Backup protections are not allowed to initiate the Auto transfer. Because under voltage may exist in the bus. Similarly protection in LV of the transformers are not initiating the Auto transfer scheme. There are three metrics used in Auto transfer scheme and 2/3 logic is adopted.
Conditions
1. Fault generator (86BG or 86A1).
2. Fault in UT.
3. Fault in GT.
4. Fault in 220 kV bus (SUT).
5. Fault in SUT (86M).