To calculate the cos phi of the plant, it is necessary to have data on the consumption of Active Energy (kWh) and Reactive Energy (kVArh), or Active Power (kW) and Apparent Power (kVA).
These values can be found in the electricity bill or through a network analysis. If in possession of kWh and kVArh
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To define the required reactive power of an automatic PFC system, it is essential to have:
• Active Power (kW)
• Initial Cos φ (also deduced from the Active and Reactive Energy consumed, see above)
• Desired Cos φ
Formula: Q = P * k
• Q: Necessary reactive power (kVAr)
• P: Active Power (kW)
• K: Cos φ coefficient from table >> attachment 1
Plant with active power 650 kW and initial Cos φ 0.75, to be brought to 0.95.
What is the necessary reactive power?
500 kW *0.553 (coefficient K for cos phi from 0.75 to 0.95)
= 276 kVAr
It is advisable to oversize the necessary reactive power by 15-20% in order to maintain an average Cos φ of 0.95 even with load variations and/or future extensions.
Compensation of MV/LV Trafo
For economic reasons, it is advisable to compensate the reactive power that the Transformer absorbs for the magnetisation of the core and for the winder reactors.
The choice of Reactive power can be made based on table >> attachment 2
Compensation of Asynchronous motors
The reactive power necessary for the power factor correction of Asynchronous Motors is chosen from the following table >> attachment 2
It is always advisable in these situations to take into account the possible self-excitation of the capacitors, which is why the installation of an automatic PFC system rather than a fix one is preferred.
It is always advisable to take into account possible operation of the Motor as a self-excited generator, and this can result in voltages that are considerably higher than those of the network.
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