smaller leakage current. Gas chromatography and mass spectrometry suggested that the enhanced suppression of the leakage current for ATH-filled HTV-SIR can be attributed to migration of the linear silicone plasticizer onto the surface contaminants. It was concluded that the addition of silicone plasticizer is useful for improving the leakage current suppression ability of HTV-SIR without reducing its tracking and erosion resistance in the above laboratory tests     

Electroluminescence technique to evaluate the effect of impulsetests on high voltage cables

EL (electroluminescence) in XLPE (crosslinked polyethylene) subjected to impulses of various amplitudes, shapes and repetition rates has been investigated. Defects that could occur in the polymeric insulation of underground HV cables were simulated by embedding semicon protrusions in XLPE to create highly divergent electric fields. Time resolved measurement of the EL pulses emitted during positive, negative or alternating polarity impulses has shown that the EL pulses are grouped into two distinct time periods between which there is no EL activity. The EL emission probability increased with fall time of the impulse voltage but was independent of the impulse rise time. A study of electroluminescence characteristics from unaged and aged XLPE, subjected to a pair of positive and negative impulses, indicated that the charge trapped in the polymeric insulation during aging plays an important role during impulse application and could affect insulation failure     

Thermal conductivity of filled silicone rubber and its relationship to erosion resistance in the inclined plane test

Silicone rubber samples having various concentrations and mean particle sizes of either alumina tri-hydrate or silica filters, prepared by room temperature and heat cured under pressure (hot pressed), are tested for erosion resistance in the ASTM D2303 inclined plane tracking and erosion test. Their corresponding thermal conductivities are determined using a transient temperature technique in which an infrared laser is employed as the heat source and a thermal imaging camera as a temperature detection device. Scanning electron microscope observations show greater filler bonding to the silicone matrix in the hot pressed samples than in the room temperature vulcanized samples leading higher thermal conductivity and increased resistance to erosion, for both ATH and silica filled samples. The correlation study shows a strong relationship between the erosion resistance and the thermal conductivity of the tested samples, highlighting the importance of an outdoor insulating material to have high thermal conductivity in order to withstand dry band arcing. The results can be used to provide guidance on filler selection for silicone rubber compounding for outdoor insulation applications.     

Correlation of damage, dry band arcing energy, and temperature in inclined plane testing of silicone rubber for outdoor insulation

Silicone rubber samples, filled with 30% alumina trihydrate, subjected to the ASTM inclined plane test, show good correlation between the measured harmonic power components of dry band arcing and the surface temperature recorded with a thermovision camera. The simultaneous study of the low frequency fast Fourier transform and variations in temperature shows that when the temperature rises above 100/spl deg/C there is a well defined increase in the low frequency harmonic components of the measured power; however, this trend is not verified by the fundamental component. The eroded mass of the samples is measured independently by a laser approach and a model to rank the tested samples using the inclined plane test is developed. The model is validated by applying it to both the recorded harmonic power components and temperature, resulting in the correct ranking of damage on the samples tested in the inclined plane test.     

Development of the DC superimposition bridge method for the measurement of high insulation resistance of medium voltage XLPE cables in operations

The method for measuring insulation resistance by superimposing the dc voltage has potential for detecting the deterioration of cable insulations. The deterioration of cable insulations can be detected by measuring current which flows through a grounded shielding wire. However, the earth current also flows from the cable sheath into the grounding wire as noise. Thus, to assess the deterioration properly, it is necessary to separate the earth current from the total current. On the other hand, to improve detection sensitivity, it is desirable to superimpose the highest possible dc voltage. In this case, provision must be made so that there are no adverse effects on the GPT. The authors, placing special emphasis on the method of measuring high insulation resistance by configuring the bridge, have conducted an investigation on how to detect the insulation deterioration with high accuracy and sensitivity without impairing the functions of the GPT. The following issues were investigated and the results are reported in this paper: (1) It is possible to measure insulation resistance as high as 100,000 MΩ by superimposing 50-V dc through the GPT after cancelling the earth-current and by configuring a bridge for determining insulation resistance from voltage ratio. (2) Even if 50-V dc is superimposed through the GPT, it is possible to prevent the magnetic saturation of GPT if the current injected by such superimposition is limited (i.e., below max. 5 mA). (3) In lieu of a variable resistor, a variable dc voltage supply is employed in the bridge. The variable dc voltage supply requires no mechanically driven part, thus making it possible to realize a fully automatic monitoring system.     

A basic study on the effect of voltage stress on a water droplet on a silicone rubber surface

Observations and analyses have been conducted on the behavior of water droplets deposited on the surface of polymer insulators. As an index to characterize the repellency of water droplets on the polymer surface, the contact angle was measured for different situations. Experiments were conducted to see the behavior of water droplets of different volumes and their size distribution on silicone rubber. Electric field strength calculation was performed to analyze the effect on water droplets. Finally, effects of the Maxwell stress and the Coulomb?s force were compared, to conclude that, even if there is some true charge on the water droplet, the Maxwell stress is predominant.     

A novel neutral point potential stabilization technique using the information of output current polarities and voltage vector

This paper proposes a new neutral point potential control technique for the neutral-point-clamped three-level inverter. Utilizing the phase current polarity information, this technique distributes the redundant voltage vectors in a manner to obtain stable neutral point voltage under all operating conditions including the zero-power-factor condition. Detailed analysis and computer simulations show the superiority of the proposed method. The feasibility of the proposed method has been proven via laboratory experiments.     

Potential of cryogenic liquids for future power equipment insulation in the medium high voltage range

Cryogenic liquids show a noteworthy impact on the concept of improved future power equipment with superconductors. Superconducting materials eliminate any Joule heating but are based on very low operating temperatures. A high overall current density in the equipment may be presupposed. Cooling however must be guaranteed by cryogenic liquids, i.e. liquid helium or liquid nitrogen, respectively. The cooling liquids are interesting basic insulating fluids with a reasonable dielectric performance in the medium high voltage domain. Most exciting is the chemical inertness. Hazards such as fire risk in case of severe faults can be ruled out, thus reducing insurance cost drastically. Thermal aging of the liquids can also be disregarded because no chemical reaction must be taken into account during service, even in the case of temporary overload operation. Dielectric aging due to partial discharges may be of little concern because the liquids can be fully reprocessed easily by vaporizing and recondensing in a closed cooling circuit; no memory effects may degrade the liquid at all. On the other hand, overload operation may yield a large amount of thermal bubbles which can affect the dielectric strength seriously. Finally, the liquids are produced from gases which are found in the open atmosphere. There is no danger of any poisonous reaction in case of uncontrolled leakage, and the disposal of insulation systems at the end of operational life is straightforward without the need of costly processing.     

一种用于测量快前沿高压脉冲的电阻分压器

分析了电阻分压器中电感和对地电容对分压器响应特性的影响,在此基础上研制了一种用于测量快前沿高压脉冲的同轴型电阻分压器,介绍了分压器的制作过程,对其进行了方波响应标定,并应用于实验测量中。实验结果表明,该电阻分压器能满足测量要求,可用于幅值10kV左右、ns前沿的短脉冲测量。     

An experimental investigation of the feasibility of using silicone and gallium arsenide solar batteries on space vehicles for receiving energy of laser infrared emission

The feasibility of transmitting electric power in space to solar batteries of space vehicles via the channel of laser infrared emission is shown. Evaluation of the efficiency of solar batteries for the given type of power transmission has been made. Possible methods of optimizing the design of space solar batteries in regard to conditions of detection of energy of laser infrared emission have been discussed.     

A novel way to quantify the magnitude of voltage sag

The magnitude of the voltage sag is accepted as one of the most important parameters for its characterization, classification, and the detection. Hence, the selection of most suitable method to quantify the magnitude of voltage sag is a basic requirement of the power quality (PQ) monitoring. This paper first discusses the three different existing methods, namely root mean square (RMS), peak, and fundamental voltage component methods, to determine the magnitude of the voltage sag. Here, these three methods are applied on test waveform, as provided by the PQ event characterization working groups, to assess the various characteristics of the voltage sag. Further, these characteristics have been compared to highlight the shortfalls of method applied. Additionally, an attempt has been made to propose a hybrid method to quantify the magnitude of the voltage sag. The proposed hybrid method is tested on the same test waveform and compared with the three existing methods available to quantify the magnitude of voltage sag. The results of comparisons show that the proposed hybrid concept reduces the overestimation in sag duration at high-threshold voltage and gives precise detection of overshoot just after the event. The effects of sampling frequency and the length of sliding window to determine the magnitude of the voltage sag are also investigated and the results are discussed in terms of depth, duration, and oscillation. Furthermore, the effect of sampling frequency on phase angle jump is discussed and minimum number of samples per cycle and minimum length of window for proper characterization of the voltage sag are suggested.     

Development of novel adaptive single-pole autoreclosure schemes for extra high voltage transmission systems using wavelet transform analysis

A novel wavelet transform analysis-based adaptive single-pole autoreclosure (SPAR) scheme of extra high voltage (EHV) transmission systems is presented in this paper. The discrete wavelet transform is adopted to analyse the fault transients caused by the secondary arc and permanent faults and the numerical results reveal that certain wavelet components can be effectively used to detect and identify the fault relevant characteristics in transmission systems. A threshold-based decision logic for the wavelet analysis coefficients is used to distinguish the transient and permanent faults, and in the case of a transient fault, to determine the secondary arc extinction time. The outcome of the study clearly indicates that the wavelet transform analysis technique can be used as an attractive and effective means of developing an adaptive autoreclosing scheme.     

A method for detecting the transition from corona from water droplets to dry-band arcing on silicone rubber insulators

The paper presents an investigation on the use of partial discharge (PD) as a means to study the development of leakage current (LC) on silicone rubber insulators in a salt-fog chamber. Simultaneous measurements of LC and PD on dead-end type silicone rubber insulators exposed to different combinations of electrical stress and salt-fog salinity are performed. The results show that for an electrical stress of 33 V/mm applied to the insulator, the average LC experiences gradual changes and reaches a critical level (around 1 mA) where dry-band arcing is observed after 20 minutes at 2300 /spl mu/S/cm, and after 25 minutes at 1800 /spl mu/S/cm. The corresponding PD levels reach hundreds of pC after 15 minutes at 2300 /spl mu/S/cm, and after 20 minutes at 1800 /spl mu/S/cm. For a salinity corresponding to 1800 /spl mu/S/cm and stress of 26 V/mm, the LC changes much more gradually and reaches the critical level only after approximately 60 minutes. The corresponding PD reaches hundreds of pC after 30 minutes and more than 1500 pC after 60 minutes. Pattern recognition techniques have been applied to investigate the transition from corona to dry-band arcing. The features extracted from the maximum and mean apparent charge trends are found to be useful to cluster and classify the two phenomena. Simultaneous visual observations of the discharges on the insulator surface support the results. The investigation, therefore, shows that PD is a sensitive method for detecting the transition from corona from water droplets to dry-band arcing and an alternative method for studying the early aging stage of silicone rubber insulators.     

压力空气绝缘起始局部放电电压特性的模拟实验研究

空气绝缘起始放电电压在不同压力下表现出不同的特征。介绍了在不同电极系统和不同压力下的空气绝缘局部放电模拟实验方法,对气体放电机理、电子崩产生过程以及空气压力对放电起始电压的影响进行分析探讨。通过模拟实验得出,在给定电极间隙下气体的起始放电电压随气体压力的增大而增大。