Converter system and method for the operation of such a converter

1. A converter system for the switching of least three voltage levels, the converter system comprising:

a converter unit having a DC side and an AC side, the DC side of the converter unit being connected to a DC circuit having a voltage zero point, and the AC side of the converter unit is connected to a shorting circuit having a plurality of thyristors and at least two shorting resistors, wherein the converter unit is configured to monitor current carried by the converter unit in relation to a threshold value and, when the threshold value is exceeded, to actuate the thyristors by the application of a closing signal, such that the AC side of the converter unit is connected to the at least two shorting resistors; and

a voltage-limiting device connected to the DC circuit and to the at least two shorting resistors, the voltage-limiting device being configured to restrict any overvoltage on the DC circuit and to turn-off the thyristors,

wherein when the threshold value is no longer exceeded, the converter unit is configured to switch the AC side of the converter unit to the voltage zero point of the DC circuit and terminate application of the closing signal, and

wherein the voltage-limiting device is configured to connect the at least two shorting resistors to the DC circuit for a configurable time interval, whereafter the converter unit is configured to isolate the AC side of the converter unit from the voltage zero point of the DC circuit once more.

2. The converter system as claimed in claim 1 , wherein:

the voltage-limiting device includes two series circuits each including a limiting resistor and a controllable power switch;

wherein the shorting circuit includes two shorting resistors; and

each series circuit including the limiting resistor the controllable power switch is connected to a corresponding one of the shorting resistors, respectively.

3. The converter system as claimed in claim 2 , wherein the shorting resistors are interconnected.

4. A converter system for the switching of two voltage levels, the converter system comprising:

a converter unit having a DC side and an AC side, the DC side of the converter unit being connected to a DC circuit, and the AC side of the converter unit being connected to a shorting circuit having a plurality of thyristors and a shorting resistor, wherein the converter unit is configured to monitor the current carried by the converter unit in relation to a threshold value and, when the threshold value is exceeded, to actuate the thyristors by the application of a closing signal, such that the AC side of the converter unit is connected to the shorting resistor; and

a voltage-limiting device connected to the DC circuit and to the shorting resistor, the voltage-limiting device being configured to restrict overvoltage on the DC circuit and to turn-off of the thyristors,

wherein, when the threshold value is no longer exceeded, the converter unit is configured to switch the AC side of the converter unit to a negative or positive voltage point of the DC circuit and terminate application of the closing signal, and

wherein the voltage-limiting device is configured to connect the shorting resistor to the DC circuit for a configurable time interval, whereafter the converter unit is configured to isolate the AC side of the converter unit from the positive or negative voltage point of the DC circuit once more.

5. The converter system as claimed in claim 4 , wherein:

the voltage-limiting device includes a series circuit including a limiting resistor and a controllable power switch; and

the series circuit including the limiting resistor and the controllable power switch is connected to the shorting resistor.

6. A method for the operation of a converter system for the switching of at least three voltage levels,

wherein the converter system includes a converter unit having a DC side and an AC side, the DC side of the converter unit being connected to a DC circuit having a voltage zero point, and the AC side of the converter unit is connected to a shorting circuit having a plurality of thyristors and at least two shorting resistors, the method comprising:

monitoring current carried by the converter unit in relation to a threshold value;

when the threshold value is exceeded, actuating the thyristors by the application of a closing signal, such that the AC side of the converter unit is connected to the at least two shorting resistors;

restricting, in a voltage-limiting device connected to the DC circuit and to the at least two shorting resistors, any overvoltage on the DC circuit;

switching in the converter unit the AC side of the converter unit to the voltage zero point, when the threshold value is no longer exceeded, and terminating application of the closing signal;

connecting, in the voltage-limiting device, the at least two shorting resistors to the DC circuit for a configurable time interval; and

after the configurable time interval, the converter unit isolates the AC side of the converter unit from the voltage zero point of the DC circuit once more.

7. The method as claimed in claim 6 , wherein:

the voltage-limiting device includes two series circuits each including a limiting resistor and a controllable power switch;

the shorting circuit includes two shorting resistors;

each series including the limiting resistor and the controllable power switch is connected to a corresponding one of the shorting resistors, respectively;

the shorting resistors are interconnected; and

the controllable power switches on the voltage-limiting device for the connection of the two shorting resistors with the DC circuit are closed for the configurable time interval.

8. A method for the operation of a converter system for the switching of two voltage levels,

wherein the converter system includes a converter unit having a DC side and an AC side, the DC side of the converter unit being connected to a DC circuit, and the AC side of the converter unit being connected to a shorting circuit having a plurality of thyristors and a shorting resistor, the method comprising:

monitoring current carried by the converter unit in relation to a threshold value;

when the threshold value is exceeded, actuating the thyristors by the application of a closing signal, such that the AC side of the converter unit is connected to the shorting resistor;

restricting, in a voltage-limiting device connected to the DC circuit and to the shorting resistor, any overvoltage on the DC circuit;

switching, in the converter unit, the AC side of the converter unit to a negative or positive voltage point of the DC circuit, when the threshold value is no longer exceeded, and terminating application of the closing signal;

connecting, in the voltage-limiting device, the shorting resistor to the DC circuit for a configurable time interval; and

after the configurable time interval, the converter unit isolates the AC side of the converter unit from the negative or positive voltage point of the DC circuit once more.

9. The method as claimed in claim 8 , wherein:

the voltage-limiting device includes a series circuit including a limiting resistor a controllable power switch;

the series circuit including the limiting resistor and the controllable power switch is connected to the shorting resistor; and

the controllable power switch on the voltage-limiting device for the connection of the shorting resistor to the DC circuit is closed for the configurable time interval.