TECHNICAL REPORT 3 (Related Project) - Summary Report

TECHNICAL REPORT 3

TITLE : Thermal Overload Protection of Power Transformers

AUTHOR : Rich Hunt

                   Michael L.
 
LINK: GaTech_ThermalOverloadProtectionofTFmrs.pdf 

SUMMARY :

 

TRADITIONAL methods of protecting power transformers use functions based on measured current and voltage. These functions are useful in detecting short circuits and other transient electrical fault events in the transformer. However, for liquid-immersed power transformers, the temperature of the winding hot-spot is the important factor in the long-term life of the transformer. The insulating oil temperature is dependent on the winding temperature, and is used to indicate the operating conditions of the transformer. Many numerical transformer protection relays available today include protection functions that operate on insulating oil temperatures, calculated loss-of-life due to high oil temperature, and predicted oil temperatures due to load. These types of functions are not routinely applied, often since protection engineers may lack an understanding of the operating principles of these functions, and transformer operating conditions, to properly determine a settings methodology. A factor to consider when looking at these temperature-based functions is the risk of accelerated aging, and transformer failure, is increasing. Modern utility operating practices try to maximize the utilization of power transformers, which may increase the occurrence of over-temperature conditions, and transformer aging. Over-temperature conditions and accelerated aging are adverse system events that must be identified and protected against. The most common function provided for thermal protection of power transformers is the thermal overload (ANSI 49) function. To properly set this function, the protection engineer must understand the basics of the thermal performance of power transformers, and the basic design of the specific implementation of the 49 function. Northeast Utilities has implemented thermal protection of substation power transformers. The temperature protection is combined with distribution automation to manage transformer load. Thermal overload levels of the transformers force an automatic load transfer through feeder circuit re-configuration. Predictive overload alarms warn the Distribution System Operators of the pending automatic forced load transfer, to allow manual intervention. The settings criteria, control logic, and operations criteria for the thermal overload protection are discussed, as well as
an overview of the operating experience.