Hi Steve,
exactly, line filters have inductances usually less than 500uH whereas line reactors in usually in excess of 5mH.
Line inductance causes the current to lag voltage, so as the current falls to near zero, the point at which the SCR would
commutate the voltage has already increased from zero. So just at the moment that the SCR has to establish its
reverse block its already under dV/dt stress.
Almost all VFDs have a rectifier/capacitor input which causes line current to be drawn in short duration high current pulses at the
peak of the voltage excursions. It causes current 'distortion' on the line, much to the power companies disadvantage.
Line reactors cause the pulses to be of longer duration and lower peak values with reduced current distortion. In many
cases the power companies will mandate that they be fitted on large installations.
While both line filters and line reactors are for the benefit of the electrical supply they address different technologies.
Craig