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While increasing a 500 Hz voltage from 300 V to 2000 V, the charging and ac loss currents of a water-treed XLPE are observed to increase more with voltage than those of a non-degraded XLPE. The fundamental and the third harmonic of the ac loss current have been numerically analyzed based on a model of water-treed insulation. In the model, a nonlinear voltage dependence of the conductance of a water-filled channel region is assumed. Magnitudes and phase angles of the fundamental and 3rd harmonic components of the ac loss current calculated from the model are compared with the observed ones. It was found that the length of water-filled channel region of a water tree grows with increasing voltage, and that this length approaches that of the water-treed region estimated by the pulsed electroacoustic (PEA) method.

The effects of specimen thickness, testing conditions and preparation technique on the measured ramp-breakdown strength of 2 polyethylene systems have been investigated. The obtained data have been found to be highly reproducible, enabling results under different test conditions to be correlated with one another. In all cases, an increase in ramp rate results in an increase in the measured breakdown strength, as does a reduction in sample thickness. Irrespective of the test parameters, the short-term performance of a polyethylene blend containing a designed microstructure is always improved compared to that of the base low density polyethylene alone. Impulse testing leads to an identical conclusion. The effect of the blending method has been shown to have a negligible effect on the breakdown strength, despite consequent changes in nucleation density, implying that it is the local lamellar texture that is responsible for the enhanced properties of the blend.

Lining of underground pipeline networks with deformed/reformed high-density polyethylene (HDPE) liners is a trenchless technique frequently specified for rehabilitation. According to the current design practice, a liner can be structurally designed as a flexible pipe, rigid pipe, or composite pipe, depending on installation conditions. One of the most important design considerations of a confined liner is its buckling under a uniform pressure similar to that exerted by ground water seeping through the deteriorated host pipe. Because long-term testing of a lining system is both time-consuming and expensive, it becomes extremely important to establish a design criteria for long-term conditions more quickly and economically. In the reported study, over 200 short-, medium- and long-term tests were performed to examine the effects of the diameter, thickness, and deformities on the buckling characteristics of a HDPE liner pipe. Based on the findings of the study, some recommendations were proposed for the design and testing of plastic liners under short- and long-term loading conditions.

EIICHI MIYAGAWA [2] AKIRA TANAKA [*] [l] HIROMITSU UNO [1] MITSUNOBU KITAMURA [1] KOH-HEI NITTA [3] INTRODUCTION The effect of the morphology on photo-degradation was investigated for low-density polyethylene materials. [...]