Notching is a power quality event caused by the commutation of current from one phase to another. In essence, notching is a voltage disturbance resulting from instantaneous and repeated short-circuiting between two phases. This is very similar to a transient event but repetitive. (Example)
Notching is caused by arc furnaces, large DC drives, static power converters and other equal-firing thyristors, SCRs or other rectifier circuits.
Notching results in the malfunction of electronic equipment, caused by the multi-zero crossing of server notching. DC ripple current producing sub-harmonic in AC systems is another problem.
Notching may be easily seen on an oscilloscope with a minimum scanning frequency of 20-kHz. Normally these events can also be seen by a recorder that can sample at 256 samples per cycle (15-kHz for a 60-Hz power system).
Isolation transformers and/or line reactors
Best Source(s) of Information
IEEE Emerald Book