The limitation of the inrush current is done via a small three phase transformer, which is prefixed the large power transformer. The purpose of the front end transformer is to perform an inductive premagnetisation of the power transformer during the start up and thus a prevention of the inrush current. With a special circuit topology the load of the power transformer is disconnected during the start up. Like this the premagnetisation is always happening during no-load and afterwards the load is automatically connected again. The premagnetisation takes place within the first cycle, which means approximately 20 ms. After this time the prefixed transformer can be bridged and only the power transformer stays connected to the net. As the premagnetisation is a relatively short process and the prefixed transformer is barely stressed, the start up procedure can be repeated any number of times.
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