IEEE C57.91-2011 pdf download IEEE Guide for Loading Mineral- 0il-Immersed Transformers and Step-Voltage Regulators
Voltage and frequency influences should be recognized when determining limitations for loading atransformer beyond its nameplate rating. This is true even though in all probability there may be littlecontrol of these parameters during a loading beyond nameplate rating event. Subelause 4-16 in-IEEE StdC57.12.00-1993 defines the capability of a transformer to operate above rated voltage and below ratedfrequency. The user of this guide should recognize that, during conditions of loading beyond nameplate, thevoltage regulation through the transformer may increase significantly (depending on the transformerimpedance) due to the increased kilovoltampere loading and possibly dropping power factor.
A conservative guideline to prevent excessive core heating due to increased excitation is to reduce thetransformer output volts per hertz limit by 1% for every l% increase in voltage regulation during theloading beyond nameplate event For example, if the voltage regulation at rated conditions is 6% andincreases to 9% at some load above nameplate, the output volts per hertz limit might be reduced from 105%to 102%.
4.3 Supplemental cooling of existing self-cooled transformers
The load that can be carried on existing self-cooled transformers can usually be incrcased by adding auxiliarycooling equipment such as fans, external forced-oil coolers, or water spray equipment. The amount ofadditional loading varics widely. depending upon the following!
a) Design characteristics of the transformer
b)Type of cooling equipment
c) Permissible incrcase in voltage regulation
d) Limitations in associated equipment
No general rules can be given for such supplemental cooling, and cach transformer should be consideredindividually.
The use of water spray equipment for supplemental cooling is not recommended for use in normal loadingbeyond nameplate rating. Appropriate precautions should be made for application of water spray equipmentfor supplemental cooling during emergency overloads. The major problem is the build up of scale on thecooling equipment due to minerals in the water. Over the long term this buildup will hinder the coolingefficiency. The spray and steam generated can also cause phase-to-phase flashover between bushings.