GENERAL VIEWS:

In general, a.c. electrical loads are classified into two groups;

A.. Effective Loads
As heaters , incandescent lamps , etc.

B. Reactive loads or reactive + effective loads

As motors, transformers, welding units, chokes, gas discharge lamps , static converters etc.Current flowing through a reactive load is called "reactive current".Current flowing through a reactive + effective load is called "apparent current" and active current (as a geometric sum ). The ratio of "active current" to "reactive current"is called "power factor" and is expressed by the cos of angle j of the current triangle ( FIGURE 1 ) its value ranges between 0 and 1.

In order to supply power to an inductive load the power system, generators, transformers, lines, cables etc, must provide significantly more current than the required active current ( FIGURE 2 ) Represents the graph of the apparent current of the reactive + effective load. The white portion represents the reactive current and the green portion represents the active current.

The current flowing through a capacitor is called capacitive current and is geometrically the opposite of the current that flows through a purely inductive load , called inductive current. If a capacitor having current I , is connected in parallel to the inductive load having the same current I , we will see the line current going to zero.

This shows that a capacitive current nulls the inductive current. If the load shown in figure 2 is added with a capacitor having a reactive current exactly the same as of the inductive load, all the reactive current will be nulled and only the active current will remain ( FIGURE 3 )

Thus the line current is now having cos j = 1, when before it was less than one. This is known as "power factor correction "or" cos j correction" with capacitors.

The power factor correction has eliminated all the energy losses related to the reactive current in the whole power system, generator, cables, transformers , lines etc. This shows how important is the power factor correction.

Capacitor is therefore , considered as an "energy saving" component. Similarly it can also be referred to the electrical power and can be referred to the electrical power and can be calculated as multiplication between voltage and current.

HAVING
Apparent power : Vx lapp = VA (voltage ampere)
Active power     : Vx leff = W (watts)
Reactive power : Vx Ic = VAR (Reactive volt ampere)

Cos j : W / VA

CONSEQUENTLY
Active power in a mixed load : W = VA x COS j

POWER FACTOR CORRECTION SYSTEM

From the above equation it is observed that power factor correction close to 1 brings an almost complete reduction of energy losses related to reactive current , enabling the entire power system to supply more active power.

This means :

1. Reduction in energy losses.
2. More efficient utilization of the power system , generators, transformers , lines , etc.
3. Reduction in the cost of energy.

For this reason the electricity producing authorities penalize loads with low power factor. In the past cos j less than 0.8 was penalized , now it has been increased to 0.95 in many countries.

On the other hand an over correction of the power factor is also not accepted , because a reactive capacitive current is equivalent to an inductive current as far as the energy losses are concerned.

Therefore, the right selection of the power factor correction system is absolutely necessary.

1. Self healing
2. Explosion proof system
3. Current surge limiting inductor
4. Individual discharge resistor

Self healing is an implicit protection and in case of dielectric failure the discharge will vaporize the surrounding metallized layers restoring the insulation. This self healing process lasts only for a few microseconds and capacitance loss is negligible. However , when the capacitor is continuously subject to self healing events , e.g. at the end of the capacitor’s life span , or in case of frequent over voltages , the internal pressure increases due to the gases generated by the decomposition of polypropylene film in the discharge areas , this if not controlled , will cause the capacitor can to break and the winding to burn. Khawaja Electronics (Pvt.) Limited has included in its design an explosion proof system which in case of any excessive internal over pressure will disconnect the capacitor automatically.

The inductance coil which is fixed externally on each single phase capacitor limits the peak inrush current , especially when the capacitor is connected in parallel to the capacitors already energized. This brings a reduction in the electrical stress on the switches and a reduction in the switch-on voltage transients (over voltages).

Hence it can be concluded that using the limiting inductance does not only ensure better performance of the capacitors but also guarantees a higher reliability of the whole system. Each single phase capacitor is fitted with a discharge resistor which brings down the voltage to <50V in less than 30 seconds. For a three phase system, the single phase capacitors are assembled in metallic boxes with internal triangle or star connection.

These units can be easily installed , wired and maintained.