Auxiliary power supply for Solid State Transformers

In contrast to traditional 60 Hz transformer, solid state transformer (SST) offers power flow control, integration of renewables and maintaining grid stability in high renewable penetration scenario. Like a typical power converter, SST requires low voltage (24 V dc) power for its control and sensing circuits. In running condition, this power is derived from its low voltage dc bus at 400 V. But it is challenging to derive it during start-up, because the only source available during start-up is the distribution grid at 7.2 kV, 60 Hz. Due to high input voltage (7.2 kV, 60 Hz), deriving a control supply of about 150 W, even for just start-up duration of about 200 ms, presents a novel power electronics problem. In this paper, two solutions have been proposed to address this issue, by taking a 20 kVA, 6.5 kV Si IGBT and 15 kV SiC MOSFET based SSTs as the reference converters. The first solution is generic and is based on storing the required start-up energy in a dc capacitance. This is based on developing a cost-effective high voltage switch using low voltage IGBTs with self-driven functionality. The second solution, applicable only to SST topologies with high voltage ac capacitive filter, is to tap the energy from the capacitor itself. The fundamental constraints considered for both the solutions are practical feasibility at high voltage (7.2 kV ac or over 10 kV dc), power loss, size, weight and cost-effectiveness. Experimental validation of extracting continuous power for the IGBT gate driver ICs from the snubber is presented with 200 V input. And, the results of the auxiliary power derivation from the filter capacitor are shown with 5.7 kV ac input.