IEEE STD C37.91-2000: GUIDE FOR PROTECTIVE RELAY APPLICATIONS TO POWER TRANSFORMER
A power transformer, in which I think everybody would agree, is a very complicated form of electrical apparatus. While in operation, it is subjected to a quite number of threats which can be unpredictable, one of which is a fault that can be very damaging. Proper care and maintenance are always a necessary step to insure that a power transformer is prepared to face, if can’t be avoided, this kind of phenomena. Beside constant maintenance, another effective way of protecting a power transformer is through proper relaying. During events like faults, overcurrent, overloading, over/undervoltage, over/underfrequency, etc., which are always a threat to a transformer, the circuit breaker intended to protect the transformer should always be able to isolate the said device from the system at the least time possible. The longer the transformer is exposed to these events, the more likely that the transformer could be severely damaged.
By statistics, the common transformer failures that exist are rank mainly to five categories namely; winding failure, tap changer failure, bushing failure, terminal board failure, and core failure. Most likely, in order to protect the transformer from this kind of failures, we should first understand the nature of these events to come up with a way to minimize this if not avoided.
Several forms of transformer protective relaying are usually found in use today to safeguard the transformer from all these kinds of events. Each kind of relaying corresponds also to each kind of function. Protective relaying is another branch of electrical engineering which is also a very complex field. However, to simplify things, the IEEE Organization came up with the C37.91-2000 standard to serve as a guide in protecting power transformers through protective relaying applications. The objective of this document is to facilitate in the effective use of relays and other devices for the protection of power transformers. Emphasis is placed on practical applications. The general philosophy and economics of transformer protection are reviewed. The types of faults experienced are described, and technical problems with such protection, including current transformer (CT) behavior during fault conditions, are discussed. Various types of electrical, mechanical, and thermal protective devices are also described and associated problems such as fault clearing and reenergizing are discussed.
Power Transformers are not only one of the most important piece of electrical device in the power system with respect to its functions but also the most expensive among all other devices found in an electrical substation. Furthermore, once a power transformer is out of service during its operation, the electric utility can not simply replace it in a short period of time not unless one has a spare which is also, mind you, can be very impractical. An electric utility can not afford to loss a power transformer because it would also mean a loss in revenue and for a manufacturer’s perspective is a loss in production.
A power transformer, in which I think everybody would agree, is a very complicated form of electrical apparatus. While in operation, it is subjected to a quite number of threats which can be unpredictable, one of which is a fault that can be very damaging. Proper care and maintenance are always a necessary step to insure that a power transformer is prepared to face, if can’t be avoided, this kind of phenomena. Beside constant maintenance, another effective way of protecting a power transformer is through proper relaying. During events like faults, overcurrent, overloading, over/undervoltage, over/underfrequency, etc., which are always a threat to a transformer, the circuit breaker intended to protect the transformer should always be able to isolate the said device from the system at the least time possible. The longer the transformer is exposed to these events, the more likely that the transformer could be severely damaged.By statistics, the common transformer failures that exist are rank mainly to five categories namely; winding failure, tap changer failure, bushing failure, terminal board failure, and core failure. Most likely, in order to protect the transformer from this kind of failures, we should first understand the nature of these events to come up with a way to minimize this if not avoided.
Several forms of transformer protective relaying are usually found in use today to safeguard the transformer from all these kinds of events. Each kind of relaying corresponds also to each kind of function. Protective relaying is another branch of electrical engineering which is also a very complex field. However, to simplify things, the IEEE Organization came up with the C37.91-2000 standard to serve as a guide in protecting power transformers through protective relaying applications. The objective of this document is to facilitate in the effective use of relays and other devices for the protection of power transformers. Emphasis is placed on practical applications. The general philosophy and economics of transformer protection are reviewed. The types of faults experienced are described, and technical problems with such protection, including current transformer (CT) behavior during fault conditions, are discussed. Various types of electrical, mechanical, and thermal protective devices are also described and associated problems such as fault clearing and reenergizing are discussed.
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