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Extreme short circuit testing of power transformers

2016 was the first year a 700+ kV transformer underwent a full short-circuit test in our KEMA Laboratory

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KEMA Laboratories Arnhem 2017 trafo testing

Challenge
Electricity is the life blood of modern society and its supply industry is going through a drastic transformation right now. The availability of an affordable, sustainable and reliable electricity supply that can meet the ever-increasing demand is driving growth in renewable sources and international electricity trading. That in turn is leading to the rise of so-called super grids: extremely high voltage, interconnected networks that transport electricity over long distances between different regions and countries. Outages in super grids have enormous impact, therefore, the energy industry puts a great deal of effort into trying to avoid disruptions to its power supply.

Power transformers are the most expensive pieces of equipment in power systems. Interruption of service of transformers needs to be avoided at any time, given the enormous consequences. International studies have indicated that up to 20% of transformer failures in service are related to short circuit.

Development
During a short circuit, large currents lead to severe mechanical forces and stresses in the transformer. Verification to withstand such stresses is sometimes carried out through design review. Based on calculation results of idealized, homogeneous structures, it does not cover transient phenomena, it excludes several key subcomponents and it is not embedded in a strict quality surveillance system. Despite the wide application of advanced calculation methods, still around 20–30% of the transformers, submitted to a full-scale laboratory short-circuit test fail to pass the test. Also, the failure rate increases as the unit rating increases.

A more secure alternative is testing. Live short-circuit testing puts the complete transformer subject to real short-circuit current and thus to the same stresses as would occur in service. Short-circuit testing is the only complete verification method of short-circuit withstand capability of power transformers. KEMA Laboratories are now ready to test power transformers with rated voltage up to 800 kV and power above 500 MVA single phase.

Benefits
Short-circuit testing is the only complete verification method of short-circuit withstand capability of power transformers. It is considered a better means of ascertaining the real performance of equipment at short-circuit, since such a test demonstrates that both construction and design are adequate. Large utilities across the globe have not seen any service problems on the units which have been short-circuit tested. Chinese experience shows that 40 transformers put into service after passing short-circuit tests, function without problem during the monitoring period from a few months to more than 5 years.