Application of Power capacitors
Application of Power capacitors
(OP)
I have seen lots of literature on application of capacitors/banks for Power factor improvements in various industrial plants.
Any members from Petro-chem and chemical plants experience can provide any information on other application of Power capacitors in Electrical Power distribution system for a
1) Use of capacitors for Harmonic Filters - Where is it necessary to apply Harmonic Filters and how is it sized? Any guide form specifications?
2) Use of capacitors for preventing excessive voltage dip during large motor starting. Under what situation capacitors are used to prevent voltage dips and how to size them correctly?
Thanks in advance.
Any members from Petro-chem and chemical plants experience can provide any information on other application of Power capacitors in Electrical Power distribution system for a
1) Use of capacitors for Harmonic Filters - Where is it necessary to apply Harmonic Filters and how is it sized? Any guide form specifications?
2) Use of capacitors for preventing excessive voltage dip during large motor starting. Under what situation capacitors are used to prevent voltage dips and how to size them correctly?
Thanks in advance.






RE: Application of Power capacitors
To reduce the starting current of a motor with capacitors, the inductive KVARs of the motor must be canceled with capacitive KVARs. This may require a capacitor bank that may be more than 4 times the KW rating of the motor.
The capacitors must then be switched out in stages as the motor accelerates. I read a paper on the technique many decades ago but I have never seen the method used in practice.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Application of Power capacitors
One of the chemical plants we serve has a 6 pulse rectifier. This is rich in n*6 +/-1 harmonics (where n is 1, 2, 3, etc.), so you'd get 5, 7, 11, 13, etc. The amount of each is often estimated as an inverse proportion to the harmonic itself. So you'd expect 20% of 5th harmonic, 14% of 7th harmonic, etc.
We have another plant that has a 12-pulse rectifier. The expected harmonics are 11, 13, 23, 25, etc.
In the first case, the rectified load represents the majority of the load so a tuned harmonic filter was used to reduce the distortion. This arrangement was a set of inductors in series with a grounded wye capacitor bank. The bank was tuned to the 4.7th harmonic, which permits it to trap the 5th and higher order harmonics.
I don't have any guideform specs to offer but I'm sure others will chime in.
On your second question dealing with motor starting capacitors, I presume these are fairly large motors, correct? If you need them, they should be sized to match the no load kvar losses of the motor and are best employed where they are switched with the motor. A rule of thumb is that the kvar should be around 15 or 20% of the motor HP. If you know the no load kvar, then that is the prefered approach.
RE: Application of Power capacitors
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RE: Application of Power capacitors
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Application of Power capacitors
Motor starting - The caps needs to be matched to the starting kVAR of the motor. Probably somewhere around 70% to 80% of the LRA x voltage x sqrt(3). Once again, this has to be designed to the site requirements.
RE: Application of Power capacitors
Neil
RE: Application of Power capacitors