The growing challenges posed by renewable and decentralized energy producers as well as electromobility call for a change from the traditional grid operation to a smart grid that can control and optimize the production, distribution and storage of electrical energy. However, since there are still numerous distribution substations without pre-installed measurement sensors, there is a demand for a retrofittable sensor system for existing stations. For this purpose, an autonomous, non-invasive and wireless sensor is designed on the basis of a current transformer, which can be retrofitted to the individual outlets of a distribution substation. The sensor uses a single current transformer both for energy harvesting and for current measurement on the respective line and sends the measurement data to a control unit. ^In this work the electrical and magnetic behavior of a current transformer is first summarized and a realistic simulation model of a 60/1 transformer is then created for the simulation program LTSpice®. Based on this an analog circuit is designed in the simulation environment, which periodically harvests energy using the current transformer, stores it in a supercapacitor and subsequently changes to a measuring mode in which a stable supply voltage and a correct measuring signal are available. The presented solutions guarantee a faultless function without the need for a battery or other backup energy sources. The affect of the additional circuit on the measurement accuracy only leads to minor defects and the total measurement error still mainly dependents on the precision class of the current transformer. ^In this work, two different approaches are shown which allow a correct operation of the sensor in the range of 13% to 100% and 22% to 100% of the rated current of the used 60/1 current transformer. It turns out that the resulting power dissipation is a limiting factor for the dimensioning of the circuit and the adaptation to other current transformers. A final comparison of the designed circuits in LTSpice® with a real prototype verifies the desired function of the harvesting and measurement circuit.