Yasuo Suzuoki

Computer simulation on formation of space charge packets in XLPE films

The packet-like space-charge behaviour in an oxidized crosslinked polyethylene (XLPE) film doped with antioxidant is discussed through computer simulation. The model is based on the following assumptions, (1) both electrons and holes are injected from the electrodes and they migrate to the opposite electrodes by hopping transport, (2) holes are also supplied from acceptor sites depending on the field, (3) the holes are excited to the hopping level only via an intermediate level. In this model, a space-charge packet is not caused by the excess of mobile electrons but by the deficiency in mobile holes. The ionized acceptors left behind by hole migration form an apparent negative space charge packet and it propagates towards the anode as the high field region moves.

Partial discharges due to electrical treeing in polymers : phase-resolved and time-sequence observation and analysis

We have investigated the partial discharge (PD) due to electrical treeing degradation in low-density polyethylene (LDPE), ethylene - vinyl acetate copolymer (EVA) and ethylene - acrylic acid copolymer (EAA) by a computer-aided partial discharge measurement system which allowed us to obtain phase-resolved PD pulse data. The experimental results revealed that the PD magnitude was strongly affected by the instantaneous applied voltage and that the occurrence of a PD was determined by the time derivative of the applied voltage (). The PD pulse-sequence analysis revealed the following: (i) a PD occurs in a discharge path which consists of a tree trunk and branches extending from the trunk; (ii) in each discharge path at most one PD occurs per half cycle. Based on these facts, a model of PDs due to electrical treeing was proposed. The influences of applied voltage and frequency were investigated by applying a triangular voltage. The number and average magnitude of PDs increased linearly with applied voltage whereas the PD charge per cycle increased quadratically. These results are in good agreement with the model.

Packet-like space charges and conduction current in polyethylene cable insulation

Packet-like space-charge behavior in polyethylene for cable insulation was studied by utilizing the laser-induced pressure-pulse technique. Space charge observation under various conditions showed that the charge packet was formed in the specimen doped with antioxidant, especially when the specimen is oxidized. Periodic formation and transport of charge packets led to a small current oscillation. The charge packet seemed to be formed in the bulk, and the internal field of 1.2 to 1.4 MV/cm, in the region where the packet was formed was periodically enhanced from 2 to 2.5 MV/cm by space-charge accumulation. This suggests that the packet formation is caused by carrier generation under the highly enhanced field. The antioxidant deteriorated by oxidation was shown to be a possible origin of the carrier generation.

Thermally stimulated currents in polyethylene and ethylene–vinyl‐acetate copolymers

Thermally stimulated currents (TSC) from polyethylene and ethylene–vinyl‐acetate copolymers with a variety of vinyl‐acetate contents have been investigated using a TSC technique with x‐ray irradiation. Five main TSC peaks, which we believe are due to trapped carriers, were observed in these polymers. The detrapping of carriers is closely related to molecular‐chain motions, and trapping sites are assigned to physical defects such as cavities formed by local arrangements of molecular chains rather than carbonyl groups. One of the TSC peaks is associated with traps in the amorphous region, one with traps in the boundary region between the amorphous and the crystalline regions, i.e., in the folded region, and another with traps in the crystalline region. The TSC spectrum from the dipole of the vinyl‐acetate unit under a bias field shows a TSC peak accompanied by a current inversion. The average effective dipole moment for the vinyl‐acetate unit was determined to be about 1.8 D from a TSC analysis.

The contribution of discharge area variation to partial discharge patterns in disc-voids

Experimental ac partial discharge (PD) patterns are presented for a disc-void (area greater than length) with metal surfaces, one metal and one insulating surface, and with both surfaces insulating. These patterns indicate that the portion of the surface over which the charge is deposited (discharge area) plays an important role in producing the fluctuations in discharge magnitude commonly observed for such voids. A simulation model for PD patterns in voids is presented, which expressly includes the effects of the charge distribution left on the void surface by consecutive PDs. This model defines two factors that control the PD propagation. These are: a minimum field required to maintain the discharge within an existing surface path Ein and a minimum peripheral field required to extend the discharge path Ep. There is also one other factor, the occurrence probability that controls the incidence of the PD. It is shown that the model gives a variation in PD magnitude throughout the active region of phase without any stochastic factors, i.e. when only variations in the discharge areas are allowed for, but that in order to reproduce the typical (turtle-like) PD patterns observed for the disc-void with insulating interfaces the occurrence probability and surface conductivity have to be included. The current model is compared to previous models and the relationship of the model factors to the change of PD patterns with ageing is briefly discussed.

Effects of space charge on electrical conduction in high-density polyethylene

Direct probing of space charge was carried out on high-density polyethylene doped with antistatic agent using a thermal pulse method. Prominent positive space charge was found to be formed near the cathode by voltage application. The field at the cathode is enhanced by the space charge. The conduction can be quantitatively explained by Schottky-type electron injection under the enhanced cathode field. The anomalous discharge current after short-circuiting, which flows in the same direction as the charging current, can also be explained by the continued electron injection due to the remaining positive space charge.

Study of space charge in polyethylene by direct probing: effects of oxidation

By utilizing the laser-induced pressure-pulse technique, the authors have quantitatively studied the behavior of space charge in unoxidized and oxidized low-density polyethylene (PE) samples whose thickness ranged from 100 mu m to 1 mm. The distribution of space charge changes with applied field, temperature, extent of oxidation and the thickness of the samples. Prominent negative space charge was formed near the cathode in oxidized PE, indicating that oxidation enhanced the electron injection from the cathode. The amount of negative space charge increased with applied field, and this suggests that the electron injection is enhanced by the applied field. The depth of charge centroid from the cathode became larger with increasing temperature and applied field. This indicates that electrons become more mobile as temperature increases. The complicated behavior of the space charge suggests that space charge formation is determined by the following competing processes: electron injection from the cathode, transport, trapping, detrapping, and charge exchange at the anode. >

Contribution of surface conductivity to the current forms of partial discharges in voids

The current form of partial discharge (PD) in a void was divided into fast and slow components, with short and long fall times, respectively. Comparing these with the current shape of PD in a void between two metal surfaces, in which the slow component was not observed, it was suggested that the formation of fast component in a usual void was due to the instantaneous discharge between the discharge areas of the PD, i.e. the region to which the carriers in a PD process can instantaneously propagate, and the slow one might be due to the low conductivity in the regions outside the discharge areas. An equivalent circuit was proposed to describe the effect and numerical simulations were carried out. It was suggested that the fall part of the first PD after polarity reversal might reflect the surface conductivity change in aging process.

A novel physical model for partial discharge in narrow channels

In order to reveal the mechanism of partial discharge (PD) in electrical trees, an artificial single channel is studied as model for tree channels. In a long-narrow channel, the behaviour of PD is similar to that in an actual tree. When the length of the channel is decreased, the /spl phi/-q-n pattern of PD in the channel changes from wing to turtle or rabbit like, and the range of light emission in the channel increases when the applied voltage is increased. Considering the influence of a PD on the total field distribution and thus on a later PD, a physical model is put forward to describe the PD behaviour. It is assumed that PD is induced by the high field at the needle tip, then propagates along the channel, and stops propagating when the field in the channel becomes lower than a critical value due to field divergence. Mathematical analysis on an ideal needle-plane electrode system clearly explains the physical meaning of the model. And the computer simulation by using the superposition method agrees with the experimental results very well. This model is also valid to describe the behaviour of PD in electrical trees.

Effects of discharge area and surface conductivity on partial discharge behavior in voids under square voltages

The transitions of partial discharge (PD) behavior with aging in a void under ac voltages with sine wave and square wave were observed. The transition of the PD magnitude with aging under square wave showed the same tendency as that under sine wave. Moreover, the experimental results also showed some distinctive characteristics of PDs under square voltage (e.g. more than two pulses after the voltage rise time and their transition with aging). These phenomena were explained in terms of the effect of discharge area and the change in surface condition due to PD degradation