Large power transformers (usually greater than 10 MVA) come with cooling package

Question

Large power transformers (usually greater than 10 MVA) come with cooling packages connected and piped into their internal cooling fluids. Just like the radiator and thermostat in your car, these cooling packages come on in stages and maintain the oil below what is affectionately called the “hot spot” or the calculated hottest part of the transformer. C-10 mineral oil which is a typical fluid used to cool the transformer (there are many, but was the first) has a flash point of around and greater than 140 OC and so this temperature must be avoided at all costs. This lead the transformer community to develop cooling standards which resulted in a three stage setup. The stages are known as self-cooled (base), 1st stage and 2nd stage. The stages are percentages of the thermal capacity of the transformers with self-cooled at base N/P rating and 1st stage through 1.33 times base rating and 2nd stage through 1.67 times base rating. An easy way to remember these rating steps are by using the staged cooling thermal equivalents of 3, 4 and 5 MVA. The N/P of such transformers would be listed as 100/133/166 MVA ONAN/ONAF/ONAF (for oil naturally cooled internally and naturally cooled to the air via external radiators/oil naturally cooled internally and multiple fans forced cooled to the air/oil naturally cooled internally and second stage of additional multiple fans forced cooled to the air) for instance. These are known as triple rated power transformers and will be a multiple of the 3/4/5 rating series. The only way to tell if such a transformer is overloaded is the compare to the ratings and assume the cooling packages are functional! Each stage would begin when the hotspot temperature approached a 65OC rise over a 30 OC ambient with a 5 to 10 OC allowance to hot spot or a total absolute temperature of 100 to 105 OC. Each manufacturer uses a very slightly different top temperature based upon their own experiences, but the process seems to be quite successful as transformers (when properly maintained) have existed for well over 100 years. The question is, transformer models have series and shunt impedances, do you think these impedances change significantly from the self-cooled stage to and through the end of the top (2nd) stage (where alarms begin to sound)? Please be specific (HINT: Recall how these transformers are modeled in large capacity sizes.)

Explanation / Answer

Transformers are used in virtually every commercial and industrial building, from the service transformer reducing the distribution voltage to a more usable voltage for the building, to step-down transformers serving individual floors, to small transformers for individual apparatus or functions. Typically a transformer is a long-lived device that can be in service for decades

n the simplest terms, there are two components to transformer losses: core losses (also called no-load losses); and coil losses (called load losses).

The core losses originate in the steel core of the transformer, caused by the magnetizing current needed to energize the core. They are constant, irrespective of the load on the transformer (thus the name no-load). They continue to waste energy as long as the transformer is energized. No-load losses do, however, vary with the size (kVA) of the transformer, and the core steel selected; hence the emphasis on proper sizing.

The coil losses (load losses) originate in the primary and secondary coils of the transformer, and are a result of the resistance of the winding material. That's where selection of copper windings can make a difference in losses.

Proper Sizing

Transformers are sometimes placed into a speculative setting in advance of occupancy, so the engineer does not necessarily know the load that will be placed on the unit. As the installer is often not the party paying the electric bill, there can be a tendency to oversize the transformer capacity relative to the load it will actually see. Since the no-load loss is a function of the kVA capacity of the transformer, careful selection of the transformer capacity closer to the intended task will ensure lowest core loss.

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