Feb 1, 1987 - inhibited naphthenic and paraffinic transformer oils were measured as a ... the ASTM D2440 test method by heating 25 g of oil in a test tube to 110°C ... of the two oils, measurements were performed on filter- ed and unfiltered ...
IEEE Transactions on Electrical Insulation Vol.
EI-22
No.1. February
1987
57
INFLUENCE OF OXIDATION ON THE ELECTRICAL PROPERTIES OF INHIBITED NAPHTHENIC AND PARAFFINIC TRANSFORMER OILS C. Lamarre, J. P. Crine and M. Duval
Institut de Recherche d'Hydro-Quebec Varennes, Quebec Canada
ABSTRACT The dc electrical conductivity and dielectric losses of inhibited naphthenic and paraffinic transformer oils were measured as a function of the oxidation time. It is shown that both properties vary with the dissolved-copper, peroxide and soluble-acidity contents of the oils and that these variations in the electrical properties with oxidation time are directly related to the generation of the aging by-products. Empirical equations relying on these three parameters and describing the conductivity and tan6 of oxidized oil are presented. The role of the chemical structure of the two oils is briefly discussed as well as the effects of filtering the oils after sludge formation. INTRODUCTION To simulate the thermal oxidation of transformer oils it is customary to oxidize the oil under accelerated conditions [1-6]. It is well known that thermal oxidation induces significant variations in the oil's electrical properties [1-6]. However, despite the fact that the dissolved-copper content [1] and polar and acidic by-products [3-5] are known to modify the dielectric losses of oxidized oils, an exact correlation between the different parameters has never been clearly established. In this paper it is shown that the variations in tan6 and conductivity of a naphthenic and a paraffinic transformers oil with the oxidation time correspond to a number of well-identified chemical processes and products in the oils. It is also shown that tan6 and conductivity vary differently as a function of the same parameters.
EXPERIMENTAL CONDITIONS The naphthenic and paraffinic transformer oils were commercially available products containing -800 ppm of antioxidant (DBPC). They were oxidized according to the ASTM D2440 test method by heating 25 g of oil in a test tube to 110°C under a flow of dry oxygen (-1 1/h) in the presence of a known amount of copper wire. In order to evaluate precisely the influence of sludge on the various physicochemical and electrical properties of the two oils, measurements were performed on filtered and unfiltered samples. Portions of the unfiltered oil samples were transferred at 110°C from the test tubes to the measurement cells. The remaining portions were then allowed to cool to 22°C for filtering, since sludge is soluble in oil at 110°C.
The soluble-acidity and peroxide contents of the oilfiltered samples were then determined at 220C. The soluble acidity was measured by the ASTM D974 test method modified to yield more reliable results. The modification consisted in using a 0.01 N solution of KOH in isopropanol and determining the neutralization point by spectrophotometric titration [7]. Wheeler's method [8] was used to measure the peroxide (including
hydroperoxide) content.
The dissolved-copper content, electrical conductivity
cy and tan6 values were measured at 22°C in both the
filtered and the unfiltered oil. The copper content was determined in -1 g of oil by neutron activation analysis (NAA) with a precision of -+10 ppb. The electrical conductivity and tan6 were measured in
a Balsbaugh MC 100 cell. The permittivity measurements were made at a frequency of 60 Hz under an ac field of -200 V/cm with a General Radio bridge, model 1616. The
dc conductivity was measured under a field of 3X103 V/cm applied for at least 30 min, which means that the current was measured under quasi-equilibrium conditions. In addition, the contents of antioxidant, sludge, carbonyl groups and polar-oxidation products were determined; details are given elsewhere [5,9].
SOLUBLE-ACIDITY, PEROXIDE AND COPPER CONTENTS SoZubZe-acidity and peroxide contents The variations in the soluble acidity of the naphthenic and paraffinic oils with time are shown in Fig. 1. The low acidity values at short times correspond to the oxidation induction time, when the oils are still
0018-9367/87/0200-0057$01.00 (D 1987 IEEE
IEEE Transactions on Electrical Insulation Vol. EI-22
5B
No.l.
February 1987
Mills [1] also observed a peroxide peak but at a much earlier time, probably because they were not using an inhibited oil. To the best of our knowledge there are no peroxide-content measurements on inhibited transformer oil reported in the literature. Actually, the antioxidant delays not only the onset of peroxide formation but also the rapid increase of acidity and sludge.
DissoZved-copper content
96 72 Oxidation time (h)
Fig. 1: SoZubZe acidity of naphthenic (,) and paraffinic (x) oils as a function of oxidation time (arrows correspond to sZudge formation). inhibited. Actually, very little oxidation by-products are formed during the induction period [5,9]. The sharp increase in acidity [9] and polar compounds [5] occurs when no antioxidant is left in the oil. Sludge formation (arrows in Fig. 1) starts at 75 h for the paraffinic oil and at 84 h for the naphthenic oil, corresponding to the increase in acidity and formation of polar compounds [5,9]. The peroxide content is shown as a function of oxidation time in Fig. 2. Peroxides appear only at the IIANr fi.
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Fig. 2: Peroxide contents of naphthenic (e) and paraffinic (x) oils as a function of oxidation time. end of the induction period, their content reaching a maximum at sludge formation (arrows in Fig. 2). After this the peroxide content decreases rapidly and levels off, while sludge, polar compounds and acid products The fact that are formed in increasing quantities. peroxides are detected only when no antioxidant is left, is consistent with the known property of antioxidants to act as peroxide decomposers [10]. Melchiore and
Fig. 3 shows the copper content before (solid line) and after (dashed line) filtering as a function of oxidation time for both naphthenic and paraffinic oils. The copper content increases rapidly as soon as the oxidation test begins. There appears to be a first concentration peak at an early oxidation stage, but the concentration then remains constant as long as there is antioxidant left in the oil. When the antioxidant has been consumed completely, the copper content rises again to form a second concentration peak; this corresponds to the formation of peroxide and sludge and also to an increase of the soluble-acidity content (see Figs. 1 and 2). After sludge formation (arrows in Fig. 3), there is an apparent decrease in the copper content of the unfiltered oils, although this is probably an experimental artifact due to the fact that some of the dissolved copper can redeposit on the walls of the test tube and on the copper wire surfaces. To prove this, the test tube which had served to oxidize the paraffinic oil for 120 h was washed very thoroughly with chloroform. The extracted material contained significant amounts of copper. The exact amount of dissolved copper in oil after the beginning of sludge formation is therefore much higher than the values reported in Fig. 3 for unfiltered oils. After sludge filtering (dashed line in Fig. 3) the copper content of oil drops to a very low level, which means that most copper precipitates out with the sludge. In fact, some 3700 ppm of copper were measured in the sludge formed after 96 h of naphthenic-oil oxidation. A rough calculation shows that this value is consistent with the assumption that the copper has been transferred from the oil (containing 2 to 3 ppm) to the sludge. Melchiore and Mills [1] suggested that hydroperoxides were responsible for copper dissolution in oil, since dissolved copper and hydroperoxides were formed at the same time in their specific experimental conditions. On the other hand, the present work shows that copper is dissolved in oil well before any measurable peroxides are formed with the result that peroxides are unlikely to be responsible for the early copper dissolution, at least in the presence of an antioxidant. The exact mechanism of copper dissolution in this case is yet unclear and deserves more work. Meanwhile, it is quite possible that the second copper concentration peak (at sludge formation) is indeed related to the formation of peroxides. Once the antioxidant has disappeared, the situation is closer to that reported by Melchiore and Mills [1], where an uninhibited oil had been used.
Sumnary of oxidation mechanism In the first 9 to 12 h copper is rapidly dissolved by a yet unclear mechanism. No peroxides are detected and the soluble-acidity content is very low.
Until ;60 to 70 h the antioxidant content decreases steadily to zero while the acidity remains at a very low value; there are few peroxides and the copper content is almost constant.
59
Lamarre et al.: Electrical properties of transformer oils
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Fig. 3: Copper content of naphthenic (-,o) and paraffinic (x,+) oils as a function of oxidation time after (- - -) and without (-) filtering. Note that the apparent scatter and decrease of the copper content values after sLudge formation are possibly an experimental artefact (see text). When the antioxidant disappears a sharp peroxide peak is observed together with increased copper dissolution, sludge formation and soluble-acidity content. At longer oxidation times, the copper and peroxide tents level off and the acidity content increases.
con-
TAN6 AND DC CONDUCTIVITY RESULTS The variations in tan6 as a function of the oxidation time are shown in Fig. 4 for both oils. A first peak in tan6 is observed in the first 20 h, followed by a plateau up to -60 to 70 h, after which a second increase is measured for both unfiltered oils. The second tan6 peaks correspond to the maximum peroxide content (Fig. 2). When the variations in tan6 in the first 80 to 85 h are compared with those of the dissolved-copper content (Fig. 3), both parameters appear to follow relatively similar patterns. The influence of dissolved
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Fig. 4: Variations in tan6 (220C, 60 Hz) of naphthenic (-) and paraffinic (x) oils as function of oxidation time after (---) and without (_) filtering.
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copper on tan6 has also been reported by Melchiore and Mills [1]. It is particularly evident here on the filtered oils, where the sharp drop in tan6 corresponds exactly to a similar drop in the copper content (Fig. 3). The removal of sludge and, therefore, of a large amount of copper, significantly lowers the tan6 values.
After -90 h of oxidation, the tan6 values of the filtered oils can be seen rising again. As the solubleacidity content (Fig. 1) is the only other parameter that is increasing at this stage (the dissolved-copper and peroxide contents having levelled off), it therefore seems reasonable to relate tan6 and the soluble acidity. The influence of acidity on tan6 has already been noted [3,4] and it might also be felt during the first hours of oxidation (
