Research On High-precision Open-close Hall Current Sensor For On-line Calibration

In modern power grid,due to the economic friendliness of live detection,on-line calibration of current transformers has become a trend.Although operation under electrification can be realized,the measurement accuracy of normal open-close Hall current sensors is generally low due to the existence of two air gaps.In addition to the effect of air gaps,the errors of Hall elements and the stability of the control current are also related to the measurement accuracy.This paper aims to improve the measurement accuracy of the open-close Hall current sensors,and conducts research to provide theoretical and experimental basis for further realization of its on-line calibration function from three aspects of the Hall element itself,the open-close magnetic rings and the control power.Firstly,the equivalent source model of flow-controlled Hall elements is established when considering the short-circuiting effect of flow-controlled Hall elements.The characteristic of high-sensitivity elements is proposed to reduce the influence of the unequal potential after analyzing the relationship between the carrier concentration and the sensitivity of the hall elements.In order to enhance the signal strength and filter the clutter signal,a dual operational amplifier circuit with low zero drift is introduced to process the Hall signal.And the feasibility and accuracy of the circuit are verified in Multisim.The mathematical model of open-close Hall current measurement is deduced as the mathematical basis of the experiment.Secondly,the characteristics of magnetic fields and the air-gap effect of the open-close structure are studied.Based on the comparison of the characteristics of several traditional ferromagnetic materials,the open-close magnetic rings based on nanocrystalline alloy are proposed.The structure of open-close magnetic rings is designed:the magnetic field uniformity coefficient is introduced from the cross section of the magnetic ring to improve the distribution of magnetic field in air gaps,and the sensing accuracy is improved as well.The appropriate proportion between air gaps and magnetic rings is deduced to improve the measurement sensitivity and linearity.The high sensitivity and great measurement linearity of the open-close magnetic rings are verified in COMSOL.Then,the anti-windup PID control is introduced into the double closed-loop constant-current source,and the high-precision DA/AD conversion circuits and main control program are designed to optimize the transient performance and output accuracy of the system.In the V-I conversion module,a series-parallel sampling resistor network with complementary temperature coefficients is designed to further improve the sampling accuracy.Lastly,the open-close Hall current sensing test platform is built.After conducting the 20?100A AC current measurement experiment,it is verified that compared with ordinary open-close sensors,the open-close Hall current sensor designed in this paper has a better accuracy of measurement,and the stability of it is excellent,which meets the expected requirements well.