Abstract: To study the effect of high converter transformer with no tap-changer on operating conditions of DC system, a simulation model is established according to the converter control system of real transmission project that includes valve group firing angle control and converter transformer tap-changer control. By means of simulating the full voltage operation and decompression operation of the system, the asymmetry caused by the operation and control of high-end and low-end converter transformers is analyzed and evaluated, which has important realistic meaning for studying the feasibility of converter transformer without tap-changer and the parameters designing of its main circuit.