Pascal Rain

Prebreakdown phenomena in mineral oil under step and ac voltage in large-gap divergent fields

This paper presents an experimental study of the propagation of prebreakdown phenomena in transformer oil, in large point-plane gaps (5 to 20 cm), in positive polarity under step and ac voltages. The prebreakdown phenomena are characterized via the simultaneous recordings of transient currents, charges, photocurrents, and high-speed photographs of the emitted light. In these experimental conditions, prebreakdown phenomena in oil are basically composed of weakly luminous branched filaments, most of the time not connected to the point electrode, and propagating continuously. Bright luminous fast discharges reilluminate periodically the main branch of the propagating discharge from the point electrode to the extremity of these weakly luminous filaments. Below the breakdown voltage, streamers stop at an average length which increases linearly with the voltage. Above a critical length, they cannot stop any more and lead to breakdown. The main propagation velocity remains constant throughout the propagation. These observations, which present a number of similarities with leader phenomena in gases, are then discussed. >

Space charge distribution measurement methods and particle loaded insulating materials

In this paper the authors discuss the effects of particles (fillers) mixed in a composite polymer on the space charge measurement techniques. The origin of particle-induced spurious signals is determined and silica filled epoxy resin is analysed by using the laser-induced-pressure-pulse (LIPP), the pulsed-electroacoustic (PEA) and the laser-induced-thermal-pulse (LITP) methods. A spurious signal identified as the consequence of a piezoelectric effect of some silica particles is visible for all the methods. Moreover, space charges are clearly detected at the epoxy/silica interface after a 10-kV/mm poling at room temperature for 2 hours.

Transient and steady-state currents in epoxy resin

Charging and discharging currents have been measured in a diglycidyl ether of bisphenol-A epoxy resin with and without silica fillers, below and above its glass transition temperature Tg = 65 °C. Both transient and steady-state current densities have been analysed. The average applied fields ranged from 3 to 35 kV mm−1 with a sample thickness of 0.5 mm. Above Tg, transient currents suggested a phenomenon of charge injection forming trapped space charges even at low fields. Steady-state currents confirmed that the behaviour was not Ohmic and suggested Schottky-type injection. Below Tg, the current is not controlled by the metal–dielectric interface but by the conduction in the volume: the current is Ohmic at low fields and both transient and steady-state currents suggest a phenomenon of space-charge limited currents at high fields. The field threshold is similar in the filler-free and the filled resin. Values in the range 12–17 kV mm−1 have been measured.

Field and temperature dependence of space charge injection in a silicone rubber

The generation of space charges in a silicone elastomer has been characterized under dc voltage as a function of the temperature using the laser induced pressure pulse (LIPP) method. The analysis of the LIPP spectra has been detailed thanks to acoustical considerations, taking into account the reflection, attenuation and dispersion of the pressure wave. Both heterocharges and homocharges have been observed depending on the field magnitude (5 to 20 × 106 V m−1), polarization duration and temperature (20, 40 or 60 °C). With temperature increasing, charges are observed at lower fields. Heterocharges appear in less stressed conditions. Then, homocharges take the place of these heterocharges. Positive homocharges are observed in lower stressed conditions than negative homocharges. Mobility of these charges has not been observed below 60 °C.

Silicone rubber subjected to combined temperature and humidity effects

Room-temperature-vulcanization silicone rubbers have been subjected to short duration, (1sec) negative corona discharges in different temperature and relative humidity environments. We have investigated the time dependence of the surface potential decay, in situ, in a climatic chamber. This study has revealed a strong influence of the initial deposited surface charge density, temperature and relative humidity. Bulk conduction mechanisms appear to be preponderant. A non homogeneous evolution of surface potential decay is observed at increased relative humidity.

Experimental model of a silica/epoxy interface submitted to a hygrothermal aging: a dielectric characterization

In order to investigate the influence of a hygrothermal aging on the organic/mineral interfaces of a silica filled epoxy resin, a macroscopic experimental model of a silica/epoxy interface was designed and realized. It consisted in coplanar interdigitated electrodes deposited on a quartz substrate moulded in epoxy resin. The dielectric properties have been followed at 80°C and 80% relative humidity. These properties have been compared with the evolutions of sheets and mouldings of unfilled epoxy resin submitted to the same conditioning. Measurements evidenced the formation of an interphase at the quartz/resin interface which appeared precisely when the water reached the quartz after its diffusion through the resin. The water bound to the resin during the conditioning of the unfilled samples induced a drop of resistance of nearly 3 decades. In the same time, due to the interphase which has grown at the organic/mineral interface, the resistance between the interdigitated electrodes fell of five decades, i.e. two more decades. This interphase is characterized by a surface resistivity of 80.106 Ω. This data may be used to evaluate prebreakdown phenomena resulting from a hygrothermal aging of silica/epoxy insulations.

A comparison of electric and dielectric properties of standard and high-crystallinity polypropylene films

The dielectric properties of two grades of Bi-oriented isotactic polypropylene (BOiPP) are studied, using a variety of techniques: breakdown field measurements, dielectric spectroscopy, thermally stimulated depolarization currents, dc conduction currents. Standard (STPP) and high-crystallinity (HCPP) polypropylene films are investigated. Measurements are carried out over a wide temperature range, up to (-150°C/ +125°C). Breakdown field in both materials show a very small difference. On the other hand, dielectric losses and dc conduction currents are significantly lower in HCPP. Both materials show a decrease of dielectric losses versus temperature in the range (20-90°C), favorable for the application to ac power capacitors. The analysis of dc currents allows to evidence two main conduction mechanisms: (i) below 80°C in both materials, a hopping mechanism due to motion of electrons occurring in the amorphous phase; (ii) above 80°C, an ionic conduction in HCPP and a hopping conduction in STPP.

Dielectric and physicochemical behavior of aged PVC insulated cables

Radial field, medium voltage cables operated at 6.6 kV are the object of this study. Dielectric properties of those aged PVC insulated cables have been measured as a function of temperature and voltage in a frequency range of 1 mHz to 1 kHz. The range of temperature tested was between 30°C to 110°. The dielectric material shows a resistive behavior at low frequency. Deduced conductivities are in good agreement with DC conductivities measured at the considered temperatures. The conductivity follows an Arrhenius law over the range studied with an activation energy of 0.96 eV. The conductivity of some sample is high enough to induce an increase of the losses at industrial frequency. The increase of the low frequency capacity is revealing an interfacial polarization. This increase of capacity is due to local accumulation of electric charges in the insulation, particularly at the electrodes. Physicochemical characterizations have been done to understand this decrease of resistivity. A migration of plasticizers did not occur as evidenced by IR micro-spectroscopy. Therefore, the decrease of resistance cannot be attributed to such mechanism. Physical aging is evidenced by DSC, which leads to the working temperature of the cables. TMA has shown modifications of thermo-mechanical properties along aging. These evolutions are attributed to the formation of double bonds resulting from dehydrochlorination.

Evolution of structural properties of glass-mica-epoxy insulations along accelerated thermo-oxidative aging

The stator windings of large air-cooled rotating machines are insulated with mica-glass fabric impregnated with epoxy. These composites are submitted to thermal, mechanical and electrical stresses. A specific aging arrangement has been setup allowing artificial thermo-oxidative aging of dielectric samples thanks to the application of a dry O2 pressure at a controlled temperature. Two different materials are considered: a class H material, designed to sustain temperatures up to 180degC, and a high conductivity (HC) tape material, which is a class F insulation. The samples were followed in mass and DMA (dynamic mechanical analysis). Similar profiles of mass evolution with various temperature and oxygen pressure contribute to validate the artificial aging procedure. DMA allowed determining the glass transition temperature (Tg) of the epoxy resin and observing the evolution of the network density of the polymeric matrix along aging. Delaminations of a few tens mum thick were observed within the samples after aging. These characterizations allowed the comparison between both materials.

Relationship between structure and dielectric properties of Bi-oriented isotactic polypropylene films for capacitors

This work reports on the relationship between structure and dielectric properties of biaxially oriented polypropylene. The morphology of semi-crystalline bioriented isotactic polypropylene (BOiPP) films is investigated using Wide Angle X-ray Diffraction (WAXD) and Polarized Optical Microscopy (POM). A β-orthorhombic structure, with a crystallinity ratio of about 46%, and a “Crater” morphology of the β-form is identified. Dielectric properties are measured by Broadband Dielectric Spectroscopy (BDS) over a wide temperature range (-150°C to 125°C). Since the dissipation factor of the PP is very low, special care was taken to obtain valid data. Two main relaxation processes are observed: a α-relaxation peak associated to the glass transition temperature (Tg) at low frequency and temperature about -7°C, and a broad β*-relaxation at high frequency at about -60°C, attributed to CH orientation. The variation of the dissipation factor versus sample thickness (from 3.8 μm to 11.8 μm) is correlated and explained by the increase of crystallinity ratio at larger thicknesses.