Electrical breakdown and space-charge effect in polyphenylene sulfide films

Electrical breakdown of polyphenylene sulfide (PPS) films, one of the high-temperature engineering plastics, has been investigated. The results show that the dc breakdown strength is lower than the impulse one, and that the difference in electric strength between dc and impulse becomes wider as the sample thickness decreases. The result suggests space-charge accumulation when dc voltage is applied. Further, the impulse breakdown strength in the presence of a dc prestressing voltage is measured in order to elucidate the space-charge effect. The data on the contribution of the prestress voltage to electrical breakdown are used to estimate the amount of space charge accumulated in the specimen during the dc prestressing.     

Measurements of acoustic waves generated by electrical breakdown in a polypropylene film

Electrical breakdown of a polymer film generates a pressure wave that is believed to include information about the breakdown initiation point. We measured the breakdown pressure wave and the space charge distribution up to the electrical breakdown field by using the pulsed electro‐acoustic method in a 30 µm thick polypropylene film. We discuss electrical breakdown phenomena based on the breakdown pressure wave and the dependence of the space charge distribution on the applied field and temperature. At room temperature, the observed breakdown pressure wave had a pulse‐like shape with a width that depended on the polarity of the applied field. Positive space charge accumulation was observed near the cathode as a hetero space charge near the electrical breakdown field. At 60 °C, the width of the breakdown pressure wave showed no dependence on the applied field polarity and positive space charge accumulation was observed inside the film near the electrical breakdown field. These experimental results suggest that electrical breakdown phenomena are affected by hetero space charge accumulation and that the initiation point of electrical breakdown corresponds to the position of hetero space charge accumulation in 30 µm thick polypropylene film. ©1999 Scripta Technica, Electr Eng Jpn, 126(3): 1–8, 1999     

Sparks in a small gap between HV spherical metal electrode andcorona-charged organic film

Sparks between a spherical metal electrode and corona-charged organic film were studied in air at room temperature. dc and impulse high voltages (HV) were applied to the spherical metal electrode, the polarity of which was opposite to that of the charge on the film. The maximum surface voltage of the corona-charged film was ±2 kV. The gap length was studied in the range 0.2 to 2.5 mm. Films of PTFE, PP, PS and PET of thickness 30 to 50 μm, were tested. In the case of negatively charged films, the electric field strength depended on the film material; the spark field strength decreased in the following order: PTFE⩾PP>PS⩾PET. The dependence of spark field strength on the material of negatively charged film was observed under both dc and impulse voltage applications. In the case of the positively charged films, the spark field strength showed less dependence on the film material. The experimental results are explained in terms of the degree of electron emission from negatively charged PTFE being higher than that from negatively charged PET     

AC breakdown strength of aromatic polymers under partial dischargereducing conditions

Partial discharge occurring at HV in AC electric breakdown field measurements on insulating polymeric films strongly reduces the breakdown field strength of the material and conceals the intrinsic breakdown process. By performing AC electric breakdown field measurements on polymer films in phthalic diesters, it was possible to suppress partial discharge and obtain breakdown field data of more intrinsic character. This is valuable for the evaluation of the influence of the structure and the properties of polymers on their insulating behavior. The AC electric breakdown field strength of five aromatic polymers at different film thicknesses was measured in dibutyl phthalate. The results from these measurements showed about a twofold increase in the breakdown field strength at sample thicknesses >50 μm, compared to data obtained in transformer oil. The breakdown field strength for thick samples showed a linear decrease with increasing sample thickness for all polymers except PEEK, which showed a constant breakdown field strength in the thickness range investigated. The breakdown field strength of thin samples was independent of the polymer structure. Measurements of the breakdown field strength of the polymers under dc conditions show no obvious correlation between chemical structure or polymer property and the breakdown field strength     

Impulse breakdown superposed on ac voltage in XLPE cable insulation

This paper discusses tree inception and breakdown voltage characteristics of XLPE cable insulation subjected to impulse voltages superimposed on ac voltage. The tree initiation tests were performed on laboratory-molded specimens equipped with needle electrodes, whereas the breakdown tests were conducted on a full-sized cable. The impulse tree initiation stress was found to be dependent on the magnitude of the pre-applied ac stress and the relative polarities of the impulse and the ac peak at the instant of their superposition. Although the impulse polarity has an effect on the tree inception, the general behavior is that the tree inception stress always decreases with an increase of the pre-applied ac stress. This phenomenon is discussed in terms of the space charge effect and the influence of the impulse voltage application itself. The impulse strength of a full-sized cable insulation was found to be independent of the pre-applied ac stress as long as that stress did not exceed the operating stress of a 500 kV cable insulation. However, subjecting cable insulation to higher ac stresses before impulse application caused a reduction of its breakdown strength as compared with the insulation without ac prestressing     

High repetition rate PEA system for in-situ space charge measurement during breakdown tests

In order to investigate transient space charge phenomena, it is essential that the space charge profile be observed at a high repetition rate. We have developed a new space charge measurement system using the pulsed electroacoustic (PEA) method, which can measure the space charge profiles every 10 /spl mu/s. It employs the most recent digitising oscilloscope model and a semiconductor switch. The effect of prestressing on impulse breakdown voltage of a low-density polyethylene sheet was investigated by using the new system. Experimental results suggest that positive charge injection was dominant immediately before the breakdown, and charge injection during the prestressing causes distortion of the electric field near the electrode, and enhances the subsequent charge injection due to the impulse voltage.     

Self-controlled pre-breakdown discharges in planar symmetry

In measurements on Teflon FEP films charged in ⩽50 μm air gaps by microsecond impulse voltages, a uniform charge deposition on the films was observed. A regular increase of the film surface potential from a threshold value of the peak impulse voltage was found. However, for 300 μm air gap it was observed that abrupt charging occurs at a lower threshold voltage, indicating breakdown, and the charge deposition on the film becomes nonuniform. The behavior in air gaps <50 μm is explained here using Townsend's theory of pre-discharges. It is shown how and why the interposed insulating film acts to make the system self-controlled, thus avoiding breakdown in the air gap, despite the large values of the applied peak impulse voltage     

空间电荷对SF_6气体正雷电冲击击穿的作用

利用两种实验方法对比测试了预注入单极性空间电荷时,SF_6气体极不均匀场间隙正雷电冲击击穿特性。分析表明;直流叠加冲击电压法不适用于观察空间电荷的作用,而切换直流/冲击电压法能有效地反映空间电荷对冲击击穿的作用。     

空间电荷对低密度聚乙烯电气击穿特性的影响

为解决聚乙烯用作电线电缆绝缘材料时所受空间电荷问题的困扰,采用在低密度聚乙烯(low density po-lyethylene,LDPE)试品上施加直流预电压使其中积聚一定量的空间电荷,然后测量试品击穿强度的方法,研究了空间电荷对LDPE击穿特性的影响。结果表明,与未经过预电压处理的LDPE的击穿强度相比,在经过较低场强(50 kV/mm)预电压处理后,预电压与击穿电压极性相同时击穿强度提高了约9%,极性相异时击穿强度降低约14%;而经过较高场强(150 kV/mm)预电压处理后,预电压时LDPE中出现空间电荷包现象,预电压后同极性击穿强度提高约19%,而异极性击穿强度反而上升约16%。分析认为空间电荷包在LDPE中的运动导致了部分空间电荷的中和,使得空间电荷积聚量减少,同时LDPE中可能的缺陷得到了一定程度的老炼而使介质得到了均匀化,从而使LDPE的击穿强度得到了提高。     

Flashover from surface charge distribution on alumina insulators invacuum

Surface charge distributions of disk type aluminas held between a backside electrode (alumina holder) and a needle electrode to be excited by an impulse voltage (rise time 64 μs, wave tail 700 μs) were measured. The measurement of surface charge distribution on the whole surface area of alumina YSA998 and UHA99 after impulse voltage application revealed that the surface charging can be initiated either from the anode or from the cathode triple junction. The charging initiated from the anode triple junction (for positive polarity) produced positive charge at the anode region and the density is dependent on the applied voltage, while the charging initiated from the cathode triple junction region (for negative polarity) produced negative charge around the cathode region. For positive polarity, the critical values of charge density to the flashover for alumina YSA998 and UHA98 are 5.70 and 17.2 μC/m², respectively     

Effects of manufacturing technology on electrical breakdown and morphology of thin films of low density polyethylene blended with polypropylene copolymer

Extruded films prepared from blends of low-density polyethylene (LDPE) and random copolymer of ethylene and propylene (EP) with the T-die method were studied with respect to electrical properties and morphology. Comparisons with data on blown films are made. These blends are of interest as improved LDPE for making XLPE for insulated power cable. In the high temperature region (90/spl deg/C), a specimen with a slightly higher EP content had higher impulse breakdown strength than that with a lower EP content, but no improvement of DC breakdown strength by blending could be found. The improvement of impulse breakdown strength (90/spl deg/C) is explained in terms of morphological changes by blending such as the orientation of chains in a film and the size of spherulites on the assumption of the thermal breakdown. In comparison, a T-die film had higher impulse breakdown strength than that of a blown film for the same composition. The impulse breakdown strength also increased with the use of the higher density LDPE. In the current versus electric field characteristics at 30/spl deg/C, the blend polymer with EP content of 5-10% showed a transition from LDPE behavior at low field region to EP behavior at high field region. However, no appreciable difference in current behavior among the specimens was observed at 90/spl deg/C, which suggests an incompatibility between the two materials that exists at 30/spl deg/C but not at 90/spl deg/C.     

The influence of surface charge on lightning impulse breakdown ofspacers in SF6

We have investigated the influence of surface charges on the discharge development across particle contaminated spacer surfaces under SF₆ for lightning impulse (LI) voltages (1.2/50 μs). Surface charges may be generated by dc, ac or lightning impulse stress. The discharge characteristic shows a strong reduction of the insulation strength if the applied voltage and the surface charge have opposite polarities. The investigations were performed with a needle protrusion attached to the bottom electrode to give severe field distortion. The bottom electrode was biased positively. The influence on the discharge process is observed by measuring the surface charge distribution and predischarge currents. The results reveal changes in streamer onset voltage, streamer to leader transition, and leader development     

The electrical breakdown characteristics of oil-paper insulationunder steep front impulse voltages

Disconnecting switch operations in gas insulated equipment cause transient voltages with risetimes as steep as 5 to 20 nanoseconds and magnitudes as high as 2.5 pu. There is very little information on the effect of these transients on oil-paper insulated equipment. There have been reports, however of transformer and bushing failures caused by these transients. The electrical breakdown characteristics of oil-paper insulation under steep front impulse were studied in this project, which was co-sponsored by the Canadian Electrical Association and BC Hydro. V ₅₀ (50% breakdown probability voltage) breakdown data was obtained with steep front (10 ns/2500 μs), lightning and switching impulse waveforms. Insulation breakdown voltage vs breakdown time (V-t) data and multiple impulse breakdown data were obtained with the steep front impulse waveform. The V₅₀ results showed that the breakdown strengths were lower for steep front impulses than for lightning impulses. The multiple impulse breakdown results showed that oil-paper insulation breakdown strength can be lower than 100 kV/mm. These results are alarming, since they suggest that oil-paper insulated equipment subjected to steep front transients will fail at voltages below the lightning impulse design level (BIL). The volt-time data had a discontinuity. The breakdown process at risetimes below about 50 ns was different from the breakdown process at risetimes above 50 ns     

Laser-induced breakdown characteristics of nonuniform dc coronadischarge fields

Laser-induced breakdown of nonuniform field gaps in atmospheric air was caused by an XeCl excimer laser. This laser beam was focused on the center of a rod-to-plane, needle-to-plane or hemisphere-to-plane gap with dc voltage. The laser energy dependency of breakdown voltage was investigated for positive and negative applied voltages and gap lengths of 15 to 25 mm. It was found that the positive breakdown voltage was higher than the negative one in the hemisphere-to-plane gaps or the rod-to-plane gaps with low voltages, where there is little or no space charge. However, the effect of polarity is reversed in the rod-to-plane gaps with large spacings and the needle-to-plane gaps. On the basis of the dependence of the discharge mode on this polarity effect, it can be seen that it was caused by the difference in development of positive and negative streamers in the corona discharge     

Puncture breakdown characteristics of protected rubber insulatinggloves

The results of puncture breakdown tests on rubber gloves are presented. A positive polarity switching surge voltage (275/2500 μs) or a 60 H alternating voltage was applied to the glove to cause breakdown. The effect of mechanical pressure on glove breakdown behavior and the influence of using a leather protector were investigated. It was found that protected gloves are more likely to break down at a position other than where the needle electrode has been placed. In both AC and switching surge breakdown tests, the standard deviation in breakdown voltage for protected samples was greater than that of the nonprotected samples by 25%. Under AC conditions, with mechanical stress applied, a bare glove will break down at a voltage which is approximately 22% below the nonstressed condition. A protected glove which is mechanically stressed will break down at a value 12.75% below the nonstressed condition. It is suggested that the results obtained will help to determine the operating limits and reliability of the glove insulation     

Space charge formation in γ-irradiated low densitypolyethylene

Energetic ionizing radiation can alter the chemical structure of polymeric materials and also may give rise to the presence of trapped charge within the material, the trapping characteristics of which may be influenced by these radiation-induced structural alterations. In the present work, the formation of space charge in γ-irradiated LDPE (low-density polyethylene) was investigated using the technique of a LIPP (laser induced pressure pulse). Specimens of LDPE, ~350 μm thick, were irradiated in a ⁶⁰Co γ-source in room air to various doses and the results indicate that space charge distributions are dependent on both dose and electric stress, including the length of time during which the stress is applied. At low doses (⩽10 kGy), there is a large amount of positive charge adjacent to the cathode, leading to stress enhancement at the interface. Higher applied stress serves to extend the positive charge towards the anode. At high doses (⩾50 kGy), the charge distribution is more complicated than that for low doses. Here there is initially negative charge present adjacent to the cathode and a substantial positive charge adjacent to the anode. With the passage of time, however, there is a polarity change from negative to positive at the cathode and a decrease in the density of the positive charge at the anode, with negative charge in the middle of the sample     

Dynamics of voltage polarity reversal as the controlling factor inspace-charge induced breakdown of insulating polymers

Our investigation is focused on the understanding of high-field phenomena in polymers. In highly insulating materials such as polyethylene, space charge has a strong influence on both the short and long term breakdown strengths. The aim of this report is to demonstrate and discuss the importance of the dynamics of voltage polarity reversal at the injecting needle electrode on space charge induced breakdown. Evidence of space charge injection is provided by the observation of local breakdown generated by grounding of the sample after the polarization (so-called `grounding' tree). The effect which is polarity dependent, can be observed only when the sample is short-circuited immediately after charging. Field computation is performed versus injected charge and a critical charge density is derived in order to explain polymer breakdown upon grounding. The time dependence of the effect is explained by space charge relaxation. Polymer degradation under ac voltage is discussed on the basis of this interpretation     

Electrical properties of biodegradable polylactic acid film

We measured the basic electrical insulation characteristic of biodegradable polylactic acid (PLA), and the following results were obtained. The volume resistivity, dielectric constant, and dielectric loss tangent measured at room temperature were almost the same as those of crosslinked polyethylene (XLPE) currently used as insulating material for cables and electric wires. The mean impulse breakdown strength of PLA was about 1.3 times that of XLPE. Also, space charge accumulation in PLA and XLPE was measured. The amount of space charge accumulation in PLA was one-half that in XLPE when a DC voltage was applied for a short time.     

Impulse breakdown of insulators and air gaps of similar electrodeconfigurations: effects of temperature

Comparative measurements are reported on the breakdown, under lightning and switching impulses, of a concentric electrode arrangement in which the inter-electrode space is spanned either by an insulator surface or by air. Temperature in the system has been varied in the range 10m, where 0.81.0 for negative polarity at the highly stressed electrode. Variations are the same, in the presence or absence of the insulator surface, but absolute positive flashover voltages are lower across the surface, than in air, while negative flashover voltages are higher. Times to breakdown decrease with increasing temperature     

High-electric field conduction phenomena in poly-2-chloro-p-xylylene/poly-p-xylylene composite thin films

The high-field currents in poly-p-chloro-xylylene (PCPX) thin films are due to hole injection from anode metal and collision ionization. PPX thin films showed the negative temperature dependence of high-field currents. Since the chlorine atoms in PCPX thin films act as carrier scattering centers, high-field currents in PCPX thin films are suppressed in the low-temperature region. This paper discusses the high-field currents of PCPX/PPX composite thin films; these films are PCPX thin films (d < 150 nm) deposited on PPX thin films (d > 200 nm). They were strongly suppressed at room temperature, but not at low temperature. Therefore, PCPX/PPX composite thin films showed a remarkable negative temperature dependence of high-field currents. High-field currents were suppressed more in PCPX (60 nm)/PPX (230 nm) composite thin film than in PCPX (130 nm)/PPX (230 nm). This can be explained by the suppression of the hole injection from anode by homo space charge at the interface between PCPX/PPX.