Design of a combined digital global differential and volt/hertz relay for step-up transformers

The author presents the specifications and design principles of a digital relay covering the global differential and volt/hertz functions for a generator step-up transformer unit. It is the second and last relay developed in the project for an integrated digital generator protection system at Hydro-Quebec. The design of this system is based on a number of premises: use of low-cost standard bus cards together with digital signal processors to achieve sufficient computing power, grouping of many protection functions in the same independent crate, and implementation of functional redundancy by distribution of the same function between at least two independent crates. It is estimated that the ultimate systems will cost less than existing generator protection systems and offer substantial benefits