Thermal Protection or Transformers Under Overload Conditions

The overloads recommended in the AIEE transformer subcommittee´s report,¹ ``Emergency Operation of Transformers,´´ are based on considerably higher transformer hot-spot temperatures for short-time operation than those given in and used to derive the emergency overloads recommended in the American Standard Guides for Operation of Transformers and Regulators C57.3. Overloads may be obtained in close agreement with the recommended emergency overload curves by the use of a relay which is controlled by a specially designed heating coil or by a specially designed relay with a conventional heating coil. Since a relay operates at about the same temperature irrespective of time to avoid too high hotspot temperature for ultimate conditions, the relay and heating coil must have such characteristics as to permit the transformer hot-spot temperature to reach considerably higher values for short times than for ultimate steady-state conditions. A detailed study of this problem has shown that this may be accomplished satisfactorily by the use of a lagged heating coil which has a relatively large time constant in comparison with that of the transformer winding or by the use of a heating coil, the time constant of which is about the same as that of the winding for operating a relay that is compensated for the changes in ambient temperature. This relay which has been developed can be set to perform three separate functions: control fans, give a warning signal, and sound an alarm or, if desired, trip the transformer off the circuit.