Electrical systems

1. An electrical system for connecting a rotary electric machine to dc networks operating at different voltages V and W where V>W, the electric machine having N≥2 polyphase winding sets each having a respective index n=(1, . . . , N), the electrical system comprising:

a first set of N ac-dc converter circuits connected in a modular multilevel configuration, each ac-dc converter circuit having a respective index n=(1, . . . , N) and an ac interface for connection with a corresponding nth winding set, and in which the modular multilevel configuration has P=N+1 dc outputs each having a respective index p=(1, . . . , P) wherein the potential difference between the pth output and the (p+q)th output is qV/N, where q=(0, . . . , P−p);

a set of N dc-dc converter circuits each having a respective index n=(1, . . . , N) and being configured to convert dc power between a voltage V/N at a first dc interface and a voltage W at a second dc interface, wherein, for all n, a first dc interface of the nth dc-dc converter circuit is connected with the p=nth and p=(n+1)th dc outputs of the modular multilevel configuration;

a second set of N ac-dc converter circuits each having a respective index n=(1, . . . , N), wherein for all n, a dc interface of the nth one of the second set of ac-dc converter circuits is connected with a second dc interface of the nth dc-dc converter; and

a second electric machine having N polyphase winding sets, each having a respective index n=(1, . . . , N), wherein for all n, the nth winding set of the second electric machine is connected with an ac interface of the nth one of the second set of ac-dc converter circuits.

2. The electrical system of claim 1 , in which 0.5 W≤V/N≤2 W.

3. The electrical system of claim 2 , in which 0.8 W≤V/N≤1.3 W.

4. The electrical system of claim 1 , in which N=4.

5. The electrical system of claim 1 , in which W is 540 volts.

6. The electrical system of claim 1 , in which V is from 1 kilovolt to 10 kilovolts.

7. The electrical system of claim 6 , in which V is from 1 kilovolt to 3 kilovolts.

8. The electrical system of claim 1 , in which the first set of N ac-dc converter circuits are bidirectional ac-dc converter circuits.

9. The electrical system of claim 8 , in which the first set of N ac-dc converter circuits comprise H-bridges.

10. The electrical system of claim 8 , in which the set of dc-dc converter circuits comprise phase-shifted full bridges.

11. The electrical system of claim 1 , in which the second set of N ac-dc converter circuits are bidirectional ac-dc converter circuits.

12. The electrical system of claim 1 , in which the second set of N ac-dc converter circuits comprise H-bridges.

13. A gas turbine engine having a low-pressure spool and a high-pressure spool, and further comprising the electrical system of claim 1 , in which the first rotary electric machine is connected with the low-pressure spool and the second rotary electric machine is connected with the high-pressure spool.

14. An arrangement comprising:

a first gas turbine engine having a first spool;

a second gas turbine engine different from the first gas turbine engine, and having a second spool; and

the electrical system of claim 1 , in which the first rotary electric machine is connected with the first spool and the second rotary electric machine is connected with the second spool.