Capacitor's on High Voltage Motors

[streeta]

We are using 1500KW 3.3KV motors on our mills. Currently in circuit we are using 123.7uF, 1.905KV, 133KVAr capacitors. With the recent power dips and power outages, we have gone through a number of capacitors, The lead time for these capacitors is up to 24 weeks. What i am after is can i put another size capacitor in circuit, If so how do you work out the sizing for capacitors to be used???
Another problem is that we wish to put capacitor banks in for PF correction over the whole plant, How do you work at what size capacitor banks we would need???
Any help on working out sizes Ect: would be grateful

Thanks
Greg
gstreet@bglgh.com (if contacting me would be easier)

 

[jbartos]

Suggestion: Reference:
1. Donald Beeman "Industrial Power Systems Handbook" First Edition, McGraw-Hill Book Company, Inc., 1955,
Chapter 8 Power-Factor Improvement
kvar at original pf=kw x tan(theta1)
kvar at improved pf=kw x tan(theta2)
The capacitor required to improve the power factor:
ckvar=kw x [tan(theta1)-tan(theta2)]

 

[jbartos]

Suggestion: Visit

http://www.willett.ca/PFCapacitors.htm

http://www.electricalengineering-book.com/subject-covered-part4.html

etc. for more info

 

[busbar]

Have you ruled out harmonics causing capacitor failures?

 

[GusD]

Hello Streeta

I believe you could be talking about Surge Capacitors or perhaps not ,and they are in fact Power Factor Correction capacitors.As for an overall Power Factor correction system for the plant,it will depend on the Degree of ixisting current overall PF and how much you want to raise it by.A .85pf is a common level of PF correction.This will allow for Plant load changes without overcorrecting.IF PowervFactor correction Caps are used in individual motors ,they have to be properly sized to minimize the possibility of over-boosting Voltage levels high enough to damage motor windings or the Caps themselves.
According to Richard Nailen (Managing Motors)IF the total Cap KVA connected to the motor terminals does not exceed the motor's magnitizing KVA,there will be no self -excitation and no dangerous Over-voltage.Calculate the magnitizing KVAs as simply 1.73 xmotorKVx"idle"Amps and compare this with the Capacitor involved.

GusD