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Different technologies have been used for more than one hundred years for designing and manufacturing protective relays. Many techniques previously used in electromechanical and solid-state relays are now used in numerical relays. An appreciation of the previous technologies is important for knowing the roots of protection and avoiding the phenomenon of re-inventing the wheel. The development of electromagnetic, electromechanical, solid-state, and numerical technologies is reviewed in Section 2 of this report. Modern numerical relays perform many functions other than protection. Some of the more commonly performed functions are identified in this section as well.

The third section addresses the issues concerning the protection of transformers.

The fourth section is devoted to monitoring transformers.

Control of transformers is addressed in the fifth section of this report.

Section six provides a brief description of techniques used in numerical relays for processing sampled and quantized data. While a very large number of techniques are reported in the literature, many techniques that are presently being used or have the potential of being used in numerical relays are described.

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