Distributed Control of a Fault-Tolerant Modular Multilevel Inverter for Direct-Drive Wind Turbine Grid Interfacing

Modular generator and converter topologies are being pursued for large offshore wind turbines to achieve fault tolerance and high reliability. A centralized controller presents a single critical point of failure which has prevented a truly modular and fault-tolerant system from being obtained. This study analyzes the inverter circuit control requirements during normal operation and grid fault ride-through and proposes a distributed controller design to allow inverter modules to operate independently of each other. All the modules independently estimate the grid voltage magnitude and position, and the modules are synchronized together over a controller area network (CAN) bus. The CAN bus is also used to interleave the pulsewidth modulation switching of the modules and synchronize the analog to digital converter (ADC) sampling. The controller structure and algorithms are tested by laboratory experiments with respect to normal operation, initial synchronization to the grid, module fault tolerance, and grid fault ride-through.