Research on Fiber Optic Large Current Sensor

0 Introduction

With the rapid development of the electric power industry, the capacity of the power transmission system is increasing, and the operating voltage level is getting higher and higher. We have to face the difficult problem of measuring strong current. The reliability requirements of electrical insulation and information transmission between primary and secondary instruments may make traditional measurement methods useless. In high-voltage, high-current and high-power power systems, the current sensor (CT for short) based on the principle of electromagnetic induction used in conventional current measurement technology has exposed a series of serious shortcomings: potential dangers of catastrophic accidents caused by explosions; large fault currents leading to magnetic saturation of the iron core; iron core resonance effect; hysteresis effect; open circuit at the output leading to high voltage; large size, heavy weight, and high price; high accuracy; and susceptible to electromagnetic interference. Traditional CTs can no longer meet the needs of the development of new-generation power systems, such as online detection, high-precision fault diagnosis, and power digital networks. The fiber optic current sensor (OCS for short) that introduces fiber optic sensing technology into current detection has become the best way to solve the above problems.

Since AJ Rogers first proposed the idea of ​​optical current sensing in 1973, fiber optic sensing technology has been developed for more than 20 years. Compared with ordinary electromagnetic transformers, fiber optic current sensors have the following advantages in high-intensity current measurement applications: fiber optic current sensors do not have magnetic saturation phenomenon, and unlike ordinary electromagnetic transformers, the dynamic working range is not limited by the magnetic saturation effect; fiber optic current sensors resist high electromagnetic interference and have low environmental requirements; fiber optic current sensors can produce high linearity response within a wider frequency band; fiber optic current sensors are relatively small in size and are relatively easy to install and use.

In summary, fiber optic current sensors have many advantages, especially their good insulation performance, small size, low cost, wide bandwidth, short response time, and can be used to measure DC, AC and pulsed large currents at the same time. Therefore, they are expected to become ideal sensors for measuring large currents under high voltage.

1 Sensor principle and optical path design

The fiber optic current sensor uses the Faraday effect of magneto-optical materials. In an optically isotropic transparent medium, an external magnetic field H can rotate the polarization plane of plane polarized light propagating along the magnetic field in the medium, and the deflection angle can be determined by the analyzer. The principle is shown in Figure 1, where B is the angle between the two polarizers and θ is the deflection angle of the plane light after passing through the magneto-optical crystal.

The rotation angle θ is proportional to the intensity H of the magnetic field on the magneto-optical material where the light propagates and the length L of the magneto-optical material: when H is constant, the rotation angle θ is:

θ=vHL (1)

Where: v is the Verdet constant; H is the magnetic field intensity; L is the length of the magneto-optical glass. The magnetic field formula for a long straight wire with current is:

H = I / 2πr (2)

The incident light intensity is P0, and the outgoing light intensity is obtained from Malus's law:

From formula (3), we can get:

From formula (4), we can get: when P = P0, Imax = 2πrB/vL. The larger the angle B is, the larger the maximum current value that can be measured is. Therefore, in experiments, the method of increasing angle B is often used to increase its measurement range. However, in practice, when angle B increases to a certain value, it will make the focusing of the optical path more difficult and make it more difficult to measure small signals. In previous experiments, B = 45° or a value with a small difference is generally taken. In this experiment, a 2 mw laser is selected as the adjustment light source. During the first focusing, the output light of the magneto-optical crystal is projected to a place 1 m away to eliminate possible birefringence. Optical glue is used to seal each joint surface to make the optical path adjustment easier to operate. Therefore, angle B is selected to be 80°. In the above formula, angle B is a fixed value after the sensor is completed. Therefore, as long as P and P0 values ​​are measured, the current value can be obtained.

In the optical path design, the structure shown in Figure 2 is used. The dark gray arrow line indicates the transmission path of the light in the sensor: the light emitted by the light source enters the sensor through the optical fiber with a self-focusing lens, is reflected by the right-angle prism with a vapor-deposited reflective film, and is changed into linearly polarized light and enters the magneto-optical crystal. The linearly polarized light with the polarization plane modulated by the magnetic field passes through the analyzer and the corresponding right-angle prism and enters the photodetector through another optical fiber with a self-focusing lens.

2 System Structure

From formula (4), we can know that by obtaining P0 and P, we can get the measured current value I. The system block diagram is shown in Figure 3.

The laser is driven by constant current, providing a constant current of 32 mA. The optical power feedback is realized through the automatic optical power control circuit, and the detected photoelectric signal is compared with the driving current of the laser to achieve the purpose of timely adjusting the laser working optical power jitter.

The light detection and amplification circuit realizes the light/electricity conversion, and amplifies and filters the electrical signal, as well as separates the DC signal and the AC signal. The principle block diagram is shown in Figure 4.

The signal acquisition and processing part collects and processes the DC signal and AC signal separately. The DC signal U0 when no current is flowing is recorded as the reference value. U is the detection value with DC and AC information of the current. After calculation, the DC component and AC component of the measured current I are separated.

3 Experimental results and analysis

In the experiment, the sensor head has an inner aperture of D1 = 2 cm, an outer ring diameter of D2 = 5 cm, and a base (i.e., the plinth in Figure 2) thickness of h = 1.1 cm; the collimator is made of an optical fiber that can pass 635 nm red light and a focusing lens; the magneto-optical crystal thickness is d = 2 cm; the Verdet constant v = -1.17 × 10-3 rad/A; the light source output power is 1 mW; and continuous current and pulse current are used to detect it in the experiment. During the experiment, one end of the device is connected to the output end of the light source, and the other end is connected to the input end, and the current-carrying wire is passed through the gap to start the test.

3.1 Experiment with continuous alternating current

In the experiment of continuous alternating current measurement, the voltage output by the fiber optic current sensor is compared with the current measured by the standard device, and the obtained voltage value is equivalent to the current value of the fiber optic current sensor. In the experiment, the standard device used is a current transformer. Table 1 is the data measured in a certain test institution.

The current value in Table 1 is used as the abscissa and the voltage value is used as the ordinate to draw a curve, as shown in Figure 5. It can be seen that the voltage and current are approximately proportional.

From the data in Figure 5, we can see that in the range of 100 to 3 000 A, the system has good linearity.

3.2 Experimental measurement using pulse current

Since general testing institutions rarely test AC currents above 10,000 amperes, Figures 6 and 7 are waveform diagrams displayed on an oscilloscope during a pulse current experiment in a testing institute. The upper curve is the waveform obtained by the fiber optic current sensor, and the curve in the figure is the waveform obtained by the current transformer.

Figure 6 is an image obtained from a pulse current experiment with B=45° and a peak value of 32 kA. It can be seen from the figure that the waveform of the fiber optic current sensor suddenly sags downward at around 10 kA. After inspection, it was found that the measured current exceeded its maximum measurement range, that is, saturation distortion occurred.

Figure 7 is an image obtained from a pulse current experiment with B = 80° and a peak value of 32 kA. In the figure, the current waveform curve measured by the optical fiber current sensor is not distorted and is well consistent with the waveform line measured by the current transformer, indicating that the current of 32 kA is within its dynamic range and the response time is less than 10 μs.

Based on the above experiments, the system has good linearity and stability in low current testing. In the high current test, saturation distortion occurred in the first test. The second test solved the distortion problem by increasing angle B, and the dynamic range was large and the response time was short. In fact, it can be seen from formula (4) that the measurement range can be made larger by increasing the verdet constant v or the magneto-optic crystal length L, but this requires re-customizing and processing the magneto-optic crystal, which makes the cost higher and the production time longer. Therefore, the method of increasing angle B was chosen after the saturation distortion occurred for the first time.

4 Conclusion

In this paper, magneto-optical crystal is used as material, and the sensor head of the optical fiber current sensor is designed and manufactured based on the principle of Faraday rotation effect. The designed sensor head is used to build an experimental system and conduct large current detection experiments. The experimental results show that the designed sensor can accurately measure 32 kA current under high voltage. Moreover, the device has a simple structure, is easy to use, has a short response time, and has good practical value.