The so-called saturation of the transformer, in fact, is the saturation of the core of the transformer. We know that the reason why the transformer can change the current, is because the primary current in the core to produce a magnetic flux, and then wound in the same core on the secondary winding in the electric potential U = 4.44f * N * B * S × 10-8. f for the system frequency, HZ; N for the number of turns of the secondary winding; S for the core cross-sectional area, m2; B for the core of the magnetic flux density. If the secondary circuit at this time for the path, will produce secondary current, complete the current in a secondary winding in the transmission. When the flux density in the core reaches the saturation point, B with the excitation current or magnetic field strength tends to be insignificant. That is to say, in the case of N, S, f determined, the secondary induced potential will remain basically unchanged, so the secondary current will be basically unchanged, one or two currents in proportion to the characteristics of the change. We know that the essence of the saturation of the transformer is the core of the magnetic flux density B is too large, more than the saturation point caused by. And how much flux in the core is determined by the size of the current to establish the flux, that is, the size of the excitation current Ie. When Ie is too large to cause excessive magnetic flux density, will make the iron core saturation. At this time, the excitation impedance of the transformer will drop significantly, resulting in an increase in the excitation current, which further exacerbates the increase in magnetic flux and core saturation, which is in fact a vicious cycle of the process.