Fault Currents, Circuit Breakers, and a New Method for X/R Calculation in Parallel Circuits Part II: Analysis and Application

In Part II of the paper the complete transient component of the fault current in two sample circuits is analytically calculated using the time domain equations of the system. It is illustrated that the transient current consists of multiple dc and, in some cases, of decaying sinusoidal components. This transient current exhibits characteristics that cannot be represented with only one decaying dc term and could subject a circuit breaker to a substantially larger fault current. To properly capture the impact of the actual transient current on a circuit breaker a variable X/R is defined whose value depends on the contact parting time of a circuit breaker. The details of derivation of the fault current's Ordinary Differential Equation (ODE) and its analytical solution, described in Part I of the paper, are also provided in two Appendixes in this Part.