The basis for power and steam system design is knowing how the systems will react to disturbances. For green field projects, or for large extensions to existing facilities, simulations must be carried out since it is not possible to make measurements. The power system simulations must calculate the transient and steady-state frequency, active and reactive power flow, and busbar voltages before, during and after disturbances. In many cases it is necessary to conduct simulations over a long period of time to ensure that the power system remains stable after a major disturbance. Short-time simulations may show that the power system recovers correctly after a contingency only to find that after several seconds, the power system actually collapses.