In modern smart electric grids, power cables are supervised by a large number of monitor equipments. In many field application cases, current transformers (CT) are employed to extract electrical power from these high voltage cables to supply monitoring circuits. In this paper, in order to harvest maximum power from cables, key parameters of the rectifier with current transformer circuits are analyzed, and the design considerations are discussed. A high frequency PWM control scheme is proposed to regulate the output voltage by regulating the current circulation duty ratio. Theory analysis and simulation are given. Experiments based on a 20 W prototype verify the validity of the proposed methods.