The propagation of surge voltages through large turbo-alternators with two parallel windings

The paper extends an earlier study of the propagation of a surge through an alternator with a single winding per phase to include large turbo-alternators with two windings in parallel¿a type of winding used in machines carrying currents exceeding about 700 amp. It is usual to construct machines of this type with a short-chorded winding, in order to reduce the proportion of harmonics in the generated voltage. From a consideration of the winding arrangements in a machine of this type, it is shown that the impulse-voltage distribution in the two halves of the unimpulsed phases is not the same with a single-phase impulse. Similarly, the two halves of either of the impulsed phases have a different voltage distribution when an impulse is applied to two phases. It can, however, be shown that in these cases one half-winding from one phase may be paired with a halfwinding from the other phase. These deductions are confirmed by test results on a 50-MVA alternator winding. Test results for other machines are given to illustrate the relationships which were derived from theoretical considerations.