Measurement of electric current by the optical current sensor based on magneto-optic crystal

Abstract

Measuring equipment is very important in smart grid, for example, the real-time accurate current measurement is necessary for over-current protection, leakage detection. Faraday Effect causes light polarization to rotate when the fiber is exposed to a magnetic field in the direction of light propagation. Thus, the magnetic field strength can be determined from the light polarization change. By applying Ampere’s law, we can get the current by measuring the light rotation. In this paper, optical current sensor (OCS) based on magneto-optic crystal has been developed. The sensing principles, optical and electronic design, as well as its characterization have been described. The weak current signal detection technique is further discussed by means of spectral analysis and lock-in amplifier methods. The performance of the prototype was tested experimentally, the sensor has a high sensitivity for currents and is capable of achieving weak electric current detection with accuracy of 1mArms (50 Hz). A linear response is obtained for current amplitude as low as several mArms at an AC frequency of 50 Hz. For the direct current (DC) current measurement, a lock-in amplifier is used in our scheme; the detection limit of the magneto-optic crystal current sensor is less than 1 mA. There is a wide range of applications for the magneto-optic crystal current sensor, which will be mainly used to monitor currents both on photovoltaic grid-connected system and insulator operating state.