Effects of dynamic power electronic load models on power systems analysis using ZIP-E loads

TL;DR

Power systems increasingly host power-electronics loads, which are not adequately captured by traditional ZIP models at fast time scales. The authors propose ZIP-E, a composite model that combines a static ZIP with a dynamic power-electronic E-load, and evaluate its impact on small-signal and electromagnetic transient stability using the IEEE WSCC 9 Bus test case with both static and dynamic line models. They find that the constant-power portion of ZIP loads can overstate instability, while incorporating E-load dynamics dampens high-frequency transients and improves convergence, especially when line dynamics are modeled. The study yields practical guidance on load-model selection near stability boundaries and delivers an open-source tool for reproducible ZIP-E simulations, highlighting implications for stability assessments in grids with high power-electronics penetration.

Abstract

Power grids are seeing more devices connected at the load level in the form of power electronics: e.g., data centers, electric vehicle chargers, and battery storage facilities. Therefore it is necessary to perform power system analyses with load models that capture these loads' behavior, which has historically not been done. To this end, we propose ZIP-E loads, a composite load model that has a ZIP load with a dynamic power electronic, or E, load model. We perform small signal and transient analysis of the IEEE WSCC 9 Bus test case with ZIP and ZIP-E load models. For small signals, we conclude that ZIP loads destabalize networks significantly faster than corresponding ZIP-E loads. In stable cases, transient results showed significantly larger oscillations for ZIP loads. Further, we find that a higher network loading condition is correlated with a higher sensitivity to load model choice. These results suggests that the constant power portion of the ZIP load has a large destabilizing effect and can generally overestimate instability, and that attention should be drawn to load model choice if operating near a stability boundary.

Figures (5)

• Figure 1: Modified IEEE 9 bus test system. SM at bus 1 (reference bus), VSM GFM at bus 2, GFL at bus 3. Note that we do not use a slack bus for our simulations.
• Figure 2: Network eigenvalues with $dynpi$ line model at $load\ scales$ of 0.2, 0.5, and 0.8 increasing to the right.
• Figure 3: Eigenvalues at $load\ scale=1.0$ for $dynpi$ (top) and $statpi$ (bottom) line models. ZI-E loads are in blue and ZIP loads are red in 10% increments as given by the heatmap bar on the right.
• Figure 4: Bus 3 inverter current magnitude after a branch trip on line 4-5 with $dynpi$ lines.
• Figure 5: Transient simulation of Bus 3 inverter current magnitude after a branch trip of line connecting Bus 4 and Bus 5. $load\ scale = 0.25$.