Sizing capacitor banks

Is there any "rule-of-thumb" as regards with the size of an automatic power factor correction capacitor banks? I would like to know what is the ratio with respect to the installed transformer kVA capacity against a capacitor bank kVAR capacity - if there is any - without knowing what loads may be connected to the system downstream. That is, the overall system power factor is not yet determined. (Knowing that this is an automatic power factor sensing corrective equipment, the power factor can vary anytime and it shall function proportionally.) And without applying vectorially the sums of effective and reactive power.

I am new in the field and my immediate supervisor says a 25% factor may be used. That is, for example, I have a 1000kVA main transformer in a substation, all I need to do is take 25% of that and the size of the capacitor bank should be 250kVAR at certain number of steps.

Is it practical (as in acceptable to Electrical Engineering practice) to connect this APFC bank at the LV mains (i.e., as one of the main LV switchgear branches)?

Follow-up question(s):

Is it required that, during emergency generator power changeover, the capacitor bank shall be disconnected from the bus (the LV switchgear main mentioned above is an ATS where incoming power comes from a utility via transformer and a standby genset.)

How is the number of steps of a capacitor determined (is there any ruling where a certain size of capacitor module shall be used with a certain number of steps)?

Thanks.

 

[jbartos]

Suggestions marked ///\\\:
Is it practical (as in acceptable to Electrical Engineering practice) to connect this APFC bank at the LV mains (i.e., as one of the main LV switchgear branches)?
///Yes, since the current through the upstream transformer will be smaller thus causing a smaller voltage drop across the upstream transformer and also smaller losses RI**2).\\Follow-up question(s):
Is it required that, during emergency generator power changeover, the capacitor bank shall be disconnected from the bus (the LV switchgear main mentioned above is an ATS where incoming power comes from a utility via transformer and a standby genset.)
///NFPA 70-1996 Art. 230-83 Transfer Equipment. It states that "Transfer equipment, including transfer switches, shall operate such that all ungrounded conductors of one source of supply are disconnected before any ungrounded conductors of the second source are connected." This means that there is no mention of PF capacitor disconnection. If you happen to have a static disconnect switch that operates in 2 to 4 milliseconds then the capacitor disconnection would not be fast enough, anyhow.\\How is the number of steps of a capacitor determined (is there any ruling where a certain size of capacitor module shall be used with a certain number of steps)?
///Industry standards could be followed. E.g.
18-1992 IEEE Standard for Shunt Power Capacitors
Also, Reference:
1. Beaty H. W., Fink D. G., "Standard Handbook for Electrical Engineers," 13th Edition, McGraw-Hill, 1993, Section "Power Capacitors" on page 10-115.
Essentially, the number of capacitors needed per capacitor step is dependent on nature of load blocks and steps. If you have one major load, you may only need one step.\\\

 

Sir,
A task is given to me related to power factor improvement. The system load is 8MW, 11000V, 50Hz,. The system power factoe is 0.81. I have to improve this pf to 0.95. What should be the KVAR, Current, and capacitance ratings in case I connect the capacitor bank in delta and or in star on 11KV three wire system and what are the suitable connections. The system has high harmonics load and no data is available. If there is an option that we can either increase or decrease the capacity by introducing or removing some banks, would it effect the overall performance i.e capacitance or voltage rating?

 

[jbartos]

Suggestions marked by ///\\\:
The system load is 8MW, 11000V, 50Hz,. The system power factoe is 0.81.
///PF=0.81 => phi=35.9°, 5.791MVAR (assumed inductive), 9.876MVA\\ I have to improve this pf to 0.95.
///PF=0.95 => xi=18.19°, 2.6287MVAR (assumed inductive), 8.421MVA\\ What should be the KVAR,
///5.791-2.687=3.1623MVAR capacitive\\ Current
///capacitor bank line current = 166A for y or delta\\, and capacitance ratings in case I connect the capacitor bank in delta
///Cdeltaleg=Cyleg/3=0.122/3=0.0407 Farads per delta leg\\ and or in star
///Cyleg=0.122 Farads for each star leg\\ on 11KV three wire system and what are the suitable
connections
///Star is preferred for 11kV. Delta is for low voltage systems.\\. The system has high harmonics load and no data is available
///A harmonic mitigation scheme may be needed depending on the actual harmonic contamination size.\\. If there is an option that we can either increase or decrease the capacity by introducing or removing some banks, would it effect the overall performance i.e capacitance or voltage rating? ///Yes. It depends how big capacitor removals or additions will be.\\\

 

[jbartos]

Addition/correction (I beg your pardon)
Cyleg=3162300VA/[2 x pi x 50Hz x (11000V)**2]=83 microFarads
Cdeltaleg=Cyleg/3=0.083/3=27.7 microFarads

 

[tgott]

Guest062601: If you can get a copy, check out Rural Utilities Service Bulletin 1724D-112 The Application of Capacitors on Rural Electric Systems.
Section 2.5 Capacitor Sizing on page 19 states, "Quick approximations for capacitor kVARs needed are fairly simple to make because the power factor angles of most uncorrect loads are around 30 degrees (a p.f. of approximately 87 percent). In a 30-60 degree right triangle, the side opposite the 30 degree angle is 1/2 the hypotenuse. As a "rule of thumb," this means that the reactive power (kVARs) is approximately half the apparent power (kVA) at 87 percent power factor. ....,it is prudent to install less capacitor kVAR than one half the kVA because residential air conditioning load power factor actually improves near peak load."
Given that your p.f. is less than 87 percent, I would probably go with half of your MVA, or approximately 5 MVAR.