Electro-optical transmission line current monitor

In this paper, a current level monitor for overhead transmission lines is presented. The current monitor is designed to be robust, low cost and light weight. It is based on an electrooptical transmitter using an LED diode. The line-current is picked up by an iron core transformer clamped over the line. The LED diode is connected to the secondary winding. The diode emits light to the receiving end through a plastic optical fiber. At the receiver, a photo-transistor converts incoming photo-current back to an appropriate voltage level. The transformer works as a current transformer with high resistance load on the secondary side. This mode of operation induce additional measurement error in current and in phase. In the most part, these errors are systematic and can be reduced numerically at the receiving end. Nevertheless, the unit shows nonlinearity and temperature dependence. However, it seems to be a feasible solution providing a low cost line-failure detection for many existing installations. For new installations, better and more expensive solutions may be appropriate. The unit can be mounted over a transmission line during normal operating conditions and due to its light weight there are no additional requirements regarding the line mechanical load.