Where pS(t) is the row vector of substation state probability,
and As is the substation transition intensity matrix, Off-s
diagonal terms (i, j) of matrix Ashave the failure/repair rate
from state i to state]j, and diagonal terms are the negative sum
of failure and repair rates of all transitions from the current
state to an adjacent state (transition rates found in the same
row). The sum of all elements in each row of the transition
matrix is zero. The initial state is assumed to be at the state 1
in Table 1, in which every component works in the normal
status. The solution to the differential equations gives the
probability of each substation states at any instant of time.