Effect of electrode polarity and additives on the direct breakdown voltage of silicon oil under highly non-uniform fields

IEEE Transactions on Electrical Insulation Vol. 23 No. 6, December 1Q88 Q61

Effect of Electrode Polarity and Additives on the Direct Breakdown Voltage of Silicone Oil under Highly

non-Uniform Fields

A. A. Zaky

College of Engineering, Sultan Qaboos University, Muscat, Oman.

1. Y. Megahed a n d S . Y. Yehia Faculty of Engineering, University of Alexandria,

Alexandria, Egypt

ABSTRACT

The direct breakdown voltage against gap length (50 to 900 pm) characteristics were obtained using point-sphere electrodes for degassed silicone oil, oxygen-saturated oil and oil containing varying concentrations (0.005 to 1.0 molar) of 1- methylnaphthalene as additive. The characteristics for degassed and oxygen-saturated methylnaphthalene were also obtained. Both stainless steel and aluminum were used for the sphere electrode. The results indicate that the shape of the characteristics for both polarities of the point depends on the material of the sphere electrode, on the presence or absence of oxygen and on the nature of the liquid itself. In silicone oil the crossover gap length, at which the breakdown voltage is the same for both polarities of the point electrode, was found to be increased greatly by the presence of oxygen, whereas oxygen greatly reduces this gap in hydrocarbon liquids. In both degassed and oxygen-saturated oil the presence of the additive caused a shift in the crossover gap to smaller lengths, this shift being especially large in the case of oxygen-saturated oil. This indicates that in silicone oil the additive strongly counteracts the effect of oxygen and that oxygen interacts preferentially with the additive.

0018-9367/88/1200-961$1.00 @ 1988 IEEE

962 Zaky et al.: Effect of electrode polarity and additives under highly non-uniform fields

INTRODUCTION

T is well known that the breakdown strength of in- I sulating liquids in general is influenced, among other factors, by the material of the electrodes, the presence of oxygen, and aromatic additives. The effect of these factors on the breakdown voltage of silicone oil using a sphere-sphere electrode geometry has been reported previously by the authors [l]. Preliminary results using a point-to-sphere geometry have also been reported briefly [2]. The present paper is an extention of the preceding investigation and gives the experimental results obtained with point-sphere electrodes on the effect of electrode material, dissolved oxygen, and varying concentrations (0.005 to 1.0 M) of the diaromatic 1- methylnaphthalene (MN) on the direct breakdown voltage as a function of gap length and point polarity for 50 cSt degassed and oxygen-saturated polydimethylsiloxane for gap lengths ranging from 50 to 900 pm. In or- der to assess the effect of the additive, polarity reversal V - d characteristics were also obtained for the additive itself, both degassed and oxygen-saturated.

Previous work on the breakdown voltage of transformer oil and liquid paraffin [3,4] showed that the shape of the V - d characteristics is strongly dependent on the material of the electrodes as well as on the presence of oxygen. The present results confirm this dependence in the case of silicone oil, but also show that the shape is dependent on the nature of the liquid.

It is now well established that for various dielectric liquids tested under highly nonuniform fields the V - d characteristic obtained for both polarities of the point electrode show the existence of a critical crossover gap length at which the breakdown voltage is the same for both polarities of the point. For gaps shorter than the critical length, the breakdown voltage for the positive point is higher than that for the negative point whereas for longer gaps the negative point gives a higher breakdown voltage. The crossover gap length depends on the type of liquid and the addition of oxygen or aromatic additives to transformer oil and liquid paraffin produces a decrease in the critical gap length [5,8,9]. Yoshino et al. [lo] have linked the length of the crossover gap to the electron mobility in the liquid. The larger the mobility, the shorter the critical length. The electron mobility of the strongly aromatic 1-MN is expected to be larger than in the long- chained silicone oil. It therefore appears that the study of the crossover gap length as a function of 1-MN concentration could be informa- tive on electron mobility and thus electron trapping in silicone oil. Hence the role of space charge formation on

the shape of the V - d characteristics could be better estimated, as discussed later.

EXPERIMENTAL DETAILS

HE tests were carried out on polydimethylsiloxane T (silicone oil) of viscosity 50 cSt and on l-methylnaphthalene (CH3.CHI0H7). The liquids were well degassed (0.1 Pa) and filtered through a 0.3 pm Millipore filter in a totally closed glass system. For experiments with 02-saturated liquids dry and filtered oxygen was bubbled slowly through the degassed liquid which then remained in contact with the liquid a t atmospheric pressure for 16 h. All tests were carried out at atmospheric pressure and room temperature (23 "C)

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Figure 1. V - d characteristics for degassed and 02-saturated silicone oil; A1 sphere.

The point electrode was a nickel-plated steel sewing needle of tip radius 25 pm. The other electrode was a 5 mm diameter etched aluminum of polished stainless

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Figure 2 . V - d characteristics for degassed and 02-saturated silicone oil; stainless steel sphere.

steel sphere. A diverter circuit was used in all tests. Each breakdown value reported is the average of ten breakdowns. New electrodes and a fresh liquid sample were used for each gap setting. Stress conditioning was carried out by raising the voltage to 60% of the expected breakdown value and kept constant for two hours. Af- ter conditioning, the applied voltage was increased a t a constant rate of about 1 kV/s from the prestressing value until breakdown occurred. The coefficients of variation were as follows: degassed oil 1.0 to 5.8%; degassed MN 3.2 to 10.7%; oxygen-saturated MN 2.3 to 8.8%; degassed oil + MN 1.52 to 14%.

R ES U LTS

DEGASSED AND OXYGEN-SATURATED SILICONE OIL

IGURE 1 shows the V - d characteristics for degassed F oil and oil saturated with oxygen using an aluminum

sphere electrode and Figure 2 shows the characteristics using a stainless steel sphere. For the degassed liquid there is a crossover gap for both sphere electrodes but the shape of the characteristics for an aluminum sphere is different than that for a stainless steel one. For an aluminum sphere and the point negative, the characteristic is convex with respect to the gap axis while with the point positive, the characteristic changes from concave to convex for gaps longer than the crossover gap. For a stainless steel sphere the characteristics are concave with respect to the gap axis for both polarities of the point and throughout the gap range examined. The effect of oxygen in the case of an aluminum sphere is to change the shape of the V - d characteristics for both polarities of the point to concave with respect to the gap axis. Also, for both aluminum and stainless steel spheres there is a large increase in the crossover gap length to about 1100 pm. With the point positive and an aluminum sphere there is a marked increase in the breakdown voltage compared with that of the degassed oil, but in view of the convex shape of the characteristic in the latter case, it would appear that this effect will reverse at large gaps. This is apparent in the case of the negative point where oxygen increases the breakdown voltage initially but then decreases it for gaps greater than about 800 pm. The same effect is observed with a stainless steel sphere but to a lesser extent, with the change over point for the point negative occurring at a gap length of about 325 pm.

DEGASSED AND OXYGEN-SATURATED 1-METHYLNAPHTHALENE

HE V - d characteristics for degassed and 02-satu- T rated 1-MN are shown in Figure 3 for an aluminum sphere and in Figure 4 for a stainless steel sphere. There is no change in the shape of the curves with a change in the material of the sphere electrode. Comparison with Figures 1 and 2 shows that with an aluminum sphere the shape of the curves for the degassed 1-MN is markedly different from that of the corresponding curves for silicone oil. The presence of oxygen has two pronounced effects: the crossover point is shifted to a gap length smaller than 50 pm and the breakdown voltage is markedly reduced over the whole gap range for both polarities of the point electrode.

SILICONE OIL WITH 1-METHYLNAPHTHALENE AS ADDITIVE

V - d characteristics for degassed silicone oil with additive concentrations varying from 0.005 to 1.0 molar


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were obtained for both polarities of the point. Figures 5 and 6 show a typical set of characteristics for the min- imum and maximum concentrations and for aluminum and stainless steel as the sphere electrode. With an aluminum sphere it was found that with increasing additive concentration there is a gradual change in the shape of the characteristics for both polarities of the point electrode. The convex portions of the curves become more and more concave and at an additive concentration of 1.0 M there is no significant difference in the shape of the characteristics for a stainless steel or an aluminum sphere.

The effect of various concentrations of MN on the V-d characteristics of oxygen-saturated oil are shown in Figure 7 for a stainless steel sphere; those obtained with an aluminum sphere were very similar. The presence of the additive reduces the breakdown voltage for both polarities of the point and there is marked decrease in the length of the crossover gap with incrcasing additive

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V - d characteristics for degassed and 0 2 -

saturated 1-methylnaphtalene; stainless steel sphere.

concentration.

Figure 8 shows the variation of the critical gap length with additive concentration under various test conditions. In degassed oil the decrease in the crossover gap length with increasing additive concentration is relatively small. However, in oxygen-saturated oil a small additive concentration produced a very large decrease in the crossover gap length. In the degassed oil an additive concentration of 0.05 M produced a reduction of about 50 pm in the critical length whilst in oxygen-saturated oil a similar concentration produced a reduction of about 700 pm. The crossover gap disappeared at concentrations of 0.1 M and above, indicating that oxygen has a synergistic effect on the additive, greatly enhancing its action.

Figure 9 shows the effect of additive concentration on the breakdown voltage for a gap length of 700 pm for positive and negative point polarities and for aluminum

IEEE Transactions on Electrical Insulation

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Figure 5. V - d characteristics for degassed silicone oil + 1-methylnaphtalene; A: 0.001 M; B: 1.0 M. A1 sphere

and stainless steel electrodes. The results, in general, are similar to those reported by Mathes [6]. Although the small peak observed with the point negative and a stainless steel sphere may not appear to be statistically significant, it has nevertheless been drawn as such since the presence of such a peak is pronounced in the case of oxygen-saturated oil as shown in Figures 10 and 11 and occurs for both polarities of the point.

DISCUSSION N view of the complexity of the breakdown mecha- I nism in insulating liquids and of the multiplicity of

factors which can influence or even change this mech- anism, it is difficult to provide a coherent explanation for all the results reported here. It is well'known that for complete breakdown to occur it is necessary to sat- isfy two conditions: The initiation of the discharge process and the subsequent propagation of this discharge to

Vol. 23 No. 6, December 1988 965

300 500 700 900 100

GAP LENGTH, m i c r o n s

Figure 6. V-d characteristics for degassed silicone oil + 1- methylnaphtalene; A: 0.001 M; B: 1.0 M ; stainless steel sphere.

bridge the gap between the two electrodes. Both the discharge initiation phase and propagation phase can be in- dependently influenced by the test conditions. Thus the enhancement or quenching of one phase need not be ac- companied by a similar effect on the other phase. For ex- ample, space charges may promote the discharge phase by enhancing the local field at the electrode surfaces but a t the same time retard the propagation phase by reduc- ing the field in the bulk liquid. Space charges play an important role in determining the breakdown process, especially when tests are carried out under dc voltages. The density and distribution of space charges will depend on a number of factors. Initially homocharges will be formed at the vicinity of each electrode. At the cathode electrons will become trapped a t acceptor-like states of the liquid molecules whilst at the anode electron tun- neling will occur from donor-like states to the electrode. Both positive and negative charges will then migrate to the opposite electrodes where they may be totally or

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Zaky et al.: Effect of electrode polarity and additives under highly non-uniform fields

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Figure 7. V - d characteristics for 02-saturated silicone oil + 1-methylnaphtalene; A: 0.001 M; B: 0.05 M; C: 1.0 M; stainless steel sphere.

partially neutralized, depending upon the conditions a t the surface of the electrodes. The strength of the final space charge formed at each electrode will depend upon the blocking properties of the surface oxide film which is invariably present on all electrode surfaces. An additional source of space charges will be the dissociation of any impurities present in the liquid.

It is evident that due to the formation of space charges the field distribution between the electrodes will differ greatly from the geometric distribution, whatever the geometric configuration of the electrodes.

Although the presence of space charges will deter- mine the actual field a t the surface of the electrodes and in the bulk liquid, there are other factors which must be now taken into consideration and are known to greatly influence the breakdown process. The two most important factors are the presence of dissolved oxygen and the presence of additives.

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Figure 8. Variation of crossover gap length with l-methylnaphtalene concentration in degassed and 0 2 -

saturated silicone oil for A1 and stainless steel electrodes.

EFFECT OF OXYGEN HERE are a number of different ways in which oxy- T gen can contribute to alter the breakdown strength

of dielectric liquids, especially under direct voltages or pulse voltages of long duration.

1. Due to its high electron affinity oxygen will trap electrons emitted at the cathode to form additional negative ions. This process in itself helps to quench any discharge initiated a t the cathode. However, the negative oxygen ions establish a negative space charge at the anode due to the presence of an oxide film on its surface which hinders their neutralization. Such a space charge will create a region of intense field a t the anode; ionization takes place in this region thereby facilitating the initiation of a backstreamer and lowering the breakdown voltage. Moreover as a result of the ionization there will be a feedback of positive ions towards the cathode. Because of the presence of an insulating oxide film there as well, these positive ions

IEEE Transactions on Electrical Insulation

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Figure 9. Variation of the breakdown voltage with l-methylnaphtalene concentration for degassed silicone oil, using A1 and stainless steel electrodes. Gap = 700 fim.

will intensify the field there leading to a lowering of the breakdown voltage.

2. The electron-trapping property of the oxygen reduces the probability of collision ionization in the bulk liquid and this leads to an increase in the breakdown strength.

3. Positive ions generated a t the anode by process (1) and returning to the cathode may be neutralized either directly or indirectly by becoming trapped in a cloud of negative ions. Emission at the cathode will thus be reduced and the breakdown strength raised.

4. Irrespective of its electron trapping properties, oxygen as a dissolved gas in the liquid, may promote the formation of the gaseous phase which is known to facilitate the breakdown process.

5. Oxygen dissolved in organic liquids is known to be an efficient quencher of the excited triplet state of the liquid molecules [7]. The relatively long lifetime of the excited triplet states are known to lead to chemical transformations of the liquid itself. Such transformations may influence the final breakdown voltage of the liquid.

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Figure 10. Variation of the breakdown voltage with l-methylnaphtalene concentration for 02-saturated silicone oil, using a stainless steel sphere. Gap = 700 ,urn.

The above-mentioned effects of oxygen should be common to all insulating liquids in general. Previous experimental results indicate this is so for all hydrocarbon liquids ranging from the simple liquids (either paraffinic or aromatic) to the more complex ones such as transformer oil, liquid paraffin and, as reported here, 1-MN. We are here particularly interested in two phe- nomenological effects which are brought about by the presence of oxygen: the change of the shape of the breakdown voltage versus gap length characteristics for both uniform [l] and nonuniform fields and the shift in the critical gap length associated with the polarity reversal characteristics with point-sphere electrodes.

The change in shape, previously found in hydrocarbon oils, also occurs with silicone oil; in both types of oil the change in shape follows a similar trend and would appear to be associated with the material of the


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electrode. However, the results for MN (Figures 3 and 5) using an aluminum sphere seem to indicate that the nature of the liquid also has a bearing on the shape of the V - d characteristic. The liquid appears to have the same effect on shape as oxygen does. Since both 0 2 and MN are electron trapping one can deduce that the shape of the characteristic is determined by both the material of the electrode and the electron trapping properties of the liquid or additive. This is consistent with the explanation put forward by the authors [3] to explain the change in shape.

EFFECT OF ADDITIVE RoMATIC additives in general are known to have a A beneficial effect on the breakdown strength of in-

sulating liquids and this is attributed mainly to their gassing inhibiting properties. The 1-methylnaphtalene

used in the present work has three main characteristics: large electron trapping cross-section, a low ionization potential and a high resistance to degradation (low gassing). Moreover it has a relatively large triplet lifetime of 2.1 to 2.5 s. (lowest triplet energy 2.6 eV). When used as additive in silicone oil the results indicate that its effect is a complex one. A result which may be generalized is that for both uniform [l] and nonuniform fields the breakdown strength increases with increasing additive concentration for small gaps whereas this effect is reversed for large gaps. A critical length a t which that reversal occurs is well defined for uniform fields irrespective of the electrode materials [l]. This concentration dependence indicates that the additive in- fluences both the breakdown initiation process and its subsequent propagation. The reported increase in the zero-gap intercept with increasing additive concentration indicates the formation of insulating films on the electrode surfaces. Such films will form due to some chemical reaction between the additive molecules and the silicone molecules. In particular it is suggested that this chemical reaction is brought about by the long-lived excited triplet states of the additive molecules adjacent to the electrode surface. This is supported by the fact in the presence of oxygen, which is known to be an efficient quencher of the excited triplet state, the zero-gap intercept is greatly reduced. A correlation between this quenching action of oxygen and the excitation of triplet states of the additive may also be inferred from the fact that in the presence of both oxygen and additive in the liquid silicone it is the effect of the oxygen on the additive which is predominant. Thus whereas the presence of oxygen increases the critical gap in silicone oil and decreases it to less than 50 p m in 1-MN, this critical gap length is decreased to below 50 prn in 02-saturated silicone oil with an additive concentration above 0.05 Mol.

THE CROSSOVER POINT OSHINO et al. [lo] did not investigate the influence Y of the material of the sphere electrode but all their

results, obtained using a stainless steel sphere and degassed liquids, show that all the V - d characteristics are concave with respect t o the gap axis. The results obtained using perfluorocarbon liquids indicate that the crossover gap is much longer than for the corresponding hydrocarbon liquids (e. g. hexane and perfluorohexane) and that its length increases with increasing perfluo- rination. As already mentioned, these authors linked the length of the crossover gap to the electron mobility. The electron mobility referred to is apparently that of the fast negative carriers reported to exist in liquid di- electrics by Minday et al. [ll] and by Schmidt and Allen

I E E E Transactions on Electrical Insulation Vol. 23 No. 6 , December 1988 969

[12] and shown to be strongly dependent on the molecular structure of the liquid. Such a conclusion appears to be in agreement with some of the present results. Thus the critical gap length for pure degassed 1-MN is smaller than that for degassed silicone oil, as expected. More- over, with increasing concentrations of 1-MN in silicone oil there was a reduction in the length of the critical gap (Figure 8) as compared with that for the pure degassed oil.

The large increase in the length of the critical gap, brought about by the presence of oxygen in silicone, may appear to be in agreement with the large decrease in mobility due to the electron scavenging properties of oxygen. In fact Schmidt and Allen have reported that the fast current carriers in dielectric liquids (excess electrons) disappeared when either oxygen or SFc were dissolved in the liquid. On the other hand Minday et al. have reported that the presence of oxygen (or water) in hexane does not lead to the complete suppression of the fast carriers but only to increase in the ratio of ions to electrons. The above interpretation however, is in- compatible with the fact that the presence of oxygen in 1-MN produces a marked decrease in the critical gap length, a result which confirms the earlier finding that oxygen has a similar effect in the case of mineral oils- transformer oil and liquid paraffin [2,5]. It would appear therefore that electron mobility per se is not the only factor which affects the length of the critical gap. The critical length will depend on the extent of propagation of positive and negative streamers from the point electrode and negative or positive backstreamers initiated a t the sphere electrode.

CONCLUSIONS ROM the experimental results reported in this paper F the following conclusions may be drawn.

(1) For degassed silicone oil the shape of the V - d characteristics depends on the material of the sphere electrode. For a particular material the shapes of the characteristics are similar to the corresponding ones obtained for degassed transformer oil and liquid paraffin. For 02-saturated silicone oil and an A1 sphere there is a marked change in the shape of the V - d characteristics for both polarities of the point electrode when compared with those for degassed oil.

(2) The shapes of the V - d characteristics for degassed 1-MN and an A1 sphere are similar to those obtained for 02-saturated oils indicating that the electron

trapping properties of 1-MN and oxygen in conjunction with the nature of the electrode material (and hence space charges) influence the shape of the V - d characteristics. This is supported by the gradual change which takes place in the shape of the V - d characteristics obtained with a gradual increase in concentration of 1-MN in degassed silicone oil with an A1 sphere.

(3) The addition of 1-MN to degassed silicone oil decreased the crossover gap length whereas the addition of oxygen produced a large increase in this length. Both these results appear to be in agreement with the hy- pothesis that the crossover gap length varies inversely as the electron mobility.

(4) In 1-MN, transformer oil and liquid paraffin the presence of oxygen produced a decrease in the crossover gap length; this result is inconsistent with the mobility hypot hesis.

(5) The addition of varying concentrations of 1-MN to degassed silicone oil produced a small decrease in the crossover gap. However, the addition of 1-MN to oxygen-saturated oil produced an unexpectedly sharp decrease in the crossover gap. This indicates that the oxygen interacts preferentially with the 1-MN greatly enhancing its effect.

REFERENCES

A. A. Zaky, I. Y. Megahed and S. Yehia, “The Direct Breakdown Voltage of Silicone Oil Under Uniform Fields”, IEEE Trans. Electr. Insul. Vol. 20, pp. 333- 337, 1985.

S. Yehia, A. A. Zaky and I. Y. Megahed, “Some Fac- tors Affecting the Direct Breakdown Voltage of Sil- icone Oil Under Nonuniform Fields”, IEEE Trans. Electr. Insul. vo1.18, pp. 86-88, 1983.

I. Y. Megnhed and A. A. Zaky, “Effect of Elec- trode Material, Oxygen and Organic Additive on the Breakdown Strength of Mineral Oil Under Nonuni- form Fields”, J . Electrostatics, Vol. 12, pp. 345-351, 1982.

A. A. Zaky, A. Nosseir, I. Y. Megahed and C. Evan- gelou, “Electrical Breakdown of Mineral Oil Under Uniform Fields”, J. Phys. D: Appl. Phys., Vol. 10, pp. 1761-1767, 1977.

9 70 Zeky et al.: Effect of electrode polarity and additives under highly non-uniform fields

[5] A. A. Zaky, A. Nosseir, I. Y. Megahed and C. Evan- gelou, “Electrical Breakdown of Mineral Oil Under Nonuniform Fields”, J . Phys., D: Appl. Phys., Vol. 9, pp. 2287-2293, 1976.

[6] K. N. Mathes, “Impulse and Surge Voltage Break- down in Oil - The Influence of Wave Shape and Oil Composition”, paper presented at local section of JIEE, Osaka, Japan, Sept. 1975.

[7] D. 0. Cowan and R. L. Driska,Elernents of Organic Photochemistry, ‘Plenum Press, N. Y., 1976.

[8] C. H. Gosling and H. Tropper, “The Direct Voltage Breakdown Strength of Purified Mineral Oils”, IEE Conference on Dielectric and Insulating Materials, London, 1964.

[9] L. Angerer, “Effect of organic Additives on Electrical Breakdown in Transformer Oil and Liquid Paraffin”, Proc. IEE, Vol. 112, pp. 1025-1034, 1965.

[lo] K. Yoshino, K. Ohseko, M. Shiraishi, M. Terauch and Y. Inuishi, “Dependence of Polarity Effect of Dielec- tric Breakdown on Molecular Structure of Liquids”, J. Electrostatics, Vol. 12, pp. 323-332, 1982.

[ll] R. M. Minday, L. D. Schmidt and H. T. Davis, “Ex- cess Electrons in Liquid Hydrocarbons”, J . Chem. Phys., Vol. 54, pp. 3112-3125, 1971.

[12] W. F. Schmidt and A. 0. Allen, “Mobility of Elec- trons in Dielectric Liquids”, J . Chem. Phys., Vol. 52, pp. 4788-4794, 1970.

Manuscript was received on 2 Sep 1987, in revised form 9 Feb 1988.

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Effect of electrode polarity and additives on the direct breakdown voltage of silicon oil under highly non-uniform fields