1. EMT/RMS-Domain Applications: Understanding the Transition

• Application: Historically, EMT-domain simulations have been used for transient studies involving equipment design (e.g., HVDC, FACTS) or renewable energy integration into weak systems.
• Approach: The primary asset under investigation is modeled in detail, while the electrical grid is simplified into a Thevenin Equivalent. This represents an ideal voltage source behind an equivalent impedance reflecting the grid's strength at the point of connection.

Traditional Use of RMS-Domain

• Application: RMS-domain (phasor-mode) simulations are widely used for wide-area dynamic studies, where the entire electrical system is modeled in detail.
• Simplification: RMS-domain approximations (phasor-mode) neglect fast electromagnetic transients and are valid as long as the system frequency remains close to nominal.
• Efficiency: Allows for larger simulation time steps (e.g., 1-10 ms), making it computationally efficient. It is particularly suited for systems dominated by slow electromechanical phenomena driven by synchronous generator dynamics.

Emerging Challenges

• Changing Landscape: The power system is evolving with increased penetration of inverter-connected generation and reduced reliance on synchronous generators.
• New Dynamics: Phenomena like converter-driven instability cannot be accurately reproduced by RMS-domain simulations.
• Example: RMS and EMT-domain simulations may yield very different results. For instance, voltage instability may not be observable in the RMS-domain. In such cases, PDT (Phasor-Domain Transient) refers to another denomination of the RMS-domain.

Source: CIGRE Technical Brochure 881

Importance of EMT-Domain

• Accuracy: EMT-domain simulations capture fast dynamics in systems dominated by inverter-based resources, which RMS-domain simulations may fail to predict.
• Critical Role: EMT-domain studies are increasingly essential for ensuring the stability of modern power systems.

2. Challenges of EMT Simulations

Time-Step Constraints

• Time Steps: EMT simulations require smaller time steps (5-50 µs) compared to RMS simulations (1-10 ms), leading to:
  • Increased Computational Complexity: Resource-intensive and time-consuming for wide-area systems.
  • Demanding TSO Needs: Transmission System Operators (TSOs) must evaluate grid stability under numerous scenarios (e.g., thousands of faults and operating conditions), making EMT simulations less practical without optimization.

3. Strategies for Wide-Area EMT Simulations

To address these challenges, various strategies can enhance computational efficiency:

A. Parallelization

• Approach: Divide the grid model into multiple sub-models.
• Benefit: Simulate each sub-model on a separate processor core to distribute the computational load effectively.

B. Model Reduction

• Focus: Detailed modeling of the "study area" while simplifying the "external grid."
• Challenges: Accurately representing the external grid's dynamics and identifying boundary buses.

C. Co-Simulation

• Hybrid Approach: Model the study area in the EMT domain and the remaining system in the RMS domain.
• Advantage: Balances computational efficiency with accuracy, leveraging the strengths of both domains.

4. The Path Forward

The increasing prevalence of inverter-connected generation underscores the urgency of transitioning from RMS to EMT-domain simulations for wide-area studies.

Interim Solutions

• Strategies like parallelization, model reduction, and co-simulation can improve computational efficiency.

Long-Term Progress

• Collaboration: Robust partnerships between TSOs and software developers are essential to drive innovation.
• Scalability: Developing scalable EMT tools for large-scale, wide-area simulations will be critical.

By addressing these challenges and advancing simulation technologies, the industry can ensure robust stability assessments and resilient operation of future power systems.

Reference

CIGRE Technical Brochure 881: Electromagnetic transient simulation models for large-scale system impact studies in power systems having a high penetration of inverter-connected generation.