500kVA Hybrid Solid State Transformer (HSST): Architecture, Functionality and Control

For the transmission system, the voltage regulation distribution transformers (VRDT) with the on-load tap-changers (OLTC) are the most conventional solutions to control the terminal voltages of the low frequency transformer (LFT). However, the OLTCs have high cost of the maintenance and the low life time, especially for the future grid which involves much more distributed sources and dynamic loads. The solid-state transformer (SST)s can be another solution, however, the high cost of the SST is still a big concern. The hybrid SST (HSST), which is a flexible, high-performance and low-cost smart transformer based on partial power processing, gets both the advantages of the SSTs and the LFTs. This paper investigates the different architectures of the HSST configuration and provides a detailed comparison of them. The configuration of low-voltage side single-winding input parallel and output series is chosen to implement the HSST with a single-stage AC/AC dual-active-bridge (DAB) converter. The proposed HSST can provide the functionalities of the seamless voltage control, advanced power flow control, and the online monitoring of the failure and fault. To control the AC/AC DAB with wide-range operations, this paper investigates the operation principles linked to the traditional PWM converters in different modes. And a systematic control strategy is proposed to cover the complete operation range with three operation modes: CCM, BCM and DCM. Finally, a 500kVA HSST prototype is developed and tested in this paper to verify the proposed concept.