Practical Solutions to Protecting Extra Large Split-Phase Autotransformers

This paper presents protection schemes for an extra large 2,000 MVA 765/345 kV split-phase autotransformer. Equipment of this size is always an integral part of a power system backbone of an interconnection, a country or a continent; and as such it should be protected applying the highest possible standards. Dedicated protection functions have been engineered and configured to cover winding, ground, bank, unit and tertiary faults. The protection system is connected to more than 50 ac signals, and is setup using numerous multi-function transformer and general-purpose microprocessor-based relays. The applied protection schemes incorporate extended supervision in order to avoid false operations that would impact power system operation and stability. The paper describes each protection function in detail including theory of operation, zones of protection, diagrams, setting recommendations, and results of simulation testing. After being fine-tuned and tested on the real time digital simulator (RTDS), the unique protection system presented in this paper is in-service for approximately one year