About Insulations HIGH TRACKING, CORONA & ARC RESISTANCE When an electric potential difference is applied between the two electrodes in air, an electric field sets in between the electrodes. The intensity of this field between the two electrodes may be uniform, non-uniform or strongly non uniform depending on the type of electrodes and the distance of separation. Any electric charge carrier i.e. electron and ion present in this field starts moving, electron moving towards the anode and the positive ions moving towards cathode. With the increase in field, the energy associated with moving carrier increases and starts accelerating. When the kinetic energy is sufficiently high, particularly with the electron, which has higher mobility, it can cause further ionization in the gas by the process of collision, which is known as collision ionization. Subsequent collision ionization result in the creation of charge separation and an electron avalanche is formed. The electrons finally gets deposited at the anode leaving behind the trail of positive ions which also get collected at cathode. This process of ionization is called Primary Ionization resulting in a feeble electric current. If the electric field is sufficiently high such that there can be production of secondary ionization at the cathode either by the bombards of the positive ions or by the photons from the electron avalanche, then the charge separation process and their migration towards opposite electrode becomes very intense. This results into formation of a spark which is known as Electric Breakdown of gaseous medium. There are factors like pressure, temperature, distance of separation and applied voltage etc. which governs this breakdown. When the shape of the electrode and distance of separation is such that it creates a strongly inhomogeneous field, then the condition of electrical breakdown is available only locally near the electrode with small radius of curvature. When the local field exceeds the ionising field strength, then collision ionisation and local breakdown occurs which is known as Partial Discharge as against complete breakdown of the full length of gap. These partial discharges have considerable practical significance particularly for the case of stress by alternating voltages. A weak partial discharge may be intermittent in nature associated with short duration pulse current. When the field strength is sufficiently high, the train of pulses continue almost for the entire cycle resulting in what is known as electric Corona. Corona is associated with a hissing sound, formation of visible glow, formation of ozone and also resulting in electric power loss. These forms of partial discharges described may be termed as external partial discharge which takes place near the electrode. In high voltage equipment, partial discharges can also occur at a distance from the electrode surface particularly in gas inclusions in solid or liquid insulating materials (cavities, voids, gas bubbles). This form of partial discharge is known as internal partial discharge. There is a risk of damage to the di-electric as a result of these internal discharges during continous stress. The heating and erosions due to discharge may ultimately develop into breakdown channels. With the application of sufficiently high overvoltage between the electrodes, which otherwise develops an inhomogeneous field distribution, the partial discharge paths may extend to bridge the entire gap and create a spark. Whether it is a breakdown at homogenous field or high over voltage for inhomogenous field, both of which results in spark by bridging the gap, the current associated depends on the strength of the voltage source. In many cases the sparking is an intermittent phenomena and the current involved in spark discharge ranges from a few milli-ampere to about 1 ampere. In cases where the strength of electrical power source is very high, hundreds or even thousands of ampere may result followed by a sparking in which case the discharge current continues to flow after the breakdown. This type of breakdown is known as arcing. Extremely high temperature in the region of 6000 - 12000 K may develop in the discharge column and it turns into a state of plasma by the process of thermo ionization. The arc voltage is associated with drop in voltage across the discharge channel which becomes less with increase in current strength. Both the spark and arc discharges are self-sustained discharge but the former is intermittent because of low power associated with it whereas the arc is a continuing discharge channel. 5b. TRACKING By definition, tracking is the formation of permanent conducting path across the surface of an insulation and in most cases the conduction results from degradation of the insulation itself. It is therefore necessary for organic insulation to be present if tracking is to occur. The three essentials of the tracking phenomena are : 1) the presence of a conducting film across the surface of the insulation, 2) a mechanism whereby the leakage current through the conducting film is interrupted with the production of sparks, 3) degradation of the insulation must be caused by the sparks. The conducting film is usually moisture from the atmosphere absorbed by some form of contamination such as salt in coastal areas, carbonaceous dust from the fuel, industrial deposits or cellulose fibres. Conducting path can also arise from the deposition of metal dust. Interruption of moisture films is caused by drying of the surface following the heating effect of the leakage current. Sparks are drawn between the separating moisture films, which act as extension to the electrodes and causes the damage to start. This represents a significant difference between tracking and discharge failure. For the discharge to occur there must be a voltage of 380 volt at Standard temperature and pressure in air whereas tracking can occur at well below 100 volt - it does not depend on gaseous breakdown. Degradation of the insulation is almost exclusively the result of heat from the sparks and this heat either carbonizes or volatilizes the insulation if tracking is to occur. For all practical purposes tracking can occur only with organic insulation - it should not be confused with flashover due to polluted surfaces. Failure of solid insulation due to tracking does not have to be on the surface only - with laminates such as resin bonded paper board and pressboard, instances are known where failure took place between layers caused by moisture and ionic contamination in the paper. Degradation may be accelerated by extraneous process, such as physical weathering, ultra violet radiation and chemical attack. Thus ozone and oxides of nitrogen generated by discharge may degrade the insulation and provide sources of contamination. Prevention of tracking must aim at clean, dry and undamaged surfaces and the material by its own virtue should be track resistant. Cleaning is not often practicable. It is employed in some cases together with coatings of water-repellent grease. Design can help by limiting access of dirt and avoiding its accumulation in areas between conduction. Increase in creepage paths will, to some extent, prevent tracking but in most practical cases moisture films can eliminate the designed creepage path. The property of a solid insulating material against tracking is represented by a numerical figure known as the Comparative Tracking Index (CTI). This is obtained by using a standard test method. Higher value of CTI Indicates better performing material against tracking. BACK