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1. (WO2017199016) DC/DC CONVERTER FOR HIGH POWER DC GRIDS
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1. A DC/DC converter comprising:- a first bridge (Bridge 1) for connection via an inductor (LI) to a first DC power source (VI);

a second bridge (Bridge 2) for connection via an inductor (L2) to a second DC power source (V2);

an AC transformer connecting the first and second bridges;

wherein the first bridge comprises :- a first branch having a pair of bi-directional switches, each including at least one IGBT semiconductor element, connected across the first power source and to a first terminal of the primary winding (Lacl) of the AC transformer;

a second branch having a pair of bi-directional switches, each including at least one IGBT semiconductor element, connected across the first power source and to a second terminal of the primary winding (Lacl) of the AC transformer; and

wherein the second bridge comprises :- a first branch having a pair of bi-directional switches, each including at least one IGBT semiconductor element, connected across the second power source and to a first terminal of the secondary winding (Lac2) of the AC transformer;

a second branch having a pair of bi-directional switches, each including at least one IGBT semiconductor element, connected across the second power source and to a second terminal of the secondary winding (Lac2) of the AC transformer; and

control means (100) for providing square wave control signals to the semiconductor elements to enable a switching sequence of the semiconductor elements to provide differing current paths according to the type of first power source and second power source and direction of power transfer.

2. A DC/DC converter as claimed in claim 1 wherein the control means varies the

frequency of the control signals to provide control of power transfer when interconnecting two VSC-HVDC systems.

3. A DC/DC converter as claimed in claim 1 wherein the control means uses fixed frequency phase-shift square-wave control to provide control of power transfer when interconnecting LCC and a VSC-HVDC systems.

4. A DC/DC converter as claimed in any preceding claim wherein the AC transformer has either a magnetic or air-core mutually coupling the primary winding and secondary winding.

5. A DC/DC converter as claimed in any preceding claim wherein AC capacitors are connected in parallel with the primary winding and the secondary winding of the AC transformer.

6. A DC/DC converter as claimed in any preceding claim wherein one or more said bi-directional switches comprises a pair of series connected IGBT semiconductor element, each having a diode in parallel.

7. A DC/DC converter as claimed in claim 7 wherein every bi-directional switch comprises a pair of series connected IGBT semiconductor element, each having a diode in parallel. one.

8. A DC/DC converter as claimed in any one of claims 1 to 5 wherein one or more said bi-directional switches comprises a diamond configuration of diodes with one IGBT semiconductor element connecting opposing apexes of the configuration.

9. A DC/DC converter as claimed in claim 8 wherein every bi-directional switch comprises said diamond configuration of diodes with one IGBT semiconductor element connecting opposing apexes of the configuration.

10. A method of operating a DC/DC converter as claimed in any preceding claim, the method comprising the steps of:- providing square wave control signals to the semiconductor elements to enable a switching sequence of the semiconductor elements to provide differing current paths according to the type of first power source and second power source and direction of power transfer.

11. A method as claimed in claim 10 further comprising varying the frequency of the control signals to provide control of power transfer when interconnecting two VSC-HVDC systems.

12. A method as claimed in claim 10 further comprising using fixed frequency phase-shift square-wave control to provide control of power transfer when interconnecting LCC and a VSC-HVDC systems.