Shoji Hirabayashi

Thermal depolarization current study of polypyromellitimide

Thermal depolarization current (TDC) of polypyromellitimide film was measured in order to obtain knowledge of the charging and discharging mechanisms of carriers in polypyromellitimide. Two peaks are observed at 173 (α) and 50 °C (β) in the dried film. The β peak attributed to dipole orientation polarization is probably associated with residual reactive groups in polypyromellitimide. The α peak shows nonlinear dependence on polarization electric field Ep. The sign of the current changes with small collecting electric field Ec which is applied during the TDC measurement. The α peak depends on electrode materials. The α peak is considered to be attributed to space‐charge polarization due to trapping of electrons injected from the cathode in the vicinity of the polymer film surface. The nonlinear Ep dependence of the α peak can be described by a two‐site hopping model.Thermal depolarization current (TDC) of polypyromellitimide film was measured in order to obtain knowledge of the charging and discharging mechanisms of carriers in polypyromellitimide. Two peaks are observed at 173 (α) and 50 °C (β) in the dried film. The β peak attributed to dipole orientation polarization is probably associated with residual reactive groups in polypyromellitimide. The α peak shows nonlinear dependence on polarization electric field Ep. The sign of the current changes with small collecting electric field Ec which is applied during the TDC measurement. The α peak depends on electrode materials. The α peak is considered to be attributed to space‐charge polarization due to trapping of electrons injected from the cathode in the vicinity of the polymer film surface. The nonlinear Ep dependence of the α peak can be described by a two‐site hopping model.

Thermal depolarization current study of polystyrene composites containing mica flakes

Thermal depolarization current (TDC) of a cross‐linked polystyrene composite whose conductive phase is mica has been measured in order to study in detail interfacial polarization (Maxwell‐Wagner effect) in polymer composites with stratified structure. The volume fraction of mica flakes in the composite was varied from 0.16 to 0.37. A peak characteristic of the composites is observed at a temperature above the Tg of the composites. The maximum current of the peak increases with the fraction of mica flakes. The behavior of the peak is different from that of TDC due to dipole polarization and the space‐charge polarization of injected electrons. The beginning of a dielectric dispersion corresponding to the peak is observed below 1 Hz at 164 °C. The TDC peak can be explained by Maxwell‐type interfacial polarization. It is proposed that the TDC peak is attributed to interfacial polarization due to trapping or localizing of conductive ions in mica flakes at boundaries between the polymer matrix and mica flakes.

Phenomena and Mechanisms of Tree Inception in Epoxy Resins

The inception of ac treeing has been investigated in various cast epoxy resins, using a needle-plane test configuration. Mechanical stress due to shrinkage in the curing process is present at the needle tip, and in some cases causes micro-cracks. Lower tree inception voltages were observed in mechanically stressed specimens indicating that the micro-cracks act as weak points. In less stressed samples which have higher tree inception voltages light emission, probably electroluminescence, was observed at a voltage much lower than that of tree inception. In those cases, a small discolored area (about 10 ¿) appeared prior to tree inception as the ac applied voltage was increased.

Mechanical Fatigue Characteristics of High Voltage Generator Insulation

This paper presents the deformation and fracture behavior of stator windings subjected to static and cyclic bending loadings, and the reliability of newly developed insulation systems. The flexural deformation characteristics are revealed on the curved end portion of the stator windings for a 1300 MVA water cooled generator. The fatigue characteristics are examined on the test bars with three different kinds of the developed insulation systems. Comparing those results with calculated thermal strain, it has been confirmed that the insulation systems have sufficient mechanical strength to withstand thermal cycling in service.

Partial discharge deterioration of epoxy resin for electronic parts

Recently many electronic parts are widely used for the power electric field, such as power transistor module, diode and solid state relay. These are encapsulated with solid insulating materials such as epoxy resin or silicon rubber. In these category of parts, majority of electrical failures has been caused by inadequate material selection or workmanship error for encapsulation systems. It is suspected that the cause of the electrical failures is due to partial discharge (PD) in voids of insulating materials or in cracks in the vicinity of conductors. Accordingly, the test on PD has been performed for these products. Although PD quantity (e.g. PD current, Qmax), decreases gradually during voltage application with ordinary electrical PD measurements, while solid insulating materials degrade along with time and finally result in electrical breakdown [1,2].

A New AC Current Testing Method for Non-Destructive Insulation Tests

A new nondestructive diagnosis method is developed mainly for stator coil insulation of high voltage rotating machines as a result of analysis and experience on the behavior of a current of a sample under ac voltage application. A newly-defined parameter Y, measured in this method, equals tan, plus y, where y closely relates to void volume ratio in a sample. A new apparatus to measure Y automatically is also developed which shows experimentally several advantages in a practical test. Y-V patterns and y-V patterns by this method are verified to correlate well to insulation condition and residual breakdown voltage of a sample.

Improved Potential Grading Methods with Silicon Carbide Paints for High Voltage Coils

This paper presents surface potentials and the accompanying thermal analysis on, two improved potential grading methods with silicon carbide (SiC) paints for HV generator coils: (1) double-coating of SiC layers along a coil and (2) combination of a conducting intersheath and a SiC paint. Approximate equations describing the thermal behavior for both methods are derived on the assumption of ideal nonlinear conductivity. The double-coating method shows effective regulation of local heating, though total heating is not suppressed. A simplified model of the combination method predicts a remarkable heat suppression effect, which also is verified with numerical calculation.

A new corona suppression method for high voltage generator insulation

1. The breakdown voltage obtained of a new system with SiC double layers is 40% higher than that for single layer of commonly used system.

Suppression of Local Heating on a Silicon Carbide Layer by Means of Divided Potentials

A new method was investigated for suppression of local heating of a silicon carbide layer used for field grading on stator coil ends in HV rotating machines. By means of synchronous divided potentials at proper positions on a SiC layer, surface current bypasses through the electrodes of the divided potentials, which prevents thermal runaway of the SiC layer during ac dielectric strength tests of coil insulation. Since SiC shows highly nonlinear resistivity, approximate formulas of heat distributions are discussed in detail on the assumption of ideal nonlinear resistivity. The formulas are compared with the results of numerical calculation and experiments.

Development of new mica paper epoxy insulation systems for high voltage rotating machines

Better insulation performance is persistently required for high voltage rotating machines with the trend of increasing capacity and rated voltage. Resistance to thermal shock, mechanical reliability and dielectric strength of insulation are especially important. As for the impregnating varnish, asphaltic compound has been replaced by synthetic resins with progress in the chemical industry. Unsaturated polyester and epoxy resins are the most popular resins, the latter becoming more important because of their overall suitability. On the other hand, the main barrier insulating material tends to move toward reconstituted mica from mica splittings. In this paper, electrical and therm omechanical performances of newly developed epoxy insulation systems based on mica splittings and reconstituted mica are compared with asphaltic compound and unsaturated polyester insulation systems based on mica splittings by extensive thermal cycle tests. Superior characteristics of epoxy insulation systems are proved.