Transformer Inrush Restraint Settings Reference Matrix (SEL, GE, Siemens)

Transformer differential protection must discriminate between high-magnitude inrush currents—generated during no-load energization due to core saturation—and genuine internal faults, which pose immediate equipment risk. Inrush currents exhibit distinctive characteristics: high second-harmonic content (typically 20–60% of fundamental), asymmetry, and intermittent zero-crossings, whereas fault currents are predominantly fundamental-frequency and symmetrical. Modern numerical relays implement multiple restraint algorithms—including second-harmonic blocking, harmonic ratio restraint (e.g., I₂/I₁), waveform symmetry detection (e.g., cross-point or wave-shape analysis), and adaptive thresholding—to achieve this discrimination. SEL relays (e.g., SEL-387, SEL-787) emphasize configurable harmonic blocking with independent I₂/I₁ thresholds per phase and optional fifth-harmonic supervision; GE’s UR series (e.g., UR-DIF, UR-F1) uses multi-cycle harmonic analysis with programmable harmonic restraint curves and optional flux-restraint logic; Siemens relays (e.g., 7UT61, 7SJ62) employ a combination of second-harmonic blocking, harmonic band restraint (I₂/I₁ and I₅/I₁), and transient inrush detection via dI/dt and RMS stability monitoring. Proper configuration requires site-specific validation using recorded inrush waveforms, CT saturation analysis, and coordination with upstream/downstream devices to ensure selectivity without compromising sensitivity.