Fault Currents, Circuit Breakers, and a New Method for X/R Calculation in Parallel Circuits Part I: Concepts and Approaches

This paper first provides an analysis of the asymmetrical characteristics of fault currents in power systems and reviews the fundamental concepts in the ANSI/IEEE symmetrical rating method for circuit breakers. Next, the paper examines the problem of determining a system's X/R when there are parallel circuits at the fault, where there is no single X/R precisely describing the transient current. The paper develops a rigorous method to construct and analytically solve the differential equation of the fault current in higher order systems, which provides its accurate transient components. In Part II of the paper, two sample circuits are analyzed with the proposed method where transient fault current demonstrates substantially different characteristics that cannot be modeled by a single X/R. The concept of a variable X/R is then developed to consider the actual impact of the transient fault current on circuit breaker interrupting capability.