Field Investigation of Voltage Disturbances, VFD Sensitivity, and Harmonic Effects in a 230 kV Grid-Connected Steel Plant

This paper presents a comprehensive field-based investigation of power quality phenomena in a large integrated steel plant supplied at 230 kV. The study focuses on voltage sag-induced rolling mill interruptions, variable frequency drive (VFD) sensitivity, and harmonic related transformer stress associated with Electric Arc Furnace (EAF) operation. Digital Fault and Disturbance Recorder (DFDR) data collected at the point of common coupling (PCC) are analyzed to distinguish between national grid disturbances, internal plant faults, and furnace-related effects. The rolling mill is driven by multiple high-power VFDs with limited voltage sag ride-through capability, tolerating only 18-20% voltage dip for durations of approximately 18-20 ms. Field observations confirm that rolling mill VFD trips are not associated with EAF operation, as a Static Var Compensator (SVC) installed at the 33 kV furnace bus effectively maintains voltage magnitude and power factor. Instead, VFD trips occur exclusively during national grid voltage sags or internal plant faults that propagate rapidly to the medium-voltage rolling mill bus. Separately, EAF operation introduces significant current harmonics that propagate upstream toward the 230/33 kV power transformer, causing abnormal acoustic noise and mechanical vibration. In addition, a localized voltage drop of approximately 1.2-1.3 kV is observed at the primary of the 60 MVA EAF transformer during arcing, despite stable voltage at the main 33 kV bus.