Stator Coil Insulation Research Articles

Charge Transport and Trapping of Surface Modified Stator Coil Insulation of Motors

This paper presents the surface modification and charge transportation mechanism of thin polyimide films. Here, we demonstrate that the surface modification of these films, by using the fluorine gas-phase, improves the charges mitigation and, thereafter, suppress the space charges. The experimental results indicate that the thermal step current for fluorine coated films reduce significantly in comparison to pure polyimide films. The thin layer of fluorine coating helps to release the surface charge, reduce the charge injection from the electrode, provide hindrance for charges to penetrate into the bulk of the films and the higher degree of charge disappearing helps to reduce the space charge growth.

Surface modified nano-SiO2 enhances dielectric properties of stator coil insulation for HV motors

Nano-SiO 2 particles were doped into epoxy resin (EP) to fabricate the main insulation used for high voltage (HV) motors. The EP-based nanocomposite with different mass ratios of nano-SiO 2 particles (0.5, 1, 2.5, and 5 wt%) were prepared. Surface morphology and conductivity of samples were tested, and AC breakdown strength was measured at different temperatures. The mechanisms for the tailoring of dielectric properties of EP were discussed. The optimal doping scheme (0.5 wt%) was selected and used as the base material in preparation of a stator coil insulation for 6 kV motors. The dielectric properties, including dielectric loss and leakage current as well as partial discharge, were tested, and the potential application of nano-SiO 2 particles in the stator coil insulation of HV motors was discussed. The test results showed that the doped nano-SiO 2 particle into dielectric composite used as the main insulation material for HV motors can be an optimal candidate to improve the above mentioned properties especially dielectric losses and electrical breakdown strengths. The results in this paper offer references and a guiding significance for the application of nano-SiO 2 in the insulation of HV motors.

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.

Aging Properties of Mica Insulation Systems Impregnated with Solventless Resin for Generator Stator Coils

In these days, the solventless resin-impregnated mica insulation systems are extensively adopted to the large powergenerator stator-coil insulation systems as a result of rapid development of synthetic resin in recent years. Various destructive and nondestructive tests with these samples were made. The electrical properties were compared between the initial coils (which were measured by the electrical properties at the acceptance test) and the used coils (which were taken out from the generator winding that was used for about ten years). It is said that the winding insulation systems will change during long running times because of a number of factors such as thermal, mechanical, and electrical. Studies by the present authors show that used coils were not deteriorated by mechanical stress because electrical breakdown voltages did not decline in comparison with those of the initial coils, and also they were not deteriorated by electrical stress because the breakdown voltages of coils at the line side were lower than at the earth side. Therefore, some changes of the electrical properties in the nondestructive test seemed to result from thermal effects. As mentioned above, it is proved that solventless resin-impregnated mica insulation systems have kept good electrical properties in service for over ten years.

Pyrolysis of Stator Coil Insulation in Air and Hydrogen

The study of changing properties of generator stator coil insulation on long-time exposure at elevated temperature is an accepted part of insulation development. This aging, however, has usually been done in air, while the windings of modem turbine generators operate in hydrogen atmosphere. Samples of synthetic resin bonded insulation were pyrolyzed in air and hydrogen at 200°C and one atmosphere. The gaseous, liquid, and residual products were analyzed by chromatography and infrared spectroscopy. Heated in air, the gases contained C3, C4, and C5 straight chain hydrocarbons, but the products in hydrogen contained, in addition, cyclohexane. The liquid and residual products indicate the sample pyrolyzed in air degrades by depolymerization and oxidation, while the sample in hydrogen depolymerizes, followed by some degree of hydrogenation.

Physical Effects of Thermal Cycling on Stator Coil Insulation of Turbine Generators [includes discussion

Physical Effects of Thermal Cycling on Stator Coil Insulation of Turbine Generators [includes discussion

An examination of high-voltage d.c. testing applied to large stator windings

The merit of a proposed non-destructive method of assessing the breakdown voltage of stator-coil insulation by means of high-voltage direct current is examined. The proposal is to measure the steady-state leakage current at successive increments of applied voltage and predict a breakdown value from the trend of the plotted results.Laboratory tests applied to a range of specimens of sheet insulation, including a number with artificially-formed faults, indicate that the only breakdown which can be predicted by this method is that across air paths external to the specimen.Tests carried out on a complete high-voltage stator winding and on single stator coils give similar results. No assessment of slot-insulation quality is possible: the only increase in current, suggestive of approaching failure, results from end-winding leakage and discharge effects. Surface contamination of the end-winding and even quite extraneous discharges, when present, have a marked influence on the current measurements.It is concluded that the high-voltage d.c. method of testing investigated has no valid basis for non-destructively indicating the service-ability of the insulation of high-voltage machines.

Routine Insulation Testing of Synchronous Machines

THE CONTINUOUS operation of large synchronous machines and freedom from failures in service is of vital interest to every electric utility company. One of the major causes of unpredicted and unscheduled outage is the failure of stator coil insulation.

Routine insulation testing of synchronous machines

THE CONTINUOUS operation of large synchronous machines and freedom from failures in service is of vital interest to every electric utility company. One of the major causes of unpredicted and unscheduled outage is the failure of stator coil insulation.