Bidirectional unisolated DC-DC converter based on cascaded cells

1. A DC-DC converter for high voltage direct current (HVDC) power transmission and distribution and for converting a first DC voltage into a second DC voltage or vice versa and comprising:

a first phase leg having a first, a second, and a third connection terminal, the first phase leg comprising:

a first variable voltage source and a second variable voltage source being arranged for providing a first voltage (U 1 (t)) and a second voltage (U 2 (t)), respectively, each voltage having an AC voltage component and a DC voltage component, each variable voltage source having a first pole and a second pole, the first pole of the first variable voltage source being connected to the first terminal of the first phase leg, the second pole of the first variable voltage source being connected to the first pole of the second variable voltage source, and the second pole of the second variable voltage source being connected to the second terminal of the first phase leg, wherein the first DC voltage (U DC1 ) is provided between the first terminal and the second terminal of the first phase leg, and the second DC voltage (U DC2 ) is provided between the third terminal and the second terminal of the first phase leg,

a first capacitor being connected between the first terminal and the second terminal of the first phase leg, and being arranged between an input end of the first phase leg of the DC-DC converter and an output end of the first phase leg of the DC-DC converter, wherein the input end of the first phase leg is provided with the first DC voltage (U DC1 ) and the output end of the first phase leg of the DC-DC converter is provided with the second DC voltage (U DC2 ), and

an alternating current, AC, filter being connected between the first pole of the second variable voltage source and third terminal of the first phase leg,

and

a control unit configured to control the first variable voltage source and the second variable voltage source of the first phase leg so as to circulate an AC current (I AC (t)) within a loop of the first capacitor, the first variable voltage source, and the second variable voltage source, of the first phase leg.

2. The converter according to claim 1 , wherein the control unit is arranged for controlling the first variable voltage source and the second variable voltage source of the first phase leg so as to maintain a phase difference between an AC component of the first voltage and an AC component of the second voltage to be close to π.

3. The converter according to claim 1 , wherein the first phase leg further comprises a first inductor connected in series with the first capacitor.

4. The converter according to claim 1 , wherein each variable voltage source comprises a plurality of converter cells connected in series, each converter cell comprising two switching elements and an energy storage element connected in series with the switching elements.

5. The converter according to claim 1 , wherein the AC filter is an inductor.

6. The converter according to claim 1 , further comprising:

a second phase leg being identical to the first phase leg, the second terminal of the second phase leg being connected to the second terminal of the first phase leg, wherein a third DC voltage (U DC3 ) is provided between the first terminal and the second terminal of the second phase leg, and a fourth DC voltage (U DC4 ) is provided between the third terminal and the second terminal of the second phase leg,

and wherein the control unit is further configured to control the first variable voltage source and the second variable voltage source of the second phase leg so as to circulate an AC current (I AC (t)) within a loop of the first capacitor, the first variable voltage source, and the second variable voltage source, of the second phase leg.

7. The converter according to claim 6 , wherein the control unit is further configured to control the first variable voltage source and the second variable voltage source of each phase leg so as to maintain a phase difference between the AC current of first phase leg and the AC current of second phase leg to be close to π.

8. The converter according to claim 3 , wherein each variable voltage source comprises a combination of half-bridge and full-bridge type cells.

9. A DC-DC converter for high voltage direct current (HVDC) power transmission and distribution and for converting a first DC voltage into a second DC voltage or vice versa and comprising:

at least two phase legs connected in parallel, each phase leg having a first, a second, and a third connection terminal, wherein each phase leg comprises:

a first variable voltage source and a second variable voltage source being arranged for providing a first voltage (U 1 (t)) and a second voltage (U 2 (t)), respectively, each voltage having an AC voltage component and a DC voltage component, each variable voltage source having a first pole and a second pole, the first pole of the first variable voltage source being connected to the first terminal of the phase leg, the second pole of the first variable voltage source being connected to the first pole of the second variable voltage source, and the second pole of the second variable voltage source being connected to the second terminal of the phase leg, wherein the first DC voltage (U DC1 ) is provided between the first terminal and the second terminal of the phase leg, and the second DC voltage (U DC2 ) is provided between the third terminal and the second terminal of the phase leg, and the third terminals of the phase legs are interconnected, and

a control unit configured to control the first variable voltage source and the second variable voltage source of each phase leg to maintain a constant phase difference between the AC voltage components of the phase legs, where the phase difference between two adjacent phase legs is equal to 2π/n, where n is the number of phase legs, so as to circulate an AC current (I AC1 (t), I AC2 (t), I AC3 (t)) within a circuit comprising the first variable voltage source and the second variable voltage source of each phase leg, wherein a constant phase difference between the respective AC currents (I AC1 (t), I AC2 (t), I AC3 (t)) of the phase legs is maintained.

10. The converter according to claim 9 , wherein the control unit is further configured to control the first variable voltage source and the second variable voltage source of each phase leg so as to maintain a phase difference between an AC component of the first voltage and an AC component of the second voltage to be close to π.

11. The converter according to claim 9 , wherein the phase difference between the respective AC currents (I AC1 (t), I AC2 (t), I AC3 (t)) of two adjacent phase legs is equal to 2π/n, where n is the number of phase legs.

12. The converter according to claim 10 , wherein each variable voltage source comprises a combination of half-bridge and full-bridge type cells.

13. The converter according to claim 11 , wherein each variable voltage source comprises a combination of half-bridge and full-bridge type cells.

14. A method of a DC-DC converter for high voltage direct current (HVDC) power transmission and distribution and for converting a first DC voltage into a second DC voltage or vice versa and comprising:

a first phase leg having a first, a second, and a third connection terminal, the first phase leg comprising:

a first variable voltage source and a second variable voltage source being arranged for providing a first voltage (U 1 (t)) and a second voltage (U 2 (t)), respectively, each voltage having an AC voltage component and a DC voltage component, each variable voltage source having a first pole and a second pole, the first pole of the first variable voltage source being connected to the first terminal of the first phase leg, the second pole of the first variable voltage source being connected to the first pole of the second variable voltage source, and the second pole of the second variable voltage source being connected to the second terminal of the first phase leg, wherein the first DC voltage (U DC1 ) is provided between the first terminal and the second terminal of the first phase leg, and the second DC voltage (U DC2 ) is provided between the third terminal and the second terminal of the first phase leg,

a first capacitor being connected between the first terminal and the second terminal of the first phase leg, and being arranged between an input end of the first phase leg of the DC-DC converter and an output end of the first phase leg of the DC-DC converter, wherein the input end of the first phase leg is provided with the first DC voltage (U DC1 ) and the output end of the first phase leg of the DC-DC converter is provided with the second DC voltage (U DC2 ), and

an alternating current, AC, filter being connected between the first pole of the second variable voltage source and third terminal of the first phase leg,

wherein the method comprises:

controlling the first variable voltage source and the second variable voltage source of the first phase leg so as to circulate an AC current (I AC (t)) within a loop of the first capacitor, the first variable voltage source, and the second variable voltage source, of the first phase leg.

15. The method according to claim 14 , further comprising:

maintaining a phase difference between an AC component of the first voltage and an AC component of the second voltage to be close to π.

16. A method of a DC-DC converter for high voltage direct current (HVDC) power transmission and distribution and for converting a first DC voltage into a second DC voltage or vice versa and comprising:

at least two phase legs connected in parallel, each phase leg having a first, a second, and a third connection terminal, wherein each phase leg comprises:

a first variable voltage source and a second variable voltage source being arranged for providing a first voltage (U 1 (t)) and a second voltage (U 2 (t)), respectively, each voltage having an AC voltage component and a DC voltage component, each variable voltage source having a first pole and a second pole, the first pole of the first variable voltage source being connected to the first terminal of the phase leg, the second pole of the first variable voltage source being connected to the first pole of the second variable voltage source, and the second pole of the second variable voltage source being connected to the second terminal of the phase leg, wherein the first DC voltage (U DC1 ) is provided between the first terminal and the second terminal of the phase leg, and the second DC voltage (U DC2 ) is provided between the third terminal and the second terminal of the phase leg, where the third terminals of the phase legs are interconnected, and

wherein the method comprises:

controlling the first variable voltage source and the second variable voltage source of each phase leg to maintain a constant phase difference between the AC voltage components of the phase legs, where the phase difference between two adjacent phase legs is equal to 2π/n, where n is the number of phase legs, so as to circulate an AC current (I AC1 (t), I AC2 (t), I AC3 (t)) within a circuit comprising the first variable voltage source and the second variable voltage source of each phase leg, wherein a constant phase difference between the respective AC currents (I AC1 (t), I AC2 (t), I AC3 (t)) of the phase legs is maintained.

17. The method according to claim 16 , further comprising:

maintaining a phase difference between an AC component of the first voltage and an AC component of the second voltage to be close to π.

18. The method according to claim 16 , wherein the phase difference between the respective AC currents (I AC1 (t), I AC2 (t), I AC3 (t)) of two adjacent phase legs is equal to 2π/n, where n is the number of phase legs.