Modulation of ac/ac mmc

A method of decoupled modulation of a direct AC/AC MMC between a first AC network having a first waveform and a second AC network having a second waveform, the MMC having a double-star topology with a plurality of phase legs, each phase leg having a first branch and a second branch, each of the first and second branches comprising a plurality of series connected bipolar cells, the method comprising:

• performing a first modulation based on a reference signal of the first AC network, independently of a reference signal of the second AC network, to generate, for each phase leg, a first integer command signal corresponding to a first combination of cell states in the first and second branches of the phase leg needed for generating the first waveform;
• performing a second modulation based on the reference signal of the second AC network, independently of the reference signal of the first AC network, to generate, for each phase leg, a second integer command signal corresponding to a second combination of cell states in the first and second branches of the phase leg needed for generating the second waveform;
• based on the first and second integer command signals, mapping to each branch a number of cell states to be used for concurrently generating both the first and second waveforms, generating branch-level command signals to a capacitor voltage balancing algorithm;
• and
• based on the mapping and the balancing algorithm, sending firing signals to the plurality of cells of each branch;
• wherein the first AC network is a three-phase AC network;
• wherein the second AC network is a single-phase AC network.

A direct AC/AC MMC configured to be connected between a first AC network being a three-phase AC network and having a first waveform and a second AC network being a single-phase AC network and having a second waveform, the MMC having a double-star topology with a plurality of phase legs, each phase leg having a first branch and a second branch, each of the first and second branches comprising a plurality of series connected bipolar cells, the MMC further comprising a controller comprising:

• processing circuitry;
• and
• storage storing instructions executable by said processor circuitry whereby said controller is operative to:
• perform a first modulation based on a reference signal of the first AC network, independently of a reference signal of the second AC network, to generate, for each phase leg, a first integer command signal corresponding to a first combination of cell states in the first and second branches of the phase leg needed for generating the first waveform;
• perform a second modulation based on the reference signal of the second AC network, independently of the reference signal of the first AC network, to generate, for each phase leg, a second integer command signal corresponding to a second combination of cell states in the first and second branches of the phase leg needed for generating the second waveform;
• based on the first and second integer command signals, map to each branch a number of cell states to be used for concurrently generating both the first and second waveforms, generating branch-level command signals to a capacitor voltage balancing algorithm;
• and
• based on the mapping and the balancing algorithm, send firing signals to the plurality of cells of each branch.