The increasing application of SF6 as an insulating gas has led to many studies on SF6 decomposition in gas-insulated equipment. In the presence-of an electric arc, spark or corona, SF6 decomposes to a wide variety of chemically active products which possess completely different properties from SF6. The accumulation of these decomposition products in the equipment has caused concerns regarding personnel safety and material compatibility problems. This paper reviews previous research in SF6 decomposition relating to the operation of gas-insulated switchgears, gas-insulated transmission lines, and electrostatic accelerators. Results on the qualitative and quantitative determination of the by-products and their formation ion rates in various modes of electrical discharges are summarized. The mechanisms leading to the formation of transient and stable products are described. In particular, the influence of discharge energies and impurities on the formation of SOF2 and SO2F2, the two dominant stable by-products, is discussed. The effects of the by-products on personnel safety and equipment ent dielectric integrity are presented. The application of SF6 gas analysis as a tool for diagnosing the internal condition of gas-insulated equipment is assessed. Based on the results of many phenomenological observations, future research activities are suggested to address the issues of safety, compatibility and equipment aging. More fundamental studies on electron, ion, and neutral reaction rates in an SF6 discharge are required to gain a better understanding of the decompositon mechanisms and the influence of the products on equipment operation.

SF6 Decomposition in Gas-Insulated Equipment

A critical review is given of the degradation of cellulose in the low-temperature region (below about 300°C) of power transformer operation. The large number of kinetic studies, under a variety of environmental conditions from Kraft paper in insulating oil, to cotton and paper in oxygen, are considered in terms of a first-order polymer chain scission model. In many cases, the data are replotted to suit the model. A common activation energy of 111±6 kjmol−1 is calculated and it is shown that the pre-exponential factor, rather than the activation energy, is sensitive to the oxidizing nature of the environment and the susceptibility to degradation of the material. The chemical mechanisms of degradation are reviewed, and conclusions and recommendations are made regarding chemical condition monitoring and life prediction of electrical insulation.

Kinetics and mechanisms of the low-temperature degradation of cellulose

This paper covers the basic phenomena involved in the aging of capacitors constructed using polymer films as the major dielectric material. A wide variety of aging phenomena can be identified, ranging from simple electrical, dielectric, thermal, radiation, chemical and physical aging, to aging originating from the specific construction of the capacitor. Some phenomena occur quickly, such as some morphological changes in the polymer, others occur progressively, and others occur later in the life of the capacitor. The polymers covered range from biaxially-oriented polypropylene, which is widely used in commercial utility and industrial capacitors, to polymers like polycarbonate, polysulfone, polyimide, and polyvinylidene fluoride, which are used in high-energy density, outer-space, high-temperature, or other special applications. The paper addresses two constructions of polymer film capacitor where the aging phenomena are significantly different due to the differences in construction: polymer/foil capacitors and metalized film capacitors. Metalized polymer film capacitors have the quality of clearing or self healing that is not present in film/foil capacitors. The discussion of aging in this paper is given from the perspective of these two designs. >

The fundamentals of aging in HV polymer-film capacitors

A link is drawn between paper hornification and low temperature thermally-accelerated ageing of paper based on a review of existing research. Similarities between treatment temperature, humidity, resultant physical properties, and possible degradation mechanisms are enumerated. Hornification can be explained by irreversible intra-fibre hydrogen bonding, whereas accelerated ageing is normally attributed to chain scission and cross linking. However, in the light of the similar experimental protocols used to investigate each, the possibility of both mechanisms occurring in both experiments should be considered. The short timescales of hornification would seem to preclude chain scission and cross linking, but it seems possible that irreversible hydrogen bonding occurs during dry accelerated ageing.

A Review of the Relationship Between Thermally-Accelerated Ageing of Paper and Hornification

An axial flow turbine provided with a stage composed of a turbine nozzle and a turbine rotor blade arranged in an axial flow direction. Both end portions of a nozzle blade of the turbine nozzle are supported by a diaphragm inner ring and a diaphragm outer ring, and a flow passage is formed to have its diameter expanded from an upstream stage to a downstream stage. In such axial flow turbine, trailing edges at ends of the nozzle blade supported by the diaphragm inner ring and the diaphragm outer ring are curved as a curvature to an outlet side, and an intermediate portion between the trailing edges is formed to be straight.

Axial flow turbine

Insulating papers and pressboards must have high dielectric strength and high tensile strength. However, these materials are gradually degraded due to thermal stress, oxygen, and moisture. Because the tensile strength decreases owing to degradation, the degradation of insulating material may affect the life of transformers. In our laboratory, through accelerated tests using models of oil-impregnated insulating systems, changes in characteristics of insulating cellulose materials have been investigated. These data were compared with data obtained from insulating papers of transformers with long service life. From these investigations, good correlation was found between the amount of gas generated from insulating papers in insulating oil and the retention of tensile strength and of degree of polymerization. Using this correlation, the degree of degradation of insulating papers in transformers may be known from the amount of gas. The average characteristic curves of insulating papers from transformers coincides with the degradation curve of 90 ° C ob tained in experiments. And if the life of insulating papers is considered to be equal to the life of transformers, the life of transformers is between 20 and 40 years.

Degradation of Insulating Materials of Transformers

Two dielectric relaxation peaks, the crystalline and the primary relaxations, were observed in the temperature-dielectric loss curves in isotactic polypropylene. Among them the primary relaxation s was studied in detail using different kinds of polypropylene films and their films purified with acetone. It was found that s-relaxation consists of two peaks, denoted s1 and s2, being consistent with the dynamic mechanical characteristics. The lower temperature peak s1 is assigned to the amorphous phase, which is rich in atactic chains, and the upper temperature peak s2 is ascribed to an antioxidant contained in the amorphous phase which is rich in isotactic chains. The antioxidant also causes an increase in dielectric loss in the higher temperature region. The extraction of impurities from polypropylene films with acetone reduces the magnitude of the primary relaxation peaks.

Impurity Effect of the Dielectric Properties of Isotactic Polypropylene

The moving blades are configured so as to meet a condition: (L+3D)×f≧14,500 or L×(L+D)×f 2 ≧1.55×10 7 , where L is an effective length of the moving blade measured in inches, D is a blade root circle diameter of the moving blades measured in inches, and f is the rated operating speed of the turbine measured in Hz. The moving blades are formed of a titanium alloy. The dovetail has an Christmas-tree shape, and is a curved-axial-entry. The moving blades are configured and arranged so as to meet a condition: 3.5≦P/W≦7.0 or 0.8≦P/C≦1.0, where w is a width of a section of a tip of the moving blade measured along an axis of the rotor wheel, P is pitch of the moving blades, and C is a chord length of the tip section of the moving blade. Adjacent moving blades are connected to each other in such a manner that the adjacent moving blades being capable of relative movement while the adjacent moving blades being connected to each other.

Moving blades for steam turbine

In an axial-flow turbine, at least one of front and rear side contact surfaces of shrouds (3a or 3b) of blades (1a or 1b) with respect to the turbine rotational direction is formed at certain angle with respect to a radial line connecting the rotor center and the contact surface. The shroud (3a) of the blade (1a) of a first kind is formed in a trapezoidal shape converging radially outward in cross section taken in a plane perpendicular to the turbine axial direction, and the shroud (3b) of the blade (1b) of a second kind is formed in an inverted trapezoidal shape converging radially inward in the cross section. Further, half of an angle (2α) between the front and rear side contact surfaces of the shrouds (3a or 3b) is made smaller than a static frictional angle of the contact surface. Since the shroud contact surfaces of two adjacent blades can be kept in pressure contact with each other under all operating conditions, a large dynamic stress reduction and superior damping properties can be obtained without producing excessive initial stresses at the blade airfoil and blade dovetail attachment portion.

Rotor blade damping structure for axial-flow turbine

According to one embodiment, a video server includes a first storage area, a second storage area, and a controller. The controller is configured to control to execute a first standby operation for reading the first content item from the first storage area in accordance with a standby instruction from a host apparatus, to execute a second standby operation for reading the second content item from the second storage area in advance when the first content item is deleted to change the first content item in a state of the first standby operation, to delete the first content item when the second standby operation is complete, and to skip deletion of the first content item when the second standby operation is not complete.

Toshio Suzuki

Papers

Degradation of Insulating Materials of Transformers

Impurity Effect of the Dielectric Properties of Isotactic Polypropylene

Moving blades for steam turbine

Rotor blade damping structure for axial-flow turbine

Video server and standby method