System Services in an Inverter-Dominated Grid: Insights from EMT Modelling of the NEM through to 2034

(e.g., those related to voltage and frequency stability), once inherently provided by synchronous machines, are now being re-evaluated due to the nuanced and selective capabilities of IBRs.

This shift necessitates a comprehensive reassessment of which services are essential and how they can be reliably delivered.

This work is structured around four key areas:

• Revisiting what constitutes a service: Identifying a broader set of essential technical needs of the system, ranging from slow dynamics like system balancing to fast dynamics such as fault current performance. • Modelling the Evolving NEM: Developing a reduced-order wide-area Electromagnetic

Transient (EMT) model of the NEM to study how technical needs change as IBR penetration increases. • IBR-Only System Analysis: Using EMT simulations, exploring the operation of the

NEM without synchronous machines (N.B. without synchronous condensers too), identifying stability challenges and necessary interventions. • Cross-Validation with Phasor Domain Transient (PDT) Models: Use PDT models to identify representable phenomena from EMT studies and highlight any PDT representation limitations. The work completed emphasized identifying technical needs rather than determining whether they should be addressed through market mechanisms or mandatory standards. Key outcomes include:

• A new, expanded power system technical needs breakdown tailored for high-IBR systems. • Development of simplified 67-bus NEM models projecting system evolution from 2024 to 2034, incorporating major network projects and renewable energy zones (REZs). • Determination that a minimum 30% grid-forming (GFM) IBR ratio is required to maintain system stability, with immediate deployment recommended. • Discovery of two new phenomena in high-IBR systems:

o Widespread phase angle jumps during faults, suggesting a system-wide response to disturbances.

o Overvoltages during remote faults, likely linked to phase angle shifts. • Recognition that while fault current magnitude may remain adequate due to the number of IBRs online, its quality, such as waveform distortion and variability, may degrade. • While PDT models generally aligned with EMT results, there were several important limitations found, particularly in detecting remote overvoltages and sensitivity to system loading. These findings lay the groundwork for future investigations into how critical system services should be delivered in a high-IBR environment and delivered publicly accessible EMT network models for any party wishing to investigate further. Continued research will be essential to ensure the NEM remains stable and resilient as it transitions to a low-emissions future.