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Protective Relaying, 4th Edition by CRC Press

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Chapter 5

Relay Input Sources

5.1 Introduction

Protective relays require reasonably accurate reproduction of the normal, tolerable, and intolerable conditions in the power system for correct sensing and operation. This information input from the power system is usually through current transformer (CT) and voltage transformer (VT). An exception is for temperature-type relays that receive their information from thermocouples or temperature indicators.

These devices—CTs, VTs, formerly potential transformers (PTs), and coupling capacitor voltage transformers (CCVTs)—provide insulation from the higher-system voltages and a reduction of the primary current and voltage quantities. Thus, primary source of the transformer is connected into or to the ...

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