Asymmetric Topology Design and Quasi-Zero-Loss Switching Composite Modulation for IGCT-Based High-Capacity DC Transformer

Medium-voltage large-capacity dc transformer (DCT) is the key energy conversion link for building intelligent distribution grid. Increasing the voltage and capacity of single module in DCT can effectively improve the power density and reduce the system cost. Aiming at the integrated gate commutated thyristor (IGCT)-based high-capacity dc transformer (IGCT–HDCT), the asymmetric topology design and quasi-zero-loss switching composite modulation are proposed to achieve the construction of high efficiency and power density DCT. Through designing the snubber circuit and phase-shift ratio trajectory ingeniously, the zero-voltage switching-
on
and zero-current switching-
off
operation of the IGCT–HDCT can be maintained, which greatly reduces the switching loss and gives full play to the extremely low
on
-state loss characteristics of IGCT. Meanwhile, the full-range soft-switching operation (from no-load to full-load) can be achieved in the composite modulation without adding any auxiliary component. And the design of the snubber circuit is greatly simplified, which cannot only effectively limit the
dv
/
dt
and
di
/
dt
of power semiconductors, but also improves the power density and operating efficiency of IGCT–HDCT. Finally, the theoretical analysis of the proposed system solution is verified by building a 1.25 MW IGCT–HDCT engineering prototype, and the maximum operating efficiency reaches 98.6%–99.1%.