CONVERTER AND METHOD OF OPERATING A CONVERTER

Abstract

A converter has an AC side to connect to an AC network. A converter control is configured to set a current reference limit to limit a converter current. The converter control is configured to reduce the limit in case a fault is detected in the AC network. There is also described a combination of the converter in a power system with a renewable power source and a corresponding method of operating a converter in a power system with a renewable power source.

Claims

1. A converter, comprising: an AC side for connection to an AC network; and a converter control configured to set a current reference limit in order to limit a converter current of the converter; said converter control being configured to reduce the current reference limit in case of a fault in the AC network.

2. The converter according to claim 1, wherein the converter control is configured to reduce the limit as a function of a voltage reference value.

3. The converter according to claim 2, wherein the function is a non-linear function.

4. The converter according to claim 3, wherein the function has a maximum function value of 1 for voltage reference values exceeding 85% of a nominal AC voltage of the AC network.

5. The converter according to claim 3, wherein the function has a maximum function value of 1 for voltage reference values exceeding 90% of a nominal AC voltage of the AC network.

6. The converter according to claim 1, wherein the converter is a modular multilevel converter.

7. A power system, comprising: a renewable power source, a converter, and an AC network connecting said converter to said power source; said converter having an AC side for connection to said AC network and a converter control configured to set a current reference limit in order to limit a converter current; said converter control being configured to reduce the current reference limit in case of a fault in the AC network.

8. A method of operating a converter, the converter having an AC side to connect to an AC network and a converter control, the method comprising: setting a current reference limit to limit a converter current; and in case of a fault in the AC network, reducing the reference limit.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0021] FIG. 1 shows a schematic view of a power system with a renewable power source and a converter; and

[0022] FIG. 2 shows a diagram representing a non-linear function for the converter control.

DETAILED DESCRIPTION OF THE INVENTION

[0023] Referring now to the figures of the drawing in detail and first, in particular, to FIG. 1 thereof, there is shown a power system 1 comprising a renewable power source 2 (a wind park with a plurality of wind turbines) connected to a converter 3 via an AC network 4. A network impedance is denoted as Zgrid. The power generated in the power source 2 is fed by the converter 3 into an HVDC (high voltage direct current) line and further transferred to a power supply grid with multiple loads. The converter 3 is a voltage sourced converter (VSC), in particular a modular multilevel converter having a DC side 5 to connect to the HVDC line and an AC side 6 to connect to the AC network 4. The DC voltage on the DC side 5 of the converter 3 is denoted by Vdc.

[0024] A converter control 7 is configured to control the semiconductor switching components of the converter 3. The converter control 7 comprises a block or module 8 configured to set a current reference limit to limit a converter current. A current reference value lac* is provided at the output of the block 8. The current reference lac* is compared with a measured actual current lac, wherein an alternating current control 9 provides a voltage reference V* based on the difference between lac* and lac. The voltage reference V* is forwarded to a modulator 10 configured to provide switching signals for the switching modules of the converter 3 (respectively for the semiconductor switching units of the switching modules). The converter control 7 further comprises an AC voltage dependent current limitation block 11. The reference voltage V* is provided as input to the AC voltage dependent current limitation block 11. The AC voltage dependent current limitation block 11 provides as its output a limiting factor F which is a function of a value of the reference voltage V*. The limiting factor F is forwarded to the block 8 to adjust the converter current limits.

[0025] FIG. 2 depicts a diagram showing an example of a non-linear functional dependence between the limiting factor F and the reference voltage V*. The value of the factor F is equal to 1 at a voltage value V1, wherein V1=0.9. Below V1, i.e., at voltage levels below 0.9, the function value of F decreases non-linearly. The functional dependence shown in FIG. 2 is linear by sections, i.e., it is formed and composed of a plurality of linear sections.