Current-Limiting Control for Fault Ride-Through of LLC-based Solid-State Transformer in Data Centers

Abstract

Solid-State Transformers (SSTs) are increasingly proposed as the interface between distribution grids and data centers due to flexible power flows and fast dynamic response. However, when a short-circuit fault occurs in a load branch, the SST with a voltage-source-type DC-DC stage is forced to shut down due to fault currents. Therefore, current-limiting strategies are strongly needed to prevent catastrophic equipment damage and cascading blackouts by instantly restricting massive current spikes and offering sufficient currents for protection devices to act at the faulted branch. This paper proposes a coordinated DC load fault-tolerant current-limiting and recovery strategy embedded directly in the control of the SST DC-DC stage, avoiding additional hardware cost. Specifically, the fault mechanism of an example LLC resonant converter is studied. Accordingly, a fault detection framework is implemented, a closed-loop current controller is proposed to limit the DC current to a designated value within microseconds by surging the switching frequency and adjusting the duty cycle, and a ramped recovery stage will then restore the DC bus after the fault isolation without inrush currents. Experiments on an LLC converter prototype have verified the feasibility of the proposed current-limiting strategy, enabling faster and lower-cost fault response suitable for resilient data center power architectures.