Electrical systems

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.