M. Abou Dakka

Space charge evolution in polypropylene containing synthetic and natural organoclays

Fully synthetic tetrasilisic mica from Topy Co., Ltd. and Cloisite® 20A powder of Wyoming natural montmorillonite clay from Southern Clay Products were used to manufacture poly-propylene-based nanocomposites with organoclay concentrations of up to 8-wt%. Both types of nanofiller reduce the amount of space charge accumulated under a dc field as compared with the base material without any nanoparticles but their concentrations have to be kept below ∼4-wt% to achieve this effect. For the same particle concentration, nanocomposites containing natural clay, which has significantly smaller aspect ratio of the individual platelets than the synthetic clay, acquire less space charge than the nanocomposites containing fully synthetic clay.

On-site Diagnostic Tests on Polymer Insulated Cables Using Depolarization Current Measurements

Amodified technique of depolarization current measurements encompassing monitoring of high speed depolarization processes was used to perform diagnostic tests on polymeric cable insulation operating under diverse environmental conditions and voltage levels. The high frequency characteristics were compared with those of the slower depolarization processes commonly obtain during on-site tests. Both the high and low frequency measurements showed a consistent correlation with the operating conditions of the cable insulation. It was observed that the low frequency characteristics were dependent on the duration of the dc poling time but only if the insulation was deteriorated, especially with water trees. Unaged and relatively new insulation did not show such behavior. The majority of the field tests were performed with a relatively low dc voltage of only 1 kV or less and under very adverse environmental conditions, with temperatures as low as ¿30°C.

Threshold of space charge injection and electroluminescence in polymeric insulation

Results of EL emission in flat-sheet samples of polymeric insulation (polyethylene), subjected to a uniform field, from two laboratories, are compared. The EL technique has been employed to detect space charge injection, trapping and recombination centers in the material. The voltage has been varied over a range often found in practice, and the frequencies used cover a part of the spectrum used in electrical and electronic apparatus. The aim of this collaborative project is to enhance the understanding of the aging mechanisms of dielectrics in service, determine the threshold field of space charge injection and provide the electrical and electronic industry with guidelines that would reduce the probability of catastrophic insulation breakdowns.

Space Charge Distribution with the Phase Resolved PEA Method in XLPE Subjected to a 50 mHz AC Field

Manuscript received on 9 January 2007, in final form 11 July 2007. Space Charge Distribution with the Phase Resolved PEA Method in XLPE Subjected to a 50 mHz AC Field Space charge distribution in cross-linked polyethylene (XLPE) subjected to a uniform field at 50 mHz is determined with the Phase Resolved PEA technique. It is shown that at almost all phase angles of the ac waveform more space charge is injected into XLPE at 50 mHz than at 50 Hz. The amount of charge due to only one ac cycle at 50 mHz is larger than that due to 1000 cycles at 50 Hz. At 50 mHz the local field in XLPE is in synchronization with the applied field. Space charge, ac voltage, PEAtechnique, phase angle Index Terms — * Permanent Address: Numazu College of Technology, Numazu, Japan AS electronic and electrical equipment become smaller and more compact, the insulating materials used in these devices experience increasingly higher electrical stresses. Under enhanced electric fields charging and charge trapping phenomena become very important factors, which need to be considered when designing insulation systems. Space charge in polymeric insulation has a major effect on electrical conductivity and usually leads to the deterioration and breakdown of the insulation. Several methods, such as the Thermal Step, Laser Induced Pressure Pulse and Pulsed Electro Acoustic (PEA) techniques [1-3], have been used to investigate space charge distribution in insulating materials. In the PEA method narrow electric pulses are applied to the sample and the interaction between the electric pulses and the internal charges in the sample creates an acoustical wave in the insulation which is detected by a piezoelectric transducer. The response signal carries the information of the space charge distribution but is a convoluted function of space and time and requires the use of appropriate mathematical methods to obtain space charge and electric field distributions. The PEA method has been extensively used to measure space charge distribution in polymeric insulation subjected to dc electric fields. Recently [4], a modified version of this technique, named the Phase Resolved PEA, was employed to measure space charge distribution in insulation subjected to an ac field at power frequency. This paper describes the space charge distribution in XLPE subjected to a uniform ac field at 50 mHz as compared to the power frequency at 50 Hz. The Phase Resolved PEA technique is used to measure space charge distribution at various phase angles of the ac waveform. Pellets of polyethylene were compression molded and crosslinked at a temperature of 180°C and 5 MPa into flat sheets 130 ± 5 μm thick. The specimens were then treated in a vacuum oven at 60°C for one week in order to decrease the cross-linking byproducts to a negligible level. Gold electrodes (30 nm thick) were sputtered on each side of the sample. Each specimen was individually subjected to an ac field of different magnitude and frequency in the PEA test cell and the space charge distribution was measured with the Phase Resolved PEA technique described earlier [4]. Narrow (5 ns) pulses having an amplitude of 400 V (3 kV/mm) were applied at various phase angles of the ac waveform. Fig. 1 shows ac waveforms of two different frequencies with narrow electric pulses applied at various phase angles of the applied waveform. The sine wave always started and ended at the zero crossing. At 50 Hz, 3 pulses were applied at each 15° step of the ac cycle as the phase angle was increased from 0° to 360°. The space charge distribution was obtained at various phase angles and the response signal was averaged 2 EXPERIMENTAL

Correlation between space charge development and breakdown in polymeric insulation under dc field

The correlation between space charge accumulation and insulation failure has been investigated in two types of cross-linked polyethylene and ethylenepropylene rubber subjected to a positive DC field of 50 kV/mm for periods up to 3% years. One type of XLPE films was equipped with tree-retardant additives. A buildup of space charge in the films was monitored periodically during aging with Thermal Step Method, which measures space charge distributions under no voltage condition. Not a single parameter that would consistently correlate with times to insulation failure could be derived from the space charge measurements. However, the space charge density peaks that appeared initially near the electrodes moved closer towards one another during aging and the area under the space charge density peaks showed a noticeable increase shortly before insulation breakdown. These observed properties of the material behavior under dc fields could be used to screen new materials for practical applications.

Polarization and depolarization current response of XLPE insulation subjected to wet-aging

This paper describes the polarization and depolarization current responses from XLPE cable insulation with and without water trees. Comparison of the polarization current with the time of polymer aging suggests that the analysis of the polarization current alone could be used to determine the operating state of the insulation. The advantage of this approach is that the polarization current provides consistent results and eliminates the need to acquire and analyze the depolarization currents as a function of the poling time.

Space charge distribution measurements in XLPE and EPR subjected to a uniform DC field

Space charge distributions in crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) subjected to dc fields of /spl sim/25 and 50 kV/mm in a dry environment at room temperature is investigated. To simulate the insulation of high voltage cable, the specimens were equipped with semicon electrodes. Aging with dc field generates considerably more space charge in EPR than in XLPE and the local electric stresses associated with the accumulated space charge can be significant. EPR develops heterocharge at both stress levels whereas XLPE develops heterocharge at the lower but homocharge at the higher stress.

On-site diagnostic tests on medium voltage, XLPE-insulated power cables

The dc depolarization current measurement technique was used to test 20 medium voltage, underground XLPE-insulated cables in the local utility power distribution network between January and March 2006 and then in March 2007. The cables varied in length (30 m to ~1600 m), voltage rating (9 to 28 kV), and age (4 to 36 years). Unlike a typical depolarization current measurement where only a low frequency portion of the current is monitored with an electrometer, the described measurements have also included the high frequency component of the depolarization current, occurring right after switching off the poling voltage. The measurements in 2007 also included the dielectric loss factor measurements between 10 mHz and 500 Hz to correlate the tan delta values with the parameters of the depolarization currents. A reasonable correlation was found between cable age and other operating conditions and the depolarization current characteristics. A one year period between 2006 and 2007 produced noticeable changes in the depolarization current characteristics for some cable and almost no change in others. No consistent time effect on the insulation of various cables could be identified.

Space charge development and breakdown in cross-linked polyethylene under DC fields

Space charge development in cross-linked polyethylene (XLPE) was investigated using the Thermal Step Method (TSM). The samples were subjected to DC fields of /spl sim/25, 50 and 70 kV/mm and, several times during aging, the TSM, which is a nondestructive technique, was used to measure insulation response current. The distribution of the corresponding space charge was computed using the newly developed inverse matrix technique. The insulation response current increased systematically with the time of voltage application. Under similar aging conditions some specimens were found to develop space charge faster than others. These specimens have consistently failed sooner than those with slower rate of space charge buildup. It is proposed that the initial rate of space charge development in polymeric insulation could provide an indication of its performance under DC field.

Space charge distribution measurements in polyethylene aged in dry and wet environments using the thermal step method

The paper presents space charge distribution measurements in various types of polyethylene subjected to accelerated electrical aging in wet and dry environments. It is shown that dry aging, which produced partial discharges of picocoulombs magnitude, introduced significantly more charge into the material than wet aging, during which partial discharges were not present. Space charge distributions in wet-aged insulation gave good correlation with the length of vented water trees and thus could be used to assess the extent of water treeing more easily than by microscopic observations and tree length measurements.