Why P.F. Not maintain Unity

Every electrical system has a natural resonant frequency based on the KVA size and impedance. Adding capacitors in parallel causes the resonant frequency to drop down towards the dominant 5th and 7th harmonics. The higher the impedance of the transformer the less KVAR it takes to drop down to trouble.

If you have a 1000KVA transformer with 5-6% impedance (most common I see at 480V), you can add up to 20% KVAR to KVA ratio (200KVAR) and stay well above a resonance, but you will still amplify (maybe double) the existing 5th and 7th harmonics. Once that ratio gets to 30% or more, then the resonant frequency has dropped to the 5th and 7th harmonics.

The problems are that the capacitors won't last and unstable voltages. Drives are effected. That's why when I'm called into a facility to find out where the gremlins are, I always ask to see any capacitors on line.

If you don't have any existing harmonics, you won't have problems with adding capacitors providing they aren't large fixed banks, which will cause the base voltage to rise to sometimes unacceptable levels.

If have harmonics and you need to add capacitors and the ratio of KVA vs. KVAR is more than 30%, you should use de-tuned filtered capacitors. That way you improve the power factor and actually reduce the existing harmonics.

You can google "power factor correction" and view very detailed formulas for my rules of thumb.