Arrangement, method and computer program product for limiting circulating currents
An arrangement, method and computer program product are provided for limiting circulating currents in a converter converting between AC and DC. The converter has a number of AC and DC terminals and includes a number of converter arms, where a first and a second converter arm are connected in parallel between a first DC terminal and a first AC terminal. Each converter arm includes a string of series-connected converter cells. The arrangement includes a control unit that obtains a current of the first converter arm and a current of the second converter arm, forms an average of the two converter arm currents, forms a first and a second voltage control signal based on the average and uses the first and second voltage control signal in the control of the voltage provided by the first and second converter arm.
1. An arrangement for limiting circulating currents in a converter converting between alternating current, AC, and direct current, DC, said converter having a number of AC and DC terminals and comprising:
a number of converter arms, where a first and a second converter arm are connected in parallel between a first DC terminal and a first AC terminal and two other converter arms are connected in parallel between a second DC terminal and the first AC terminal, each converter arm comprising a string of series-connected converter cells; and
a control unit configured to:
obtain a first arm current running in the first converter arm and a second arm current running in the second converter arm sensed by current sensors in the first and second converter arms;
form an average of the two converter arm currents;
form a first error signal between the first converter arm current and the average;
form a first voltage control signal based on the average using the first error signal;
form a second error signal between the second converter arm current and the average;
form a second voltage control signal based on the average using the second error signal;
add the first voltage control signal to a modulation reference in order to obtain a first converter arm control signal;
use the first voltage control signal in the control of the voltage provided by the first converter arm comprising controlling the first converter arm using the first converter arm control signal and thereby controlling the cells of the first converter arm to provide a voltage on the AC terminal corresponding to the first converter arm control signal;
add the second voltage control signal to a modulation reference) in order to obtain a second converter arm control signal; and
use the second voltage control signal in the control of the voltage provided by the second converter arm comprising controlling the second converter arm using the second converter arm control signal and thereby controlling the cells of the second converter arm to provide a voltage on the AC terminal corresponding to the second converter arm control signal.
2. The arrangement according to claim 1 , wherein the control unit is configured to perform proportional control of the first error signal in order to obtain the first voltage control signal and perform proportional control of the second error signal in order to obtain the second voltage control signal.
3. The arrangement according to claim 1 , wherein the control unit is further configured to filter the converter arm currents before being used.
4. The arrangement according to claim 1 , further comprising at least one inductor between the first and second converter arm.
5. The arrangement according to claim 1 , further comprising the converter.
6. The arrangement according to claim 2 , wherein the control unit is further configured to filter the converter arm currents before being used.
7. The arrangement according to claim 2 , further comprising at least one inductor between the first and second converter arm.
8. The arrangement according to claim 3 , further comprising at least one inductor between the first and second converter arm.
9. The arrangement according to claim 2 , further comprising the converter.
10. The arrangement according to claim 3 , further comprising the converter.
11. The arrangement according to claim 4 , further comprising the converter.
12. A method for limiting circulating currents in a converter converting between alternating current, AC, and direct current, DC, said converter having a number of AC and DC terminals and comprising a number of converter arms, where a first and a second converter arm are connected in parallel between a first DC terminal and a first AC terminal and two other converter arms are connected in parallel between a second DC terminal and the first AC terminal, each converter arm comprising a string of series-connected converter cells,
the method being performed in a control unit of a circulating current limiting arrangement and comprising:
obtaining a first arm current running in the first converter arm and a second arm current running in the second converter arm sensed by current sensors in the first and second converter arms;
forming an average of the two converter arm currents;
forming a first error signal between the first converter arm current and the average;
forming a first voltage control signal based on the average using the first error signal;
forming a second error signal between the second converter arm current and the average;
forming a second voltage control signal based on the average using the second error signal;
adding the first voltage control signal to a modulation reference for obtaining a first converter arm control signal;
using the first voltage control signal in the control of the voltage provided by the first converter arm comprising controlling the first converter arm using the first converter arm control signal and thereby controlling the cells of the first converter arm to provide a voltage on the AC terminal corresponding to the first converter arm control signal;
adding the second voltage control signal to a modulation reference for obtaining a second converter arm control signal; and
using the second voltage control signal in the control of the voltage provided by the second converter arm comprising controlling the second converter arm using the second converter arm control signal and thereby controlling the cells of the second converter arm to provide a voltage on the AC terminal corresponding to the second converter arm control signal.
13. The method according to claim 12 , wherein the forming of the first voltage control signal comprises performing proportional control of the first error signal in order to obtain the first voltage control signal and the forming of the second voltage control signal comprises performing proportional control of the second error signal in order to obtain the second voltage control signal.
14. The method according to claim 12 , further comprising filtering the converter arm currents before use.
15. The method according to claim 13 , further comprising filtering the converter arm currents before use.
16. A computer program product for limiting circulating currents in a converter converting between alternating current, AC, and direct current, DC, said converter having a number of AC and DC terminals and comprising a number of converter arms, where a first and a second converter arm are connected in parallel between a first DC terminal and a first AC terminal and two other converter arms are connected in parallel between a second DC terminal and the first AC terminal, each converter arm comprising a string of series-connected converter cells, the computer program product comprising a non-transitory data carrier with computer program code configured to cause a control unit of a circulating current limiting arrangement to:
obtain a first arm current running in the first converter arm and a second arm current running in the second converter arm sensed by current sensors in the first and second converter arms;
form an average of the two converter arm currents;
form a first error signal between the first converter arm current and the average;
form a first voltage control signal based on the average using the first error signal;
form a second error signal between the second converter arm current and the average;
form a second voltage control signal based on the average using the second error signal;
add the first voltage control signal to a modulation reference in order to obtain a first converter arm control signal;
use the first voltage control signal in the control of the voltages provided by the first converter arm comprising controlling the first converter arm using the first converter arm control signal and thereby controlling the cells of the first converter arm to provide a voltage on the AC terminal corresponding to the first converter arm control signal;
add the second voltage control signal to a modulation reference in order to obtain a second converter arm control signal; and
use the second voltage control signal in the control of the voltages provided by the second converter arm comprising controlling the second converter arm using the second converter arm control signal and thereby controlling the cells of the second converter arm to provide a voltage on the AC terminal corresponding to the second converter arm control signal.