Protection of Rotating A-C Machines Against Traveling Wave Voltages Due to Lightning

The problem of providing protection for rotating a-c machines against lightning overvoltages has two parts: 1. The protection of insulation to ground, which is mainly a question of the magnitude of the overvoltage, and 2. The protection of turn insulation, which is primarily a matter of wave front. To meet the first requirement, the overvoltage to ground must be limited to a safe value. This is assumed to be approximately equal to the crest value of the 60-cycle high potential test. To fulfill the second requirement, the turn voltage gradient must be limited to conform to the turn insulation. To secure these results, a protective scheme is proposed, which includes arresters 2,000 feet and 500 feet out on the line, or other means, to limit the incoming voltage to 2.5 times the crest value of the terminal arrester rating, a special arrester at the machine terminal to limit the voltage to the crest value of the machine high potential test, and a capacitor at the machine terminal to slope the wave front, thus limiting the turn-to-turn stresses. The capacitor also serves to limit internal overvoltages due to reflection at the neutral. For neutral grounded solidly or through resistance equal to or less than the combined surge impedance of the machine windings 0.1, ??f capacitance is sufficient; while for isolated neutral, capacitance up to 0.5 ??f may be necessary. Ground wires over the line for the initial 2,000 feet are recommended to protect against direct strokes.