This paper investigates how the choice of Low-Voltage-Ride-Through (LVRT) test equipment significantly affects certification outcomes for both Grid-Following (GFL) and Grid-Forming (GFM) inverters. As renewable energy penetration increases, accurate LVRT certification is critical for ensuring grid stability, yet current testing standards rely on equipment that may not faithfully replicate real grid fault conditions.

Using a unified Thevenin equivalent circuit framework, four test configurations are systematically compared: a Hardware-in-the-Loop (HIL) system representing a realistic double-circuit transmission topology, a Programmable Voltage Source, a Short-circuit Impedance Virtual Synchronous Generator, and an Autotransformer-based VSG. A Monte Carlo-style parametric sweep across realistic grid parameters (short-circuit ratio, X/R ratio, fault location) quantifies the statistical distribution of equivalent impedance magnitude, angle, and voltage phase shift across configurations.

For GFL inverters, equilibrium-point analysis of the Phase-Locked Loop synchronization reveals that realistic double-circuit fault conditions can eliminate stable operating points entirely, leading to inevitable loss of synchronization. Even when a stable equilibrium exists, the angular displacement exceeds 140 degrees, compared to less than 3 degrees for all three test equipment types. This demonstrates that conventional test equipment creates artificially favorable stability conditions.

For GFM inverters, reactive current measurements at the same operating point differ by more than 50 percent across test configurations, due to differences in equivalent impedance characteristics and voltage phase shifts inherent to the double-circuit topology. Furthermore, two different current-limiting strategies are evaluated, confirming that this measurement discrepancy is systematic and independent of the chosen control approach.

These findings, based on analytical modeling and simulation, demonstrate that current LVRT certification procedures using conventional test equipment may not accurately predict field performance. The results support integrating HIL-based testing with realistic grid topologies into future certification standards to improve the reliability and representativeness of LVRT compliance assessment.