The relationship between power factor and hormonics
Posted 06 May 2002 - 02:19 PM
Posted 06 May 2002 - 10:22 PM
Firstly, by definition, power factor is the ratio of KW to KVA and gives an indication of how much of the current drawn by a load is being efficiently utilised by the load. A power power factor does not necessarily affect the end user, but definitely has an affect on the supply. The power meter still turns at the same rate indicating the KW used. A poor power factor will increase the loses in the distribution system. Perhaps if the power authorities charged for amps (or KVA) rather than KW, then this power factor would take on a different meaning.
There are two contributing components to a poor power factor
- Inductive current
- Harmonic current
Inductive current is effectively passenger current that increases the total supply current but does not contribute to the KW consumed by the customer. If the customer has a usage of say 100KW and a pf of 0.5, then the KVA would be 200 and so the current draw is double what it would be if the power factor was 1.0 The result of this is that his power meter still runs at the same speed, but the power lost in the distribution system is four times what it should be. It also means that the transformer supplying this customer needs to be larger than necessary. (transformers and generators are rated in KVA not KW) This all contributes to a lower overall efficiency in the distribution system. Inductive current can be due to motors, transformers etc. Inductive current can be easily neutralised by adding capacitive current. - this can introduce other problems such as resonance but that is a different story.
Harmonic current is due to distortions in the current waveform and in todays world, this is very common. Typical sources of harmonic curents are solidstate circuits such as computers, variable speed drives, UPS's, flourescent lightind and other forms of discharge lighting. Almost anything electronic connected to the supply is going to cause harmonic current to flow due to the rectifiers in the powersupply. Harmonic currents are cumulative and will lead to increased supply losses also.
The inductive and harmonic currents are totaly independant curents and can exist in isolation, but can also coexist on a supply depending on the connected load.
A poor power factor could be the result of harmonics from lights and computors, plus inductive currents from motors. Some equipment such as flourescent lighting can contribute both harmonic currents due to their discharge nature, and inductive current due to their inductive balast.
Power factor correction by adding capacitors will reduce the inductive current, but will not reduce the harmonic current. Harmonic current correction is much more complex and is best done as part of the equipment design. Electronic correction units are available, but expensive.
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Posted 14 June 2010 - 10:43 AM
please can i know how to calculate harmonic distortion? and if there is any program to calculate it?
Posted 14 June 2010 - 05:22 PM
There are three (3) entities identified as Power Factoró
- True Power Factor
- Displacement Power Factor
- Distortion Power Factor
True Power Factor is the product (multiplication) of the Displacement Power Factor x Distortion Power Factor
Another way of stating this is: True Power Factor is the ratio of Average Power to Apparent Power (Avg. Pwr / Apparent Power)
or Pavg/(Vrms x Irms)
A technical paper produced by EPRI (Electric Power Research Institute) ... entitled : HARMONICS AND HOW THEY RELATE TO POWER FACTOR
discusses the subject in greater detail. This paper can be found at the below link.
You will need to have the means for reading a PDF file, i.e. Adobe Acrobat PDF Reader
(free download from...... http://acrobat.9-pdf-software.com/Visit My Website)
Additionally, I think you will find the below reference links to be helpful to you as well.
Distortion Power Factor
".......the question is the first step to acquiring knowledge."
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