Overview

Thomas H.Ortmeyer

Clarkson University

Protective systems are designed to sense faults and initiate fault clearing in a timely manner while minimizing the affected area. Protective relays are used to sense the faults and initiate circuit breaker tripping. Alternatively, fuses are used on the distribution system to sense and clear faults.

Protective relays sense system current and voltage levels to determine both the presence and location of faults. Current transformers (CTs) and voltage transformers (VTs) are used to scale the system levels to levels suitable for the relays and measurement instruments. Voltage transformers are most commonly connected with wye secondaries, with 120Y/69 V being a standard secondary-voltage rating. They are selected through their voltage and volt-amp requirements. Current transformers carry both a continuous current rating and a burden rating. Their application is discussed in the next section.

Recently, digital protective relays have become available and are popular in many applications. These relays offer flexibility, self-checking, and ease of installation and often can provide additional functions over traditional electromechanical relays. Settings calculations for many of these relays are straightforward and are outlined in the relay s applications manual. In order to make these calculations, knowledge of peak-load current, minimum and maximum fault currents, and the CT and VT ratings is required.

This section provides a brief overview of some of the most common protective relaying applications. A much fuller discussion of these topics is available elsewhere, including Anderson (1999), Blackburn (1987), Elmore (1994), and IEEE Buff Book. Relay manufacturers commonly supply application...