To some extent, a similarity can be drawn between flow of water and flow of electric current. Water requires a difference in height to cause a flow. Electricity, like – wise, requires a difference in potential between to points for the current to flow. We call this difference in potential as voltage of one point with respect to earth or just a voltage between to points. Electric current flows in three difference ways.

These are thousand of loosely attached electrons in conducting metals like copper, aluminium, silver, etc. even a small voltage between to points – say of wire- will drive these electrons from a higher level to lower level and cause a current flow. We can measure this current in Amperes (Amps.). In its flow, some metals will have too many obstacles – which take a higher voltage to drive and which dissipates heat in the metal – like that in an electric bulb. This heat is termed as Ohmic Heat.

These are a class of materials called "Insulators" which have very few loose electrons. They present a near perfect wall, blocking flow of loose electrons, if a voltage is applied across these. Some of these insulators have free dipoles at the end of their obstacle wall.

These dipoles get charged positively and negatively during each half cycle of and A. C. supply. A whole array of such dipoles between two conducting plates under an A. C. voltage carry, positive and negative charges from one plate to another during each half cycles. This transformer of charge – also a current – measured in amps, forms dielectric or capacitive current. It is entirely different from the Ohmic Current.

3. Inductive Current.

Just as dipoles transfer reactive power under an applied field, magnetic fields produced by a current flowing through a wire, grow and collapse twice in each A.C. cycle and transfer energy. They transfer energy from a high level to lower level. Not only they transfer electrical energy in to mechanical energy as in an electric motor. These currents can be termed as Inductive Currents. Without Inductive Currents, we would not have used electricity to the extent that we use it today. They have a side effect, they lower the power factor and cause wastage of power during transmission and distribution.

The very purpose of employing capacitors to produce dielectric currents is to improve the power factor and reduce losses, while the inductive currents are doing their useful work.

By the nature of generation, in a A.C. circuit, the capacitor gets charged as the current flows. When the flow stops, the current is zero and the capacitor is charged to full voltage. in other terms, the current leads the voltage.

On other hand, a choke coil which has built up full magnetic field, starts sending out the current as the field collapses gradually. Here the voltage leads the current or the current lags behind the voltage.

Both the currents produce static or magnetic fields - but do not do useful work like heating a bulb. However when they flow through the wire or a transmission line, they cause power loss. As such, their magnitudes should be minimum possible. Since both flow of positive times with respect to voltage, they nullify one another. Then we produce leading current by using capacitors to cancel out the lagging currents. This is termed as Power Factor Correction - which finds large capacitor application.

5. Some Technical Terms.

An ideal dielectric material will be one with an absolute resistance to passage of free electrons and a large number of dipoles at its molecular end. There is n such material. Solid, liquid and gas type of dielectric materials have been used for making capacitors. Some free electrons are to be found in small quantities in these materials. But the biggest source of free electrons and ions comes from impurities, moisture, free air etc. a dielectric material can block effective passage to free electrons upto its strength limit. Beyond this, some electrons penetrate the resistance wall and wander through. The ohmic passage cerates heat and under the continuos attack at higher and higher voltage levels and heat, more and more ohmic current flows - till a continuous path is established from one conductive plate to the other - through the dielectric. In other words, there is a puncture and a short circuit. Following technical terms are associated with this phenomenon.

1. The measure of free dipoles or the capacity to carry charged is termed as the dielectric constant. Paper has a dielectric constant of 4 to 5 and carries more dipoles than polypropylene with a dielectric constant of 2.2.
2. The measure of free electrons available for an ohmic flow of current is called tangent of loss angle or tan delta. Free electrons can come from a bad dielectric material itself or from the contaminants accumulated during manufacturing process or due to imperfect removal or air and moisture during the process or re-entry of these in service, due to hermetic seal braking down. The broken down wall of a dielectric with broken molecules also supplies large number of free electrons.
3. The voltage level upto which a given thickness of insulating material holds back effectively passage of free electrons is called the dielectric strength of the material. This 480 - 600 volts D.C. per micron of polypropylene film as against 180 - 200 volts/micron for paper with oil in it. It becomes lower as we go from solid to liquid and to gas dielectric. For the presently used PXE oil it is 70 - 80 volts/micron. For epoxy potting used, it is 10 - 12 volts/micron and for SF-6 gas at atmospheric pressure, it is 8 -10 volts/micron.
4. The voltage level at which the wall is breached and electrons start flowing in, is called the partial discharge beginning or inception level. If this level is reduced gradually, there is a point below which no more dangerous electrons wander through. It is called as the partial discharge extinction (or put - out) level.

A capacitor or dielectric system may be likened to a medieval fort under siege. Assume that under steady service conditions, the outside enemy is maintaining a blockade and a steady pressure. A well chalked out fort will hold out for ages. The enemy now changes his tactics and sends wave after wave of soldiers to ram down the gates - just as voltage surges and harmonic will do to a capacitor. It might break open the door and a few bands of enemy will rush in - creating hot spots. Come the night, and surge levels drop down. A good general within, will brick - up the breach overnight and throw the inside enemy soldiers in the moat to the mercy of the scavengers there.

He is ready for the next onslaught. Repeated onslaught will eventually destroy all the defence potential and the fort will vanish in to history. The on - slaught could be tackled on the outside in difference ways. Just like the fort, capacitor can have an inner layer of second and even third layer of defence. For the given cost to build up the fort, and defend it, one has to select the best of all buildings blocks, use them wisely, train and discipline the soldiers during formation and above all, look for the contaminating, inside enemy. It takes a capable general to organise this.

1. Short falls in Design and Manufacturing :

1. Inadequate - Dielectric Thickness.

- Margins at ends.

- Insulation to body.

2. Ionic impurities in liquid portion, moisture and air incompletely removed.
3. Defects in welding, in soldering of bushing to material. Other mechanical

1. Short falls in transit :

c) Short falls in service conditions :

1. Heavy inrush current during - starting or paralleling.
2. Resonance conditions during starting or during operations.
3. Server voltage fluctuations, high surges, arcing back across faulty switches and fuses on capacitors, resulting in high surge voltages, arcing and bus bar shorts in vicinity of the capacitor.
4. High harmonic magnitudes in supply.
5. Inadequate ventilation, oil leakage, loose connection, burn out, discharge resistors.
6. Over - correction leading to `leading' power factors arising mostly due to non - switching of capacitors when not required.