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Saturday, August 3, 2019

Fixed or automatic power factor details

Which are better, fixed or automatic power factor correction capacitors?
Fixed banks are used extensively in utility distribution systems where a fixed amount of reactive power needs to be generated to support the voltage or correct for constant inductive loads like the magnetizing inductance of all the distribtion transformers connected to some feeder (as one example of many). Because they are not switched, the banks can be made larger and placed strategically such that the voltage regulation on the feeder will meet certain limits (±5% typically) through the entire load range of the feeder.

Switched banks are then added as needed wherever the voltage regulation starts to exceed the design limits. These banks are typically sized smaller because they are switched more frequently and each time they are switched they cause the voltage to raise or lower which people will complain about if it is too large of a change. This is know as light flicker. The typical size of switched banks is typically based on a maximum change in the steady-state voltage of 3%. Below this level, people won't complain as long as the number of switching cycles per hour is limited to be less than the objection level given by the flicker curve which is less than 3 switches per hour.
There are numerous applications of power capacitors and way too many to try and discuss here. I hope my short answer sheds some light on distribution applications.

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