A.T. Bulinski

Degradation of polymeric insulation due to photoemission caused by high electric fields

The mechanism of electrical tree initiation in polyethylene subjected at AC, DC, and half-rectified AC voltages is studied by an optical technique which is at least two orders of magnitude more sensitive than partial discharge detection. For DC voltage up to 46 kV, no light emission or tree inception was detected in the polymer containing the normal and decreased concentrations of air in its free volume. For AC and half-rectified AC voltages, tree inception only occurred if light, in the visible and ultraviolet ranges, was detected. Photodegradation of the polymeric insulation by ultraviolet radiation can occur when the voltage applied to the polymer exceeds the threshold voltage of light inception such as during switching and lightning overvoltages. This can account for the gradual degradation of the polymeric insulation which ultimately leads to the formation of an electrical tree. By comparing the light emitted from the polymer subjected to AC voltage having half-cycles of different amplitudes to DC superimposed on AC it is shown that the space charge injected into the polymer causes the zero-crossing of the local field in the polymer. This is required for light emission and subsequent tree inception. >

Oxidation and Water Tree Formation in Service-Aged XLPE Cable Insulation

The insulation from six 5 kV power cables, which has been in service underground for 6 to 8 years, was examined by infrared (IR) spectroscopy and oxidation induction time (OIT) analysis. Sections of insulation containing water trees were found to contain high levels of ionic contaminants. All insulation samples showed evidence of oxidative degradation in service and frequently there was a higher than average level of oxidation in the treed regions of the insulation. Sections of the insulation containing water trees had appreciably shorter OITs than untreed regions, indicating that they were more prone to subsequent oxidative degradation. A model for water tree formation and electrical breakdown of the insulation is described where oxidative degradation during prolonged service reduces the ability of the insulation to withstand stress concentrations at defects, and water trees are initiated. Some localized oxidation may accompany the tree propagation step. Extensive localized oxidation then takes place in the treed regions, catalyzed by ionic contaminants, and insulation failure occurs.

Evidence of near‐ultraviolet emission during electrical‐tree initiation in polyethylene

The spectra of light emitted during electrical‐tree initiation in low‐density polyethylene used in high‐voltage cables were investigated. Light in the visible and in the near‐ultraviolet range was detected. Low‐density polyethylene subjected to highly divergent fields at voltages below the light‐inception level did not develop an electrical tree, and it is suggested that the light‐inception voltage is the threshold for insulation degradation. The ultraviolet light emitted at voltages above the light‐inception level could cause degradation of the polymer. A model is proposed to explain the mechanism of electroluminescence. Ultraviolet stabilizers added to the polymer prolonged the time to electrical‐tree initiation by preventing photodegradation of the polymer.

Electric field calculations with the boundary element method

The boundary element method is used to calculate the electric field profiles at needle tips commonly used for electrical treeing tests. Field distributions are also obtained for polyethylene containing a space charge, at the needle tip, and are compared with the values previously obtained by the finite difference method. >

Polymer oxidation and water treeing

In order to determine whether or not oxidation of polymer influences water treeing, more than 200 vented and bow-tie trees from 31 field-aged cables were investigated. Micro-IR spectroscopy analysis has not shown any consistent excess of carbonyl content in water trees as compared with the adjacent non-treed regions of the insulation. Although the levels of carbonyl content of the bulk of the polymer within the treed regions are similar to those in untreed regions of the polymer, vented trees are more susceptible to oxidation if subjected to high temperatures in the presence of oxygen. It was observed that vented trees initiate at similar rates in XLPE in either a nitrogen or air atmosphere. This indicates that tree initiation is rather independent of the presence of oxygen. However, the tree growth rate is slower in nitrogen than in air, the actual difference being affected by the type of ionic solution used. This further suggests that some, not yet known, chemical reactions between oxygen, XLPE and ions play an important role during water tree propagation. The IR absorption band at (1585 cm/sup -1/), typical of carboxylates, was detected in some water trees. It should be noted that the large absorption band of water at (1640 cm/sup -1/) often masks the smaller carboxylate band. Under laboratory conditions carboxylate groups were detected on oxidized nontreed XLPE surfaces. These results do not imply that carboxylates are responsible for tree propagation but confirm only that carboxylate groups are formed during XLPE oxidation. Thermal pre-oxidation of XLPE, to the levels measured in typical field-aged cables, has little or no effect on the initiation and growth of vented water trees. Very high levels of pre-oxidation (as determined by the carbonyl content), at least 80/spl times/ the average oxidation level measured in typical field-aged cable, retard the growth of vented water trees. However, these high levels of oxidation negatively affect the dielectric properties of the insulation.

Space-charge distribution in XLPE by TSM, using the inverse matrix technique

TSM (Thermal Step Method) is a nondestructive technique to determine space charge distribution in thin and thick dielectric specimens. Although the experimental procedure is simple, the numerical techniques required for analyzing the data are complex and time consuming. Fourier analysis is commonly employed to analyze the TSM data. This paper proposes the use of the inverse matrix technique to simplify the analysis of the signals acquired by TSM. Numerical simulation by the inverse matrix technique is compared with the Fourier analysis technique. Corona charged as well as DC poled XLPE specimens are tested to confirm the validity of the inverse matrix technique. It is shown that this technique gives better results and requires less computing time than other techniques usually employed for the TSM.

Threshold voltage of luminescence and electrical tree inception in low‐density polyethylene

The characteristics of luminescence observed during the initiation of an electrical tree have been studied for polyethylene subjected to high electrical stresses. Use of a light detection system which is at least three orders‐of‐magnitude more sensitive than conventional partial discharge detectors shows that luminescence occurs during tree initiation and this luminescence is not due to partial discharges. Applying a constant high voltage to the polymer or interrupting the voltage several times does not change the behavior of luminescence. The light starts at about the same voltage and its intensity returns to the same value as before the interruption. This behavior is contrary to that observed for partial discharges. The luminescence inception voltage is the threshold voltage at which the polymer starts to degrade. Thus, it is important that the insulation of electrical equipment be protected from transient overvoltages which exceed the threshold level.

Degradation mechanism at XLPE/semicon interface subjected to high electrical stress

In HV devices such as power cables, electrical stress enhancement can occur at the interface of semiconducting shields (semicon) and polymeric insulation. In this study, points of electrical stress enhancement are simulated by embedding semicon protrusions into the polymer. XLPE with two different concentrations of crosslinking byproducts and impregnated with various gases has been used. It it shown that, prior to electrical tree inception, electroluminescence occurs at the semicon tips and visible and ultraviolet light is emitted. The characteristics and the spectra of electroluminescence at semicon protrusions embedded in XLPE are similar to those previously observed at metallic needles embedded in LDPE. The ultraviolet light, emitted at points of electrical stress enhancement, can photodegrade the polymer, cause bond scission and lead to electrical treeing. Also, the results for XLPE impregnated with different gases indicate that, as in the case of LPPE/metal interface, oxygen in the free volume of the polymer plays an important role in the deterioration of XLPE insulation subjected to high electrical stress. >

The Role of Polymer Interface During Tree Initiation in LDPE

Based on charge transfer mechanisms at the polymer interface, it is bhown that the injected charges are responsible for the light emitted during tree initiation in LDPE subjected to highly divergent fields. Light was not only detected d in degassed specimens as reported by other workers, but also in air-impregnated (normal) specimens as well as in those impregnated with other gases. The photoemission characteristics of degassed specimens were found to be different from all others and probably are due to radiation from electron-hole recombination. In all other specimens a combination of this and other radiative phenomena could be responsible. It is proposed that at low voltages the emitted light, which is mostly in the visible spectral range, is not immediately harmful to the insulator. It is only at high voltages that the emitted light would include more energetic and intense ultraviolet (UV) radiation and this UV could cause rapid degradation of the polymer and give rise to an electrical tree.

Threshold voltage for electrical tree inception in underground HV transmission cables

Experimental evidence is provided to show that irreversible aging which leads to electrical treeing in polymeric insulation such as low-density and crosslinked polyethylene used in HV transmission cables, does not start at local fields between 5 and 20 kV/mm. It is shown that a much higher stress is required for electrical tree inception and that the electroluminescence inception voltage is the threshold voltage at which the polymer starts to degrade. >