The duration of an EMT simulation depends on the complexity of the network and integration step size that is configured in the initialisation command (*.ComInc).

The integration step size depends on the transient that is being investigated. Many transients require time steps in the range of 1 ns to 10 µs. The selection of a adequate integration step size should be based the criteria that are defined in the attached pdf document.

PowerFactory provides the possibility to adjust the integration step size during the simulation by means of step size adaptation. The step size adaptation can reduce the duration of the simulation significantly.

There are two possibilities for step size adaptation:

1. Automatic step size adaptation:
   PowerFactory adjusts the step size by internal routines within a range. The range (min. step size and max. step size are define in the *.ComInc). Further information can be found in the user manual.
2. Step size adaptation via event:
   The EMT simulation is carried out with a fixed / constant step size. The user defines events (Set integration step size (EvtStep)) to change the step size during the simulation.

The second option (variable step size by means of events EvtStep) is quite powerful:

• The EMT simulation can be started with a large step size (e.g. 0.1 ms) to reach a steady state in the simulation before the power system transient is triggered (e.g. by a switch event or lightning flash).
• Shortly before the transient occurs, the step size is reduced to fullfill the criteria defined in the attached pdf file
• The step size is increased again, once the high frequency transients are damped

The attached example contains three studies cases for different approaches for the EMT simulation:

1. Fixed integration step size during the complete simulation
2. Automatic step size adaptation during the complete simulation
3. Variable integration step size during the simulation. The step size is changed by means of events:
   1. Start of simulation at t_sim = - 100 ms with a step size of t_step =0.1 ms
   2. EvtStep: Adaptation of step size from step = 0.1 ms to step = 0.1 µs at simulation time t_sim = 1 ms
   3. EvtStep: Adaptation of step size from step = 0.1 µs ms to step = 0.1 ms at simulation time t_sim = 20 ms

In the attached example, the simulation duration can be reduced significantly with the adaptation of the step size using the EvtStep.

Note: The small step size of 0.1 µs during the transient is not required. The small step size was selected in order to show the impact of the different solver options.