Yasuo Sekii

Experimental study on the causes of electrical breakdown of EHV XLPE insulation using 'prebreakdown partial discharge detection method'

The causes of electrical breakdown in EHV XLPE (cross-linked polyethylene) cables were clarified by carrying out the prebreakdown partial discharge (PBPD) detection test under electric stresses from 40 to 70 kV/mm. Ten causes of electrical breakdown were observed in the PBPD test. As expected, it was found that they were associated with very small contaminants and protrusions. The electrical tree inception stress was estimated to be 270 kV/mm on the average. The prebreakdown time was estimated to be on the order of 1 hour under an applied electric field of 5 kV/mm.<<ETX>>

Degradation of low-density polyethylene and cross-linked polyethylene by partial discharge

To study the degradation of low-density polyethylene (LDPE) and cross-linked polyethylene (XLPE) by partial discharge (PD), experiments were conducted in a nitrogen (N2) and (N2+O2) gas atmosphere. After being subjected to the PD, samples were examined using atomic force microscopy (AFM) and FTIR. The AFM images clearly revealed the creation of pits on the surface of degraded samples. A large difference in the pit pattern was observed between samples subjected to the PD in N2 and those subjected to the PD in (N2+O2). The FTIR spectra detected in samples subjected to the PD in (N2+O2) showed that carbonyl compounds were produced on the surface of degraded LDPE. Experiments were performed using an experimental system incorporating GC-MS to measure the gas components produced during degradation. Results of experiments showed that carbon dioxide (CO2) is produced when LDPE and XLPE are subjected to the PD in an oxygen-containing atmosphere. The amount of CO2 was found to increase concomitantly with the period of voltage application and oxygen concentration in the area where PD occurs. Based on experimental results, the processes of LDPE and XLPE degradation by PD were examined along with the antioxidant effect on degradation.

Generation and Dissipation of Negative Heterocharges in XLPE and EPR

Charge generation in cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) under dc stress are influenced strongly by inclusions. The inclusions influencing charge generation are cross-linking byproducts, antioxidants, and moisture. These inclusions particularly influence the generation of negative heterocharges observed near positive electrodes, which produce high electric stress regions in XLPE cable insulation under dc voltage. Because a high electric field might cause dielectric breakdown of high-voltage equipment, it is important to minimize negative heterocharge accumulation. Consequently, the authors performed experiments to clarify the negative heterocharge generation mechanism in XLPE and EPR. The authors first studied the influence of cross-linking byproduct and moisture on negative heterocharge generation to clarify the mechanism. The authors next performed experiments to elucidate the effect of antioxidants on heterocharge generation, which revealed that the negative heterocharge is generated in XLPE insulation containing sulfur-containing phenolic antioxidant, or sulfur-type antioxidant. This heterocharge is presumed to be created by the combined effect of the antioxidant and acetophenone. Furthermore, the authors studied the dissipation of negative heterocharges in XLPE and EPR, clarifying that the dissipation of negative heterocharges in EPR is much faster than that in XLPE. This paper presents results of these studies.

Influence of defects on insulating properties of XLPE cable

The influence of defects on the insulating properties of XLPE (cross-linked polyethylene) cables was studied using XLPE cables containing artificial defects. It was confirmed that the AC breakdown strength of the cables decreases with the increase of void size, while the existence of voids does not have any effect on impulse breakdown strength. The contaminants and the protrusions decrease both AC and impulse breakdown strengths. The value of the life exponent, n, of the V-t characteristics of the XLPE cable containing voids was estimated to be smaller than that of the cables containing contaminants or protrusions. In addition to to the influence of those defects on initial and long-term performance of the cables, the influence of impulse voltage on the long-term performance of the cable, together with the behavior of the space charge accumulated around the defects, was investigated. It was found that impulse voltage greatly affects the long-term performance of cables containing voids.<<ETX>>

Development of 500 KV Cross-Linked Polyethylene Insulated Power Cable

A 500kV cross-linked polyethylene (XL PE) insulated power cable with conductor size of 2000 mm2 and insulation thickness of 35 mm has been designed and developed for the first time in the world.

Development of New Water Tree Suppressive XLPE Cable

With a view to develop a new water tree suppressive XLPE cable, the authors have investigated the effect of introducing polar groups into crosslinked polyethylene (XLPE) dielectrics. An aliphatic carboxylic acid derivative was found to be the best additive as a water tree suppressor. New cables insulated with XLPE dielectrics containing this aliphatic carboxylic acid derivative were developed. It was confirmed that the cables, having the same physical and electrical properties as those of conventional XLPE cables, showed an excellent water tree suppressive effect and long term performance.

Effect of antioxidants on thermal degradation of EPDM and XLPE

An experimental investigation of the effects of antioxidants on thermal degradation of EPDM and XLPE was carried out. We used ldquomapping methodrdquo in our FTIR spectroscopy to evaluate the degree of thermal degradation. The obtained absorbance data of FT-IR spectra were analyzed statistically. Thereby, we confirmed a clear suppressive effect of sulfur-containing phenolic antioxidant, 4-4 thiobis (3methyl-6-tert-butyl phenol), for thermal degradation of EPDM and XLPE. We confirmed also that a similar effect is obtainable when a phenolic antioxidant is combined with a sulfur type antioxidant. In our experiment, we discovered a great difference of antioxidant effects between EPDM and XLPE. Based on the results of experiments, the scheme of suppressive effects of antioxidant on thermal degradation was discussed.

Influence of antioxidants and cross-linking on the crystallinity of XLPE dielectrics

Crystallinity of polymeric dielectrics is known to be influenced by cross-linking. To study that influence, gel fraction measurements and evaluation of the degree of crystallinity were performed under different cross-linking temperatures using XLPE samples with different DCP concentrations. Furthermore, the influence of antioxidants on XLPE crystallinity was studied. Gel fraction and crystallinity measurements were made of XLPE samples containing different concentrations of DCP and phenolic or sulfur-type antioxidants. The experimental results are used to discuss the influence of antioxidant on cross-linking and crystallinity of XLPE.

Influence of moisture on the negative hetero-charge generation in polymeric dielectrics

Under DC voltage stress, a negative hetero- charge is generated near the positive electrode in cables insulated with cross-linked polyethylene (XLPE). The generation of negative hetero-charges in XLPE-insulated cable results from cross-linking byproducts, antioxidants, and moisture. Among these inclusions, moisture is known to originate from dehydration of a-methyl styrene, which is a byproduct produced during chemical cross-linking of XLPE cable. To investigate the influence of moisture on negative hetero-charge generation, the authors first studied the moisture absorption of XLPE, EPR, and LDPE. The results show a big difference in moisture absorbing characteristics among XLPE, EPR, and LDPE. The authors next studied the effect of moisture absorption on negative hetero-charge generation in XLPE, EPR, and LDPE. Experimental results demonstrated that the charge densities of negative hetero-charges generated in LDPE and XLPE are proportional to moisture concentration in these materials. The hetero-charge generation mechanism in XLPE is discussed.