The invention provides a current transformer which comprises three leg circuits, every two leg circuits are connected in parallel, each leg circuit comprises an upper leg circuit and a lower leg circuit in serial connection, one end of each upper leg circuit is connected to a positive DC bus, one end of each lower leg circuit is connected to a negative DC bus, the connecting point of the upper leg circuit and the lower leg circuit of each leg circuit serves as the output end of the current transformer, at least one of the upper leg circuit and the lower leg circuit comprises at least one identical sub unit in parallel connection, and each sub unit comprises an inductor and two bidirectional switches. The problem that a modularized multi-level current transformer based on a capacitive sub module in the prior art cannot control the output current of a converter station unless the output voltage is controlled, namely cannot control the output current of the converter station directly, is solved.