0.8 LO 1.2 Trichlorodiphenyl
4.7.8 Epoxy Resins
Table 4.12 Dielectric Properties of Polyesters
Property Glass reinforced type Cast resins Premixed Preformed Rigid Flexible Volume resistivity 1012-1015 IO14 1012-1014 —
(ohm-cm) Dielectric constant
50 Hz 5.3-73 3.8-6.0 3.3-4.3 4.4-8.1 IO3 Hz 4-68 4-G-6-0 3.2-4.3 4.5-7.1 IO6 Hz 5.6-6A 3.5-5.5 3.2-4.3 4.1-5.7 tan S
50 Hz 0.01-0.04 0.01-0.04 0.006-0.05 0.026-0.031 IO3 Hz — 0.01-0.05 0.006-0.04 0.016-0.050 IO6 Hz 0.008-0.022 0.01-0.03 0.017-0.019 0.020-0.060 Mylor polyester film is being largely used in preference to paper insulation. At power frequencies, its dissipation factor is very low, and it decreases as the tempera- ture increases. It has got a dielectric strength of 2000 kV/cmf and its volume resistivity is better than IO15 ohm-cm at 10O0C. Its high softening point enables it to be used at temperatures above the operating limit of paper insulation. It has got high resistance to weathering and can be buried under the soil also. Therefore, this can be used for motor and transformer insulation at power frequencies and also for high frequency applications which are subjected to varying weather conditions.
Polystyrenes
Polystyrenes are obtained when styrene is polymerized with itself or with other polymers or monomers producing a variety of thermoplastic materials with varying properties in different colours. Electrical grade polystyrenes have a dielectric strength comparable to that of mica, and have low dielectric losses which are independent of the frequency. Their volume resistivity is about IO19 ohm-cm and the dielectric strength is 200-350 kV/cm. The dielectric constant at 2O0C is 2.55, and the loss tangent is 0.0002 at all frequencies up to 10,000 MHz.
Polystyrene films are extensively used in the manufacture of low loss capacitors, which will have a very stable capacitance and extremely high insulation resistance.
Films and drawn threads of polystyrene are also used for high frequency and cable insulations.
modified either by the selection of a curing agent or by the use of modifiers or fillers.
They are highly elastic; samples tested under very high pressures, up to about 180,00 psi (12,000 atm) returned to their original shape after the load was removed, and the sample showed no permanent damage. Resistance to weathering and chemicals is also very good. The tensile strength of araldite CT 200 and hardner HV 901 is in the range 5.5-8.5 kg/mm2, and the compressive strength is 11-13 kg/mm2. The dielectric constant varies between 2.5 and 3.8. The dielectric loss factor is very small under power frequency conditions lying in the range 0.003-0.03. The dielectric strength is 75 kV/mm, when the specimen thickness is 0.025 mm or 1 mil. The volume resistivity of the material is of the order of 1013 ohm-cm.
Epoxy resin can be formed into an insulator of any desired shape for almost any type of high voltage application. Insulators, bushings, apparatus, etc. can be made out of epoxy resin. It can also be used for encapsulation of electronic components, generator windings and transformers. It is used for bonding of very diverse materials such as porcelain, wood, metals, plastics, etc. It is a very important adhesive used for sealing of high vacuum joints. In any laboratory or industry in which electrical or electronic components or equipments are handled or manufactured, numerous oc- casions arise wherein epoxy resins can be used with an advantage saving time, labour and money.
In the previous sections details are given of a variety of insulating materials, commonly used for electrical insulation purposes. A good insulating material should have good dielectric strength, high mechanical strength, high thermal conductivity, very low loss factor, and high insulation resistance. The specific application of these materials in various power apparatus, electronic equipments, capacitors and cables are discussed in Chapter 5.
QUESTIONS
Q.4.1 What do you understand by "intrinsic strength'* of a solid dielectric? How does breakdown occur due to electrons in a solid dielectric?
Q.4.2 What is "thermal breakdown" in solid dielectrics, and how is it practically more significant than other mechanisms?
Q.4.3 Explain the different mechanisms by which breakdown occurs in solid dielectrics in practice.
Q.4.4 How does the "internal discharge" phenomena lead to breakdown in solid dielectrics?
Q.4.5 What is a composite dielectric and what are its properties?
Q.4.6 Describe the mechanism of short-term breakdown of composite insulation.
Q.4.7 How do the temperature and moisture affect the breakdown strength of solid dielectrics?
Q.4.8 What are the advantages of using plastic film insulation over the paper insulation?
Q.4.9 What are the properties that make plastics more suitable as insulating materials?
Q.4.10 What are the special features of epoxy resin insulation?
WORKED EXAMPLES
Example 4.1: A solid specimen of dielectric has a dielectric constant of 4.2, and tan 8 as 0.001 at a frequency of SO Hz. If it is subjected to an alternating field of SO kV/cm, calculate the heat generated in the specimen due to the dielectric loss.
Solution: Dielectric heat loss at any electric stress E [Eq. (4.S)]
£2/er tan 8 .
= r |2 W/cm3 1.8 XlO1 2
For the specimen under study, the heat loss will be
SO x SO x IQ6 x SO x4.2 x .001 1.8 x 1012
= 0.291 mW/cm3
Example 4.2: A solid dielectric specimen of dielectric constant of 4.0 shown in the figure has an internal void of thickness 1 mm. The specimen is 1 cm thick and is subjected to a voltage of 80 kV (rms). If the void is filled with air and if the breakdown strength of air can be taken as 30 k V (peak)/cm, find the voltage at which an internal discharge can occur.
Solution: Referring to Fig. 4.5(a) and Eqs. (4.7) and (4.8), the'voltage that appears across the void is given as
Vd1
"'V^o
where, d\ = 1 mm d2 = 9 mm
E0= 8.89xUT12F/m
ei = e^o = 4-0^
''-rTi
HJ (w\
•l»J
The voltage at which the air void of 1 mm thickness breaks down is 3 kV/mm x 1 mm
= 3kV
13V 13x3 39
•'•vl~ 4 " 4 " 4
= 9.75 kV (peak)
The internal discharges appear in the sinusoidal voltage 80 sin co t kV when the voltage reaches a value of 9.75 kV (see Fig. 4.6 for the discharge pattern).
Example 4^: A coaxial cylindrical capacitor is to be designed with an effective length of 20 cm. The capacitor is expected to have a capacitance of 1000 pF and to operate at 15 kV, 500 kHz. Select a suitable insulating material and give the dimen- sions of the electrodes.
Solution: The capacitance of the coaxial cylindrical capacitor is
271 £0 er '
^ W«2
^T
d\
where / = length in metres, d\ and d^ are the diameters of the inner and outer electrodes, and er = dielectric constant The dielectric material that can be selected is either polyethylene or P.T.F.E.
Choosing high density polyethylene, its dielectric constant er = 2.3, and its breakdown stress is taken as 500 V/mil or 200 kV/cm. Allowing a factor of safety of 4, the maximum stress Emax = 50 kV/cm. £max occurs near the inner electrode and is given by
*-- -^7 (2)
T1In^.
from equation (1),
dz r2 27Ce0E,/
in —7" — in — —
di T1 capacitance 2i4£- x 2.3 x 0.2
_ 36n
10Ox 10~
12= 0.02556 /. — = 1.026fi
rl From equation (2),
V . r,J^
T1= — InJ.
^max T1
= 15 50x0.02556
= 11.74 cm .'. r2 =1.026x11.74
= 12.05cm
The thickness of the insulation is 3.1 mm (refer to Tables 4.8 and 4.9 for the properties of the material).
REFERENCES
1. Of Dwyer, J.J., Theory of Dielectric Breakdown in Solids, Clarendon Press, Oxford (1964).
2. Whitehead, S., Dielectric Breakdown of Solids, Oxford University Press, Oxford (1951).
3. Von Hippel, A., Dielectric Materials and Applications, John Wiley, New York (1964).
4. Mason, J.H., "Electrical insulation", Electrical Energy, Vol. 1, 68-75 (1956).
5. Taylor, H.E., Modern Dielectric Materials (Ed. J.B. Birks), Chap. 9, Haywood, London (1960).
6. Clark, P.M., Insulating Materials for Design and Engineering Practice, John Wiley, New York (1962).
7. Bradley, A., Electrical Insulation, Peter Peregrinus, London (1984).
5
Applications of
Insulating Materials