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Polyimide (PI) films have excellent electrical, mechanical and thermal properties in aspect of resisting corona, aging and radiation. However, the pulse voltage combined with dc voltage often causes concentration of local electrical field, which makes the surface charge behaviors more complex and threatens the system. As a means of surface treatment, direct fluorination has been proved as an effective way to improve the polymer properties, without compositing the bulk characteristics. Therefore, this paper intends to reveal the effects of fluorination on surface charge coupling behaviors of the PI films under dc and pulse voltage combination. The PI samples fluorinated for 0, 15, 45 and 60 min were obtained. Surface corona discharging was stressed under the combination of dc and pulse voltage. The surface charge accumulation, decay and distribution were measured coupled with an analysis of trap distribution. The results show the surface charge dynamic behaviors are strongly correlated with the fluorination time, pulse voltage amplitude and polarities of the dc and pulse voltage combination. The trap distribution results imply that the effect of fluorination and charging condition attribute to the change of trap depth and amount. Besides, surface charge behaviors are greatly improved by appropriate fluorination. [ABSTRACT FROM AUTHOR]

As one kind of the basic insulating materials, polyimide has been widely used in turn to turn insulation and turn to ground insulation in inverter-fed motors. However, the pulse voltage from the converter often causes overvoltage and localized high electrical field in the insulation system. During applying the pulse voltage, charges accumulate on the surface of the insulator, which could affect the dielectric property and cause the breakdown of insulation. Direct fluorination is a method to modify the surface layer of the polymers without changing the bulk properties. As a result of the fluorination, hydrogen atoms are replaced by the fluorine atoms and the component of the polymer surface is changed, which can influence the surface charge accumulation and affect the electrical properties. The experiment in this paper was focused on the effect of pulse duration and pulse number on surface charge of the polyimide films. The polyimide films with the fluorination time of 0, 15, 45 and 60 min were prepared. Surface charging was performed by corona discharging using repetitive pulse power at the temperature of 20 °C with the relative humidity of ~15%. The repetition frequency was 1 kHz and the pulse duration was 50, 110 and 210 μs, respectively. The surface charge decay was measured using a specific probe coupled with an electrostatic voltmeter. The results showed that the surface charge increases with increasing the pulse number and the duration of the microsecond pulse. [ABSTRACT FROM AUTHOR]

As one of the basic insulating materials, polyimide films have been widely used in turn to turn insulation and turn to ground insulation in wind turbine generators. However, the pulse voltage from the converter often causes overvoltage and localized high electrical field in the insulation system. Surface charge could affect the dielectric property and cause the breakdown of insulation. Direct fluorination is a method to modify the surface layer of the polymers without changing the bulk properties. As a result of the fluorination, hydrogen atoms are replaced by the fluorine atoms and the component of the polymer surface is changed, which can influence the surface charge accumulation and affect the electrical properties. The experiment in this paper was focused on the effect of direct fluorination on surface charge of polyimide films under pulse voltage. The polyimide films with the fluorination time of 15, 30, 45 and 60 min were prepared, together with the unfluorinated one as a comparison. Surface charging was performed by corona discharging using repetitive pulse power at the temperature of 25 °C with the relative humidity of ~45%. The rising time of the pulse was 25 μs and the frequency was set at 200, 400, 600 and 1000 Hz, respectively. The surface charge decay was measured using a specific probe coupled with an electrostatic voltmeter. The results showed that the surface charge behavior was affected by the fluorination, corona discharging time, pulse voltage amplitude and frequency. [ABSTRACT FROM PUBLISHER]

Silicone rubber (SiR), as a basic insulating material, is widely used in insulators for HVDC transmission lines. The prominent characteristics of the SiR coating are the hydrophobicity and the hydrophobicity recovery, which can suppress the development of leakage currents and dry-band discharges, and prevent the flashover incidents. Fluorination is a method to change the chemical component of the surface layer of polymers, which could induce the corresponding change in hydrophobicity and some electrical properties, thus influencing the dc surface flashover characteristics. Room temperature vulcanized (RTV) SiR samples were fluorinated in a stainless reaction vessel at about 298K (25°C) using a F2/N2 mixture with 12% F2 by volume and 0.05 MPa (500 mbar) for respectively 5, 10, 15 and 20 minutes. The hydrophobicity of samples with different fluorination time was measured by the static contact angle tests after corona aging. The variation of hydrophobicity with the lapse of time was recorded. Dc flashover tests in different conditions were carried out between two plane electrodes on top of the material sample. The flashover voltage was found to be associated with the surface hydrophobicity status of samples. Obtained results showed the effects of the fluorination on the surface charge, hydrophobicity performance and dc flashover voltage. It is suggested that the appropriate fluorination can improve surface hydrophobicity and suppress surface charge accumulation, thus raising the dc flashover voltage. [ABSTRACT FROM AUTHOR]

Epoxy is widely used in electronic and electrical devices because of its excellent insulation properties. The addition of nanoparticles to epoxy could change the material structure and then charge behaviors are altered as a consequence. The accumulation of surface charge may cause electric field distortion, which would lead to surface flashover. In this paper, the samples were made by dispersing nano-scale boron nitride (BN) particles in Epoxy with the weight ratios of 5, 10, 20, 30, 40 wt%. The samples were charged by a dc corona triode system using needle-plate electrodes. The distribution of charges was measured with an electrostatic voltmeter and the effect of nanoparticles on charge accumulation and decay behavior was studied. A flashover test was carried out under a dc stress between two finger-type electrodes. The effect of nanoparticles on flashover characteristics and the relation between surface charging and surface flashover were also investigated. The results showed that the traps between nanoparticles and the polymer, which were introduced by the added nanoparticles, could cause surface charge accumulation. In addition, the flashover occurred at a lower voltage because of the accumulated charge on the sample surface. [ABSTRACT FROM AUTHOR]

Polymeric insulating materials exposed to pulse voltage with a nanosecond rise time in pulsed power have drawn increasing attention. Surface charge accumulation and decay on the charged polymers are very important parameters related to discharge. Nanocomposite technology has been used to improve the electrical properties of polymeric insulating materials. Therefore, it must be confirmed whether the nanosecond rise time of pulse voltage on the dynamic behaviour of surface charge on polymeric nanocomposites is different from that without nanoparticles. In this paper, we attempted to clarify the effect of the rise time on the electrical properties of epoxy/TiO2 nanocomposites focusing on surface charge accumulation and decay. Samples were prepared by dispersing nano-scale TiO2 particles into epoxy (EX) by mixing with shear force. Corona charging tests were performed at room temperature with a relative humidity of ~ 40%. The charge distribution was measured by means of an electrostatic voltmeter. The results show that the rise time plays an important role in the accumulating charge as well as the charge decay rate and varies with the concentration of nanoparticles and charge polarity. It is suggested that charge dynamics are dependent upon the rise time. [ABSTRACT FROM AUTHOR]

It seems generally accepted that HVDC technology is to become increasingly relevant in power systems. With this expected future advance of HVDC, the use of gas insulated switchgear or lines for dc application are getting increasingly interesting. As in the case of dc the electric fields are determined resistively in contrast to the capacitive fields in the case of ac, gas insulated technology has to be revisited. In many previous works surface charging of model spacers was investigated. However, the ion flow problem to properly account for the electric conduction in the gas has not yet been solved. Further, no combination with inhomogeneous electric conductivity determined for example by temperature profiles from ohmic heating in nominal condition has been performed so far. In this paper, the gas conductivity arising from ions produced by natural radiation is computed by solving the ion flow field problem, and its role is compared to spacer bulk conductivities. Further, the impact of temperature profiles on spacer charging is investigated. The results indicate that for actual disc spacers, the spacer bulk conductivity is more important than that from the gas ionized by natural radiation. Especially in the case of polarity reversal, thermally enhanced spacer charging can result in considerable increase of tangential field strengths. [ABSTRACT FROM PUBLISHER]

Effect of surface charges on dc flashover characteristics of a composite polymeric insulator is studied by means of experiments and theoretical calculations. The considered insulator consisted of a glass fiber reinforced epoxy core covered with a layer of silicone rubber and terminated by metallic electrodes with rounded smooth edges. In the experiments, the insulator surface was charged by external corona while keeping the electrodes grounded and different charging levels were realized by varying its intensity. A series of disruptive discharge tests were carried out on the charged insulator under negative dc voltages. It was revealed that negative deposited surface charges led to an enhancement of the flashover performance whereas positive ones reduced the flashover voltage level. A theoretical model has been developed and utilized for analyzing the experimental results. In the model, surface charge density profiles deduced from measured surface potential distributions were used as boundary conditions for calculations of electric fields. The measured and calculated flashover voltages were found to be in agreement indicating that the observed variations in the flashover characteristics could be attributed to the modifications of the electric field produced by the surface charges. [ABSTRACT FROM PUBLISHER]