One topic that regularly comes up in discussions with utilities, OEMs, universities, and research institutes is the choice between EMT and RMS simulation.
The distinction seems clear in theory, but in practice the boundary is becoming increasingly blurred as grids become more converter-dominated.
For example, I have seen projects where an RMS study showed acceptable system behavior, but an EMT simulation later revealed:
- Harmonic interactions
- Converter control interactions
- Resonance issues
- Fast transient phenomena during faults or switching events
Below is an example of a generating plant model validation. Current and voltage waveforms at the Point of Common Coupling (PCC) are compared between PSCAD offline simulation and a real-time EMT simulation.
At the same time, I’ve also seen studies where EMT added considerable complexity while RMS already provided the answers needed for planning, stability assessments, and contingency analysis.
The example below shows a cascading outage scenario leading to system islanding and frequency deviations across multiple islands. For this type of study, RMS simulation is often sufficient and allows larger systems and longer simulation horizons to be analyzed efficiently.
An interesting aspect is the transition from offline studies to real-time simulation.
In Europe, my observation is that:
- PSCAD is often the preferred tool for offline EMT studies.
- DIgSILENT PowerFactory is often the preferred tool for offline RMS studies.
Many organizations already have validated PSCAD or PowerFactory models and naturally want to reuse that work when moving toward HIL testing, operator training, or digital twins.
At OPAL-RT, we frequently see:
- PSCAD models reused in HYPERSIM for real-time EMT simulation.
- PowerFactory models reused in ePHASORSIM for real-time RMS simulation.
I’m curious to hear how others approach this decision:
- What is your practical criterion for choosing EMT over RMS?
- Have you encountered situations where RMS missed important phenomena?
- Have you encountered situations where EMT was not worth the additional effort?
- How do you transition from offline studies to real-time simulation and HIL testing?
- Do you see the EMT/RMS boundary shifting as inverter-based resources become more dominant?
I would be very interested to hear about real project experiences and lessons learned.