PROJECT TITLE :

Virtual Capacitor Control: Mitigation of DC Voltage Fluctuations in MMC-based HVDC Systems - 2017

ABSTRACT:

The modular multilevel converters (MMCs) have emerged as the most appropriate converter technology for HVDC applications. Besides the recognized blessings over conventional voltage supply converters, one amongst the outstanding features of the MMC is its ability to store energy in the distributed submodule capacitors. This necessary feature can be used to mitigate the fluctuations of the DC voltage, that is inherently volatile against power disturbances compared to the frequency of typical AC systems. This paper proposes a unique management strategy, known as virtual capacitor control, that permits the utilization of the energy storage capability of the MMC to attenuate voltage fluctuations of HVDC systems. With the proposed control, the MMC behaves as if there have been a physical capacitor whose size is adjustable and will be even larger than the physical capacitor embedded within the converter. This control permits the system operator to optionally modify this virtual capacitor of each MMC station and, thus, it provides a further degree of freedom to the HVDC system operation. The EMT simulations of a 401-level MMC-based mostly HVDC link system show the effectiveness of the proposed control to improve dc voltage transient dynamics.