Physical significance of sub-subtransient quantities in dynamic behaviour of synchronous machines

The recent accurate modelling of alternating-current machines sometimes requires, in addition to the known transient and sub-transient quantities, the use of sub-subtransient quantities. The sub-subtransient time constants are very small; they can, however, have a great influence on the machine performance. The physical meaning of these new sub-subtransient quantities for the stability and switch-on process of the synchronous machine is explained and illustrated with examples. It is shown that by introducing the sub-subtransient values, the effect of the solid turborotor and the behaviour of the salient-pole machine with solid poles can be treated with sufficient accuracy.