power factor

[bigheadted]

Is there a problem with setting the targer power factor of a capacitor bank to unity? bearing in mind it is one of three independently acting capacitor banks the other two are set for 0.98.
What is the minimum switching time i should be using to switch in and out capacitors? and what are the limiting factors?
i understand that changing these settings is not ideal but the transformer supplying our site is at its maximum, after contracted maintenance to our pfc i have seen the pf drop very low causing higher than usual currents on ramp up of the machine. i would like to set the pf to unity and increase the sitching time in attempt to improve the situation untill we are in a position to get a larger transformer
thanks

 

[vic3fan]

Like it or not, you need to allow some sort of inductive reactive power circulation. Almost any load, excepting capacitor banks, requires it. A PF=1 will cause a ferroresonance phenomenon which leads to a loss of stability of your system. That’s why they prefer to have PF=0.9 or (0.8 in other cases).

 

[dpc]

As long as the capacitors are not directly connected to a motor, there is no inherent problem with a leading power factor.

Switching of capacitors in and out of a system can cause some significant system voltage transients, depending on the size of the cap bank and its design. The other concern is wear and tear on the contactor or switch used for switching.

But I question the correlation between low power factor and higher starting currents. I wouldn't count on changing a pf setting from 0.98 to 1.00 to solve your starting current problems.

 

[waross]

I agree with dpc.
I would add that the National Electric Code prohibits attaching more capacitors directly to a motor than needed to correct its power factor to unity. If the capacitors are directly connected to the motor such that the combined motor/capacitor current flows through the overload devices, there can be motor protection issues.
The other problem with over correcting power factor is excessive voltages at light loads. This was an issue when it was common practice to permanently connect the power factor capacitors on line 24/7. Not a problem with a power factor controller.
You can reduce your steady state current by about 2% by going from 98% to 100% power factor. Once your power factor goes leading your current increases again.
You may be able to reduce motor starting loads by over correcting the power factor but then your steady current would be higher with the leading power factor.
The factor that impacts the life and welfare of a transformer is heat. The temperature response to a step change in loading of a transformer may be 15 minutes or more. Motor starting is over in seconds. If you can live with the voltage dip, don't worry about normal motor starting overheating the transformer.
respectfully

 

[dpc]

Capacitors are sometimes used to assist in motor starting, but it takes a heck of a lot of capacitors and these must be switched out after the motor is running.

A motor at locked rotor draws roughly 6 times normal full-load current at a power factor of maybe 0.2. Your normal pf correction capacitors don't usually make much of a dent in the var requirements for a starting motor.