Frequency-Domain Compliance Assessment of Grid-Forming Devices

TL;DR

This paper introduces a frequency-domain compliance criterion for grid-forming inverter devices, improving accuracy over traditional time-domain methods by analyzing Bode plot characteristics to ensure voltage source behavior.

Contribution

It proposes a novel frequency-domain approach for assessing GFMIs' compliance, addressing measurement issues in fast transient conditions, and demonstrates its effectiveness on models and system stability.

Findings

Frequency-domain criterion aligns with time-domain assessments.

Method effectively evaluates GFMI compliance in simulations.

GFMI compliance impacts small-signal stability of power systems.

Abstract

Grid-ForMing Inverters (GFMIs) are expected to provide voltage stiffness to the grid. Explicitly, system operators (SOs) and regulators expect GFMIs to behave like a "voltage source behind impedance (VSBI)" in the (sub)-transient time frame. SOs assess this VSBI characteristic of GFMIs during compliance by defining a pass-fail time-domain criterion. This is done by evaluating the GFMIs' active (or reactive) power/current response to step changes in voltage phase (and magnitude) at its terminals. However, this approach is prone to errors due to poorly defined measurement specifications for very fast (less than a cycle) transients. To address this, this work proposes a compliance criterion for the VSBI characteristic of GFMIs in the frequency domain based on elements of the frequency-domain Jacobian. The compliance criterion is defined in terms of the minimum expected P(s)/\theta(s) and Q(s)/V(s) Bode plot characteristics across a specific frequency range. The equivalence between the time-domain and frequency-domain criteria is established. The proposed method is demonstrated by assessing the compliance of generic NLR (formerly NREL) GFMI models in PSCAD. Furthermore, the impact of GFMI compliance on the small-signal stability of the IEEE 39-bus bulk-power system is demonstrated.