ac breakdown strength of aromatic polymers under partial discharge reducing conditions

IEEE !Transactions on Dielectrics and Electrical Insulation Vol. 1 No. 1, February 1004 80

ac Breakdown Strength of Aromatic Polymers under Partial Discharge

Reducing Conditions

Peter Bjell heim and Bertil Helgee Department of Polymer Technology,

Chalmers University of Technology Goteborg, Sweden

ABSTRACT Partial discharge occurring at HV in ac electric breakdown field measurements on insulating polymeric films strongly re- duces the breakdown field strength of the material and conceals the intrinsic breakdown process. By performing ac electric breakdown field measurements on polymer films in phthalic diesters, it was possible to suppress partial discharge and obtain breakdown field data of more intrinsic character. This is valuable for the evaluation of the influence of the structure and the properties of polymers on their insulating behavior. The ac electric breakdown field strength of five aromatic polymers at different film thicknesses was measured in dibutyl phthalate. The results from these measurements showed about a twofold increase in the breakdown field strength at sample thicknesses > 50 pm, compared to data obtained in transformer oil. The breakdown field strength for thick samples showed a linear decrease with increasing sample thickness for all polymers except PEEK, which showed a constant breakdown field strength in the thickness range investigated. The breakdown field strength of thin samples was independent of the polymer structure. Measurements of the breakdown field strength of the polymers under dc conditions show no obvious correlation between chemical structure or polymer property and the breakdown field strength.

1. INTRODUCTION structure of polymeric materials on their insulating properties have on the other hand been rather scarce [4-61. Still it has been pointed out [7] that it is necessary to study the relationships between the properties inherent to polymers and their electric breakdown in order to un- derstand the complex behavior of polymers at electric breakdown.

EHAVIOR of polymeric materials under electric stress B has been the subject of numerous investigations dur- ing the years. Several studies concerning the mechanism of electric breakdown in solids have appeared in the literature and extensive reviews [l-31 also have been published. Investigations concerning the influence of the chemical In a previous paper [8] we reported on the influence of

1070-9878/94/ 83.00 @ 1994 IEEE

90 Bjellheim et al.: Breakdown of Aromatic Polymers under Partial Discharges

the chemical structure and the sample thickness on the ac electric breakdown of aromatic polymers in transformer oil where partial discharges cannot be avoided.

In the present paper we report on the ac electric breakdown of aromatic polymers under conditions where the partial discharge (PD) that occurs a t HV has been suppressed. The PD suppression is achieved by performing the experiments in insulating liquids with permittivity ( E )

substantially higher than that for transformer oil. The overall aim of this work was to investigate the influence of the the chemical structure of aromatic polymers on their electric breakdown strength.

In order to evaluate the influence of the permittivity of the medium on the ac breakdown field for polymeric films, determinations of the ac breakdown field dependence on film thickness for polyimide (PI) films were performed in five different phthalic esters and one isophthalic ester. The permittivity of the liquids cover a range from 4.9 to 7.6. Depending upon the medium for the measurements, two types of behavior of the breakdown field data vs. film thickness were found. Esters with E = 4.9 to 5.3 showed a power law dependence of breakdown field vs. film thickness, very similar to that obtained with transformer oil [8], while esters with E = 5.6 to 7.6 showed an almost linear dependence with higher breakdown field values than those obtained with transformer oil.

Under ac conditions electric breakdown field strength (FB) of five different aromatic polymers (see Figure 1) was measured in dibutyl phthalate (DBP) ( E = 6.3 at 50 Hz) a t several film thicknesses. A decreasing, approximately linear, dependence of FB on film thickness was obtained for four of the polymers while PEEK showed a constant FB value with increasing film thickness. All the investigated polymers for film thicknesses 2 50 pm, showed an increase of FB of - 2 x when the meaaurements performed in DBP were compared to those performed in transformer oil.

2. EXPERIMENTAL

2.1 MATERIALS

HE following polymeric films were used in the experi- T ments without any pretreatment: Kapton H (PI) (Du Pont de Nemours), Ultem (PEI) (GE Plastics and Roehm AG), Hostaphan (PET) (Hoechst AG), amorphous Stabar K200 (PEEK) (IC1 Films) and Stabar SlOO (PES) (IC1 Films) (Figure 1). The film thicknesses were measured with a Mitutoyo thickness meter with resolution of 1 pm.

PI

PET

PEEK

PES

Figure 1. Chemical structures of the repeating units of the polymera used.

Diethyl phthalate (DEP), dibutyl phthalate (DBP), bis(2- ethylhexyl) phthalate (DOP) and di-iso-octyl phthalate (DIOP) were of commercial grade quality and used without further purification. Dihexyl phthalate (DHP) and dibutyl iaophthalate (DBIP) were synthesized [9] and dis- tilled under vacuum before use. Boiling points of DHP and DBIP were in accordance with literature data and NMR and IR spectra were in accordance with the structure. The transformer oil used was Nytro 1OX from Nyniis Industri AB.

2.2 ELECTRIC BREAKDOWN MEASUREMENTS

For the ac measurements of electric breakdown fields of the films, a slightly modified Hipotronics OCBOA liquid dielectric tester was used. Instead of the original electrode setup, opposing cylinders of 5 mm in diameter and with rounded edges were mounted. One of the cylinders was spring loaded to allow good contact with the tested film and provide parallelism in the electrode setup. The films were cut into 30 mm wide strips which constitut- ed the test specimens. The entire electrode setup was immersed in a vessel containing the desired dielectric flu- id. All measurements were performed using a continuous ramp in voltage of 500 V/s. All voltage values are rms values. In some cases, and in particular when thick samples

IEEE !Zkansactions on Dielectrics and Electrical Insulation Vol. 1 No. 1, February 1004 01

were tested, audible partial discharges occurred at high voltages. Each film thickness was subjected to more than 20 breakdown experiments. The sites of the breakthrough were distributed all over the electrode area. The test conditions used are in good agreement with ASTM standard D149-87. No measurements of partial discharges were performed.

Dc electric breakdown field measurements were performed a t ABB Corporate Research according to the following. The film sample was placed on a copper foil on the bottom of a jar filled with transformer oil. A 5 mm diameter brass cylinder with rounded edges was placed on top of the film. Across the electrodes, the copper foil and the brass cylinder, a dc voltage was applied with a continuous voltage ramp of 500 V/s. The breakdown voltage was determined by recording the voltage on a chart recorder. Each experiment consisted of 20 to 30 individ- ual measurements.

Data from all measurements were evaluated using Wei- bull statistics. The variance in the measurements, pre- sented as error bars in the plots, are standard deviations approximated as the ratio A / B of the Weibull parameters. The B parameters of the Weibull plots are in all cases > 8.

2.3 PERMlTTlVlTlES

The permittivities and loss factors of the phthalate diesters at 50 He have been measured a t ABB Corporate Research using a low-frequency response analyzer. The permittivity values were calculated using parallel capaci- tance data. Both the instrumentation and the measurement techniques have been described [lo].

3. RESULTS AND DISCUSSION

3.1 METHOD

N the evaluation of the ac electric field strength of poly- I meric insulating materials, PD occurred a t HV in all immersing fluids. Because PD is responsible for a part of the degradation leading to breakdown of the material, it is of interest to find conditions where PD is suppressed and more of an intrinsic breakdown strength of the m a terials is measured.

By measuring FB under ac conditions for PI films of different thicknesses in a series of phthalic diesters, a dependence of FB on the properties of the medium surrounding the test sample was found. The thickness dependence of

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Ac breakdown field data for PI vs. film thicknees. Media: 0 DEP, DBIP, V DBP, DHP, A DIOP and 0 transformer oil.

FB for PI in a series of phthalic acid diesters is shown in Figure 2. The data obtained make up two groups: Group I (DEP, DBP, DBIP and DHP) wherein FB shows a rea- sonably linear dependence on the sample thickness, and Group I1 (DIOP and DOP) wherein FB follows the power law dependence described for transformer oil [8]. (DOP is excluded from Figure 2 for clarity). The FB values for film thicknesses 2 50 p m of Group I are - 2 x higher than for Group 11. In the measurements, audible PD occurred a t substantially higher voltages for diesters of Group I compared to those of Group 11.

In efforts to explain these results, FB as been plotted against dielectric constant and viscosity of the phthalate esters without finding any simple correlations. Plots of FB vs. E* , complex permittivity, for the different phthalic acid diesters are shown in Figure 3 for PI films of different thickness. The resulting plots contain abrupt changes of FB as E* increases monotonically, including a step change in the interval 5.3 < E* < 5.6. It is striking that the breakdown fields obtained in DOP and DIOP so close- ly follow the data obtained in transformer oil, although there is a difference of 2.7 in the permittivity between DIOP and transformer oil. For the group I fluids it is shown in Figure 3, that for those with higher conductivity (DEP) and (DBIP) the FB values obtained are in the same order as the values of the low conductivity fluids. Hence changes in the conductivity of the liquid film


Table 1. Permittivity and dissipation factor at 50 Hz for different phthalic diesters and polymeric films.

Ester '01 ymer DEP DBP DBIP DHP DOP DIOP Oil" PI

PE1 PET PES

PEEK Transf

E

7.6 6.1 6.0 5.6 5.3 4.9 2.2

3.4 - 3.5 3.2 - 3.7

3.3 3.7 3.3

cmer oil

- - E*

.5.6 6.1 !4.2 5.6 5.3 4.9

-

6

3issipation factor

1.8 0.11 3.9

0.08 0.04 0.04

0.0018 0.0014 0.005 0.0018 0.0018

For ~ O W - ~ O S B fluids E = E * .

between the electrodes and the polymeric film does not affect the breakdown field strength of the polymer. Per- mittivities and loss factors of the actual phthalate esters and polymers are given in Table 1.

3.2 MEASUREMENTS ON AROMATIC POLYMERS

The a t electric breakdown field strength F ( T 0 ) of the aromatic polymers shown in Figure 1 has been studied in transformer oil medium, where &(TO) = 2.2 [8]. In Fig- ures 4 to 8, ac breakdown field F(DBP) data measured in DBP are plotted vs. film thickness for each polymer. These Figures also show ac breakdown fields in transformer oil from our earlier work and the results from dc measurements performed in transformer oil, both as func- tions of film thickness.

Upon comparison of the plots of Figures 4 to 8 of the breakdown fields of the different polymeric films vs. film thickness in DBP and in tramformer oil, it is clear that the ac breakdown strength increased greatly when DBP replaced transformer oil. The power law dependence [8] of the decrease of F ( T 0 ) with increasing film thickness of systems immersed in tratlsformer oil c ( f i l m ) > &(TO), has changed to a linear dependence of F ( D B P ) for PI, PET and PE1 in D B P ~ ( f i l m ) < &(DBP) . The F ( D B P ) for PEEK films is almost constant in the film thickness range measured, while the breakdown behavior of PES films in DBP is somewhat uncertain due to the low num- ber of points. At the same time the overall level of the

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Figure 3. Ac breakdown field data for PI vs. the permittivities of the media at different film thicknesses d . (0) 13 pm, (a) 25 wn, (V) 50 pm, (v) 75 pm and (0) 125 pm.

FB values has increased by a factor - 2 x when the permittivity of the surrounding medium changed from 2 to 6 in going from low to high E medium. These effects are at- tributed to the suppression of PD when liquids of E 2 5.6 are used.

In our previous paper [8], F ( T 0 ) was shown to follow a power law dependence on film thickness d . It was also shown that the exponent of the power law for the different polymers correlated r e a n a b l y well to the reversible reduction potentials of the repeating units of the polymers.

For PI, PEI, P E T and PES films F ( D B P ) shows a

IEEE !lkansactions on Dielectrics and Electrical Insulation Vol. 1 No. 1 , February 1994

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Breakdown field data for PI vs. film thickness, 0 dc in transformer oil, 0 ac in DBP and V ac in transformer oil.

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Breakdown field data for PET vs. film thickness, 0 dc in transformer oil, 0 ac in DBP and V ac in transformer oil.

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Breakdown field data for PEEK vs. film thickness, 0 dc in transformer oil, 0 ac in DBP and V ac in transformer oil.

tials of the repeating units (from our earlier work [B]) a correlation is indicated although PEEK and PE1 deviate from this (Table 2). At the very high ac voltages reached in the measurements of F(DBP) for the thick films, partial discharges could be heard just before breakdown. The


Polymer 25 Pm

PI 404f 12 PE1 437f20 PET 408f21

PEEK 404 f 15 PES 408 f 29

600

50 Pm

389f6 440f40 388f16 422 f 27 353f 11

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300

P P

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L I L I L I I I ' I I ' ' 20 40 60 80 i o 0 120 140

d (w) Figure 8.

Breakdown field data for PES vs. film thickness, Udc in transformer oil, 0 ac in DBP and V ac in transformer oil.

Table 2 . Regression data of FB vs. d in DBP

PEEK -1.54

cm Normal Hydrogen Electrode (NHE) Mean value and standard deviation.

action of PD at these high fields is likely to be the reason for the thickness dependence of F(DBP) .

Table 3. FB (V/pm) for 25 and 50 pm films in DBP

At the lower film thicknesses (25 and 50 pm), the voltage applied to reach breakdown is rather low and no

partial discharges could be heard. Thus, we can assume that the low amount of PD occurring does not affect the breakdown process. If this non-PD breakdown process is dependent upon the structure or the properties of the polymer, this dependence would show in a comparison of the data for the thin films of the different polymers. In Table 3 the F(DBP) values of 25 and 50 pm films are list- ed together with their standard deviations from Weibull analysis. Because these values for the different polymers show little variation within each thickness group it is r e a sonable that there is very little influence on F(DBP) of the chemical structure or other polymer properties which vary in this series of aromatic polymers. Hence the non- PD breakdown process seems to be insensitive to the type of structural differences that are present.

When regarding the ac breakdown process in low E media it follows that the dependence of FB on sample thickness is an effect of partial discharges. Consequently the previously demonstrated relationship [8] between F ( T 0 ) and the chemical structure of the polymer, through the reversible reduction potential of the repeating unit, con- cerns partial discharges. This means that the reactions taking place when an aromatic polymer is subjected to partial discharges are of charge transfer character. In the mechanistic discussion [8] the first step of electric breakdown is proposed to be the ionization of polymer seg- ments by energy-rich electrons via collisions, yielding two electrons of low energy and a cation radical (Equation (1))-

e* + P + P+. + 2e (1)

where e' stands for an energy-rich electron. The abili- t y of the polymer to trap these low-energy electrons de- pends on the ease of reduction of the polymer (Equation (2)). Electrons that are not trapped will reach high energy levels and cause further ionization and degradation of the polymer. Consequently this mechanistic descrip tion is applicable only to the partial discharge breakdown process and not to breakdown processes of more intrinsic character.

3.3 dc MEASUREMENTS

Since F ( D C ) measurements of FB under dc conditions do not involve PD it is of great interest to obtain dc data for the series of polymers in order to get a comparison with ac data measured under PD suppressing conditions. Figures 4 to 8 already include the results of F(DC) determinations in transformer oil. From these Figures it is clear that F(DC) in transformer oil for the aromatic polymers are, with one exception, higher than F(DBP) . The

IEEE TZ.ansactions on Dielectrics and Electrical Innsulation Vol. 1 No. 1, February 1994 95

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d (Pm) Figure 9.

Dc breakdown field data vs. film thickness for the aromatic polymers: 0 PI, V PEI, 0 PET, 0 PEEK, and A PES.

dc/oil and ac/DBP data for PEEK overlap and do not show a clear trend with increasing sample thickness. For PI (Figure 4) a decreasing linear dependence of F ( D C ) on film thickness is observed with a correlation coefficient of 0.998. From Figure 9 it is obvious that PI, PE1 and PES show very similar F(DC) values, which means that the dc measurements do not reveal any dependence on the chemical structure or other differences between these three polymers. In particular, it is interesting to note that the polymers PI, PE1 and PES cover a broad range of electron accepting capability, i.e. 1.4 V in terms of the reversible reduction potential, yet this has no effect on the F ( D C ) values. The dc breakdown behavior of PET and PEEK deserves further study to elucidate the re+ sons for their deviations from the behavior of the other aromatic polymers.

4. CONCLUSIONS

T has been possible to raise the inception voltage for I the partial discharges occurring a t HV by performing ac electric breakdown measurements in phthalic diesters instead of transformer oil. PD suppression is achieved when the permittivity of the diester is > 5.3.

The ac (50 HE) electric breakdown field strength of aromatic polymeric films increased by a factor of 2 when the measurements were performed in dibutyl phthalate

instead of transformer oil and the samples were thicker than 50 wm.

The ac breakdown field strength of the polymeric films in DBP showed, with one exception, a decreasing linear dependence on increasing sample thickness. However, the breakdown field strength for PEEK was found to be almost constant within the thickness range investigated.

All polymers showed approximately equal ac breakdown field strength a t 25 pm film thickness in DBP, indi- cating a breakdown process independent of the molecular structure of the polymer.

Electric breakdown due to partial discharges occurs through an ionization mechanism and is dependent on the chemical structure and the electron accepting capability of the polymer.

Ac breakdown field measurements with and without PD suppression may become a useful tool to determine the PD resistance of polymeric materials.

The dc measurements show that there is no obvious correlation between the structure or properties of the polymers and their dc electric breakdown field strength.

ACKNOWLEDGMENT

Financial support by The National Swedish Board for Technical Development and ASEA Brown Boveri Corpo- rate Research AB, Sweden is gratefully acknowledged.

REFERENCES

[l] P. P. Budenstein, “On the Mechanism of Dielectric Breakdown of Solids”, IEEE Trans. Electr. Insul., Vol. 15, pp. 225-240, 1980.

[2] J. J. O’dwyer, “Breakdown in Solid Dielectrics”, IEEE Trans. Electr. Insul., Vol. 17, pp. 484-487, 1982.

[3] J. J. O”Dwyer, “The Role of Space Charge in the Theory of Solid-dielectric Breakdown”, IEEE Trans. Electr. Insul., Vol. 19, pp. 1-9, 1984.

[4] M. Ieda, M. Nagao and G. Sawa, “Dielectric Break- down of Polyethylene and Ethylene-vinyl Acetate Composite Polymers”, Int. Conf. on Dielectric Ma- terials, Measurements and Applications 3, Inst. of Electrical Eng. Conf. Publ., Vol. 177, pp. 185-188, 1979.


[5] K. Yoshino, “Dependence of Dielectric Breakdown of Liquids on Molecular Structure”, IEEE Trans. Elec- tr. Insul., Vol. 15, pp. 186-200, 1980.

[6] V. K. Srivastava, “Evidence for Schottky-emission Dominated Dielectric Breakdown” , Phys. Rev. Lett., Vol. 30, pp. 1046-1047, 1973.

[7] M. Ieda, “Dielectric Breakdown Process of Poly- mers”, IEEE Trans. Electr. Insul., Vol. 15, pp. 206-224, 1980.

Thickness and Chemical Structure”, IEEE Trans. Electr. Insul., Vol. 26, pp. 1147-1152, 1991.

[9] Organikum, Veb Deutscher Verlag Der Wis- senschaften, pp. 499-501, Berlin 1984.

[lo] U. Gavert and B. Nettelblad, “Measurement Tech- niques for Dielectric Response Characterisation a t Low Frequencies” , Proc. Nordic Insulation Sympo- sium NORD-IS 90, 7.1:l-10, Lyngby Denmark 1990.

Manuscript was received on 30 January 1992, in final form 30

[8] B. Helgee and P. Bjellheim, “Ac Electric Breakdown Strength of Aromatic Polymers. Dependence on Film

April 1993.

ac breakdown strength of aromatic polymers under partial discharge reducing conditions

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