Tree Inception Voltage Research Articles

A consideration of potentials of low‐density polyethylene using metallocene catalyst as insulator for power cable

AbstractWe investigated the high‐field conduction, DC and impulse breakdown strength, space charge distribution, and tree inception voltage for three kinds of new low‐density polyethylene (LDPE) prepared using a metallocene catalyst (mLna, mLao, mLldao), linear LDPE prepared using a Ziegler catalyst (LLao), and LDPE prepared by a high‐pressure process (LDna). The dc and impulse breakdown strengths of LDPEs prepared using a metallocene catalyst were higher than those of LLao and LDna. The high‐field currents of LDPEs prepared using a metallocene catalyst were lower than those of LLao and LDna. A homo‐space charge was accumulated near the cathode in mLna. The tree inception voltage of mLna was higher than that of LDna. From these results, it is concluded that LDPE prepared using a metallocene catalyst has electrical insulating properties superior to the conventional LDPE and that the former has potential as a power cable insulator. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 139(4): 17–25, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.1164

Propagation of Electrical Tree in Extended Chain Crystal Polyethylene

In the present paper the electrical tree growth and tree inception voltage on oriented highly crystallised polyethylene is reported. The electrical treeing discharges were investigated with a needle to plate electrode system under a lightning impulse voltage application.

The influence of antioxidants on electrical tree generation in XLPE

AbstractIn conjunction with previous experiments studying the effect of decomposition products of dicumyl peroxide (DCP) and antioxidants (AOs) in cross‐linked polyethylene (XLPE) insulation on electrical tree generation, the authors conducted experiments investigating tree inhibition by phenolic AOs. The experiments were conducted using XLPE slabs containing four different phenolic AOs. Among the specimens containing these AOs, the specimen containing 4,4é‐thio‐bis(3‐methyl‐6‐tert‐butylphenol) was found to show the highest tree inception voltage. To examine the reasons for the significant increase in tree inception voltage in this specimen, further experiments were carried out on XLPE specimens containing both phenolic AO and a sulfur‐containing AO. The results demonstrated that the tree inception voltages of specimens containing both phenolic AO and sulfur‐containing AO were higher than those of the specimens containing only one of these two types of AOs. The roles of antioxidants in XLPE dielectrics are discussed with reference to the mechanisms of autoxidation of polymeric materials. The great increase in the tree inception voltage of XLPE specimens containing 4,4é‐thio‐bis(3‐methyl‐6‐tert‐butylphenol) appears to be due to a synergistic effect between the phenolic AO and the sulfur‐containing AO. © 2001 Scripta Technica, Electr Eng Jpn, 135(3): 1–7, 2001

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.

Correlation of optical and dielectric properties of water trees in polyethylene

Many experimental works report that the presence of water trees lowers the voltage needed for the initiation of dielectric breakdown in polyethylene. The influence of water trees on the dielectric breakdown is generally related to their length. Here, in addition to this spatial property, we consider the extinction of the light by the water trees due mainly to scattering of the incident light by water microcavities. This effect is described by means of the water tree absorbance. The absorbance itself is related to another main variable: the water content. We also take into account the existence of a structure in the water tree, i.e., of a heterogeneous distribution of water microcavities inside the trees. Water content and spatial water distribution allows us to define a local permittivity for each region in the tree. These elements are the basis of a model for the computation of the disturbance induced by the tree in the spatial distribution of the electric field (reduction of the field inside the tree, enhancement outside). The field enhancement effect is computed as a function of the water concentration. Measurements of the light transmitted by given water trees are performed; they allow us to propose an estimate of the water contained by different water trees. From these concentrations, different field enhancement coefficients are computed from the model. They are then compared with measurements of electrical treeing inception voltage performed with and without water trees. Theoretical values obtained from the model are found in good agreement with experimental data.

DC Tree Associated with Space Charge in PMMA

DC tree initiation and propagation and treeing phenomena in PMMA under ramped voltages have been investigated by a optical method using a photomultiplier and a close-up camera. The Weibull plot of the initial tree inception voltage for a positive point had a knee at about 35 kV, above which the slope was lower, while the plot for a negative point was linear. The succeeding negative streamer originated from the tips or stem of tree channel as well as from the point cathode, while the succeeding positive streamers originate from only the point anode.

Photo-Inspection between Cross-Linked Polyethylene and a Semiconducting Layer

Compatibility between a cross-linked polyethylene layer and a semiconducting polyethylene layer in a block sample, simulating a solid dielectric cable, is investigated by use of a photo-multiplier to detect light emission at the interface. The application of a photo-multiplier to detect the quality of contact at the interface provides superior sensitivity to the conventional discharge detection method which, itself, is sensitive only to actual treeing discharges and consequently is valid as a detector only after the start of electrical treeing. The semiconducting polyethylene layer used as an electrode in the experiments is sandwiched between sheets of low-density polyethylene (M. I. = 1.0) and heated with dicumyl peroxide to achieve chemical cross-linking. As determined from the output of the photo-multiplier, the treeing inception voltage increases with the quality of the contact. A temperature dependency of the contact is also detected by the photo-multiplier in the range above room temperature where light emission shows a maximum at about 60°C, suggesting that contact becomes poorest at about that temperature. The effect that mechanical drawing of the test samples has on light emission at the interface is also investigated at various temperatures by means of the photo-multiplier.

Phenomena and Mechanisms of Tree Inception in Epoxy Resins

The inception of ac treeing has been investigated in various cast epoxy resins, using a needle-plane test configuration. Mechanical stress due to shrinkage in the curing process is present at the needle tip, and in some cases causes micro-cracks. Lower tree inception voltages were observed in mechanically stressed specimens indicating that the micro-cracks act as weak points. In less stressed samples which have higher tree inception voltages light emission, probably electroluminescence, was observed at a voltage much lower than that of tree inception. In those cases, a small discolored area (about 10 ?) appeared prior to tree inception as the ac applied voltage was increased.

On Tree-Inhibition in Polyethylene

During a series of experiments aimed at developing a better understanding of the basic mechanisms of electrical treeing in polymer insulations, the dependence nce of the dc conductivity of polyethyle ( with and without additives) on temperature and field strength was examined. It was discovered that all additives, and especially acetophenone, which is well known to resist electrical treeing, substantially increase the conductivity. Due to the low conductivity of insulators, in the case of ac voltage excitation capacitive voltage division ion generally occurs. This leads to enormous stress concentrations at the sites of impurities, which finally may lead to the start of trees. Computations using the results of the dc conductivity measurements showed that these local stress concentrations ions can be diminished by the field-depen conconductivity of the polymer even for 50 or 60 Hz ac voltage, thus leading to an improved resistance to treeing. The experimental proof of the relevance of this field-grading mechanism is given by the results of treeing experiments with polyethylene containing specific additives. Furthermore, tree-inception voltages of cross linked polyethylene containing various amounts of etoacetophenone show a dependence on temperature and frequency as expected from the mutual capacitive-conductive lvoltage division by the additive.

Treeing Inception Voltage of Polyethylenes: Effect of Blending and of Crosslinking

This paper describes a better insulating material for polyethylene (PE) and cross-linked PE (XLPE) power cables. We blended low density PE with S to 20% high density PE. The treeing inception voltage increased significantly with the high density polyethylene content.