High Temperature for the excitation transformer

Please correct your question. All the figures in you question are inconsistant. For an answer one needs to know the name plate data of 1) Excitation Transformer i.e. Rated kVA, Volatge, Current, Phases. 2) Excitation Rectifier i.e. Rated Input Voltage (1 or 3) Phase, Output Voltage and Current.

MIQ

The voltage rating is 11kv/415V.

I- for current

3Phase.

If the heat is significantly higher in one location, you may have a loose connection or shorted winding turn. If the heat is generalized over the entire unit, the problem is likely related to environmental or operating conditions. If available, a thermal imager (infrared thermometer) will help pinpoint a localized problem. In a pinch I've used a zero lux camcorder (it has an infrared filter for extremely low light conditions).

If the temperature rise is not in a specific location, I would check the following:

1. If the transformer is in a protective enclosure, check the cooling louvers and filters (if any) for dirt or other obstruction.

2. Look for a buildup of dust or other grime on the windings. This will act as an insulator and prevent proper cooling of the windings.

3. Is the Excitation transformer overloaded (compare the actual current on all input phases with the nameplate rating)?

4. Is the ambient temperature higher than the nameplate rating?

5. What has changed since the unit was operating at normal temperature?

Above 410, you have exceeded the device's rating of 410kVA. The average is certain to be above 410 kVA.