The fixed voltage sources of NetworkOpened have been replaced with generators. Generator1 provides for primary frequency control, while generator2 introduces fluctuations.
Note the computation of the average sytem frequency in the global system model, basing on senders for each generator. This is needed for initialization (see initial equations in the text view). The initialization of the simulation corresponds to a black start in the real world.
The remainder of the PowerFlow library hides this mechanism in the composed generator or plant models.