Sudden reduction of Power Factor in a Motor feeder
Sudden reduction of Power Factor in a Motor feeder
(OP)
In a 3 phase, particular motor feeder, the Power factor suddenly reduced from 0.98 to 0.82. The circuit is shown below,

The company is not sending me to the plant, anyway I would like to know the cause of the trouble for my experience. I'm thinking of blowing of capacitor fuse in one phases. If so, what would be the Kvar output from the capacitor bank from the 2 capacitor banks.It means the two capacitor banks are in series with applied voltage of 6kV. Also the operator observed the noise from the 6% series reactor. What could have possibly went wrong?

The company is not sending me to the plant, anyway I would like to know the cause of the trouble for my experience. I'm thinking of blowing of capacitor fuse in one phases. If so, what would be the Kvar output from the capacitor bank from the 2 capacitor banks.It means the two capacitor banks are in series with applied voltage of 6kV. Also the operator observed the noise from the 6% series reactor. What could have possibly went wrong?






RE: Sudden reduction of Power Factor in a Motor feeder
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sudden reduction of Power Factor in a Motor feeder
Attached again.
RE: Sudden reduction of Power Factor in a Motor feeder
Voltage across each phase before the fuse blows is VLL/sqrt(3). Voltage after the fuse blows is VLL/2.
Output of each phase before the fuse blows is [VLL/sqrt(3)]²/X = VLL²·2·pi·f·C/3. Total of all three is VLL²·pi·f·C·2.
After the fuse blows, the output of each phase is [VLL/2]²/X = VLL²·2·pi·f·C/4. Output of two phases is VLL²·pi·f·C.
RE: Sudden reduction of Power Factor in a Motor feeder
A blown fuse is a very likely suspect here. If I had to guess why, I'd say it was because of the primary reactor starting method and that the PFC caps are on line during starting. As PFC caps, the kVAR you select is for correcting the vars of the RUNNING motor. Starting current and the associated low PF causing stress to the PFC caps when started across-the-line (DOL) is not very significant because the duration is very short. When using a reduced voltage starting method, the amount of time the motor is pulling higher than normal current and lower than normal PF will result in the caps attempting to contribute more VARs into the circuit than they are rated to deliver for that extended time. They over heat and swell. You may not have actually blown the fuses if the caps have a "turkey timer" pop-out button indicating that their internal safety disconnect opened, but the effect is the same, the cap(s) is not available to the circuit.
On RV starting methods, I generally recommend having a PFC Cap Contactor ahead of the caps, isolating them until after the starting is done by controlling that cap contactor with, in your case, the reactor bypass contactor.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Sudden reduction of Power Factor in a Motor feeder
RE: Sudden reduction of Power Factor in a Motor feeder
RE: Sudden reduction of Power Factor in a Motor feeder
RE: Sudden reduction of Power Factor in a Motor feeder
RE: Sudden reduction of Power Factor in a Motor feeder
RE: Sudden reduction of Power Factor in a Motor feeder
I remember a wise old head electrician. Whenever he passed a bank of caps he would lightly drag his hand across the cans. If one can was cooler he would have someone check the fuses.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sudden reduction of Power Factor in a Motor feeder
What could be the cause for reduced capacity of a capacitor?
RE: Sudden reduction of Power Factor in a Motor feeder
RE: Sudden reduction of Power Factor in a Motor feeder
RE: Sudden reduction of Power Factor in a Motor feeder
RE: Sudden reduction of Power Factor in a Motor feeder
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sudden reduction of Power Factor in a Motor feeder
RE: Sudden reduction of Power Factor in a Motor feeder
Could you please elaborate? My understanding is that when the fuses of the capacitor bank blown, the Motor simply draws the required reactive vars from the utility on that particular phase. I understand that in case of the Main fuse on the way to motor terminal blowing, negative sequence current/voltage arises. But in this case,does the negative sequence currents or voltages exists because of blown fuse of the capacitor bank.