Inrush current avoider for large transformers

Datasheet as PDF

large transformer is turned on with triple no-load current










Inrush current avoider for large transformers













small auxiliary transformer in special version for a soft start

BV 2914100 + 3250105

Customer’s demand

Our customer needs a large and powerful three phase transformer with 400 kVA. With the help of an additional circuit it may not produce any inrush current when turned on and thus it has to be possible to fuse it with the rated current on the net. Either under load or in no-load the start up should be possible and may not be limited in frequentness. Any number of start up and turn off processes has to be possible without blowing the fuse.


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.

Technical data

Power: 400 kVA
Primary: 3 x 400 V
Secondary: 3 x 440 V
Current inrush: 3 times no-load current

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