A Reliable Scheme for Full-Range of Reduced DC-Link Voltage Operation of Hybrid MMC With Zero Voltage Ride Through

A hybrid modular multilevel converter can tolerate dc short-circuit faults and also operate with lower dc-link voltage under polluted insulator conditions due to harsh atmospheric conditions in high-voltage direct current transmission applications. When the dc-link voltage falls significantly below its rated value, the ac modulation index becomes greater than unity, resulting in over-modulation conditions. During ‘moderate over-modulation’ conditions, the energy balancing between half-bridge (HB) and full-bridge (FB) sub-modules (SMs) is hampered due to the inequality in their voltage references. This paper proposes an arm voltage splitting technique to overcome this problem. However, if the dc-link voltage further drops and goes below a particular level, the arm current becomes unipolar. The HB SMs can either charge or discharge, which disrupts their energy balance. This condition is termed ‘severe over-modulation’ condition. To fix this problem, second-order harmonic circulating current (SHCC) injection along with voltage reference splitting technique is proposed. This gives an accurate quantitative value for the SHCC reference. To rectify the errors committed due to unmodeled dynamics, a simple closed-loop control technique is proposed to supplement the calculated value of SHCC reference. To illustrate the strategy's viability, different reduced dc-link operations including zero voltage ride-through are performed in a laboratory prototype.