When capacitors are to be added to an industrial plant to correct the power factor you may chose between bulk correctin and switched correction.
Historically the power factor of a plant was calculated on a monthly average and penalties applied when the monthly average dropped below 90%. The active power, KiloWattHours, and the reactive power, KiloVoltAmpReactiveHours, from the revenue meter were used to determine average monthly power factor. The monthly average could be corrected by connecting bulk capacitors and adding a set amount of KVARs or KVARHrs per month. At times of low load the power factor may be leading, and the voltage may rise enough to damage lighting. At times of heavy loading the power factor may be low, but at the end of the month the average would be high enough to avoid penalties.
A compromise was to connect as much bulk capacity to the sevice as was possible without incurring over voltage issues and then to connect capacitors to large motors so that the capacitors were switched with the motors. On the assumption that when the large motors were running there would also be a number of smaller motors running, the large motors may be over corrected. Typically the amount of capacity required to correct a motor to unity would be doubled.
A typical saw mill in the Pacific North West may run 16 Hrs per day for 5 days per week. The lumber kilns often ran 24/7.
Comparing 16 Hrs. x 5 days = 80 hours per week, with 24 Hrs x 7 Days = 168 Hrs per week it as apparent that capacitors switched with the lumber kiln fans would be twice as effective in correcting the monthly average.
Even though some capacitors may be switched with motors, this may still be classified as a bulk correction scheme.
That was then and this is now.
Metering technology has progressed and the electronic meters are able to meter any use of KVARHrs. Some utilities are able to and do penalize any and all use of KVARHrs outside of allowable limits. That is low power factor.
Switched capacitors in an industrial plant may now include a Power Factor Controller.
The PF controller monitors the power factor in real time and switches capacitors in and out so as to maintain the Power Factor of the plant at or near unity.
Bill
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"Why not the best?"
Jimmy Carter