Abridgment of voltage oscillations in armature windings under lightning impulses — I

Experience has shown that rotating machinery connected directly to overhead lines is more vulnerable to surges than many other types of apparatus. This fact, together with a desire on the part of some to connect important units to the line in this manner, has necessitated a study of the protection problem. Such a study is here described, showing oscillograms taken when steep voltage surges were applied to machine windings measuring internal voltages to ground which are 200 per cent of the voltage allowed by the terminal lightning arrester. A simple traveling wave analysis of these oscillations is developed, which has successfully explained the pecularities of over 400 oscillograms taken under various terminal conditions. Practical methods of eliminating the oscillations with neutral impedance are outlined in the light of the theory developed, and oscillographic evidence supporting their reliability is given. A generalized theory of neutral protection is proposed. The importance of wave-front, surge impedance of incoming line, arrester resistance, and other factors is discussed. Methods for protection of the turn insulation and the insulation to ground of such machines are suggested, showing that the lightning arrester limits only the potential of waves entering the machine and cannot control oscillations which may take place within the machine. The advantages of thyrite as a neutral resistor are pointed out in connection with the short-circuit protection, telephone interference, and lightning protection problems of such machines. In Appendix A, the test circuits and methods used in the laboratory are discussed in relation to actual field conditions.