Dynamic Impedance Method (DZM) Stability Screening for Large Systems

(EMT) simulations to evaluate the stability limits of IBR-dominant systems. These traditional approaches are computationally burdensome and human resource intensive, becoming difficult or unmanageable for large power systems. The challenge is greatly exacerbated by the everevolving characteristics of IBRs, which can exhibit significantly different dynamic characteristics depending on the controls software parameterization.

The new approach combines steady-state voltage stability analysis with the use of detailed EMT simulations using an impedance-based method to capture specific stability characteristics of different resources (grid-forming - GFMs, grid-following - GFLs, and synchronous machines SMs). In this paper, we briefly describe the Dynamic Impedance Method (DZM) and document the incremental efforts to quantitatively compare the accuracy and utility of the method against the more standard positive sequence dynamic simulations of most large-scale utility planning practices. The DZM utilizes an impedance analysis to quantify the volt-VAr response of a generator resource and distill it into a single impedance value that has been shown to capture important dynamic characteristics of IBR during stressed system conditions. This dynamic impedance, computed for each resource, is then applied to the representation of each resource in a large system power flow model such that steady-state analysis methods like P-V and Q-V analysis can be used to determine stability-constrained power transfer limits.

For the development of the DZM, EMT simulations with manufacturer-supplied equipment models for IBRs have been the “gold standard” for accuracy. This effort compares the stability limits determined from positive sequence phasor domain simulations against the stability limits determined from EMT and also against the stability limits predicted by the DZM. Determination of stability in large-scale systems is complex; here, the stability limit was based on six criteria:

power recovery, voltage recovery, dip, sag, sustained voltage, and damping.

The test EMT simulations were performed on a reduced network model based on a region of the Midcontinent Independent System Operator (MISO), considering many different operating conditions and disturbances. The set of 168 distinct scenarios included 14 different resource mixes (combinations of SM, GFL, and GFM at multiple locations), a variety of disturbances, and used four different product-specific IBR models from three different Original Equipment

Manufacturers (OEMs).

Key findings of the work are that stability limits from EMT and PSDS compare well for most disturbances, showing a slight conservative bias. The Dynamic Impedance Method estimates stability limits well, with statistical analysis showing a mean error of 358 MW (conservative bias) and, with 95% confidence, that optimistic errors are bounded to within 7% of the power transfer level. This study demonstrates that the DZM offers a similar level of accuracy to PSDS relative to EMT simulation, while being simpler and faster. By providing an alternative to traditional time-domain methods, the DZM could enable planners to evaluate a vast range of future grid scenarios and operational conditions more effectively.