Insulation is required to keep electrical conductors separated from each other and from other nearby objects. Ideally, insulation should be totally none conducting, and then currents are totally restricted to the intended conductors. However, insulation does conduct some current and so must be regarded as a material of very high resistivity. In many applications the current flow due to conduction through the insulation is so small that it may be entirely neglected but in some instances it is being measured and used as a test value to determine the suitability of the insulation for use in service.
Although insulating materials are very stable under ordinary circumstances, but part of the insulation of all machinery is in the form of organic compounds that contains water as part of make-up. Excessive temperatures which tend to dehydrate and oxidize the insulation cause it to become brittle.
The insulation used in machinery does not last forever at a sudden. It gradually deteriorates, slowly at low temperatures and more rapidly at higher temperatures. The greater the load, the higher the temperature and the shorter the life of the insulation. Thus how hot a machine may safely be operated can be answered only by how long a life is desired for the machine.
Available statistics indicate the life expectancy of insulation to be approximately halved with each 10°c rise in operating temperature.
1.2 Properties of Insulation:
There are many properties which determine the suitability of a material for use as an insulating material.
1.2.1 Electrical Properties:
An ideal insulating material should have
(i) High dielectric strength, sustained at elevated temperatures,
(ii) High resistivity or specific resistance, which in turn reduces leakage currents or current through insulation,
(iii) Low dielectric hysteresis, which reduces the dielectric absorption,
(iv) Low power factor, which determines power dissipation in insulation,
(v) Arc / corona resistant,
(vi) Long life under continuous electrical stress.
1.2.2 Thermal properties:
(i) Heat endurance – It should retain the insulating properties for a long life on designated temperature under normal operating conditions.
(ii) Non erasing – After few thermal cycling material should not start erasing.
(iii) Low weight loss: – Insulating material should not lose weight at elevated temperature for long period of service.
(iv) Conduct heat: – It should have sufficiently enough thermal conductivity so that it can dissipate the heat produced in conductor.
(v) Temperature change shocks: – As per requirement the conductor current changes, the temperature of insulation also changes accordingly. Under short circuit condition the change in temperature is very rapid and most of the heat generated is to be absorbed by insulating material Therefore the material must be capable of bearing the thermal shocks.
1.2.3 Mechanical Properties:
(i) High internal integrity
(ii) High tensile strength
(iii) High compressive strength
(iv) Abrasion Resistant
(v) High tear stress
(vi) Good adherence
(vii) Shock resistant
1.2.4 Environmental Properties:
(i) Acid and Alkalies resistant
(ii) Water resistant / non hygroscopic
(iii) Solvent resistant
(iv) Salt resistant
1.2.5 Specific properties:
For NPPs the most required property along with above is Radioactive Invert and long life under continuous radiation stress.
Radiation from a nuclear reactor consists primarily of gamma rays and neutrons. Gamma rays are high energy electromagnetic radiation, where as Neutrons are particles that have almost the same mass as protons, but no electric charge.
The insulating materials are elastomers, plastic (polymers) and ceramics. These materials contain molecules which are held together by strong forces. The absorbed radiation dose, which results in an amount of energy produced in the material, produces change in the structures of material. Therefore the material, chosen for insulation, must high threshold limit so it can survive for a sufficient long life. Some limits are as follows:
1.3 Dielectric Constant (Permittivity):
Dielectric constant is a measure of the capacitive properties of a dielectric material. When a dielectric material insulates an a.c. conductor, its permittivity determines how much a.c. current can pass through the insulation by alternatively charging and discharging its capacitance.
Once again the size of this current under normal condition is unimportant to the electrical operation of the equipment but it is very important to the integrity of the insulation.
There is no measurement unit of permittivity its value is always stated as a ratio to that of air or vacuum which definitely has a permittivity of 1.0. To explain further any air insulated electrical system has a certain value of capacitance. If the air in the system is completely replaced with another dielectric material, that capacitance will increase. The ratio of the two capacitance is the permittivity of the second dielectric material.
When an alternating voltage is applied across an insulator the leakage current through the dielectric consists of a resistive component and a capacitive current. The resultant current leads the applied voltage usually by a large angle. This can be expressed as power factor.
Power factor is important because it represents the electrical losses within the dielectric. At voltages these losses are insignificant but because power loss is proportional to square of voltage, they become significant in high voltage system.
1.4 Dissipation Factor:
Dissipation factor, usually expressed as tan ‘δ’, is related to power factor, as shown in figure above. In some dielectrics the dissipation factor increases with temperature. This leads to increased losses within the dielectric; further heating takes place & a cascade effect leading to break down.
1.5 Loss factor:
Loss factor is another way of expressing the same. It is equal to the dissipation factor multiplied by the permittivity and is a more direct way of comparing losses within the dielectrics.
1.6 Permissible Temperature Rise:
During the normal operation of electrical machinery, its temperature rises above that of the surrounding air. Because the ambient air temperature of operating machines seldom exceeds 40°C, this value is established as the reference temperature when the permissible rise is determined. Hence the permissible temperature rise of the hottest spot in a winding may be obtained by subtracting 40°C from the hottest allowable temperature. The actual measurement of the temperature rise may be made externally with a thermometer or internally with embedded detectors or resistance measurement. Because the internal temperature is greater than the external value, the thermometer measurement is always less than that obtained by embedded detectors or resistance measurement. Furthermore, owing to variations in the thickness of insulation, no uniformity of cooling, inaccessibility of the hottest spot, etc., the observable temperature rise may be less than the actual value. Hence allowances are made for such variation when the permissible observable rise above the 40°C ambient is determined. Unless otherwise indicated, the permissible temperature rise stamped on nameplates of electrical machinery is based on thermometer determinations in a 40°C ambient. Hence the maximum total temperature permissible is the nameplate temperature rise in degree centigrade plus 40°C.
Fig. 1.3 Life expectancy versus operating temperature for class 105 (class A) insulation.
A temperature rise of more than 10 or 15°C above the known normal operating temperature, when operating at normal loads and in a normal ambient, is an almost positive indication that the machine should be cleaned. The accumulation of dirt on the surface of the insulation and in the ventilating ducts reduces the dissipation of heat, raises the temperature, and causes thermal degrading of the insulation. Because the useful life expectancy of electrical machinery is approximately halved with each 10°C increase in operating temperature, good preventive maintenance procedures require that periodic checks be made on the operating temperature of machines, particularly those operating on a continuous basis.
Figure 1.3 illustrates the temperature life curve for class A insulation. The temperature indicated is the ambient, plus the temperature rise, plus 15° to obtain the hottest internal value. The sides of the shaded band give the maximum and minimum life expectancy for a given operating temperature, and the straight line through the center of the band indicates the average value. Thus a machine with class A insulation operating at 40°C rise and in an ambient of 40°C has its hottest internal temperature approximately equal to 40 + 40 + 15 = 95°C. The life curve of Fig.2.3 indicates that this machine has a life expectancy of 29.2 years when operating at that temperature. Yet, when it operates at a total internal temperature of 105°C, only 10° higher, its life expectancy is reduced to 15 years.
Although the temperature life curve helps to estimate the life expectancy of a machine, the mode of operation is an important determining factor. Machines operated intermittently have a longer life span than those in continuous use. Vibration, overvoltage, and other adverse operating conditions also decrease the useful life by weakening the insulation.