Review of SIR Calculations for Distance Protection and Considerations for Inverter-Based Resources

Source-to-line impedance ratio (SIR) is a parameter used in the application of distance protection. IEEE Std C37.113-2015 (Line Protection Guide) presents a method to calculate SIR for three-phase faults and single-line-to-ground faults. This method has been incorporated into short-circuit programs and is used by practitioners. However, the method uses assumptions that produce erroneous results in systems with zero-sequence mutual coupling and in systems with inverter-based resources. This paper reviews the evolution of SIR calculations (methods) and advocates the use of newer and simpler SIR calculations that remain accurate for all distance protection applications.