2002 Annual Report Conference on Electrical Insulation and Dielectric Phenomena

Weibull Statistical Analysis of Area Effect on the Breakdown Strength in Polymer Films

Saeed UI-Haq and G.R Govinda Raju Department of Electrical and Computer Engineering,

University of Windsor, Windsor, Ontario, Canada, N9B 3P4

Abstract: The area and film thickness effects on the breakdown strength were examined for different polymer films at room temperature. Electrodes of four different diameters were used for this experimental work, which were in the range of !4 to 2 inches in diameter. Materials of various thickness used for this investigation were aramid paper (NOMEX type 410), Polyimide film (KAPTON), Mylar Polyester film, and Nomex-Polyester-Nomex composites. Two-parameter Weibull distribution has been used to analyze the

. results. The experimental analysis shows that the breakdown strength decreases as the electrode area

- increases. In case of thickness of 125-pm film, change of IO-12% in the breakdown strength was observed, where as for 25-pm thickness, 30-35% change in the breakdown strength was noticed. It was also noticed that the 2.5-pm thickness films are more exposed to the mechanical damage during handling process, thus chances of lower breakdown strength of such films are more, as probability of covering the mechanically damaged area increases with the increase in the electrode area.

Introduction

Polymer films used mostly for insulation form an integral part of high voltage structures. Under normal conditions, they exhibit a wide range of dielectric strength, depending on the conditions of the environment and the testing method. The measured breakdown voltage is influenced by a large number of external factors such as temperature, humidity, test duration, applied voltage type, pressure applied to the electrodes, type and size of electrodes, discharge in the ambient medium and in cavities [I-51.

Several distinct mechanisms of breakdown in solid polymer films have been identified and treated theoretically. No singIe theory fully explains the process of breakdown and predicts the breakdown stress of a given solid insulator. It is known that the breakdown strength of liquids increases as the electrode area decreases [6-81.

In order to assess the quality of the dielectric in insulating systems two types of tests are used; static tests and dynamic tests. In static test, a number of identical samples are stressed at a constant electric filed

and the time to breakdown of each sample is recorded. In dynamic test, the electrical stress applied is a function of time, and the magnitude of the electric field at breakdown of each sample is measured. A large number of samples are required in each since a distribution of the results is expected.

Dynamic test is preferred in the laboratory because the variation in the measured results is less than that for equivalent static testing. Static testing requires critical control of the electric field as small variations in the field can cause significant variations in the time to breakdown. However, static stress is useful in the determination of the lifetime of the comparable samples I61PI.

Weibull Distribution: The Weibull distribution is a general-purpose reliability distribution used to model material strength, h i e s to failure of electronic and mechanical components, equipment, or systems [lo]. Considering breakdown strength, E as the random variable, the cumulative Weibull function is given by:

(1) p,+) = I-e[’-(@Yya!ABI

where E is the electric field in V/m. P,(E) indicates the probability of specimens tested will fail. The parameter a represents the magnitude of E for 63.2% of tested units failure. Similarly the parameter p, which is also called the “Shape Parameter”, is a measure of dispersion of E. The parameter y which is referred as a threshold parameter (V/m) is that value of E below which breakdown is not possible.

In order to plot data on probability paper it is necessary to rank them and assign a cumulative probability failure, PF, to each point. A calculation of Pp depends on the ranking scheme such as median and mean ranking. The median ranking is given by:

P&) = [(i-O.3)/(n+O.4)]xlOO% (2)

where as mean rank approximation is:

PF (i,n) = [i I @+I)] (3)

Though the mean rank function has been recommended by IEEE standards, the median rank has been shown to be more appropriate [Il-131.

0-7803-7502-5/W$17.00 @ 2002 IEEE 518

Experimental Procedure

In figure 1 below, the test circuit setup for DC application is shown. In the environmental test chamber a pair of Rogowski electrodes is used between which the polymer film is mounted. The top electrode rests on the sample by a suitable spring-loaded mechanism that can be operated fiom outside of the environmental chamber. The bottom electrode and the width of the polymer are sufficiently large to prevent a spurious value being recorded due to misalignment.

Figure I: Experimental Setup. 1-sample spool, 2- moveable upper electrode, 3-heater element, 4- retracting spool, 5-lower electrode, 6-thermocouple.

Before starting the DC test, the sample, which is either in the form of thin film or paper, is cut to 3" to 4" width ribbon. After cutting the sample it is wound on the inside spool and then another end is passed through the electrodes and finally through the slot and secured on the retracting spool which is outside. To acquire the desire temperature, PID controller is used in the test chamber. In our case, 50 tests were taken at room temperature for each electrode to get accurate results. The voltage was raised approximately 200 Vhec until breakdown is achieved. For next test, the new portion of the sample is adjusted in the electrodes with the help of the retracting spool. Same procedure was repeated for each test.

Figure 2: Electrodes of different diameters, A=0.5", %LO", C=1.5", W2.0".

Results and Discussions

Aramid papers are used extensively as high temperature insulation in motors, generators, and transformers. Two different thickness of aramid paper i.e. 75 and 1 2 5 - p were used for area effect investigation. Figures 3 and 4 show the Weibull plots for breakdown strength respectively. During the analysis, it was noticed that both thickness gives almost identical results. The change in the breakdown strength is almost 2125%.

Figure 3: Weibull plot for 7 5 - p aramid paper for different electrode diameters at room temperature.

Figure 4: Weibull plot for 125-pn aramid paper for different electrode diameters at room temperature,

KAPTON" maintains its electrid properties over a wide temperature range. It has wide applications in numerous electronic equipments. Tests were conducted on 25 and 75-pn films using DC voltages. Figures 5 and 6 show the respective plots. In case of 25-pn film approximately 35% change in breakdown strength was noticed where as for 75-pn film 11.42% was observed. Figure 7 shows the change in the scale parameter at various electrode diameters.

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Figure 5: Weibull plot for 25-pm KAF'TONTM for different electrode diameters at room temperature.

Figure 6: Weibull plot for 75-pn KAF'TONTM for different electrode diameters at room temperature.

Figure 7: Plot showing variation in dielectric strength for 75-pm KAPTONTM film with respect to change in electrode diameter

Mylar polyester films, which offer unique design capabilities to the electrical industry, 25 and 75-pm films, were selected for area effect investigation. Lower

breakdown strength values were observed during application of DC voltage. These lower strengths can be avoided by placing the test sample with great care between the electrodes. This will avoid the mechanical damages to the film, which lowers the dielectric strength. Weibull plots are shown in figures 8 and 9 respectively.

Figure 8: Weibull plot for 25-pn MYLAR film for different electrode diameters at room temperature.

Figure 9: Weibull plot for 75-pm MYLAR film for different electrode diameters at room temperature.

Nomex-Polyester-Nomex is obtained by laminating polyester on Nomex. These fibers possess good tensile strength and are used where physical toughness is required. It bas good resistance to acids, alkalis, refrigerants, ketones, alcohols and oils. However, it is susceptible to moisture absorption, which may improve physical properties but lowers insulating values. For area effect analysis 175-pm thick sample was used and only 4.18% change in dielectric strength was noticed by changing the electrode size from 0.5" to 2.0". Figure 10 shows the Weibull plot of the results obtained.

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Figure IO: Weibull plot for 175-p Nomex-Polyester- Nomex for different electrode diameters at room temperature.

During the study it was observed that the dielectric strength of polymers differs with each test, only statistical techniques can predict the probability of failure. The two-parameter Weibull distribution allows simpler procedures for parameter and confidence bound interval calculations.

AAer the analysis of large data, collected through different samples revealed that the breakdown strength decreases as the electrode area increases and thus has an inverse relation. However materials with lower thickness show high changes in breakdown strength with the increase in electrode diameter.

In case of Nomex-Polyester-Nomex higher thickness of 175-pm was selected for investigation and it was observed that the increase in electrode area bas almost negligible effect on the value of breakdown strength. As this material is mechanically very tough and small changes were observed, it is concluded that the change in breakdown strength can be associated with the material property.

Conclusion

1. As lower thickness films i.e. 25 and 75-pn are more exposed to the mechanical damages during handling and processing, it is suggested that higher thickness films should be preferred during the design of any insulation system.

It is suggested that during the design of expensive equipment the value of electrical breakdown strength should be considered in relation to the probability of breakdown (say 1%) rather than the 63.2% probability.

2.

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Author addresses: Saeed UI-Haq, Department of Electrical and Computer Engineering, University of Windsor, 401 Sunset Ave. Windsor, Ontario, N9B 3P4, Canada. Email ulhaal(auwindsor.ca.

Prof. Dr. G.R. Govinda Raju, Department of Electrical and Computer Engineering, University of Windsor, 40 1 Sunset Ave, Windsor, Ontario, N9B 3P4, Canada. Email: raiu@.uwindsor.ca.

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