In converters with regenerative active front-end rectifiers, there is the possibility to premagnetize the power rectifier transformer through the dc-bus of the converter
First the equipment connection command, the internal water cooling unit is connected.
After, the precharge of the capacitors is performed. In case there would be any the internal breaker in between the secondary of the transformer and the input of the converter, these would be closed.
Once the DC-Link is precharged, the grid side rectifier starts switching the IGBTs with the energy stored in the DC link capacitors.
Then, the voltage is ramped up, so as to premagnetize the upstream transformers softly without any inrush current, ensuring that there is not stress in the connection.
Once the upstream transformer's premagnetization is completed, and it is synchronized with the net voltage, the upstream medium-voltage breaker can be closed avoiding any inrush current.
The premagnetization is started and the medium-voltage upstream breaker is now closed. When the converter receives this feedback, the active-front end rectifier will start the regulation of the dc-bus.
Finally, normal operation through transformer and converter is achieved, inrush current and stress has been avoided thanks to this procedure, without any need to use additional auxiliaries or any premagnetization transformers, which implies saving in materials and overall lower hardware complexity.