Transformer Saturation Analysis and Mitigation for MMC-Based Grid Emulator

The modular multilevel converter (MMC)-based grid emulator emerges as a favorable solution for testing and validation of medium-voltage high-power wind turbines. The interface transformer of the grid emulator can be saturated when emulating voltage sags/swells in the faulted grids. The magnetic saturation can lead to severe inrush current, which may degrade the lifetime of transformer windings and even adversely affect the operation of grid emulator. This paper analyzes the dynamics of grid emulator under the interface-transformer saturation. It reveals the effect of the dc and harmonic components of magnetizing current on the MMC internal dynamics. An inrush current suppression technique is applied to mitigate the transformer saturation during the transient states, which guarantees the safety operation of the grid emulation system.