The Phantom Tertiary Menace

Southern Company experienced two unexpected events involving wye-wye transformers, each exposing complex and initially unrecognized behaviors driven by phantom tertiary core magnetics. These incidents prompted a deeper investigation into how such phantom effects influence system response and how they should be represented in sequence network models. The findings highlight the critical importance of accurately characterizing wye-wye transformer core types and their zero-sequence behavior during engineering studies—particularly in protective relay and unbalanced fault analysis—for both new and experienced engineers alike.

Event 1: A storm triggered widespread outages and repeated operations of a 12 kV feeder breaker supplying a 25 kV circuit via a 22 MVA autotransformer. Field crews later reported abnormal noise and overheating. Investigation revealed the Wye-wye autotransformer was single-phased due to a failed Phase B regulator. Despite the open phase, voltage persisted on the 25 kV side, with current flowing out but none entering on the 12 kV side. The unit operated at over 80°C with minimal load. This unexpected behavior was traced to an unanticipated phantom tertiary effect regenerating voltage, aided by downstream capacitor banks. This paper delves into oscillographs, field reports, and system modeling to recreate the event and explain the situation to our operators and field personnel.

Event 2: A fault on a radial 115 kV line feeding area load and a plant’s 115/4 kV reserve auxiliary Wye-wye transformer caused simultaneous trips of both line and transformer relays. The line reclosed successfully, but the plant’s transformer backfed approximately 450 amps of ground current despite having no source of generation coupled to its low-side terminals. This led to an out-of-zone trip by the transformer’s overcurrent elements. The cause of the backfed ground fault current was later identified as a phantom tertiary ground source attributed to the three-leg core form transformer design, but this behavior had not been considered in the relay settings development when the transformer was put into service. This paper explores the mitigation effort to ensure coordination is achieved for transmission system faults.