Comparison of security margin estimation methods under various load configurations

TL;DR

This paper compares two methods for estimating security margins in power systems, analyzing their differences under various load and contingency scenarios to improve reliability assessment.

Contribution

It introduces a methodology for fair comparison of PCLL and SOL, highlighting conditions where their estimates significantly diverge.

Findings

SOL can differ greatly from PCLL under certain load conditions

Longer fault clearing times increase the difference between the two margins

High penetration of constant power loads affects security margin estimates

Abstract

The post-contingency loadability limit (PCLL) and the secure operating limit (SOL) are the two main approaches used in computing the security margins of an electric power system. While the SOL is significantly more computationally demanding than the PCLL, it can account for the dynamic response after a disturbance and generally provides a better measure of the security margin. In this study, the difference between these two methods is compared and analyzed for a range of different contingency and load model scenarios. A methodology to allow a fair comparison between the two security margins is developed and tested on a modified version of the Nordic32 test system. The study shows that the SOL can differ significantly from the PCLL, especially when the system has a high penetration of loads with constant power characteristics, or a large share of induction motor loads with fast load restoration. The difference between the methods is also tested for different contingencies, where longer fault clearing times are shown to significantly increase the difference between the two margins.