Apparatus for filtering harmonics in railway contact lines and method

1. An apparatus for filtering harmonics in a power transmitting contact line of a railway, the apparatus comprising:

a multilevel converter having one single phase leg, the phase leg comprising switching cells, each switching cell comprising semi-conductor switches arranged to selectively provide a connection to a corresponding energy storage element,

wherein the multilevel converter includes a controller provided to control switching of each switching cell, wherein the phase leg is configured in two parallel branches of cascaded switching cells, the two parallel branches being interconnected in a closed circuit, and the controller is provided to monitor a content of the harmonics in the contact line, and is adapted to filter the monitored harmonics by adapting the switching of the switching cells to absorb the harmonics, and

wherein the controller is adapted to create a circulating current in the two parallel branches of the phase leg and adapted to modify the circulating current in the two parallel branches to transfer energy between the corresponding energy storage elements of at least two switching cells, upon detecting, during monitoring voltage levels of each of the corresponding energy storage elements, that at least one monitored voltage level deviates.

2. The multilevel converter according to claim 1 , wherein the controller is adapted to monitor the voltage levels of each of the corresponding energy storage elements and is adapted to transfer the energy between the corresponding energy storage elements by controlling the switching of the switching cells in dependence of the voltage levels to compensate for voltage unbalances.

3. The multilevel converter according to claim 2 , wherein the converter is adapted to compensate for reactive power to the contact line, and is adapted to maintain supply of the reactive power during the harmonics filtering.

4. The multilevel converter according to claim 2 , wherein the controller is adapted to monitor the content of the harmonics in the contact line by applying a Park's transform to a measured voltage or current signal.

5. The multilevel converter according to claim 2 , wherein the controller is adapted to employ an inverse Park's transform to provide a switching control signal for filtering the monitored harmonics.

6. The multilevel converter according to claim 1 , wherein the controller is adapted to effectuate the transfer of energy between the corresponding energy storage elements by individually varying the switching of the switching cells.

7. The multilevel converter according to claim 1 , wherein the corresponding energy storage element of at least one of the switching cells is a capacitor.

8. The multilevel converter according to claim 1 , wherein the corresponding energy storage element of at least one of the switching cell is a battery.

9. The multilevel converter according to claim 1 , wherein each switching cell comprises a full H-bridge with four semiconductor switching elements connected to the corresponding energy storage element.

10. The multilevel converter according to claim 1 , wherein each switching cell comprises a half-bridge with two semiconductor switching elements connected to the corresponding energy storage element.

11. The multilevel converter according to claim 1 , wherein the corresponding energy storage elements of a majority of the switching cells of each branch are capacitors.

12. The multilevel converter according to claim 1 , wherein the multilevel converter is adapted to compensate for reactive power to the contact line, and is adapted to maintain supply of the reactive power during the harmonics filtering.

13. A method for controlling a single phase multilevel converter connected to a railway transmission and contact line, the multilevel converter comprising one phase leg, the phase leg comprising switching cells, each switching cell comprising semi-conductor switches arranged to selectively provide a connection to a corresponding energy storage element, and the phase leg comprises two parallel branches of switching cells, the two parallel branches sharing an output connected to the contact line, the method comprising steps of:

monitoring harmonic content of power in the contact line;

compensating for the harmonic content by controlling switching of the switching cells;

creating a circulating current in the two parallel branches; and

balancing voltages of the corresponding energy storage elements, wherein the balancing includes modifying the circulating current in the two parallel branches to transfer energy between the corresponding energy storage elements of at least two switching cells, upon detecting, during monitoring of voltage levels of each of the corresponding energy storage elements, that at least one monitored voltage level deviates.

14. The method for controlling the single phase multilevel converter according to claim 13 , wherein the monitoring of the harmonic content includes employing a Park's transform for identifying a size and phase angle of each harmonic.

15. The method for controlling the single phase multilevel converter according to claim 14 , wherein the Park's transform is employed on one harmonic at a time, and the harmonics that are compensated are monitored in sequence by the Park's transform.

16. The method for controlling the single phase multilevel converter according to 13 , further comprising steps of:

monitoring reactive power of the contact line; and

compensating for the reactive power in the contact line by switching the cells to supply-a current to the contact line.

17. The method for controlling the single phase multilevel converter according to claim 16 , wherein compensating for the harmonic content is maintained during the step of compensating for the reactive power.