Saeed Ul Haq

Performance of nanofillers in medium voltage magnet wire insulation under high frequency applications

The paper presents the experimental results on accelerated aging of enameled wires with and without nanofiller in the coating under partial discharge (PD) activity. The residual life of the wires, and the surface roughness as measured by a scanning electron microscope (SEM), are used to evaluate the effects of the nanofiller. The surface erosion in the specimens without nanofiller is considerably greater, which is 1120-1270 nm, under pulse aging; whereas, in specimens with ~1%, by wt, fumed silica (SiO2) nanofiller it was measures to be 230-250 nm. An evaluation of the residual life of the wires, aged under high frequency AC waveforms, reveal that the wires with SiO2 achieved a life that is twice the life of conventional wires. The two-parameter Weibull distribution of dielectric strength data show that the value of the shape parameter, beta, increases from 2.6 to 11.4 for the coatings with nanofillers indicating that the material becomes more homogeneous and exhibits fewer defects. Finally, the paper discusses possible reasons for the reduction in surface erosion and improvements in breakdown strength of enameled wires filled with various types and concentrations of nanofillers

DC breakdown characteristics of high temperature polymer films

This paper shows the effect of electrode area on dielectric strength for different polymeric materials. In the investigation, four electrodes of different diameters are selected in the range of 12.5-50 mm. To develop a base of understanding, several commercially available dielectric materials of different thickness that have been studied are aramid paper (Nomexreg) polyimide film (Kaptonreg) Mylarreg polyester film, PTFE, Nomex-Mica, and Nomex-polyester-Nomex composites. The DC breakdown strength of these high temperature dielectric materials is investigated by using the two-parameter Weibull distribution. It is noticed that the lower thickness films are more susceptible to the mechanical damage during handling process, thus increasing the chances of having lower breakdown strength. This lower breakdown strength cannot be ignored for effective insulation design of electric power apparatus and for the high reliability that is desired in the sensitive devices such as microelectronics, fiber optics, and in outer space applications

Insulation Problems in Medium Voltage Stator Coils Under Fast Repetitive Voltage Pulses

In this paper, the degradation processes in stator-bar groundwall insulation that is subjected to steep-front voltage pulses, at different temperatures, are studied. The test conditions include pulse voltage amplitudes up to 6kVp and pulse switching frequencies up to 3 kHz in ambient temperatures of 23degC , 120degC , and 155degC . During the aging process, the surface temperature is monitored with an infrared camera to determine the regions with highest temperatures and the partial discharge (PD) activity. The visual examination of failed bars confirmed that the groundwall insulation in these regions is degraded.

Degradation of turn insulation subjected to fast repetitive voltage pulses

The paper describes the failure mechanisms of medium voltage inter-turn insulation as a consequence of space charge formation and partial discharge (PD) activity. In order to investigate the failure behavior, models of turn insulation are prepared with magnetic wires having polyimide and nano-filled coating. The ageing of such model bar samples is done under sinusoidal (60 Hz), fast repetitive unipolar voltage pulses and high frequency waveforms. The ageing tests are performed in the presence of PD activity. To establish the residual strength of bar samples, DC breakdown voltages are determined before and after the ageing. Experimental results reveal that the evaluation and analysis of such model study will be helpful for both motor and wire manufacturers to minimize ageing and to improve motor performance under medium voltage inverter-fed drive applications

Surge withstand capability of electrically and thermo-mechanically aged turn insulation of medium voltage form-wound AC stator coils

This paper investigates the surge (turn-to-turn) withstand capability of form-wound AC stator coils after subjecting them to electrical and thermal stresses imposed by voltage endurance (VE) and/or thermal cycling tests. Medium voltage (MV) form wound stator coils rated at 4.0, 10, and 13.8 kV were surge tested to failure after they were aged under VE testing in accordance with IEEE Std. 1553. Another set of sample coils rated at 3.3 kV were subjected to thermal cycling following the procedure of IEEE Std. 1310 and were then surge tested-to-failure. A third set of sample coils were subjected to the VE and thermal cycling tests before surge testing. Surge testing was performed in accordance with IEEE Std. 522 until the failure point is reached. The surge failure levels and waveforms for each sample coil were carefully analyzed to investigate the adverse effect of electrical and thermal stresses on the surge capability of the sample coils.

Aging characterization of medium voltage groundwall insulation intended for PWM applications

The paper describes an investigation into aging of medium voltage mica-based groundwall insulation when subjected to high frequency AC and steep-front pulse voltages. The test conditions adopted in this study involve 60 Hz, 3 kHz and 4 kHz AC frequencies, nanosecond rise time pulses with repetition rates up to 2000 pps, and at temperatures of 20 and 155 degC. The experimental data of time-to-failure in the accelerated life tests are presented by Weibull distributions. The parameters of the life model are developed that link the failure time to the amplitude and temperature of the applied voltages. The test method and the data processing adopted in this study are useful to characterize groundwall insulation in terms of improved vacuum pressure impregnation (VPI) and for applications at higher temperatures and frequencies

Evaluation of medium voltage groundwall insulation exposed to high frequency pulse voltages

Class-F 4.0 kV coils designed for medium voltage induction motors are aged under combined thermal and electrical stresses of 500 h at a constant temperature of 120degC and 200 h at a constant temperature of 150degC and stressed electrically at 60 Hz and with steep-front unipolar pulses of 40 ns rise time. The changes in the partial discharge activity and the dissipation factor as a function of time are reported. During ageing, the temperature profiles are recorded with an infrared camera as a means for determining the highest stressed regions. The experimental results show that pulse ageing is very severe when compared to 60 Hz ageing, despite using a lower pulse voltage, although the location where damage occurs on the groundwall insulation is different

Comparative study of IEC 60270 compliant instruments for partial discharge pattern acquisition

The petrochemical industry considers offline partial discharge (PD) measurement an important quality assessment for new medium voltage motor and generator stator windings. A recent edition of API 541 requires an instrument compliant with IEC 60270 for the measurement of PD on sacrificial stator coils, and proposes 100 pC (pico-Coulombs) for guidance on acceptance criteria until more data becomes available. Given the industrys faith in PD as an acceptance test based on a representative sample, it warrants closer scrutiny. The characteristic PD distribution pattern produced by each test instrument provides information on the origin of the discharges and therefore can be used to determine what type of defect has produced them. This paper shows how external factors such as instrument selection, testing environment, and time interval between tests can influence PD test results. Three different IEC-compliant instruments were used to measure PD from sample coils of the same design, under identical conditions. Each sample coil was modified to create the same imposed defect, with the same technique used to interpret the PD magnitude and distribution pattern produced in each case. Each instrument recorded a different charge magnitude and characteristic PD pattern. The variation in charge magnitude measured by each instrument exceeds the proposed API 541 acceptance limit, and the observed PD patterns did not accurately reflect the imposed coil defect. Offline PD measurement therefore lacks sufficient repeatability, reproducibility and accuracy to support a standard acceptance limit.

Analysis of times-to-failure of various turn insulations of form-wound coils under PWM voltage waveform

The paper presents the results of an experimental work to examine the times-to-failure of various types of turn insulations used in form-wound stator coils. A unipolar PWM waveform with a peak of 12 kV, rise time of approximately 270 ns and switching frequency of 3 kHz is used for aging. The types of insulations studied include enamel, Daglas®, mica tape with polyester film, and enamel plus mica glass tape. The time-to-failure data are analyzed using two-parameter Weibull statistics. The breakdowns are examined microscopically providing information as to the probable reason for the failures. The role of each type of insulation; enamel, Daglas® and mica polyester film tapes, and their dimensions are discussed to explain the times-to-failure under PWM aging.

Electrical conduction processes in polyimide-teflon/spl reg/ FEP films - II

Extensive investigations of the absorption and desorption currents are carried out over the temperature range of 50 to 200 %C and electric fields up to 12 MV/m in 25.5 mum polyimide film base with 12.7 mum coating of Teflonreg FEP on one side. It is believed that this is the first time that studies on this film are reported in the literature.