Study on Tunneling Magnetoresistance Based Symmetrical Air-Gap Current Sensor for Medium-Voltage DC Systems

Emerging current sensors based on tunneling magnetoresistance (TMR) were preliminarily researched in DC measurement for the smart grid. Nonetheless, the single air-gap TMR sensor will produce measurement errors by changes of the current-carrying conductor position in the magnetic circuit loop. To improve the accuracy of the measurement, this paper proposes a TMR based symmetrical air-gap current sensor. Based on the closed-loop zero-flux principle, the proposed current sensor adopts a symmetrical double air-gap magnetic circuit structure to reduce errors. The numerical simulation models of a single air-gap TMR sensor and a symmetrical double air-gap one are built on the PSCAD platform to verify its steady-state response and transient response. The test result shows that the symmetric air-gap TMR sensor reduces the amplitude error by 89.08% of the single air-gap TMR sensor when measuring DC current, and the error is reduced by 12.21% when measuring 50Hz AC current. Finally, the two types of TMR sensors are installed in the DC grid test system model of ±10kV to analyze their measurement effects in normal operation and fault conditions. The simulation results prove that the sensor proposed in this paper is more suitable for applications in medium-voltage DC grids.