Effective Detection of Incipient Faults in Dry-Type Transformers using only Negative Sequence: Practical Realization and Field Experience

Abstract

Dry-type transformers are exposed to early failures in particular during testing, commissioning, and early operation, as observed in several railway projects. These failures are strongly associated with frequent switching operations involving vacuum circuit breakers and short cable lengths, which subject transformer insulation to repetitive high-frequency transients and cumulative stress. Over time, this stress leads to inter-turn insulation degradation and development of internal faults. Due to their small ratings, such transformers are often not equipped with differential protection, allowing faults to remain undetected until catastrophic failure. This paper presents a practical field-proven approach for early detection of incipient internal faults using negative sequence current (NSC) protection (ANSI 46) available in standard feeder protection relays with appropriate pickup and time-delay settings. The study is based on the analysis of more than 20 transformer failure events in railway projects, supported by relay records. Field-recorded disturbance data show that turn-to-turn faults are consistently characterized by sustained NSC lasting several minutes prior to failure, while normal operating conditions including load unbalance, traction operation, and magnetizing inrush produce significantly lower or shorter NSC levels. The proposed approach enables early fault detection and mitigation with minimal cost and complexity, making it particularly suitable for widely deployed MV/LV dry-type transformers in switching-intensive environments.