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The work proposes an interleaved modulation method for two three-level converters connected in parallel for grid applications such as high-power active front end rectifiers, STATCOMs, and so on. The parallel converter system is controlled using a centralised predictive controller and 60° discontinuous pulse width modulation, with phase-shifted/interleaved carriers. The proposed PWM utilises the switching state redundancy of ‘0’ state of the flying capacitor topology such that instantaneous pole voltage difference between the converters is zero. The proposed method eliminates the circulating current flow between the converters, and shifts the dominant grid current harmonics to twice the switching frequency. The method also ensures the balancing of flying capacitor voltages along with a reduction in common mode voltage.

In this paper, a constant switching frequency predictive control method for a three-level flying capacitor active front end converter is presented for the first time. This predictive control scheme results in reduced switching frequency operation of the converter with reduced computation burden compared to conventional predictive control methods. For the predictive controller, the cost function is defined in terms of grid current error vector in synchronously rotating $d-q$ frame and the condition for optimal converter voltage vector is derived. The optimal converter voltage vector is then synthesized using level shifted carrier based space vector modulation. Thus the proposed method overcomes the disadvantage of variable switching frequency in predictive based control methods. The controller ensures very low harmonic distortion of line currents with minimum computational requirement of the predictive controller. The performance of the proposed predictive controller is analysed using MATLAB/SIMULINK and the results are compared with that of a conventional PI based control. It can be observed that the proposed controller results in similar performance in terms of harmonic spectrum, better performance in response time and decoupled control compared to a classical PI based controller and can be readily used for grid connected power conversion applications and distributed generation systems.