The massive deployment of renewable energy sources, such as wind and solar, into power systems is increasing the penetration rate of Power Electronics Interfaced Resources (PEIR). Such a change is challenging the traditional methods of system modeling and stability analysis: the usual phasor (also commonly denoted Root Mean Square - RMS) models are no longer sufficient as the small signal stability of the power system is no longer limited to the electromechanical phenomena.

In fact, PEIR display different dynamics than the traditional synchronous machines-based generation, on a wider frequency range. Oftentimes, the PEIR use Voltage Source Converters (VSCs) with high switching frequencies and fast controls, which introduce phenomena of a faster time scale than their familiar electromechanical counterparts. Hence, literature studies show the need for Electromagnetic Transient (EMT) models to capture the fast transients and high frequency oscillations brought by the PEIR. However, multiple works in the literature retain RMS modeling and simulations as a sufficient method to study the slow dynamics of the power system, even under high penetration rates of PEIR, assuming all types of slow dynamics can be captured by RMS models.

This article investigates the validity of RMS-based studies for analyzing slow dynamics in converter-dominated power systems. Given that Grid-Following (GFL) control schemes remain the predominant control strategy in most PEIR, this work focuses on this control mode. Following literature guidelines, a generic GFL control structure is considered, initially modeled in the electromagnetic transient (EMT) domain and then simplified into an RMS representation.

To get insight on the dynamic behavior, a state-space representation of the EMT-based GFL converter model is developed, enabling the analysis of how various control parameters influence system modes. Particular attention is given to the current control loop, which tuning is shown to affect not only fast but also slow system modes. This observation challenges the common assumption that RMS modelling, which typically neglect the fast control dynamics, is sufficient to capture all relevant slow phenomena.

Comparative EMT and RMS simulations are performed to support this claim, revealing significant differences in the prediction of slow modes. This result highlights the potential blind spots of RMS models beyond the established harmonics and fast oscillations, which is important to consider when judging the validity of RMS simulations in converter dominated systems, especially with the simplified converter RMS models provided by constructors or established by standards such as the IEC 61400-27-1.