The properties of insulating materials are enumerated and discussed as under: 1. Electrical Properties 2. Thermal Properties 3. Chemical Properties 4. Mechanical Properties.
1. Electrical Properties of Insulating Materials:
i. Insulation Resistance:
It may be defined as the resistance between two conductors (or systems of conductors) usually separated by insulating materials. It is the total resistance in respect of two parallel paths, one through the body and other over the surface of the body.
Insulation resistance is affected by the following factors:
ADVERTISEMENTS:
(i) It falls with increase in temperature. In some cases there is a marked decrease in insulation resistance; e.g., by raising the temperature of Indian rubber by 15°C, the resistance is found to be halved.
(ii) The resistivity of the insulator is considerably lowered in the presence of moisture.
(iii) It decreases with the increase in applied voltage. The insulation resistance is determined either directly by an ohm meter (or meggar insulation tester) or indirectly by measuring the total leakage current when a definite direct voltage (about 500 V) is applied, precautions being taken in each case to avoid electrostatic (capacitance) effects by taking reading one minute after the application of the voltage.
Resistivity:
ADVERTISEMENTS:
This is usually measured as insulation resistance. This term when applied to insulating materials needs qualification to indicate whether it refers, to volume or surface.
Volume Resistivity:
Volume resistivity is the resistance between opposite faces of a cube of unit dimensions; it is usually expressed in mega ohm-centimetres. The volume resistivity of most insulating materials is affected by temperature, the resistivity decreasing with an increase of temperature, i.e., the temperature co-efficient of resistivity is negative.
Surface Resistivity:
ADVERTISEMENTS:
Surface resistivity is the resistance between the opposite sides of a square of unit dimension on the surface of the materials, it is usually expressed in mega ohms per centimetre square. The surface resistivity of any square on the surface of materials however, is independent of the size of the square provided that the surface resistivity is uniform over the whole surface.
Insulation Resistance of a Cable:
In a cable useful current flows along the axis of the core but there is always present some leakage of current. This leakage is radial i.e., at right angles to the flow of the useful current. The resistance offered to this radial leakage of current is called “insulation resistance” of the cable. If the length of the cable is greater, the leakage area is also greater meaning thereby that more current will leak. In other words insulation resistance is decreased. Hence the insulation resistance is inversely proportional to the length of the cable.
ii. Dielectric Strength:
If the voltage across an insulating materials is increased slowly the way in which the current increases depends upon the nature and condition of the material as illustrated schematically in Fig. 7.37. For material I, the current increase very slowly and approximately linearly with voltage until a large, sharp increase result in what can be described disruptive dielectric breakdown.
In contrast, for material II the current increases more rapidly until current “runway” occurs. It can be shown that the voltage at which current “run way” occurs depends upon the rate at which the voltage is increased, so that a more definite though arbitrary, value of dielectric breakdown may be obtained. It is also observed that with a relatively slow increase in voltage material II may show a very marked increase in temperature and the failure is then termed thermal breakdown.
The potential gradient at which breakdown occurs is termed as dielectric strength. It is easily calculated for uniform fields by dividing the breakdown voltage by insulation thickness. Non-uniform fields are common however, as indicated by a flux plot for a cable insulation, and the higher dielectric stress must be taken into account in actual design.
The dielectric strength of an insulating material decreases with the length of time that voltage is applied. Moisture, contamination, elevated temperatures, heat ageing, mechanical stress, and other factors may also markedly decrease dielectric strength to as little as 10% of the short time values at standard laboratory condition. At radio frequency the dielectric strength may also be considerably less than 60 Hz.
Dielectric failure that occurs along the interface between a solid insulating material and air, or a liquid insulating material is termed “surface breakdown”. Allowable design values for voltages stress along a surface may be even lower than those allowed for the material itself, since effect of contamination and voltage-stress concentration may be even more important at the surface. Minimum creepage and clearance disturbance may also be needed to avoid chance shorting from external objects.
The values of dielectric strength, despite the limitations discussed, is useful in comparing insulating materials, determining the effect of environmental and operating conditions, measuring uniformity, and controlling acceptance of the material.
iii. Power Factor:
Power factor is a measure of the power loss in the insulation and should be low. It varies with the temperature and usually increases with the rise in temperature of the insulation. A rapid increase indicates danger.
iv. Dielectric Constant (Permittivity):
This property is defined as the ratio of the electric flux density in the material to that produced in free space by the same electric force. It is an important property of dielectrics for capacitors as, for a given thickness of dielectric and dimensions of plates, the capacitance is directly proportional to the permittivity; hence materials of high permittivity are preferred in capacitors where economy of space is desired.
The permittivity has an important effect on the voltage gradients and electric stresses when dissimilar insulating materials are arranged in series, the individual voltage gradient being inversely proportional to the permittivities.
The dielectric constant is a measure of the electrostatic energy stored in the insulating material per unit volume under one unit of voltage gradient. It is dependent also a temperature, moisture, exposure frequency and other factors. For design purposes it is particularly important that both power factor and dielectric constant should be determined for the condition involved in the expected application.
Power factor and dielectric constant at power frequencies can be used to compare insulating materials and determine the effect of environment and operating conditions. When measured at high voltage, power factor and dielectric constant are useful in evaluating high-voltage insulation system.
The dielectric constants of several materials commonly used for insulation are given below:
v. Dielectric Loss:
The dielectric losses occur in all solid and liquid dielectrics due to:
(i) A conduction current, and
(ii) Hysteresis.
The conduction current is due to imperfect insulating qualities of the dielectric and is calculated by the application of ohm’s law- it is in phase with the voltage and results in a power (I2R) loss in the material which is dissipated as heat.
Dielectric hysteresis is defined as the lagging of the electric flux behind the electric force producing it so that under varying electric forces a dissipation of energy occurs, the energy loss due to this cause being called the dielectric hysteresis loss. The energy is dissipated as heat.
Dielectric hysteresis is somewhat analogous to magnetic hysteresis, e.g., a varying or alternating electric stress in the dielectric causes continual changes in the orbital paths of the electrons in the atomic structure of the material and a dissipation of energy. Dielectric hysteresis cannot be measured as a separate quantity, and in practice, the total dielectric losses are measured by means of an A.C. bridge usually of the Schering type.
With alternating voltages the dielectric losses cause the total current I (which is the vector sum of the true charging current, Ic, and an inphase component, Ip, supplying the losses) to lead the voltage by an angle slightly less than 90°, i.e., the power factor, in not zero.
The vector diagram is shown in Fig. 7.38 in which OIc, represents the true charging current, OIp the inphase current supplying the losses and OI the resultant, or “apparent” charging current. The losses are given by VIp or VI cos ɸ or VIc tan δ or 2 f CV2 tan δ, where f is the frequency, C the capacitance and δ the complement of the phase angle ɸ (called the loss angle and tan 8 the loss tangent).
The dielectric loss is affected by the following factors:
(i) Presence of humidity … it increase the loss
(ii) Voltage increase … it causes high dielectric loss
(Hi) Temperature rise … it normally increases the loss
(iv) Frequency of applied voltage … the loss increases proportionally with the frequency of applied voltage.
2. Thermal Properties of Insulating Materials:
i. Specific Heat Thermal Conductivity:
In solids there is a wide variation of these properties although in solids electrical and thermal conductivity go together especially very closely in metals. In case of liquids used as coolants in transformers, these properties are of little importance due to small range of variation available in all insulating materials.
Among gases, hydrogen and helium have five to ten times the thermal conductivity and specific heat of other gases and inspite of their lower density, they are very good coolants. Hydrogen is sometimes employed for cooling large generators and when hydrogen is used for such a purpose, windage losses are reduced because of the low density of hydrogen.
ii. Thermal Plasticity:
Pressure on the wires of a wound coil varies under operating conditions because of the expansion and contraction of the parts caused by variations in temperature. Although, in practice, pressure generally is accompanied by slight vibratory motion and consequent abrasion, it is valuable to observe the flow of wire insulation at high temperatures in the absence of vibration.
For this purpose, a device is used that subjects a crossed-wire sample to pressure at an elevated temperature while 120 V direct current is applied. The material under test insulates the wires, and proper care is taken to prevent grounding or shorting elsewhere.
The results of this test are indicative of the resistance of the material to failure caused by pressure or flow of film. Actually, since a high degree of thermoplasticity may be unimportant in many applications or even of considerable benefit, it should not always be viewed with disfavour.
iii. Ignitability:
Insulating materials exposed to arcing should be non-ignitable. In case they are ignitable, they should be self-extinguishing, resistant to cracking or carbonisation of the material.
iv. Softening Point:
The softening point of solid insulating material should be above the temperature occurring in practice.
v. Heat Ageing:
Ageing is, in effect, the wearing out of an insulating material by reducing its resistance to mechanical injury. It increase rapidly with temperature, approximately doubling for each increase of 10°C to 16°C, depending upon the material.
Such increase in temperature causes dehydration of all cellulose materials and an intensification of oxidation and other chemical changes in both cellulose and varnish substances. These are the effects that lead to brittleness, cracking, shrinking, undue vibration and stress, ultimate crumbling and disintegration.
Electrically the material does not wear out until the electrical breakdown occurs; thus, ageing may progress quite far before a mechanical movement breaks the brittle insulation sufficiently for voltage puncture. The standard test for heat ageing has been a flexibility test preceded by ageing in an oven at elevated temperature.
vi. Thermal Expansion:
Thermal expansion is important because of the mechanical effects caused by thermal expansion due to temperature changes. In insulating materials it should be very small.
3. Chemical Properties of Insulating Materials:
i. Resistance to External Chemical Effects:
Insulating materials should be resistant to oils or liquids, gas fumes, acids and alkalies. The materials should not undergo oxidation and hydrolysis even under adverse conditions.
ii. Resistance to Chemicals in Soils:
Cables laid in the soil can deteriorate by the action of chemicals in soils. The suitability of insulating materials for such conditions can be decided by a long experience.
iii. Effect of Water and Tropical Tests:
Water directly lowers electrical properties, such as electrical resistance and dielectric strength. The water may be transmitted through an outside coating and cause damage inside; it may be directly absorbed by an insulating material; it may cause a chemical change of insulation itself; or it may drastically lower the surface resistance of an insulator.
Water will pass less than half as fast through a film made with the newer phenolic-oil type of varnishes than it will pass through the old type of varnishes. No varnish, enamel, lacquer, or paint film is 100 percent water impervious, however, and moisture resistance and water repellence depend a great deal upon the degree of the film, and upon the character of the film-supporting material.
The most absorbent supporting materials are cotton, paper, and asbestos, the water being soaked up by the wick action of the fibres. Under moist conditions or high humidity these materials should be avoided if possible. The effect of water absorption on electrical properties may be determined by measuring dielectric strength, insulation resistance, or power factor after immersion in water or during exposure at high humidity.
4. Mechanical Properties of Insulating Materials:
i. Density:
Electrical insulations are used on the basis of volume and not weight. Insulating material of low density is especially suitable for small portable equipment and aircraft components.
ii. Viscosity:
It is of importance in liquid dielectric. Uniform viscosity provides uniform electrical and thermal properties.
iii. Moisture Absorption:
Water lowers the electrical resistance and dielectric strength. With its absorption certain chemical and mechanical effects may result e.g., swelling, warping and corrosion.
iv. Hardness of Surface:
Hardness of surface enables the dielectric to resist surface scratching and abrasion while lower surface resistivity permits irregular moisture films to form and also contribute to corona and other surface deteriorating effects. Surface roughness is objectionable.
v. Surface Tension:
In liquid dielectrics low surface tension is desirable as it causes greater wetting of the electrical components and thus gives better cooling, impregnation and greater voltage uniformity. To improve this property some wetting agent may be added.
vi. Uniformity:
Dielectric should be uniform throughout as it keeps the electrical losses as low as possible and electric-stresses uniform under high voltage differences.
In case of solid insulators, tensile strength, compressive strength, shear strength, bending strength and impact strength are all important. Machinability and resistance to splitting are equally important.
In addition, there are certain other mechanical properties uniquely important to varnish products. These are bonding penetration and thorough curing.
Bonding is the degree to which a compound binds insulating material and wires into a solid mass. In rotating armatures, considerable stress on insulating materials is caused by centrifugal forces. The insulating material should not deform plastically and break the bond of wire under such treatment.
Furthermore heat from the windings must be dissipated through the insulation to the surroundings. Thus bonding serves two functions- (i) it binds the conductors together, thus minimizing movement and consequent abrasion, and it also improves the heat conductivity of the conductor mass. Bonding strength is typically listed as high, medium or low.
Penetration is the degree to which a compound will penetrate its supporting structure and it may be recognized as a generalised function of viscosity, surface tension, and the ability to wet the structure. Filling is related to the ability of a compound to produce a void free structure.