Tree Channel Research Articles

A Physical Model for PD in Tree Channel and its Effects on Tree Growth

A Physical Model for PD in Tree Channel and its Effects on Tree Growth

Space charge formation in water-treed insulation

Observations of the space charge distribution around water trees are reviewed, and the nature of water trees analyzed from those results is discussed. It was concluded that at least two mechanisms are responsible for the formation of space charges. One is interfacial polarization due to the high conductivity of a tree channel, and the other is carrier injection from the tree tip at high electric fields.

Treeing Deterioration Characteristics in Cross-linked Polyethylene and Epoxy Resin Composite Materials

An experimental investigation for evaluating the treeing deterioration characteristics of composite materials was carried out by preparing an interface between cross-linked polyethylene and epoxy resin. The results of various interface conditions are described in this paper. It was determined that the interface had a significant effect on the treeing breakdown under various specimen arrangements. The time until complete breakdown of the sample is longer than the time required for complete penetration of the sample by one or more tree channels. This erosion process is similar to that of epoxy resin materials. It was also determined that if the gases in tree channels were trapped in the interface, the time until complete breakdown would be much longer.

Nonlinear fillers in electrical insulation

The very low conductivities of modern insulating materials do not permit the dissipation of accumulated space charge. Space charge can therefore persist both within the bulk of an insulator and on its surface for long periods of time. This can lead to regions of intense electric field in the surrounding dielectric, producing localised breakdown resulting in the erosion of surfaces and penetration of the bulk by electrical trees. A nonlinear characteristic has been produced in the conductivity of polyester resin by the introduction of an appropriate quantity of particulate ZnO. The critical filler/resin ratio has been determined above which the insulation resistance is high at low fields but decreases as the electric field increases. Alternative fillers, such as Al2O3 and SiC, have been incorporated into the resin for comparative purposes and to elucidate the conduction mechanisms. The enhanced conductivity at high fields in ZnO-filled polyester permits the dissipation of accumulated space charge at the extremities of electric tree channels as they impinge on barriers of this material, thereby reducing the local electric field. As a result of this phenomenon, tree growth and barrier penetration are inhibited, leading to extended lifetimes.

Architectural and protocol frameworks for multicast data transport in multi-service networks

The paper presents architectural and protocol techniques for multicast transport of multi-service data (such as video, audio and graphics). A tree-structured channel in the network is employed as building block for constructing multicast transport services. Data from different sources are multiplexed over a shared tree channel for reaching destinations through intermediate network nodes and links. The choice of multicast architecture is based on the extent of sharing of common links in a tree across multi-source data flows to allow finer degree of link bandwidth allocation control and reduce the fixed cost of links for data transport. The canonical network substrate so constructed exemplifies a 'programmable network' that may be 'plugged-in' with QOS and flow parameters to instantiate the network behavior for matching the needs of each application. This philosophy is in alignment with the evolving 'Internet Service Layer' functionalities. The paper describes the functional and protocol elements of multicast architectures to support the 'programmable network' view. It also describes the salient features of an implementation embedding these architectures.

Auto-correlation function of PD pulses under electrical treeing degradation

Pulse correlation in PD signals was analyzed with auto-correlation function of PD pulses occurring in tree channels at 50 Hz ac voltage application as well as PD pulses occurring in several simple model electrodes. It was clarified that the auto-correlation function of PD pulse trains was strongly affected by void shape including tree length. It was found that even simple model electrodes, which showed very similar /spl phi/-q distribution patterns to those of tree channels, had different auto-correlation functions from those of PD pulse trains of tree channels. The results suggest that the auto-correlation function may give additional information about the PD mechanism to that given by the /spl phi/-q distribution patterns.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Interface Effects on Tree Growth of Cross-linked Polyethylene

It is well known that treeing is one of the main causes of breakdown of CV cables. It has been also considered that the interface exists between two solid insulation materials become a weak point on the electrical strength of CV cables. Therefore, it is important to reseach on the tree growing properties when there exists an interface. For evaluating the interface effect on the AC tree growth, an experimental investigation has been carried out by preparing an interface between two cross-linked polyethylene(XLPE) blocks. This paper describes the experimental results under various interface conditions. It was found that the interface had a great influence on the tree growth. The time to breakdown was reduced by the interface in the present test conditions. Painting the silicone oil on the interface, not only reduced the time to breakdown but also prevented the breakdown through the surface of spec imen(B). And the pressure applied on the interface had a slight effect on the time to breakdown but on the tree growth pattern. With the increasing of applied voltage, phenomena above mentioned were more significant It is proposed that the discharge column activities and the gas pressure reduction in the tree channels play important roles in the present situation.

Charge deposition in gas filled channels with insulating walls

Solid insulation suffers from the formation of treelike structures of narrow gas filled channels originating from points of electrical stress concentration. Although the growth of these channels, and the development of the characteristic tree structures, have been the subject of many observational studies, the mechanisms involved have yet to be elucidated. A significant observation is that, in their early stages of development, the channels have a high electrical strength. This has been assigned to the effects of charge trapping by the surrounding insulation, leading to avalanche suppression partly by the creation of back fields. The work described establishes the presence and distribution of deposited charge in asymmetrically stressed synthetic tree channels cast and moulded in polymeric insulation. Charge trapping in straight open ended channels was studied at the outset. But practical tree channels are rarely straight. They usually develop at an angle to the symmetrical axis of the field, contain numerous bends, and are closed at their extreme end by insulation. The effect of each of these aspects was studied. The results obtained help to explain some of the phenomena associated with tree growth, and go one step towards a complete understanding of the mechanisms underlying the growth of tree channels and structures in solid insulation.

Electrical and water treeing: a chairman's view

The essentials of the reports on electrical and water treeing presented at the 3rd ICPADM are summarized. Starting from these findings and including generally accepted facts of the phenomena as well as results published recently, the corresponding model concepts of the two phenomena are described. It becomes evident that bond scission is the decisive point in both deterioration mechanisms. In the case of electrical treeing very high electric fields create hot electrons and/or high energetic photons that cleave molecular bonds and produce radicals. Small voids and tree channels are generated in which gas discharges occur. In the case of water treeing the precipitation of water droplets leads to a local mechanical overstressing of the neighboring polymer chains and finally to their disruption. Depending on the availability of foreign substances, e.g., oxygen, additional chemical reactions may occur and influence further development of the trees. >

On the penguin phases and missing CP asymmetries in exclusive hadronic B decays

Within the standard model, we elucidate the picture of exclusive hadronic B-meson decays, which occur through both the tree and penguin channels. We emphasize that the weak penguin phase is entangled with the strong one and they can be parted only in special approximation. Ignoring this point will lead to significant missing of direct CP violation in either charged-B or neutral-B decays. The effects of these phases on CP-violating partial rate asymmetries are examined, and the relevant ambiguities in the previous literature are clarified.

DC Tree Associated with Space Charge in PMMA

DC tree initiation and propagation and treeing phenomena in PMMA under ramped voltages have been investigated by a optical method using a photomultiplier and a close-up camera. The Weibull plot of the initial tree inception voltage for a positive point had a knee at about 35 kV, above which the slope was lower, while the plot for a negative point was linear. The succeeding negative streamer originated from the tips or stem of tree channel as well as from the point cathode, while the succeeding positive streamers originate from only the point anode.

Characteristics of discharges inside simulated tree channels under impulse voltage

The authors attempt to characterise the electrical discharges initiated in hollow filaments of small diameter in low-density polyethylene. This study is related to electrical treeing, a pre-breakdown phenomenon in certain polymers submitted to high electrical stresses. A positive single pulse voltage was applied to a channel prepared in a polyethylene specimen. The channel diameter ranges from 1 mm to 13 mu m and the gap from 1 to 5 mm. The characteristics of the discharges growing in the channels were recorded and compared with free air discharges under the same conditions. It is shown that the channel walls disturb the discharge below a critical value of the ratio between the channel diameter ( Phi ) and the gap (d). A quicker heating of the gas is assumed to explain the behaviour of these confined volumes.

Comparison of Single and Double Needle Tests for Evaluation of Resistance to Treeing Breakdown

This paper describes results on treeing breakdown in organic insulating materials using diverging fields formed by two different types of electrode geometries. These were needle/plane (SNT) and needle/needle (DNT) systems. Characteristic voltage, voltage life, and short-time breakdown tests were performed so that a comparison between SNT and DNT could be made. Low-density polyethylene (LDPE), cross linked polyethylene (XLPE) and epoxy resin (EX) were used. The result on characteristic voltage test shows that the relationship of characteristic voltage is as follows: EX > XLPE > LDPE and DNT > SNT For voltage life tests, the same relationship as the characteristic voltage is found. On the other hand, the relationship of short-time breakdown voltage among three kinds of specimens is given by LDPE > XLPE > EX and DNT > SNT For the voltage life test with SNT and LDPE, the trees spread out with many branches to produce a bushlike shape. With DNT, the trees grow along the electrode direction with fewer branches. The tree shape in XLPE is similar to that in LDPE. For EX, the tree channels seem to be narrow tubes.

An Investigation into the Mechanism of Water Treeing in Polyethylene High-Voltage Cables

Water treeing is a well known fracture phenomenon in polyethylene (PE) and crosslinked polyethylene (XLPE) high voltage cable insulation. We studied two phenomena: (1) The insulation material is mechanically fatigued by as is shown by measuring the relevant constant, which moreover is compared with literature values on other polymers. The observed electrostriction is found to originate mainly from the Coulomb force. Using a simple model, the deformation mode at the end of a water tree channel can be described and the strain amplitude of the PE at the water interface can be calculated. (2) Under high voltage, the surface tension at the PE-water interface decreases strongly as we found experimentally. The observation that this decrease is proportional to the square of the applied field strength is explained. The joint effect of the Coulomb force and the reduced surface tension may cause a fracture phenomenon, well known in the field of polymers, called environmental fatigue failure (or thermal softening). The same assumption is made as in common environmental stress cracking of PE with aqueous detergents, viz. that the low surface tension plays an essential and not a coincidental role.

The Space Charge Behavior and Luminescence Phenomena in Polymers at 77 K

Electroluminescence phenomena were observed at the needle tip in polymer samples with a point-plane electrode system under an applied ac voltage at liquid nitrogen temperatures. This luminescence was considered to be caused by the recombination between positive and negative charges injected from the point electrode. The nature and behavior of the charges injected into the polymer became clear through a detailed study of luminescence. A degenerate rate region of polymer was also found at the needle tip after prolonged ac voltage application, which may indicate a depolymerized region caused by the attack of then injected charges. The fact that tree channels started from the degenerate region in the polymer led us to theon conclusion that the injection of charge can be hazardous for polymer solid insulation systems.

Pseudo-Spherulite Structures in Cross-Linked Low-Density Polyethylene

This paper describes evidence for the formation of secondary fissure structures, in low-density polyethylene (PE), and in cross-linked polyethylene (XLPE). These structures can be observed by optical and by scanning electron microscope techniques. The secondary fissures are not a volume effect like spherulites, but are associated with the fracture surface only. An explanation of this effect is given, as well as its relation to the observed electrical discharges in tree channels.

The Development of Electrical Discharges in Simulated "Tree" Channels

The development of spark discharges in 5 x 10-3 cm diameter cylindrical tubules of 0.5 cm length bounded by solid dielectric has been studied by making the conventional electric strength measurements and by using a self-triggered high speed camera which possesses a temporal resolution of 5 ns and a spatial resolution of about 50 line-pairs per mm to obtain sequential photographs of the prebreakdown luminescence accompanying voltage collapse. The photographs verify the deductions made from the information obtained from the electric strength measurements: the walls of the tubules interfere with the breakdown processes in the confined gas and collect considerable charge. This results in an electrostatic field which appreciably distorts the applied field. It is evident that the electric strength of tree channels under unidirectional voltage waveforms can be very high, approaching that of the solid insulation. It is concluded that the mechanisms of trapping and decay of charge in a solid insulation are important factors in assessing its resistance to tree penetration, and that the channels of electrical trees cannot be an effective extension of the electrodes into the insulation as is almost universally assumed.