With a growing number of commercial installations around the world, HVDC technology increased its presence and importance in the power systems. Among various converter topologies, the Modular Multilevel Converters (MMCs) are considered as the most suitable one for HVDC application today. Besides its recognised advantages over conventional converters, the MMC has an interesting extra degree of freedom, which is the energy stored in the distributed cell capacitors. Although the amount of this energy is relatively small, it can provide a significant contribution to the DC system stability when properly used. This paper presents experiment results that demonstrate the effectiveness of virtual capacitor control. This control, previously proposed by the authors, makes use of the above additional degree of freedom to attenuate fluctuations of the DC voltage, which tend to be inherently volatile against power disturbances compared to the frequency of conventional AC systems. Under the virtual capacitor control, the MMC behaves as if there were a capacitor on the DC side of the converter whose size is easily adjusted by the control variable and can be even bigger than the physical capacitor actually embedded in the converter. In practice, the emulation of the capacitor dynamics is realised by the auxiliary control which adjusts the exchange of the energy between the stacked cell capacitors and the DC grid during the transient. Thus, no adverse effect is imposed on the AC grid. Furthermore, the system operator can optionally adjust the equivalent capacitance of the system to achieve desired mitigation level of DC voltage fluctuation during the operation. Therefore, this additional degree of freedom can largely extend the operability of the DC systems. The feasibility and effectiveness of the virtual capacitor control is demonstrated by experimental results obtained by using a small-scale MMC prototype.

K. SHINODA, A. BENCHAIB, J. DAI, A. ZAMA, B. LUSCAN and X. GUILLAUD