The existence of modern on-line monitoring and diagnostic systems at power transformers allow increasingly diverse possibilities of monitoring electrical properties and discerning of critical conditions. But the expressiveness of results of monitoring systems is hardly sufficient to make a decision and has to be supported by an appropriate offline test. Furthermore, an increasing number of on-site repairs require tests on-site before reconnecting the overhauled power transformer or distribution transformers to the power grid. The presented approach allows mobile test systems that provide a practicable possibility to carry out routine tests at big power transformers up to 500 MVA on site, including induced voltage and applied voltage testing as well as no-load and load losses tests. It therefore enables a reliable form of testing after a repair on-site. Moreover, it will be a helpful tool for off-line diagnostics. The same approach also allows testing power transformers in the factory. Small and compact frequency converters are successfully used to replace motor-generator-sets.
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