MMC Test Bench: Laboratory-Scaled (Multi-Terminal) HVDC to Modernize the European Electricity Grid

The company

The Institute for High Voltage Equipment and Grids, Digitalization and Power Economics at RWTH Aachen University in Germany is a leading research institute for electrical power engineering. RWTH Aachen participated in the PROMOTioN (PROgress on Meshed HVDC Offshore Transmission Networks) initiative—a major EU project comprising 33 partners across 11 countries aiming to develop cost-effective, reliable meshed HVDC offshore networks to evacuate gigawatts of wind power from the North and Baltic Seas.

The challenges

• Offshore Wind Evacuation: Integrating multi-gigawatt offshore wind farms via multi-terminal HVDC introduces complex control and protection challenges for continental AC transmission grids.
• High-Voltage Testing Bottlenecks: Full-scale physical testing of high-voltage HVDC converters is practically impossible due to extreme safety risks, cost, and lack of access to grid infrastructure.
• Complex Converter Control: Modular Multilevel Converters (MMCs) feature novel topologies that require sophisticated low-level and high-level control validation before full deployment.
• System Interaction Uncertainty: Grid operators and planners lack operational experience regarding the dynamics between MMC-HVDC links, offshore wind farms, and surrounding AC grids

The OPAL-RT solution

OPAL-RT delivered and integrated a comprehensive low-voltage multi-terminal DC MMC test bench setup:

• HYPERSIM Real-Time AC & Wind Farm Simulation: An OP5707 HIL simulator running HYPERSIM emulates the surrounding AC power grid and offshore wind park dynamics.
• Modular MMC Test Benches: Installed eight laboratory-scaled MMC test benches. Each test bench consists of an OP4510 (Kintex-7 FPGA + Intel Xeon CPU) for low- and high-level converter control and six OP1210 submodule boxes (up to 11 levels, 400V, 6 kW).
• Power-Hardware-in-the-Loop (PHIL): Integrated four-quadrant 21kVA linear power amplifiers (Puissance Plus) to couple the physical laboratory MMC converters with the simulated AC environment in real time.
• DC Link Emulation: Utilized OP8600 PI sections (32 stages) to emulate up to 800 km of bipolar (or 1,600 km monopolar) DC transmission lines.

The results

• Rapid Commissioning: Successfully commissioned the complex multi-terminal HVDC demonstrator in July 2019, just 8 months after Factory Acceptance Testing (FAT) in Montreal.
• AC Grid Support Validation: Successfully analyzed advanced MMC control strategies for providing ancillary services and frequency support to AC grids.
• Harmonic & Fault Research: Enabled in-depth testing of offshore wind park harmonic resonance, black-start capabilities of diode rectifiers, and DC fault handling strategies.
• De-risked Technology Transfer: Scaled-down PHIL testing validated low-power solutions that can be directly extrapolated and applied to commercial high-voltage offshore HVDC grids.

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