Consultation Paper

Current sensor based on poly-magnetic ring structure

With the rise of various new technologies, the Internet of Things, big data and other technologies combined with industry, the big data environment in the industrial field is gradually taking shape, and data is transformed from a by-product of the production and manufacturing process into a strategic resource of great concern to enterprises. The power industry, as a national basic energy facility and the special characteristics of electric energy production and transmission, determines that the demand for big data in the power industry will greatly exceed that of other basic energy industries. The power industry has complex types of data, such as node voltage, current, power factor, temperature of equipment, etc., which are very critical to the operation and rational planning of the power grid. As an important device for collecting, converting and processing power data information - sensors - have become an indispensable and important technical tool in the field of power. Sensor technology as the cutting edge of modern technology, and computer technology, communications technology is considered to be the three pillars of modern information technology technology, but also become the 21st century mankind to compete for the high tech technology of the high point. However, compared with the other two technologies, the development of sensing technology is lagging behind, resulting in a "developed brain, five senses" situation; at the same time, the demand for sensor technology and products in various industries continues to improve, efficient, stable, adaptable new sensing technology has become an urgent need for industrial development.

Power systems need to measure the current is very much more, the amplitude and frequency of the common current in the power system, the current type in the power system, including: transmission line line current, the normal operation of the line current, the abnormal state of the short-circuit current, corona current, lightning current, harmonic currents, leakage current of electrical equipment, etc., the amplitude of the current ranges from the level of ?A level to the level of kA ranging, the frequency range covers the level of direct current to MHz. The frequency range is from DC to MHz. According to the magnitude of the current, the current measurement requirements in the power system can be divided into three categories: small and medium currents, normal operating currents and transient currents. Small and medium-sized currents mainly include harmonic currents, leakage currents of electrical equipment, which are characterized by small amplitude and frequency, and long duration; normal operating currents include AC and DC transmission lines, and normal operating currents of distribution systems, which are characterized by moderate amplitude and long duration; transient currents are characterized by large amplitude, short duration, and high frequency, such as short-circuit currents, lightning currents, and so on.

Currently, the current sensors applied in the power system mainly include: current transformers, Roche coils, shunts, fiber-optic current transformers, fluxgate current sensors, Hall current sensors, giant magnetoresistive current sensors, etc. These sensors are mainly based on the following physical principles: the law of electromagnetic induction, Ohm's law, Faraday's effect and magnetic field sensors. The following table shows a comparison of the performance characteristics of the above current sensors.

Compared with the traditional current transformer, the current sensor based on TMR effect has the advantages of being able to measure DC to high-frequency (MHz scale) current signals, wide measurement range, high sensitivity and small size, etc. In particular, the TMR current sensor is able to measure the DC current, which is extremely favorable for the monitoring of DC in the DC transmission system in the converter station; compared with the Hall current sensor, its size is small, sensitivity Compared with the Hall current sensor, it is smaller in size, higher in sensitivity, and has better temperature stability, which can adapt to the drastic changes in the ambient temperature of the power grid; compared with the new fiber-optic current sensors, Roche coils, and fluxgate current sensors, it is simple in structure, easy to manufacture, and cheap, which makes it easy to promote its use on a large scale.

1 Basic Principle
1.1 Basic principle of TMR current sensor

The measurement principle of the TMR current sensor is shown in the figure below. When the conductor inside the flow of current I, according to the right-hand spiral rule, around the wire will produce a spiral line-like magnetic field, the size of the magnetic field is proportional to the size of the current inside the conductor, the TMR chip is fixed somewhere around the conductor, and the direction of the magnetic field generated by the conductor and the direction of the sensitivity axis of the TMR chip is kept parallel to the direction to obtain the maximum output voltage. When the size of the current inside the conductor changes, the size of the surrounding magnetic field changes, the magnetic resistance inside the TMR chip changes accordingly, due to the use of bridge structure inside the chip, the change of the magnetic field is converted to the corresponding output voltage signal, so when the conductor and the TMR chip to maintain a constant position relative to the TMR chip, according to the output voltage of the TMR chip indirectly reflect the current inside the conductor to achieve the purpose of current measurement. Therefore, when the relative positions of the conductor and the TMR chip are kept constant, the current inside the conductor can be indirectly reflected from the output voltage of the TMR chip to achieve the purpose of current measurement.

1.1 Principle of closed-loop TMR current sensors

The specific working principle of the closed-loop TMR current sensor is shown in the figure below, Ip is the primary current, i.e., the current to be measured, and the number of turns of the primary loop is N1, which is usually 1; Is is the feedback current, and the number of turns of the feedback coil is N2. The magnetic flux generated by Ip inside the polytronic ring is Φp, and that of Is is Φs. The magnetic field generated by the primary current acts on the TMR chip, and an output voltage signal will be generated, which is amplified by an operational amplifier and passed through a power amplifier to obtain the feedback current Is. The output voltage is amplified by the operational amplifier and passes through the power amplifier to obtain the feedback current Is, Rm is the feedback resistance, and the feedback coil is connected to ground through Rm. When the feedback current Is flows in the feedback coil, a feedback magnetic field can be generated. Because the direction of the feedback magnetic field is opposite to the primary magnetic field, the output voltage of the TMR chip and the feedback current Is will gradually decrease until the primary magnetic field is equal to the feedback magnetic field. At this time, the feedback current Is stops decreasing and the whole system reaches dynamic equilibrium, i.e., zero flux state. Therefore, the relationship between Ip and Is can be expressed as:

When the current to be measured changes, the equilibrium state is broken, and the TMR chip generates a corresponding output voltage, which is amplified to generate a corresponding feedback current to compensate for the unbalanced magnetic field, thus reproducing the equilibrium state. The closed-loop structure has a high response speed because it takes less than 1?s to realize the equilibrium. The current to be measured can be indirectly reflected by detecting the voltage across the feedback resistor Rm in the feedback loop. In the closed-loop structure of the TMR current sensor, the compensation current Is is an exact mapping of the current to be measured Ip, which has very high accuracy and good linearity, and its response time is very short, so that irregular currents can be measured. At the same time, due to the existence of its negative feedback link, the error caused by temperature change can be offset to a certain extent. When the temperature increases, the output voltage of the TMR chip decreases, the compensation current generated by the feedback circuit decreases accordingly, and the difference between the magnetic field generated by the current to be measured Ip and the magnetic field generated by the compensation current Is increases, leading to an increase in the output voltage of the TMR chip, thus offsetting the error generated by temperature changes.

1.1 Simulation and analysis of poly-magnetic ring

The poly-magnetic ring structure can gather the magnetic field generated by the current-carrying wire to improve the sensitivity of the TMR current sensor, and the poly-magnetic ring structure is shown in the following figure. The sensitivity of the TMR current sensor with the introduction of the poly-magnetic ring structure is related to the structure and material of the poly-magnetic ring, such as the inner radius r of the poly-magnetic ring, the width rd of the poly-magnetic ring, i.e., R-r, the width d of the airgap of the poly-magnetic ring, the relative permeability of the poly-magnetic ring ? and other factors are related, so each factor is analyzed and discussed separately.
The method of simulation is used to analyze them separately. The model shown in the figure below is established, the radius of the wire is 5mm, and the initial values of other parameters are set as follows: the inner radius r of the poly-magnetic ring is 10mm, the initial value of the width rd is 2mm, the width d of the air gap of the poly-magnetic ring is 2mm, and the relative permeability of the poly-magnetic ring is set as 1000. was set to 1000. each parameter was parameterized and scanned separately to analyze its effect on the measurement of TMR current sensor.

The sensitivity of the TMR current sensors with the structure of the polymagnetic ring is mainly related to the relative permeability of the polymagnetic ring ? and the air-gap width d of the poly-magnetic ring, while the inner radius r of the poly-magnetic ring and the width rd of the poly-magnetic ring have little effect on the sensitivity of the TMR current sensor.

1.4 Simulation analysis of shielding structure

TMR current sensor is through the measurement of the magnetic field generated by the energized conductor to indirectly reflect the size of the current to be measured, has a high sensitivity to the magnetic field, so it is also susceptible to interference by stray magnetic fields, in practice, the application environment of the TMR current sensor is complex, and the stray magnetic fields from the surrounding environment will produce a certain degree of error in the current measurement results. The poly-magnetic ring structure can play a certain role in shielding stray magnetic fields. After increasing the shielding shell structure, the magnetic field distribution in the XY plane, it can be seen that, due to the integrity of the magnetic shielding shell structure, provides a magnetic flux path for the stray magnetic field, a portion of the stray magnetic field in the space along the shielding shell through the center of the air gap at this time the magnetic flux density of 0.1429mT, the shielding shell shielding efficiency is 16.9dB, with better shielding efficiency.

2. Current sensor structure design scheme

The hardware block diagram of the TMR current sensor is shown in the following figure, which consists of power supply, signal amplification, filtering circuit, bias zeroing circuit, temperature compensation circuit, power amplification circuit, feedback coil and so on.

2.1 Power Module
The TMR current sensor uses a 12V lithium battery to power the entire circuit, which has a large capacity, is safe and reliable, and provides a stable source of power for the circuit. Due to the use of operational amplifiers in the circuit to amplify bipolar signals, the need to use bipolar power supply, while the TMR chip's operating voltage of 1 ~ 7V, TMR chip power supply voltage is set to +5V, so the need to convert the 12V input voltage, the overall program as shown in the figure below.

2.2 Amplifier modules
Design of two-stage amplifier circuit, i.e., preamplifier circuit and secondary signal amplifier circuit, the output voltage of the TMR chip is in the order of mV, and the design of the two-stage signal amplifier circuit not only ensures high voltage amplification multiplier, but also does not affect the performance of the circuit; design of the cut-off frequency of 1MHz second-order low-pass filter to suppress the high-frequency noise.

2.2 Closed-loop feedback module

The closed loop feedback circuit is the negative feedback link. Since the output signal of the op-amp cannot directly drive the feedback coil, it is necessary to design the power amplifier part to drive the feedback coil after power amplification of the output voltage of the op-amp, so as to obtain the voltage signal on the feedback resistor, the closed-loop feedback circuit is shown in the following figure.

3. Current Sensor Performance Test

3.1 Current Range Test

Schematic diagram of the AC test rig using test equipment including a closed-loop TMR current transducer, Agilent 33220A function generator, YE5872A power amplifier, and Tektronix MDO 3012 dual-channel oscilloscope.

3.2 Error level test

Linearity is one of the most important static indicators of a current sensor, and is defined as the percentage of the maximum deviation (ΔYmax) between the measured curve of the sensor and the fitted straight line and the full-scale output (Y), which is also known as the "non-linear error". The measurement curve of the current sensor and the fitted curve are shown in the figure below. At a current of 70A, the maximum deviation is 0.618A, resulting in a linearity of 0.386%.

3.3 Frequency Response Test

The frequency response of the closed-loop TMR current sensor is tested. The -3dB bandwidth of the closed-loop TMR current sensor is roughly 550kHz. compared with the open-loop structure, the closed-loop TMR current sensor introduces the negative feedback link, its response speed is faster, and it has a wider bandwidth than the open-loop structure. the closed-loop structure of the TMR current sensor has a wider bandwidth, and it can satisfy the needs of the majority of the current testing in the power system.

4. Conclusion

Under the impact of various advanced technologies, the traditional power grid can no longer meet the requirements of the construction and operation of the electric power industry, and the rapid development of the smart grid has put forward higher requirements for sensing technology. As one of the most important parameters of power data, the accurate measurement of current is of great importance. The closed-loop current sensing technology based on tunnel magnetoresistance effect has certain theoretical significance and practical value. The sensitivity of the closed-loop TMR current sensor can reach 20.02 mV/A, the rated measurement range is ±120 A,-3 dB bandwidth can roughly reach 550 kHz, and the measurement accuracy class is 1.0.