NETE The Asymmetric Alternate Arm Converter: A compact Voltage Source Converter design for HVDC

Abstract

The Modular Multilevel Converter (MMC) has
become one of the most practical topologies for integration of
renewable energy power plants into the grid, through submarine
HVDC cables and HVDC lines. Nonetheless, the MMC requires
significant large DC storage capacitors to mitigate for large
power, low frequency pulsations due to the single-phase like AC
to DC conversion process in their arms. This work addresses the
recently introduced Asymmetric Alternate Arm Converter, a
hybride MMC topology which uses modular converter arms only
on the lower side of the main converter whereas the upper side
uses only standard ON-OFF valves topology. This converter is
similar to the Alternate Arm Converter (AAC) but dispenses of
the modules on the upper side and of the director switch on the
lower side. Like the AAC, the Asymmetric AAC offers a
significant reduction in DC energy storage requirements and in
the number of modules. In addition, the Asymmetric AAC offers
a less restricted operation than the AAC, in terms of operating
over a wide range of AC to DC voltage ratio, and the possibility of
achieving both distortion-free AC current and ripple-free DC
current.
The principle of operation of the Asymmetric AAC is
revisited here and a suitable control strategy to demonstrate its
operation is developed. In addition, requirements for energy
storage, voltage and current rating of converter modules and
converter losses are examined by means of simplified analyses.
Proposals are verified through simulation studies and
experiments conducted on a low power prototype having three Hbridge
modules per arm. Results confirm good operation of the
proposed converter and control scheme.