6kV/0.4kV Transformer Fed by 7kV? 1

[Sanditech]

I am at a site where a 6Kv/0.4kV 1.25MvA Cast Resin Transformer is being fed by 7kV (Utillity Co will rectify soon?). What affect would this increased voltage have on the transformer? More with respect to heating than degregation of insulation?
Its is intermittantley run at 6% of load capacity and the core temps after 8 hours were 82degC 50degC & 60degC Class F insulation. This seems high to me for basically off-load conditions.

Sanditech.

 

[jbartos]

Suggestion: Please, would you elaborate on decimals at 6kV/.4kV and 7kV. They do count.

 

[busbar]

At greater than 16% overvoltage, iron is likely well into saturation, {higher V/Hz} so iron losses will be very high, along with increased heating and probably noise, too. Primary-current waveforms are doubtless pretty ugly. VAR losses in transformers increase proportional to the fourth power of voltage.

 

[busbar]

From IEEE C37.91 If saturation occurs, substantial exciting current will flow, which may overheat the core and damage the transformer severely. The waveform will be distorted and again the wave will have harmonic content and current zero periods.

 

[jbartos]

Comment on Sanditech (Electrical) Mar 30, 2004 marked ///\\I am at a site where a 6Kv/0.4kV 1.25MvA Cast Resin Transformer is being fed by 7kV (Utillity Co will rectify soon?). What affect would this increased voltage have on the transformer?
///There is a chance of the transformer being overloaded. Therefore, the load should be decreased or made mutually exclusive.\\ More with respect to heating than degradation of insulation?
///These go together to some extent.\\Its is intermittantley run at 6% of load capacity and the core temps after 8 hours were 82degC 50degC & 60degC Class F insulation. This seems high to me for basically off-load conditions.
///The ambient temperature is an important factor. The NEMA Class F insulation is good enough up to 155°C that consists of 40° ambient temperature maximum and 115°C temperature rise maximum.\\

 

[alehman]

"...82degC 50degC & 60degC"
Are you saying the three legs of the core are 82, 50 and 60degC? I would be concerned about that much difference. Is that actually core (iron) temperature or winding temperature?

7kV is certainly pushing the limits of a 6kV transformer. Are there any higher taps available on the primary?