Enhanced flat-topped modulation for MMC control in HVDC transmission systems

Flat-topped modulation is a member of the family of triplen-series injection techniques that have been extensively utilized in PWM inverter systems to increase the DC-link, and hence semiconductor, utilization. We propose the use of an optimized flat-topped modulation scheme for the modular multilevel converter (MMC) control. The optimized flat-topped waveform minimizes the magnitude of the triplen harmonics, particularly compared to the popular space-vector modulation (SVM) technique, while fully utilizing the DC voltage. This has particular advantages if the converter-side of the interfacing transformer is earthed. Under such conditions, the zero-sequence earthing current is affected by the triplen series injected into the sinusoidal modulating functions. Therefore, it is critical to minimize the injected triplen harmonics. The operating principle of the flat-topped scheme is presented and the Fourier coefficients are compared with the SVM technique. Additionally, the influence of the proposed control scheme on MMC performance is evaluated mathematically. Simulation of a point-to-point HVDC link using average model demonstrates the effectiveness of the proposed MMC operational schemes. The third harmonic of the flat-topped modulation is reduced by 33%, which lowers the potential zero-sequence current flowing to earth. Compared to conventional sinusoidal modulation, the submodule capacitance is reduced by 25%. This significantly lowers submodule cost, volume, and weight. Station conduction losses are expected to reduce by 11%, yielding higher efficiency and lowering cooling system capacity. In addition to the improvement under normal operation, the proposed control scheme also reduces the fault current by 13.4%.