New decentralised and mostly renewable energy sources and the increase in the e-mobility domain cause new challenges for the grid infrastructure. Especially, the low-voltage grid will be affected. A necessary step towards a resilient operation of future grids is to equip the low-voltage grid with sensors, which will provide sufficient monitoring data for the implementation of intelligent planning and control algorithms. Since the infrastructure is very durable and not all distribution substations are equipped with sensors, this paper presents a Plug Play monitoring system that can be easily integrated into existing substations. The system consists of a local control box and fully wireless, energy self-sufficient current sensors. The sensors are attached to the individual phases of secondary feeders, measure the line current and transmit the data to the controlbox using a time synchronized wireless communication protocol. The controlbox measures the secondary voltage directly, calculates all relevant power values of the substation and transmits them to a backend (e.g., SCADA). In order to demonstrate the functional principle of the proposed monitoring system, a prototype of the current sensor and the backend is presented in this paper.