3.3 CONDUCTION AND BREAKDOWN IN PURE LIQUIDS
3.4.4 Stressed Oil Volume Theory
In commercial liquids where minute traces of impurities are present, the breakdown strength is determined by the "largest possible impurity" or "weak link1'. On a statistical basis it was proposed that the electrical breakdown strength of the oil is defined by the weakest region in the oil, namely, the region which is stressed to the maximum and by the volume of oil included in that region. In non-uniform fields, the stressed oil volume is taken as the volume which is contained between the maximum stress (£max) contour and 0.9 Emax contour. According to this theory the breakdown strength is inversely proportional to the stressed oil volume.
The breakdown voltage is highly influenced by the gas content in the oil, the viscosity of the oil, and the presence of other impurities. These being uniformly distributed, increase in the stressed oil volume consequently results in a reduction in the breakdown voltage. The variation of the breakdown voltage stress with the stressed oil volume is shown in Fig. 3.7.
Stressed oil volume (CC)
Fig. 3.7 Power frequency (50 Hz) a.c. breakdown stress as a function of the stressed oil volume
• With steady voltage rise x One minute withstand voltage 3.4.5 Conclusions
All the theories discussed above do not consider the dependence of breakdown strength on the gap length. They all try to account for the maximum obtainable breakdown strength only. However, the experimental evidence showed that the breakdown strength of a liquid depends on the gap length, given by the following expression,
Vb = Adn (3.6)
where, d = gap length, A = constant, and
n a constant, always less than 1.
The breakdown voltage also depends on the nature of the voltage, the mode in which the voltage is applied, and the time of application. The above relationship is of practical importance, and the electrical stress of a given oil used in design is obtained frcm this. During the last ten years, research work is directed on the measurements of discharge inception levels in oil and the breakdown strengths of large volumes of oil under different conditions.
It may be summarized that the actual mechanism of breakdown in oil is not a simple phenomenon and the breakdown voltages are determined by experimental investigations only. Electrical stresses obtained ior small volumes should not be used in the case of large volumes. As a general guideline, the properties of good dielectric oils for electrical purpose are tabulated (see Table 3.1), and the designer should satisfy himself on all the properties before acceptance.
Breakdown stress (kV/cm)
Table 3.3 gives the typical breakdown strengths for highly purified liquids and the design stresses actually used. A factor of safety of about 10 is used as can be seen from the data in the table. The reasons for such an approach can be understood from the various factors considered in the breakdown theories discussed.
Table 3.3 Power Frequency Design Field Strengths and Breakdown Field Strengths for Highly Purified Dielectric Liquids—a Comparison Dielectric liquid Used in Design field Breakdown
(equipment) strength (MV/m) Strength (MV/m) Transformer oil Transformers 2-5 100 n-hexane Cables 13-20 132 Polybutane Capacitors 10-25 109 (Synthetic
hydrocarbon)
In the next 20 to 30 years, transformer oils, which are derivatives of petroleum crude, may be in short supply. Therefore, various synthetic insulating oils are being examined as potential insultants in high voltage apparatus. Among the synthetic oils, polybutane liquids have been used for some years in cables and paper capacitors. They are superior to transformer oils in various electrical properties including dielectric strength. Fluorocarbon and silicone liquids are also used in special apparatus. How- ever, their use will require further evaluation of their properties. Further details can be found in refeences 7,9 andlO cited at the end of this chapter.
QUESTIONS
Q.3.1 Explain the phenomena of electrical conduction in liquids. How does it differ from that in gases?
Q3.2 What are commercial liquid dielectrics, and how are they different from pure liquid dielectrics?
Q.3.3 What are the factors that influence conduction in pure liquid dielectrics and in commercial liquid dielectrics?
Q.3.4 Explain the various theories that explain breakdown in commercial liquid dielectrics.
Q.3.5 What is "stressed oil volume theory", and how does it explain breakdown in large volumes of commercial liquid dielectrics?
WORKED EXAMPLES
Example 3.1:. In an experiment for determining the breakdown strength of trans- former oil, the following observations were made. Determine the power law deen- dence between the gap spacing and the applied voltage of the oil.
Gap spacing (mm) : 4 6 10 12 Voltage at break-
down (kV) : 90 140 210 255
The relationship between the breakdown voltage and gap is normally given as V= Kef1
or, log V= log K+ n log d i.e., log V - log K = n log d
log!/-log*
or, n = — ,—^ — log d
= slope of the straight line as shown in Fig. E.3.1.
= 0.947 From Fig. E.3.L,
K= 24.5
.•. Relationship between the breakdown voltage and the gap spacing for the transformer oil studied is
V= 24.5 d0'947 where V is in kV and d is in mm.
Gas spacing, d (mm)
Fig. E.3.1 Breakdown voltage as a function of gap spacing
Breakdown voltage, kV
REFERENCES
1. Adam Czewski, L, lonization, Conduction and Breakdown in Dielectric Liquids, Taylor and Francis, London (1969).
2. Gallager, TJ., Simple Dielectric Liquids, Clarendon Press, Oxford (1975).
3. Hawley, R. and Zaky, A.A., Conduction and Breakdown in Mineral OU, Peter Peregrinus, London (1973).
4. Alston, L.L, High Voltage Technology, Oxford University Press, Oxford (1968).
5. Lewis, TJ., Progress in Dielectrics, Vol. 1, Heywood, London (1959), pp. 97-140.
6. Sharbough, A. and Watson, P.K., Progress in Dielectrics, vol. 4, Heywood, London (1962), pp. 199-248.
7. Code of practice for maintenance and supervision of insulating oil in equipment, IS:
1866-1983-, also IS: 6262-1971 (for tan 6 measurement) and /5: 6792-1972 (for measurement of electrical strength).
8. "Specifications for new insulating oils for transformers and switchgear", IEC No.
269,1969.
9. "Oxidation tests for inhibited oils",/£C No. 474,1974.
10* Wilson/ A.C.M., Insulating liquids: Their uses, manufacture and properties, Peter Peregrinus and IEE, London (1980).
11. "BEE Colloquim on New Dielectric Fluids for Power Engineering" IEE, London (1980).
4
Breakdown in Solid
Dielectrics