Coordination of mode switching control and SMES unit for low‐voltage ride‐through fulfillment of power electronic transformer

Power electronic transformer (PET) should have a satisfying low-voltage ride-through (LVRT) ability to cope with different kinds and severities of voltage sags. To solve the LVRT issue, this paper proposes to coordinate mode switching control (MSC) and superconducting magnetic energy storage (SMES), and it is to realize the targets of promoting voltage recovery, keeping DC-link stabilization, and alleviating current inrush. First, the theoretical modeling of a PET incorporating with a SMES unit is implemented, and the PET's fault transient characterizes as well as the SMES's potential supportability is analyzed. Then, the coordination philosophy of the MSC and the SMES is presented. For that the voltage sag reaches a certain threshold, the MSC is enabled to remove the PET's normal control, and the SMES collaboratively operates to lower the DC-link overvoltage, while avoiding the PET damage. In the process of the coordination control handling the voltage transients, a proper reactive current injection is carried out at the PET's output stage to assist the voltage recovery, and the PET can possess a better LVRT margin. Using MATLAB, a 3 MW PET with an 8 H/1.2 MJ SMES unit is modeled, and different voltage sag scenarios and utilization ways of the MSC and the SMES are simulated. The comparative analysis results show that the proposed approach realizes a satisfying LVRT for the PET.