The transformer pass through DC means short circuit. Because the one side of transformer access DC, the transformer winding has no resistance for DC. The coil is equipped with allowing DC and preventing AC characteristic. So the transformer access DC means swift power off. According to different voltages, there is also existed dangerous such as winding and power were burned. The transformer another side would not destroy, because of DC would not create changing magnetic induction line. The principle of transformer is electro-magnetic induction. The primary coil access DC voltage out of unchanging magnetic field, so the secondary would not react voltage.
The DC resistance of transformer is tiny. When the secondary with no load is basically a pure inductance, its impedance is related to frequency. The secondary impedance is the parallel connection of the secondary reflected impedance and inductance. When connect with direct current, the impedance is the resistance of the transformer due to the frequency f=0. If the same voltage as the rated AC voltage is applied, because the impedance is very small and current is enough to burn the transformer.