If the nature of your question was with regards to how the two issues coincide, they DON'T! They usually collide.
PFC caps are best when used as close to the motor as possible. That provides the maximum benefit in terms of reducing loses in conductors etc. In addition, you do NOT want PFC caps to be on line when the associated motor is not, otherwise you end up over correcting your entire system, making a leading power factor which can be just as bad as a lagging one. For those reasons, the best method of connecting PFC caps is to have them wired down stream of the motor starter, at or near the motor.
That is where they conflict with soft starters. When a soft starter begins firing into a circuit with capacitors, the fast rise time of the capacitor charging current looks like a short circuit to the SCRs in the soft starter. Current flows too quickly through them, creating a condition called dI/dt which stands for delta I (current) over delta t (time), a way of describing a steep change in current. That can cause the SCRs to "self commutate" or turn themselves on without a gate signal, meaning they will fire when you don't want them to fire, and that leads to their becoming damaged / shorted. At the same time, until the SCRs short, they are firing in what is called a "phase angle" method, which creates a lot of harmonics. Normally it is so short as to not bother a motor, but a capacitor on the other hand will absorb those harmonic currents as heat, which can swell the capacitors and cause them to fail. So when PFC caps are located down stream of SCRs, such as in a soft starter, it's usually a race to see which one fails first; the caps or the soft starter.
You can still use PFC caps with soft starters, but the trick is to have a separate contactor tapping power off up stream of the soft starter to feed the capacitors, then only close that PFC cap contactor AFTER the soft starter has finished ramping.
