CHANGES I N PHYSICAL AND ELECTRICAL PROPERTIES OF A MINERAL INSULATING OIL, HEATED I N CONTACT WITH AIR* BY HUBERT H. RACE
I. Object To obtain a clearer insight into the changes in a mineral insulating oil resulting from heating in contact with air, the following properties were studied in portions of the same oil which had been oxidized under different conditions. A. D-C. conductivity at I ISOC. B. Dielectric losses at 3oOC. for frequencies between 103 and IO^ cycles per second. C. Oil spreading on water using Langmuir's method. D. Ultramicroscope particle count. E. Viscosity using modified Ostwald viscometer. F. Refractive index using a Pulfrich refractometer. G. Acid number. These chemical, physical and electrical tests were all made on the same samples so as t o correlate the results of the different types of measurement and determine whether sensitive electrical equipment could be advantageously used to supplement the usual chemical and physical tests. 11. Preparation of Samples TABLEI Physical Data for No. 5317 Oil (I) Flash Point 2 2 5°C. (2) Fire Point 265'C. (3) Acid value 4 x IO+ grams KOH per grams of oil (4) Density at 3oOC. 0.935 grams per cc. (5) Viscosity a t 3oOC. IO poises A portion of commercial cable oil having the general characteristics shown in Table I was divided into nine equal samples which were aged in contact with air in a G. E. Life Test oven according to the schedule shown in the first three columns of Table 11. This procedure was used so that the results could be interpreted directly in terms of standard practice in the Cable Plant where resistivity measurements before and after heating 96 hours a t I IS'C. are used
* This paper is based in art upon data presented at the fourth annual conference of the committee on electrical insufation, division of en ineering and industrial research, National Research Council, held a t Harvard University, Earnbridge, Mass., Nov. 13-14, 1931. See Elec. Engineering, 51, 33 (1932).
CHANGES IN PROPERTIES OF A MINERAL INSULATING OIL
I929
in factory control. The time schedule for heating at the different temperatures was chosen so that corresponding samples a t the different temperatures would show approximately the same change in conductivity. This was estimated from preliminary data on changes in conductivity with heating temperature. 111. Experimental Procedure and Data
.
.
A.-D. C. Conductivity ut 116°C. About two years ago the small oil conductivity testing set shown in Fig. I was developed for measuring the direct current conductivity of insulating oils. The wiring diagram for this set is shown in Fig. 2. Its main advantages are ease of operation and good control of the temperature of the sample under test. The extremely good electrical properties of these insulating oils require special care in the design of the testing cell. Fig. 3 shows such a cell, made of stainless steel
FIG.I Oil Resistivity Testing Set,
N O WLT, COCVCLC SarRTE
FIG.2 I. 2.
3.
4.
5. 6.
7.
8. 9. IO. I I. 12.
13.
14. 15. 16.
Wiring Diagram Two-circuit Heater Switch Safety Switches in Doors. Transformer Switch. Ten-second delay Relay. Filament Transformer Y 2006. Plate Transformer U P 1016. Two Mercury Rectifier Tubes-PJ 28. Two 800 volt, I microfarad Capacitors. roB-ohmwire wound Resistor. Special Galvanometer Switch. Portable Insulation Testing Galvanometer. Quartz insulated Oil Testin Cell. Heater (not used when a ce% is placed on an oil storage tank). Textolite insulation between Cell and Heater. Thermostat for controlling Heater (not necesaary). Variable Resistance for low Heat Control.
I930
HUBERT H. RACE
TABLE I1 Summary of Experimental Results obtained by heating No. 5317 Oil in Contact with Air A B C Conductivity Max. loss Spreading at I 1j0c. Factor Area Oxidation Y 6“m A, (nu- Sq. em. per Time TEmp. mho. per em. cube meric) gram of oil hours C. ~~
Sample No.
x
X
10-12
103
D E F -~~ Particle count Viscosity Index of First Second at 3oOC. Refraction at 30°C. No. per No. per (r) C . C . of oil C.C. of oil poises (numeric)
‘
x
IO^
X IO^
1.02
2.11
3.3
10.7
1.5098
2.20
12.0
4 5
I39
2.62
56
10.4
I . jog8
11.8 0.84
2.74
26.0
33
51
10.0
1.5102
9.7
I . ~ I O I
2.17
2.26
11.0
8.35
2.6
1.5095 1.5108
0.75 1.45
2.15
4.55
2.15
3.4 6.7
67
22.0
89 166
253 41 4j
2.27
3.0 7.0
136 32
5 75
9.j 11.9
2.52
21.0
103
3
11.9
10.7
1.;108 1.5110 I
with fused quartz insulation. The inner electrode ( I ) is a hollow cylinder to which hemispherical ends have been welded using an atomic hydrogen flame. The bottom of the outer cylinder (2) has a central conical hole into which the fused quartz post (3) fits. This post is held by a friction fit in a central hole in ( I ) but it may be removed for cleaning. The top of the inner electrode is centered by a fused quartz collar (4) which is also made with a sliding fit so that the inner electrode may be easily removed for cleaning. The small cylinder ( 5 ) serves as a high lead to the cell, as the top centering post and as a container for a thermometer to indicate the temperature of the inner electrode. The major advantages of this cell are as follows: (a) I t is made of materials which are not attacked by the oil and do not act as catalysts on the oil. (b) It is easily taken apart for cleaning, and (c) Its calibration remains constant even though it is repeatedly taken apart for cleaning. With the above equipment the conductivity of each sample wasdetermined at I I ~ O C . , as is shown in Table 11. The curves in Fig. 4, plotted from these data, indicate that, the longer the aging FIG.3 - - time, the greater i s the effect of OilResistivityTestingCell high temperature on the conductivity.
f
CHANGES IN PROPERTIES O F A MINERAL INSULATING OIL
‘931
FIG.4 Variation of Conductivity resulting from heating 5317 Oil in Air a t different Temperatures
B.-High Frequency Measurements of Electrical Properties. Previous work on the high frequency electrical properties of insulating oils has suggested the presence of polar molecules in oil and t,he possibility of estimating their It has been found convenient to interpret the high frequency measurements in terms of the equivalent parallel circuit shown in Fig. 5 . The com-
FIG.5 Parallel circuit electrically equivalent t o Sample
plex expression for the reciprocal impedance of the measuring circuit is given by the relation z-1 = G, + iwC, = wc, (E”-k id) in which G, = the equivalent parallel conductance of the sample C, = the equivalent parallel capacitance of the sample w = m f , where f = electrical frequency in cycles per second /i = Y - I C, = capacitance of the same geometric arrangement of electrodes in vacuum. D. W. Kitchin and Hans Muller: Phys. Rev., ( 2 ) 32, 979 (1928). H. H. Race: Phys. Rev., ( 2 ) 37, 430-446 (1931).
I932
HUBERT H. RACE
For comparative purposes it is convenient to study unit quantities which may be defined by comparing the two forms of equation ( I ) as follows: (C,/C,) = capacitance factor* (Dielectric Constant) (GJwC,) = loss factor sin tan+ (
