DC leakage current measurements have been carried out regularly to diagnose the insulation of XLPE cables. The degree of the deterioration of the cable insulation is determined by the DC leakage current value. On the measurement at the installation of XLPE cables, the DC leakage current sometimes shows a large value with a peak. To clarify the causes of the DC leakage current peak, we investigated the DC leakage current of 6.6kV dry-cured XLPE cables at different temperatures. The results show that the residue of the crosslinking reaction influences the peak of the DC leakage current. To discuss mechanism of the peak, we also measured the DC leakage current repeatedly at the same cable, and investigated the space charge distribution in the cable insulation by the Pulsed Electroacoustic Method. It is expected that the DC leakage current peak depends on the space charge relative to the residue.

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The pulsed electroacoustic (PEA) method is widely known to be an excellent method for the measurement of the space charge distribution which accumulates in thick dielectric materials (1-5mm). However, because of its lower resolution in the depth direction, conventional application of this method is less suitable for thin specimens than the laser-induced pressure wave method. The authors developed a new PEA measurement system which can be applied to thin dielectric materials (_??_ 100μm). The resolution in the depth direction is improved to less than several μm by detecting signals in a wide frequency range. In addition, the system employs a deconvolution technique with FFT in order to compensate for the frequency characteristics of the piezoelectric sensor. The new PEA measurement system is expected to be useful for quantitative and nondestructive measurement of the space charge distribution in dielectric films.

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Space charge measurement was done with water tree degraded XLPE cables using the Pulse Electroacoustic Method. A DC voltage was applied on the E-T type (inner semiconducting layer extruded-outer shield tape wound) XLPE cable in which vented water trees generated from the outer shield. A space charge of the same polarity as that applied on the outer shield accumulated in the vicinity of water tree region. This space charge remained stable after short-circuiting the sample, however, its decay was accelerated by applying an AC voltage. The acceleration of space charge decay was theoritically analized assuming ionic conduction. A good correlation was seen between the space charge intensity and the degree of water tree degradation. Space charge measurement under AC voltage showed a difference between 50Hz and 0.05Hz application. The space charge under lower frequency reaches further through the vented water tree.

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Pulsed electro-acoustic method is applied to the measurement of space charge distribution formed in dielectric materials. In this method, charge distribution is detected as the acoustic wave signals which are generated by the charges existing in the sample under an applied pulse voltage. In the soft polymer sample such as low-density polyethylen, however, the attenuation and dispersion distort the acoustic waveform during the propagation through the sample. To obtain the undistorted waveform, so-called "deconvolution processing" using "Jacobi method" is employed. In this report, the principle of the deconvolutipn processing is discussed and some examples of the recovery is introduced.

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To clarify the space-charge distributions in multi-layer low-density polyethylene (LDPE) and cross-linked polyethylene (XLPE) which were measured in terms of a pulsed electroacoustic method by Li et al, the computer simulation was made on the transport of both bulk-excited and electrode-injected carriers from anode and cathode by using hopping model. The following items are taken into account in the analysis; the bulk-excited carrier, electron injection from cathode, hole injection from anode and carrier transport.
The hetero-space charge distribution was observed at polymer/polymer interface in multilayer LDPE with physical contact. The simulation shows that the hetero-space charge appears whenthepotentialbarrieratpolymer, /polyerinterfacewashigherthanthatinthebulk. On the other hand, the homo-space charge was observed at the hot pressed polymer/polymer interface of two-layer XLPE. If the potential barrier of polymer/polymer interface is smaller and the carrier injection from the electrodes exists, the homo-space charge formation appears at polymer/polymer interface in the simulation.

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It is important to have an information of the space charge behavior in thick insulated samples in order to understand the DC characteristics in polymer insulated DC cables. By using the pulsed electro-acoustic method which is effective for above purpose, several factors of the space charge formation in 2mm-thick of polyethylene (PE) was investigated and following results were obtained.
As for measurement factors, (1) Polymeric semiconducting electrode gives more accurate measurement than metal electrode because of good matching of acoustic impedance with PE. (2) Within DC electrical stress range of several tens kV/mm, the space charge distributions under and after DC voltage application are almost the same, due to a comparatively long time of space charge decay. (3) The space charge distribution of a plate sample agrees with that of acable sample of the same insulation thickness.
As for insulating material factors, (1) The amount of space charge in crosslinked polyethylene (XLPE) is much larger than that in low density PE (base of XLPE). The space charge of XLPE continues to increase even after DC voltage application of 24 hours, while that of LDPE becomes equilibrium within a few hours. (2) The above-mentioned space charge difference between XLPE and LDPE is assumed to be caused by ionic impurities in XLPE, but not by additives themselves (acetophenone and cumylalcahol as byproducts of crosslinking agent and antioxidant).

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It was found that polyethersulfone (PES) has two thermally stimulated current peaks in the temperature range from 0°C to 230°C, consisting ofβ peak whose peak temperature moves towards a higher temperature side with an increase in the poling temperature and α peak which appears around 210°C. It was also found that PES shows a 'spontaneous current peak' around 210°C, even when the sample was not poled. Both the spontaneous current peak and α peak were found to disappear when the sample had been heated up to 230°C, keeping the external circuit closed. From such similarity of disappearance between the two peaks and the corresponding change in the spatial distribution of space charges inside the sample examined by the pulsed electroacoustic method, it is concluded that the spontaneous current peak and a peak are ascribed to the same origin and that nonuniform distribution of positive charges, probably due to K₊ ions, is responsible for the two peaks.

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We have been investigating effects of oxidation, antioxidant, interface and other factors on the space-charge behavior in low-density polyethylene (LDPE) for cable insulation by utilizing the laser-induced-pressure-pulse (LIPP) technique. The present study concerns the effect of oxidation in LDPE doped with antioxidant. The dependence of conduction current on the degree of oxidation is different from that in LDPE without antioxidant and it can be classified into three oxidation regions;i) the conduction current increases with the degree of oxidation in the low oxidation region, ii) it decreases with the degree of oxidation in the intennediateoxidation region and iii) it increases again with the degree of oxidation in the high oxidation region. In LDPE with antioxidant, remarkable homo space charge is formed close to both electrodes.
We constructed a model to explain the above characteristics by considering the distribution of antioxidant, that is, the concentration of antioxidant is higher near the surface than in the bulk. 1) In the low oxidation region, oxidation of LDPE is inhibited and the deteriorated antioxidant on the surface enhances carrier injection. 2) In the intermediate oxidation region, oxidation of the surface is still inhibited but the bulk starts to be oxidized. The oxidation products in the bulk act as carrier traps and suppress the conduction. 3) In the high oxidation region, antioxidant is fully deteriorated and LDPE starts to be oxidized from the surface. The oxidation products on the surface enhances carrier injection and thus conduction. The model is confirmed by using the LDPE samples with different distributions of antioxidantand deterioratedantioxidant.

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