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Power Factor Guide 2018

The Guide to Power Factor Correction by BEAMA

Determining the Power Factor

Power Factor may be shown on your Energy Bill in the format of a number between 0 and 1. Excess reactive power charges are levied if your power factor is less than 0.95 and are applied for each half hourly consumption period. Whether provided or not it is recommended that a site survey be conducted to determine the true Power Factor of the site over a period of time. 

The relationship between the voltage and current can be shown using vectors and this approach allows easy calculation of the rating of Power Factor Correction equipment. 

For a 3 phase power supply: kVA = line Volts x Amps x 1.73 / 1000

This is converted to kilowatts (kW) by the formula: 

kW = line Volts x Amps x 1.73 x PF /1000 

This can instead be expressed as: 

kW = kVA x PF (N.B. 1.73 is the square root of 3) 

So as the power factor worsens from say 0.98 to 0.5, the generator has to supply more kVA for each kW you are using. 

Take the example of an industrial site, which may typically have an uncorrected Power Factor of about 0.8. If the site has a load of 1000kW, with an uncorrected power factor of 0.8 the electricity supplier would have to supply 

1000 ÷ 0.8 = 1250kVA (1740A per phase), 

and you would require an authorised supply capacity of 1250kVA minimum. 

However with the introduction of Power Factor Correction, applied to the same 1000kW, with a corrected power factor of 0.99 the electricity supplier would only have to supply 

1000 ÷ 0.99 = 1010kVA (1414A per phase). 

With this improvement you could now set your authorised supply to 1010kVA. A reduction of 240kVA (326A per phase). 

How this power is wasted can be shown graphically since in 3phase power supplies "power" can be represented and measured as a triangle. The diagram in the figure below  shows the relation between Active Power and Reactive Power and the resulting Apparent Power.

Power Factor Correction  calculation
This article has been extracted from "The Guide to Power Factor Correction" by BEAMA 

Download the Guide Here

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