MVDC and MVAC Comparison for Stability and Frequency Support in Expanding Distribution Grids with Intermittent Resources

The integration of distributed energy resources (DERs) into medium-voltage alternating current (MVAC) grids presents significant challenges, particularly in hosting capacity limitations. As decentralization efforts increase, MVAC grids become saturated, necessitating expansion to maintain stability and efficiency. This study examines two possible expansion strategies: reinforcing the existing MVAC infrastructure or transitioning to a hybrid network incorporating medium-voltage direct current (MVDC) links. A key focus is on frequency stability, specifically the impact on the rate of change of frequency (RoCoF) and nadir during primary frequency response. To address this, small-signal models are developed and validated, enabling sensitivity analysis to identify key parameters that influence RoCoF and nadir. These models are further validated through Electromagnetic transient (EMT) simulations to demonstrate their accuracy and reliability. An assessment was performed to evaluate the effectiveness of MVDC integration in improving frequency stability relative to traditional MVAC expansion.