How is the external grid modelled for RMS simulation?
It is modelled as a voltage source with an internal impedance. The mechanical equations are equivalent as those for the synchronous machine.
The Acceleration Time Constant (in s) represents the combined inertia of all the synchronous machines in the system. The overall inertia is the sum of the synchronous machine inertias in the system.
In addition, the Secondary Frequency Bias (in MW/Hz) characterises the dependency of the active power on the frequency. If the frequency of the system is reduced transiently, the active power will be increased to simulate the behaviour of the control of generation and frequency (load-generation control).