Frequency converter with DC link capacitor and method for pre-charging the DC link capacitor

A frequency converter for operating an electrical machine on an electrical supply system, comprising:

• an AC-DC converter having an AC input connected to the electrical supply system and a DC output,
• an inverter having a DC input and an AC output connected to a load, and
• a DC link having a first circuit branch and a second circuit branch interconnecting the DC output of the AC-DC converter and the DC input of the inverter, wherein the DC link comprises
• a DC link capacitor connecting the first circuit branch and the second circuit branch across the DC input of the inverter and a first commutation capacitor,
• wherein the first circuit branch comprises a choke having a first terminal connected to a DC input terminal of the inverter and a second terminal connected to a first terminal of a freewheeling diode and to a first terminal of a semiconductor switch, wherein a second terminal of the semiconductor switch is connected to a DC terminal of the AC-DC converter in the first circuit branch, wherein a second terminal of the freewheeling diode is connected to the second circuit branch,
• wherein the first commutation capacitor connects the second terminal of the semiconductor switch with the second circuit branch, and
• wherein the semiconductor switch is configured to control a magnitude of an electrical current flowing from the AC-DC converter into the DC link capacitor in accordance with a control signal and to also interrupt the electrical current flowing from the DC link to the inverter in the event of a fault,
• the frequency converter further comprising a second commutation capacitor haying a first terminal connected to the second circuit branch and a second terminal connected in series to a second switch, with the series connection connecting the first and the circuit branches, and
• a fourth diode connected between the second commutation capacitor and the first terminal of the choke, wherein the fourth diode blocks current flow from the first terminal of the choke to the second commutation capacitor.