  This paper analyzes the mechanisms of the inner-group machine. It is first
clarified that the inner-group machine is created from the difference between
the equivalent system and the original system. The inner-group machine
stability is analyzed based on the machine paradigms. In particular, strict
correlation between the inner-group machine trajectory and the inner-group
machine transient energy conversion is established through the I-CR system
modeling. Then, the transient characteristics of the inner-group machine are
analyzed. It is clarified that the inner-group motions might be inseparable or
separable, and the inner-group machine DLP will occur later than the EDLP and
IDLP. Simulation results show that the severity of the original system cannot
be simply evaluated through its equivalent system once any inner-group motion
becomes fierce.
