Hiromasa Fukagawa

Simultaneous Measurement of Microscopic Image and Discharge Pulses at the Moment of Electrical Tree Initiation

This paper reports some experimental results concerning the tree initiation from a semiconductive needle electrode in polyethylene under a 50 Hz AC voltage. With a newly developed measuring system, the correspondence between partial discharge pulses and tree initiation and growth up to 10-20 µm in length was clarified for the first time. Tree observation and pulse measurement were made simultaneously. At the moment of tree initiation, positive pulses (current from the needle towards the grounding electrode) of 0.05-0.1 pC were first observed. A very small shade of the initial tree was observed when continuous positive pulses appeared; it grew rapidly to about 10 µm in length, along with the positive and negative pulses. After the tree branched, the growth rate dropped for a while. In many cases, the negative pulses disappeared during this stage.

The influence of morphology on electrical tree initiation in polyethylene under AC and impulse voltages

AC and impulse voltages were applied to needle electrode specimens modeling the interface of the semiconducting layer and insulation of XLPE (cross-linked polyethylene) cables. TEM (transmission electron microscope) photographs of minute trees ( >

TEM Observation of Electrical Tree Paths and Microstructures in Polyethylene

The starting point of AC electrical trees in polyethylene and the surrounding microstructures were observed by a transmission electron microscope (TEM) for the first time. To minimize the size of tree paths, the authors developed a new apparatus for preparing specimens in which the tree growth was suppressed to less than 10 µm in length. The tree was started from a needle electrode made of carbon-filled polyethylene. The AC tree was a hollow pipe of about 100 nm diameter immediately after its generation. There were constrictions inside the tree, resulting in a corrugated surface. The starting point of the AC tree was a microscopic convex of several tens to one hundred nanometers, which was based on the cohesion of carbon particles of the electrode. The AC tree path was virtually parallel to the lamellae.

Thermal Brealdown and Thermal Runaway Phenomena in EHV Cables

The paper describes a new method of estimating thermal breakdown in EHV oil-impregnated paper-insulated cables with some experimental data. In the analysis it is shown with the computed results that the heat generation in the insulation can be exactly approximated by the average temperature of the insulation. In addition this method can calculate the critical voltage at which EHV cables go into thermal breakdown. The values here calculated compare agreeably with the breakdown voltages of 500 kV OF cables.

Breakdown Voltage of XLPE Cables Pre-Heated by Excess Current

Figure 1 shows the densities which were measured by a density gradient tube (Shibayama Scientific InstruFig. 1: Density of XLPE as a function of pre-heating ment Manufacture Co. Ltd.) against treatment temperatemperature. ture (oil temperature). Since the melting point of XLPE insulation material was about 1000C by DSC, it is found from Fig. 1 that the density increases with preheating temperature up to the melting point. However, the density decreases to the initial value when the The same considerations apply to the results shown pre-heating temperature exceeds the melting point. in Fig. 1. When XLPE is pre-heated at high temperature, but below the melting point, recrystallization Two of the authors reported [1] that a subpeak will cause an increase in density. If the pre-heating corresponding to the pre-heating temperature appears temperature is higher than the melting point, all of on the DSC thermogram of a pre-heated XLPE sample. We the crystals will melt, so the density will be the explained in the report that since PE or XLPE partially same as that of the initial state. melts and crystallizes during the pre-heating, the DSC thermogram shows subpeaks of partial melting at the pre-heating temperature. When the pre-heating temperature is higher than the melting point, all of the crystals will melt, and thus no subpeak remains in the DSC thermogram.

Transient Oil Pressure Rise at EHV Pipe-Type Cable Faults

Since various phenomena resulting from earth faults in pipe-type cables have not yet been sufficiently clarified, the authors made fault tests on pipes of 105 and 254 mm in diameter and 20 to 60 m in length. As a result, arc voltages were found to be nearly constant, independent of the magnitude of fault current. Moreover, the mechanism of transient pressure rise and the relation between arc energy and oil pressure rise were elucidated. Calculated results agreed fairly well with experimental data.

Transient Oil Pressure Rise in Accessories at EHV Pipe-Type Cable Faults

In the previous paper [1], the mechanism of transient pressure rise at the faults in oil-filled pipe-type cables was discussed in detail, and the relations between arc power and pressure rise were analyzed. This paper deals with the transient pressure rise in accessories such as semi-stop joints, normal joints, and trefoil boxes.