*2.3.1. Line conductor overvoltages due to over-excitation and nonlinear resonance [6,7]*

The transient recorder in Fig. 3(a) recorded 2 abnormal overvoltages (at 56 Hz and 45 Hz, respectively) after the last 345 kV-transmission line connecting to the NPP was tripped on the remote end which turned the NPP into an electrical island. As will be explained in the next section, the 1st overvoltage was caused by the over-excitation of the motors (e.g. recycle water pump) in the plant who, with terminal voltages supported by large line capacitance, now operated as induction generator after loss of external power.

The 2nd overvoltage is caused by a different mechanism. After a few cycles the low voltage relays tripped many of the plant motors leaving only 2 biggest motor (now operating as induction generator) still connected and were supported by a comparatively much larger capacitance leading to not only over-excitation but also magnetic saturation of both the motors and transformers. This created a condition very close to ferroresonance resulting in even bigger overvoltage.
