Hisayasu Mitsui

Mechanical Degradation of High Voltage Rotating Machine Insulation

Several kinds of insulation systems, which were developed for high voltage coil insulation for motors and generators, are examined for insulation damage caused by mechanical stresses. Tests have been carried out on electrical properties under bending stress, i. e. the critical deformation characteristics which are defined as the maximum deformations able to withstand a fixed proof voltage, the fatigue characteristics under repeated bending stresses, and the voltage life characteristics under bending stress. Breakdown voltage, BDV, decreases with an increase of bending stress. Moreover, a remarkable increase of the base tan6 value, tan6o, and tan6 tip-up can be seen due to the delamination of mica-splittings, especially in the case of the insulation using mica splitting. An insulation system in which a flexible binder is used has a far greater critical deformation. The decrease of residual breakdown voltage RBDV, and the increase of tan6o and tan6 tip-up are seen at less repeated bending stress in the insulation system with mica-splitting tapes than in the insulation system with only mica-paper, because of the weakness of mica-splittings under repeated bending stress. Effects of the temperature on the repeated bending fatigue characteristics from -55 to 1550C are greatly influenced by the binder of the mica tape and/or the impregnating resin. Moreover, voltage life under a static bending stress becomes extremely short beyond a certain stress. Lastly, experimental equations are proposed for estimating the relationship between RBDV and the numbers of repeated bending cycles, and the voltage life under a static bending stress.

Electrical Failure Properties of Cast Epoxy Resins

Impulse and ac breakdown properties of transparent cast epoxy resins are reported having either a castin needle electrode or a needle-shaped recessed gold film electrode deposited by vacuum evaporation or sputtering, given a divergent electric field. Dielectric breakdown always initiates from the needle tip where the electric field strength is the highest. The dielectric breakdown process, which consists of tree initiation, tree propagation, and the complete puncture of the cast epoxy resin is investigated. This process varies considerably with the wave form of the applied voltage. Chemical analysis by Fourier transform infrared spectroscopy (FTIR) indicates that chemical reactions caused by voltage application take place in the epoxy during the induction period until the initiation of the tree. The dielectric breakdown strength is considerably influenced by the electrode configuration, and also by resin formulation.

Thermal Cyclic Degradation of Coil Insulation for Rotating Machines

Thermal cyclic degradation of epoxy micaceous insulation for ac high voltage generators and motors was investigated. From the experimental results, it is made clear as follows. Thermal cyclic degradation begins with a separation of the innermost main insulation layer from the strand insulation at the end of the iron core. It propagates in a mode of mica delamination and the formation of cracks. This degradation condition agrees well with the measurement results of non-destructive insulation tests using divided electrodes. The compatibility between a strand insulation and a main insulation greatly affects thermal cyclic degradation. Thermal degradation cannot be ignored under certain thermal cycle condition. Thermal cyclic degradation increases with an increase in the peak conductor temperature. Even in the case of a post impregnated insulation system where the coil insulation adheres to the iron core, the thermal cyclic degradation is little for a core length below 1.4 m and a 160 °C hottest conductor temperature. Non-destructive insulation diagnoisis of hydrogen filled water-cooled turbine-driven generator insulation shows little thermal cyclic degradation after approximately 15 years normal operations. Lastly, it is proposed that a thermal cylce test under voltage application is important hereafter, to grasp the degradation of actual machines precisely.

Water Treeing Phenomena in Humid Air

Water treeing is a phenomenon which causes organic insulating material to degrade under electrical stress when moisture is present. Papers have been published on water treeing, for instance, those on laboratory experiments through acceleration test methods. In all these reports, some part of the specimen was exposed to water. In the experiment described in this paper, specimens were exposed only to water vapor, not water in the liquid state. A part of the experimental results already has been reviewed and presented by Nunes and Shaw [1]. This paper discusses the problem of water treeing in humid air in detail.

Some Considerations on AC Water Trees in Crosslinked Polyethylene

This paper first confirms that moisture absorption of XLPE (crosslinked polyethylene) obeys Ficks law. The effects of moisture absorption and temperature on water treeing in XLPE are then discussed. The results of physical and chemical tests of the sites of water trees are presented. Infrared Fourier Transform Spectro-scopy (FT-IR) test results show the chemical aspects of the deterioration. Finally, the authors suggest that the water tree is due to a shift in chemical equilibrium (free energy) of the insulating materials due to the impressed electric field.

Effect of epoxy cracking resistance on the stability of impregnated superconducting solenoids

The stability and quench positions of superconducting test solenoids impregnated with three types of epoxy resin, with different crack resistances, have been studied. Voltage signals and acoustic emission (AE) were monitored during training tests. Premature quenches in the solenoids almost occurred at the innermost layer of windings; voltage spikes and AE appeared just prior to the premature quench initiation. This suggested that premature quenches were due to friction or debonding between ground insulation and winding. The solenoid with a release film between ground insulation and winding showed high stability and reached critical current at the second charging. From the results, the effects of epoxy cracking resistance on the stability of impregnated superconducting test solenoids are discussed.

Influence of Mica Tapr Application on Insulation Characteristice of High Voltage Rotating Machinery Coils

This paper studies the influence of mica tape application methods on the insulation characteristics of HV rotating machinery coils generally. Resin impregnability , electrical failure properties and mechanical properties were tested on specimens prepared by sheets or tapes with different widths. From these test results, it was clarified that these insulation characteristics were influenced significantly by the mica tape application methods. This influence was caused by resin impregnating the interfaces between the mica tapes and thus weakening the resistance of the tape to electrical and mechanical stresses.

Several Properties of Impregnating Epoxy Resins Used for Superconducting Coils

In order to find the most appropriate vacuum-pressure impregnating epoxy resin for superconducting coils, three types of epoxy resin were experimentally investigated from various points of view. Those epoxy resins were amine curing type A, anhydride curing type B with tertiary amine accelerator, and anhydride curing type C without accelerator.

Insulation Effects of Hydrogen Gas for Cooling Turbine-Driven Generators

Endurance against surface discharge of epoxy resin and epoxy resin-impregnated mica composites were examined. Voltage endurance properties of epoxy micaceous insulation with open (ventilated) voids and closed voids were examined. Effects of compressed hydrogen gas on creeping discharge suppression were also examined in a high voltage rotating machine winding. Furthermore, thermogravimetric analysis (TGA) was done on epoxy micaceous insulation and wire enamel in an atmosphere of hydrogen, nitrogen, and air. From these test results, advantageous effects of compressed hydrogen gas for cooling turbine-driven generators were clarified.

Proposal of GF (Gas Filled) Insulation for Rotating Machines

Gap or void discharge and Creeping discharge were measured in several kinds of gases at different pressures with model specimens, such as a gap model between two parallel plates of glass, a model of single plate of glass, epoxy-mica insulation coils, and a motorette degraded by a series of functional test., From the experimental results, it is made clear that discharges in open type (ventilated) void and creeping discharges are remarkably suppressed with increasing the gas pressure, and discharges in high voltage rotating machine winding are also suppressed in compressed gas due to the transition of included voids from closed type to open type. The latter was confirmed by the measurements of a turbine generator after long service. These effects are most distinguished in SF6 gas and hydrogen gas above 1.5 atg was superior in the discharge suppression effect to the air at O atg. Moreover, the breakdown voltage of epoxy mica insulation containing many ventilated voids rises remarkably with increasing gas pressure, approaches to that of epoxy vacuum-pressure impregnated coil when SF6 gas at 6 atg is filled. Based on these data, the authors proposed the availability of GF (gas filled) insulation system in the rotating machine.