POWER CONVERTER AND METHOD FOR OPERATING A POWER CONVERTER

Abstract

A power converter has at least one series circuit of switching modules that each have semiconductor switches and an energy storage unit. A method of operating the power converter includes a step of making a prediction for at least one voltage value of the power converter and carrying out switching operations on the switching modules based on the prediction in order to regulate a switching frequency. There is also described a power converter that is configured to carry out a method according to the invention.

Claims

1. A method for operating a power converter, the power converter having at least one series circuit of switching modules each with semiconductor switches and an energy storage unit, the method which comprises: making a prediction for at least one voltage value of the power converter; and carrying out switching operations on the switching modules based on the prediction in order to control a switching frequency.

2. The method according to claim 1, wherein the at least one voltage value is a setpoint voltage value for an arm voltage of the power converter.

3. The method according to claim 2, which comprises making the prediction on a basis of at least two previous voltage values of the arm voltage.

4. The method according to claim 3, which comprises making the prediction by calculating a discrete time derivative of the previous voltage values.

5. The method according to claim 1, which comprises making the prediction on a basis of at least two previous voltage values of an arm voltage of the power converter.

6. The method according to claim 1, which comprises making the prediction by generating change information about whether an expected subsequent future voltage value is higher or lower than a currently valid voltage value.

7. The method according to claim 6, which comprises preventing a switching operation that causes a change in the arm voltage that is contrary to the change information.

8. The method according to claim 7, which comprises preventing the switching operation only if an absolute difference between the subsequent future voltage value and the currently valid voltage value is greater than a predetermined threshold.

9. The method according to claim 1, wherein the step of controlling the switching frequency comprises a tolerance-based modulation.

10. The method according to claim 1, wherein the voltage value is an arm voltage of a power converter arm of the power converter and the method comprises determining a setpoint voltage value for the arm voltage taking the prediction into account.

11. A power converter, comprising: at least one series circuit of switching modules each having semiconductor switches and an energy storage unit; and a regulating device configured to carry out the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0030] FIG. 1 is a schematic illustration of an exemplary embodiment of a power converter according to the invention; and

[0031] FIG. 2 is a schematic flowchart of an exemplary embodiment of a method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Referring now to the figures of the drawing in detail and first, in particular, to FIG. 1 thereof, there is shown a power converter 1 which may also be referred to as a modular multilevel converter (MMC). The power converter 1 comprises six power converter arms 3-8 having series circuits RS1-RS6 that each comprise a plurality of switching modules SM and having an inductance L (arm inductor). In the example illustrated, the MMC is designed to convert an AC voltage of an AC voltage grid, to which the MMC can be connected for example by way of connections A1-A3 and of a grid transformer, into a DC voltage UDC (or vice versa). The power converter 1 can be connected to a DC voltage grid or a DC voltage line by way of the connections D1, D2. The power converter 1 also comprises a regulating device 2, or controller 2, that is designed for power converter control. In this case, current, voltage, power and frequency can be regulated by means of the regulating device 2. By way of example, an arm voltage Uarm can be regulated by means of the regulating device 2. The arm voltage Uarm indicates the voltage present at the first power converter arm 3.

[0033] In the example illustrated in FIG. 1, all the switching modules SM are of identical design. In principle, however, it is also conceivable for switching modules of different designs to be used in one and the same power converter, for example half-bridge switching modules and full-bridge switching modules. The switching module SM comprises a capacitor branch in which there is arranged a first semiconductor switch S1 with an antiparallel freewheeling diode F and, in series therewith, an energy storage unit C. A second semiconductor switch S2 is arranged with an antiparallel freewheeling diode F in a bridge branch between two connections X1, X2 of the switching module SM. A switching module voltage Usm that corresponds to the capacitor voltage Uc or else a zero voltage can be producible at the connections X1, X2 by way of suitable actuation of the two semiconductor switches S1, S2.

[0034] FIG. 2 illustrates a flow chart 100 for a method for operating a power converter, such as the power converter 1 in FIG. 1, for example. In a first method step 101, two past or previous setpoint voltage values S(t1) and S(t2) are provided for an arm voltage of a power converter arm. Each of the past setpoint voltage values is assigned a past time t1 or t2, wherein t1<t2. For predicting future setpoint voltage values, the following procedure is adopted. In a second method step 102, a discrete derivative DeltaS/Deltat is calculated: DeltaS/Deltat=(S(t2)−S(t1))/(t2−t1). In a third method step 103, the absolute value |DeltaS/Deltat| and change information are stored and provided, wherein the change information indicates whether DeltaS/Deltat>0, DeltaS/Deltat<0 or DeltaS/Deltat=0.

[0035] In a fourth method step 104, a predetermined, defined threshold or a predetermined threshold value is provided. In a fifth method step 105, the threshold is tested against the absolute value from step 103, i.e., a check is performed to determine whether the absolute value is greater than the predetermined threshold value. If the absolute value is below the threshold (or does not exceed the threshold), according to a sixth method step 106, all of the future switching operations ascertained or determined by the remaining switching frequency regulation are permitted. If the absolute value is above the threshold, according to a seventh method step 107, those switching operations determined by the remaining switching frequency regulation that would cause a change in the arm voltage that is contrary to the change information are prevented.

[0036] If, for example according to the change information, it holds true that DeltaS/Deltat>0 (ΔS/Δt>0), then it is assumed that a subsequent future setpoint voltage value increases relative to the last valid setpoint voltage value. Accordingly, those switching operations that cause the arm voltage to reduce are prevented (or stopped using closed-loop control technology) within a predetermined period of time.