The Effect of Power Factor On An Electrical System

Definition of Power Factor (PF)

In a purely resistive AC circuit, the voltage and current wave-forms are in step (or in phase), changing polarity at the same instant in each cycle (see diagram one). Where reactive loads are present, such as with capacitors or inductors (like electric motors, strip heaters, cooking stoves, lamp ballasts, etc.), the energy stored in the loads results in a time difference between the current and voltage wave-forms, as the stored energy is not available to do work at the load it is termed apparent power. This is known as a lagging power factor (which is less than 1.0)

Difference Between True Power - kW and Apparent Power - kVA and Reactive Power

In an AC system, such as inductive motors, transformers and solenoids, internal electrical energy is required for magnetization of items such as a motors field coils. This internal power stored and discharged within an inductive piece of equipment is referred to as reactive power and measured as volts x amps reactive (VAR). Without internal magnetization the AC equipment would not function. The more reactive power required for magnetization of the internal inductive load, the greater the unusable power and increase in apparent power (kVA) requirements within the electrical system. As shown in diagram one, the greater the value of apparent power (kVA) the lower the power factor (P.F.) and by ratio the lower the real power available, given in kWs. In layman’s terms, P.F. has as more to do with the internal inductive loads of AC electrical equipment and the resultant true power kW available.

A system designer endeavors to select equipment and design a system that reduces the drop in PF. A system with a low P.F. increases the energy lost in the system and requires a much greater input than can be used effectively to power equipment. Generator sets are normally rated for power factors between 0.8 and unity. In summary apparent power kVA is the power required to serve the equipment’s internal reactive load power requirements and true power kW is the power available after reactive power has been satisfied.

Adverse Effects and Why to Avoid Low Power Factor

A system load with a low P.F. will draw more current than a system with a higher P.F.. A system designer considers the following:

• A Low P.F. draws a higher internal current and the excessive heat generated will damage and/or shorten equipment life

• Increased reactive loads can reduce output voltage and damage equipment sensitive to reduced voltage

• Low P.F. requires equipment to be constructed heavier to absorb internal energy requirements

• Low P.F. will result in a more expensive system with equipment able to absorb internal loads and larger load requirements

• A system designer looks to increase P.F. to lower system costs, increase reliability and increase the system’s life cycle

• Utilities will charge a higher cost to industrial and commercial clients having a low P.F

Methods to Increase Power Factor and Load Types

Electrical system designers endeavor to increase the PF to as near as 1.0 as possible by incorporating P.F. ‘corrector’ devices within the system. P.F. correction methods adopted depend on whether the load is termed linear or non-linear.

Linear Load - These are loads such as induction motors and transformers and can be corrected with the addition of a passive network of capacitors or inductors. Capacitors store electrical power that can be used to excite the internal magnetic fields and reduce the required apparent power kVA. (see diagram three)

Non- Linear Load - These are loads include equipment that has components such as rectifiers, some form of arc discharge such as fluorescent lamps, electric welders, arc furnaces, etc. This type of load will distort the current drawn into a system. The current in non-linear loads is interrupted by switching devices within the equipment. Switching causes the current to contain frequency components that have multiple power factor frequencies. For non-linear loads captive or passive power factor correction can be incorporated to counter the distortion and elevate the P.F.. P.F. correction devices can be installed either at a central substation, spread throughout the distribution system, or built into the power-consuming equipment

Non Reactive Loads - These loads are purely resistive such as heater elements and incandescent lights and do not effect P.F..