IEEE C37.91:2021 pdf download.IEEE Guide for Protecting Power Transformers.
The scope of this guide includes general philosophy, practical applications, and economic considerations involved in power transfbrmer protection.
Emphasis is placed on practical applications. General philosophy and economic considerations in protecting transformers are reviewed. Types of’ faults in transf’orrners are described. Technical problems with the protection systems, including the behavior of current transformers (CTs) during system faults, are discussed. Associated problems, such as fault clearing and reenergization. are discussed as well.
1.3 Word usage
The word shall indicates mandatory requirements strictly to be followed in order to conform to the standard and from which no deviation is permitted (shall equals is required to).”2
The word should indicates that among several possibilities one is recommended as particularly suitable, without mentioning or excluding others: or that a certain course of action is preferred but not necessarily required (should equals is recommended that).
The word nav is used to indicate a course of action permissible within the limits of the standard (may equals is permitted to).
The word can is used for statements of possibility and capability, whether material, physical, or causal (can equals is able to).
1.4 Purpose
The purpose of this guide is to provide protection engineers with information to assist in properly applying relays and other devices to protect transformers used in transmission and distribution systems.
1.5 Device numbers
The device numbers used in this guide are listed in Table 1.
3. Definitions, acronyms, and abbreviations
3.1 Definitions
For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause.
differential current: The sum of all currents entering and leaving a section of’ the electrical system; measured with consistent polarity. Also referred to as an operating current. See. restraining current.
digital relay: A static relay or relay unit in which the designed response is developed by converting the inputs into numeric form and manipulating them mathematically or logically.
harmonic sharing: A modification of the general harmonic blocking technique for preventing misoperations of a transformer differential relay during inrush; where the magnitude of the harmonic content of the operating currents in all three phases is summed before it is compared in ratio with that of the fundamental current in each phase element. Operation is blocked if the ratio exceeds a prespecified threshold.
microprocessor-based relay: A relay in which the designed response is developed by using one or more microprocessors. See: digital relay.
protective relay: A relay whose function is to detect defective lines or apparatus or other power sysLein conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action.
NOTE—A protective relay may be classified according to its operating principle or performance characteristics.
restraining current: A current in a low-impedance differential relay which is derived from all currents of the differential zone to make the difTerential function more secure when current transformers are saturated due to external faults. It is used in conjunction with a percentage differential characteristic to desensitize the effect of the differential current and reflects the severity of external faults. See: differential current.
restricted earth fault protection: A sensitive ground relay system that is designed to detect ground faults in a well-defined protective zone.IEEE C37.91 pdf download.