MMC Test Bench

  Picture during the operation of the MMC Test Bench Copyright: © Martin Braun

The integration of point-to-point and multi-terminal HVDC systems into existing AC transmission systems present novel challenges to transmission grid operators, grid planners and manufacturers. A major obstacle towards the realisation of HVDC networks and complex integrated AC/DC systems is the limited experience regarding their operation and control as well as their interaction with the surrounding AC systems, such as continental transmission grids or offshore wind power plants.

The laboratory-scaled (multi-terminal) HVDC demonstrator – the MMC Test Bench – serves to address these issues. It is a unique laboratory, which is used for the investigation and demonstration of Modular Multilevel Converter (MMC) controllability in integrated AC/DC systems. At the heart of this testbench are eight laboratory-scaled MMCs, which can be configured as half- and full-bridge or mixed cell converters. The characteristics of the converters are as follows:

  • Rated DC voltage: 400 V
  • Rated power per MMC: 6 kW
  • Cells per arm: 10
  • CPU-based control for high-level controls (e.g. grid controls)
  • FPGA-based control for low-level controls (e.g. submodule management)

The DC networks can be represented by cascaded Pi sections, which can be configured as symmetrical and asymmetrical monopole as well as bipole. To investigate the interactions between DC and AC networks, such as offshore wind power plants and AC transmission systems, the MMCs are embedded in a real-time simulation of the surrounding AC systems by employing four-quadrant linear power amplifiers in a Power-Hardware-in-the-Loop (PHiL) setup. The parameters of each power amplifier are as follows:

  Schematic representation of an exemplary arrangement of the MMC Test Bench Copyright: © IAEW
  • Rated power: 21 kVA (7 kVA per phase)
  • AC voltage range: 0-800 V
  • DC voltage range: 0-1120 V
  • Bandwidth (full scale): DC – 5 kHz
  • Bandwidth (small signal at -3 dB): 50 kHz

The real-time simulators used in the system are:

  • Grid simulator (OP5707) for the grid and MMC simulations with HYPERSIM (EMT)
  • Centralised DC grid control (OP4510) connected to each laboratory MMC and the grid simulator via optical fibre (SFP)