1.5 Times Overload Capability

You are correct.

The absolute best source of transformer overload capacity will come from the manufacturer.

Contact them immediately!

loading with harmonics too should be taken into account.

eg a transformer badged as 100kVA ie its continuous service rating, must be able to support and survive unharmed 150kVA of loading for a period of time.

The trafo manufacturer's data sheet will tell you what it's overload capacity is and how long AND how frequently it can be overloaded.

It means the statement is open-ended and requires more data. You need to specify the duration and frequency of the overloads, plus be cognizant that each overload cycle causes insulation damage that is cumulative and reduces the service life of the transformer.

Bottom line is that if you're thinking that you can overload the transformer by 50% on a daily basis, you'll be replacing the unit decades before you expected to. And it also assumes that you don't mind the extra voltage drop, poor voltage regulation, poor power quality, and increased harmonics that are the result of oversaturating the transformer.

It used to be that utility companies and other industries operated all of their power transformers in a constant overloaded state in order to maximize efficiency.

This was directly related to poor transformer design and the materials used.

There have been and continue to be, many instances wherein the overloaded transformers have catastrophically failed causing critical power distribution system disturbances and/or failures which resulted in "brown-outs" and "Black-outs" of large areas.

Due to these power system disturbances and failures there are now new regulations pertaining to transformer efficiency which you need to review and comply with.

The transformer OEM should have all of the critical information and they should know all of the new regulations.

If the OEM does not know or will not issue a statement of conformity to all current regulations, I strongly suggest you go with a different transformer supplier that will.

"...It used to be that utility companies and other industries operated all of their power transformers in a constant overloaded state in order to maximize efficiency..." Maybe in the utility that you worked for but not mine, few utilities want to burn up hundreds of millions of dollars of assets that are supposed to have 30 year lifetimes. You should also be aware that pushing equipment into overload also moves it beyond the most efficient operating point so it costs more to provide the next increment of energy.

Granted every utility is required to have emergency ratings on every piece of equipment critical to meeting short term overload conditions, but those ratings were calculated internally and reviewed by their respective manufacturers', and if the line was to be crossed it was only with everyone's approval including the insurance companies that had to make good if the units failed within the guidelines.

Even though deregulation changed many of the ways that utilities operate, their main charter is to keep the energy flowing until the point where continuing to do so will cause equipment failure that would jeopardize their operations once the emergency passes.

In my opinion there are conditions in which an utility has to allow overload on its power transformer. For instance according to Denver Utility:

PERMISSIBLE LOADING OF OIL-IMMERSED TRANSFORMERS AND REGULATORS

http://www.usbr.gov/power/data/fist/fist1_5/vol1-5.pdf

"Loads in excess of normal rating may be carried under certain conditions when necessary"

Normal life expectancy:

1) If the ambient temperature is less than rated-see Fig.1

2) If initial load was less than 100% rated-See Fig.8,9,10,11,12,13

Remark: the overload may be applied not more than once in 24-hour period.

Or for loss of life see other figs.