What is Power Factor Correction?  

 
Power Factor is simply a name given to the ratio of “actual” power (active power) being used in a circuit, expressed in watts or more commonly kilowatts (kW), to the power which is “apparently” being drawn from the mains, expressed in volt – ampere or more commonly kilo volt – ampere (kVA).
All modern industries utilize electrical energy in some form or other. Two basic categories of load are encountered in alternate current (AC) networks.

 
1.  Resistive Loads

Devices containing only resistance e.g. incandescent lamps, heaters, soldering irons, ovens, etc. 

The current drawn from the supply is converted into heat or light. Since the voltage is assumed to be constant, the actual power (kW) being used is identical to the apparent power (kVA) being drawn from the line.  The power factor is therefore unity or 1. In these purely resistive circuits, the current and voltage sine wave peaks occur simultaneously and are said to be “ in phase”.

 
2. Inductive Loads

All motors and transformers depend on magnetism as the basis of their operation.  Magnetism is a force and in the physical sense is not consumed.  In AC motors and transformers, magnetic forces are only required periodically.  Consequently, a permanent magnet cannot be used and the necessary magnetism is produced by electrical means. The electrical current needed for this purpose is not fully utilised.  Having produced the magnetic force, the current flows back to the power station again.  This current is called the reactive current in contrast to the active current which performs work and is fully utilised in so doing.  Although the reactive current is not utilised, it imposes a load on the electrical distribution system and supply authorities demand payment for this load according to specific tariffs.  

The current drawn from the supply is made up of two separate kinds of current “power producing current” and “magnetizing current”.  Therefore the current flowing in an AC circuit (unless corrected) is generally larger than is necessary too supply the power being by the expended point.

 

What does Cosφ mean?

 
Reactive power and active power flow through the motor or transformer.  Geometrical calculation of these two powers yield the apparent power.  The ratio of the active and apparent power factor is denoted by cosφ and indicates what fraction of apparent power flowing is actually used by the motor.

 

As can be seen from Fig. 1, the apparent power is greater than the active power and hence the power factor is a value considerably less than unity.

 

Disadvantages of low power factor

 

1.     Increased authorities cost since more current has to be transmitted and this cost is directly billed to consumers on maximum demand kVA systems.

2.     Causes overloaded generators, transformers and distribution lines within a plant, resulting in greater voltage drops and power losses, all representing waste, inefficiency and needless wear and tear on industrial electrical equipment.

3.     Reduces load handling capability of the plants electrical system. Most electrical supply authorities have changed to kVA demand system from the inefficient kW demand system. Consumers are now billed and penalized for their inefficient systems according to the apparent power being used.  In future, customers will be penalized for plants with power factor below a pre – determined value.

 

Improving power factor
The most practical and economical power factor improvements device is the capacitor.  As stated previously, all inductive loads produce inductive reactive power (lagging by a phase angle of 90º).  Capacitors on the other hand produce capacitive reactive power, which is the exact opposite of inductive reactive power.  In this instance, the current peak occurs before the voltage peak, leading by a phase angle of 90º.  By careful selection of capacitance required, it is possible totally cancel out the inductive reactive power when placed in circuit together.

Cosφ1 is the kVA used before Power Factor Improvement equipment was added to the network.

Cosφ2 is the kVA used after Power Factor improvement equipment was added to the network. 

To prevent the continual flow of reactive current back and forth between the load and power station, which is in effect a reactive current storage device, is connected in parallel with the load.  The reactive current supplied by the power station and used for the magnetic force when the load is switched on does not now return to the power station but instead flows into the capacitor and merely circulates between the latter and the load.  Consequently the distribution lines from the power station are relieved of the reactive current. 

Capacitors can therefore be utilised to reduce kVA and electrical costs. Improved power factor results in: 

1.     Reduced kVA charges.

2.     Improved plant efficiency.

3.     Additional loads can be added to the system.

4.     Reduced overloading of cables, transformers, switchgear, etc.

5.     Improved starting torque of motors.

6.     Reduce fuel requirements to generate power due to lower losses.

 

  Power Factor Correction using Capacitors

 

 Two methods of improving power factor using capacitors are:

a)     Individual motor compensation (static capacitors)

b)    Centralised compensation (automatic capacitor banks)

 

a) Individual Motor Compensation

 

Most effective correction is obtained by connecting individual capacitors directly to the terminals of each motor.  The motor and capacitors can be controlled jointly by the motor switchgear.  The capacitor rating should be matched as closely as possible so that the power factor of the entire plant can be corrected to the optimum value, irrespective of the number of motors switched on.

The size of capacitor required may be determined from Table 1 by taking the motor kW and speed into consideration.  Table 1 is a guide only and no guarantee of correct power factor.  The correct method of maximum capacitor rating can be determined by using the following formula:

               Qc  = 0.9 IoV√3

Where    Io   = motor magnetizing current

              Qc  = capacitor power in Var

If the magnetizing current is not known, 95% of the motor no – load current can be used as an approximate value.  Care should be taken not to exceed the value calculated to avoid dangerous over voltages and possible self excitation of motors at switch – off.

Over compensation can cause higher supply voltages which can cause consequent break down of motor insulation and flashover at motor terminals. To be safe, rather use standard capacitor sizes (as indicated below).  For this reason, individual motor compensation is not recommended for motors which are rapidly reversed e.g. cranes, hoists, etc.

b) Centralised Compensation

( Automatic Power Factor Correction )

 

In large industrial plants where many motors are generally in use or, when the main reason for power factor is to obtain lower electricity bills then centralized compensation is far more practical and economical than individual motor compensation.

In this instance, large banks or racks of capacitors are installed at the main incoming distribution boards of the plant and are sub – divided into steps which are automatically switched in or out depending on specific load requirements by means of an automatic control system, improving the overall power factor of the network.

 
Generally an automatic power factor system consist of:

a)     a main load – break isolator (or circuit breaker)

b)    an automatic reactive control delay

c)     power factor capacitor backed by suitable fuse protection

d)  suitably rated contactors for capacitors switching

 
The automatic reactive control delay monitor the total network and will switch – in the required capacitor banks at pre – determined intervals compensating for capacitor discharge times and load dependant requirements.
 
As capacitor switching subjects components to exceptionally high stresses it is imperative to correctly size and rate all components utilised in a system.

 

Substantiating Power Factor Correction Costs

 

This question can best be answered by an example.  Assuming a plant has a total load of 500 kW and a power factor (cosφ) of  0.75 lagging. Supply authorities kVA demand charge is approximately R 50.00 per kVA.

 
Total cost @ R50.00/kVA = R33,300.00/ month

 
By installing capacitors to improve power factor (cosǿ) to 0.98 lagging new costs are:

 

Total costs @ R50.00/kVA = 25,500/ month

Therefore savings monthly  = R 7800.00

 
A complete system required to effect power factor from 0.75 to 0.98 (as in above example) would require a system of 360 kVAr.

 
Power factor correction usually pays for itself well within 12 months (Annual maintenance check ups are advised) of the initial purchase and continues saving indefinitely.  It therefore stands to reason that more significant savings can be anticipated with the ever increasing escalation costs of electricity in the future.