Stability analysis of the high voltage DC link between the FEC and DC-DC stage of a transformer-less intelligent power substation

The advent of medium voltage blocking SiC devices has resulted in the development of 3-phase Solid State Transformers (SSTs) to inter-connect the 13.8 kV distribution grid with the 480 V utility grid. One such 3-phase SST is the Transformerless Intelligent Power Substation (TIPS). TIPS is tied with the 13.8 kV grid through an ac-dc Front End Converter (FEC) stage. The isolation stage is a dc-dc Dual Active Bridge (DAB) converter. The FEC and DAB High Voltage (HV) side converter are composed of 15 kV SiC IGBTs. The dc bus at the interface between the FEC and DAB stages is rated for 22 kV. The stability of this dc link is of great concern. The closed loop input and output impedances of these stages affect the stability of the system depending on the direction of power flow. This in-turn depends on the adopted control strategies, dc bus capacitance and also on the operating conditions such as dc bus voltage, grid voltage and power levels. This paper does a thorough analysis of the stability of the TIPS HV dc link which could be extended to other similar systems.