Modeling and protection of hexagonal phase-shifting transformers--Part II: protection

Owing to several advantages over traditional phase shifters, applications of hexagonal phase-shifting transformers (PSTs) keep expanding. These transformers are built with unique connections of their windings, neither delta nor wye. Modeling and protection methods are not yet fully established for hexagonal phase shifters. This work is divided in two parts. In Part I, a digital model of a hexagonal PST has been developed and validated. This part presents protection techniques for hexagonal PSTs. Several novel protection schemes have been developed based on ampere-turn balance equations of a hexagonal PST. These schemes include linear and nonlinear current-based protection methods, a power-based technique, and a phase comparison method. Protection schemes that require position of the tap changer for maximum sensitivity as well as a method to work without this extra information are presented. The developed techniques have been validated with digital and physical made-to-scale transformer models, and implemented on a general-purpose microprocessor-based relay platform. This material is of value to protection engineers facing the problem of protecting hexagonal PSTs. Index Terms--Differential power protection, differential protection, hexagonal winding connection, phase comparison protection, phase shifting transformer (PST), short-circuit protection.