Electrical systems

1. An electrical system for connecting rotary electric machines with gas turbine spools, the electrical system comprising:

a first rotary electric machine mechanically coupled with a first gas turbine spool and a second rotary electric machine mechanically coupled with a second gas turbine spool,

each rotary electric machine having an identical even number N≥4 of phases, each phase having a respective index n=(1, . . . , N), and

each phase comprising an identical number P≥1 of coils wound in a configuration in which adjacent phases are radially separated by 2π/NP mechanical radians;

a first set of N bidirectional converter circuits for conversion of alternating current (ac) to and from direct current (dc),

each converter circuit of the first set of N bidirectional converter circuits having an ac side and a dc side, the ac side of each converter circuit of the first set being connected with the first rotary electric machine, and

each converter circuit of the first set having a respective index n and being connected at an ac side with the P coils in the nth phase of the first rotary electric machine;

a second set of N bidirectional converter circuits for conversion of ac to and from dc,

each converter circuit of the second set of N bidirectional converter circuits having an ac side and a dc side, the ac side of each converter circuit of the second set being connected with the second rotary electric machine, and

each converter circuit of the second set having a respective index n and being connected at an ac side with the P coils in the nth phase of the second rotary electric machine;

an R-channel electrical network; and

a switching arrangement having a number R of dc outputs for connection with the R-channel electrical network,

wherein each dc output has respective index r=(1, . . . , R),

wherein R satisfies 1≤R≤N/2, and

wherein for all r, the switching arrangement connects the rth dc output with the dc side of the nth converter circuits in both the first set and second set, for which n(mod R)≡r is satisfied.

2. The electrical system of claim 1 , wherein for each rotary electric machine, the identical even number N=4 phases.

3. The electrical system of claim 1 , wherein for each phase, the identical number P=2 coils, such that the coils are wound in a duplex configuration.

4. The electrical system of claim 1 , the number of dc outputs for connection with an R-channel electrical network is R=2.

5. The electrical system of claim 1 , wherein, for all n, the switching arrangement connects a dc side of the nth converter circuit in the first set is connected with a dc side of the nth converter circuit in the second set to facilitate dc power transfer between the first gas turbine spool and the second gas turbine spool.

6. The electrical system of claim 5 , wherein

the switching arrangement comprises N switch arrays, each having a respective index n, and

for all n, the nth switch array is configured to connect the nth converter circuits in the first and second sets to a corresponding dc output.

7. The electrical system of claim 6 , wherein each switch array comprises:

a first input connected with one of the converter circuits of the first set of converter circuits;

a second input connected with another of the converter circuits of the second set of converter circuits;

a switch array output for connection to a corresponding dc output, and to which the first input and the second input are connected;

a first switch between the first input and the switch array output;

a second switch between the second input and the switch array output;

a bus tie across the first input and the second input, the bus tie comprising a third switch.

8. The electrical system of claim 7 , wherein the switch arrays are configured to operate in:

a non-fault condition in which the first switch and the second switch are closed, and the third switch is open;

a first fault condition in which the first switch is closed and the second and third switches are open;

a second fault condition in which the second switch is closed and the first and third switches are open;

a third fault condition in which the third switch is closed and the first and second switches are open.

9. The electrical system of claim 7 , wherein the first, second, and third switches all have the same electrical rating.

10. The electrical system of claim 6 , wherein each switch array comprises:

a first input connected with one of the converter circuits of the first set of converter circuits;

a second input connected with another of the converter circuits of the second set of converter circuits;

a switch array output for connection with a corresponding dc output;

a node with which the first input, the second input and the switch array output are connected;

a first switch between the first input and the node;

a second switch between the node and the switch array output.

11. The electrical system of claim 10 , wherein the switch arrays are configured to operate in:

a non-fault condition in which the first switch and the second switch are closed;

a first fault condition in which the first switch is closed and the second switch is open;

a second fault condition in which the first and second switches are open.

12. The electrical system of claim 10 , wherein the first switch has half the electrical rating of the second switch.

13. The electrical system of claim 6 , wherein the switches are dc contactors.

14. A gas turbine engine comprising:

a low-pressure spool;

a high-pressure spool; and

the electrical system of claim 1 , wherein

the first gas turbine spool is the low-pressure spool, and

the second gas turbine spool is the high-pressure spool.

15. An aircraft having an R-channel electrical network and comprising:

the gas turbine of claim 14 .

16. An arrangement comprising:

a first gas turbine engine having a spool;

a second gas turbine engine having a spool; and

the electrical system of claim 1 , wherein

the first gas turbine spool is the spool in the first gas turbine engine, and

the second gas turbine spool is the spool in the second gas turbine engine.

17. An aircraft having an R-channel electrical network and comprising:

the arrangement of claim 16 .