Under emergency loading conditions, many utilities loading practice could operate units at 130% of nameplate rating in summer and 150% of nameplate rating when ambient air temperature is les than 10oC (50oF). For short-term loading, the top capacity is limited to 200% of nameplate rating.
For continuous loading, the ANSI Standard C57. 92 permit to load the transformer increasing the rating 1% for 24 hr air temperature bellow 0oC<T< 30 oC. Also decrease the nameplate rating 1.5% for 24 hr average air temperature between 30oC>T>50oC.
For short time rating of 15 to 20 minutes per day the transformer should be operating OK within the normal life expectancy. However, to minimize the guessing, please consider the following suggestions:
- Estimate the daily loaded cycle and the site ambient seasonal temperatures.
- Check the data and graph on C57.91 & 92, for short-term overload operation.
- Consult the transformer manufacture for recommendation.
- Monitoring closely the top oil temperature, hot-spot and oil characteristics Typical values are as follow:
o Top oil max…………………….110 oC
o Hottest Spot conductor temp….180 oC
Taking temperature reading is OK . However, having online oil monitoring could be a lot easy. Check the following site:
- Explore supplementary cooling to operate the unit within the hotspot temp allowance.
- Explore if is cost effective use a loading software. See sample attached