When operating with Ut= 0.95U_н, an increase (change) in the load by 10% leads to a two-fold field forcing (excitation forcing). With a further increase in the load, the system opens up. The same phenomenon occurs when the load decreases. The minimum excitation limit triggers, the system also opens up, and there is no longer any basis to speak of its stability.

A simplified expression for the transmitted power of a generator operating onto an infinite bus system:

U_t — voltage at the machine terminals.

U_a — voltage on the buses of the infinite bus system.

During a short circuit (s.c.), the voltage Ut decreases, and P decreases proportionally. To maintain U_t and P at a given level, it is necessary to achieve the ceiling excitation value by the exciter in the shortest possible time. When the short circuit is cleared, the resistance x_Σ increases, and a repeated rapid action of the excitation system is required.

The control system is tuned in such a way that the forward paths contain predominantly forcing (lead) blocks. The feedback paths contain damping blocks to ensure steady-state stability when the control loop is closed.

Thus, the excitation control system ensures the transmission of active power even under disturbances.

As was shown in Section 8.1, the excitation voltage rise rate when the voltage changes by 10% is equal to 51 unit exc / s.

When Ut changes by 50%: