Mixed time-frame simulation

The use of a single time frame throughout the simulation is inefficient for studies involving widely varying time constants. A typical example is multimachine transient stability assessment when the system contains HVDC converters. In such cases the stability levels are affected by both the long time constant of the electromechanical response of the generators and the short time constant of the converter's power electronic control. It is, of course, possible to include the equations of motion of the generators in the electromagnetic transient programs to represent the electromechanical behaviour of multimachine power systems. However, considering the different time constants influencing the electromechanical and electromagnetic behaviour, such approach would be extremely inefficient. Electromagnetic transient simulations use steps of (typically) 50 μs, whereas the stability programs use steps at least 200 times larger.