Significance of Thermal time constant of a transformer below 5 MVA 2

[Rwelagedara]

Dear All,
Most of the transformer technical specifications are requesting for the value of thermal time constant from the manufacturers to specify. Is there any good or bad in thermal time constant being high or low , for a customer? Why this value is significant. Please advice.

 

[RRaghunath]

If a transformer has higher thermal time constant, it means it has better short time overload capability which is desirable.

 

[prc]

I believe customer has no benefit with high or low time constants. There are two time constants ( the time it takes for reaching 63.2 % of ultimate stabilized temperature under loading) Oil time constant ( 1-3 hours depending on rating and type of cooling) and winding time constant ( 5-10 minutes).These are used for calculating temperatures under transient heating and cooling conditions. Time constants are required to calculate temperatures under short time overloads ( of time less than time constants) but not adding much to overload capability or arriving at temperatures under over load conditions. Designer has no choice in selecting time constants, but it depends on the amount of active materials used, decided based on performance requirements ( like efficiency) See C57.91-2011 overloading guide.