Optical current sensor by self-compensating the ...

Optical current sensor by self-compensating the Faraday effect 1

Hugo C. Beltran,1 Jorge L. Flores, 2José A. Ferrari, 1Guillermo Garcia-Torales, 1Javier Cabrera 1 Electronic Engineering Department, University of Guadalajara, Av. Revolución #1500, CP. 44840, Guadalajara, Jal., México 2 Instituto de Física, Facultad de Ingeniería, J. Herrera y Reissig 565, 11300 Montevideo, Uruguay ABSTRACT We present a new optical current sensor architecture, which is based on a polarimetric configuration and a control system for self-compensation of the Faraday effect taking place at the sensor head. After passing through a bulk Faraday sensor head, the light travels through the free space reaching a Faraday modulator placed some distance away from the conductor carrying the current. The first device acts a current transducer and the second one acts as a magneto-optical element operated in a closed-loop mode to compensate the angle of rotation of the polarization introduced by the sensor head. The control system operates in closed loop feedback through a simple current-driven solenoid, and this way, the optical output from the current sensor is maintained at a constant intensity. Considering that the optical and electrical parameters of the sensor head and the Faraday modulator are known, the electrical current applied to the solenoid can be measured, and thus the current flowing through the conductor can be calculated. Experimental results demonstrate the feasibility of the proposed device to measure remotely the current carried by the conductor. Key words: Optical current sensor, Faraday effect, closed loop feedback

1. INTRODUCTION An accurate electric current transducer is one of the key components in power system instrumentation. Assuming, that it provides information for revenue metering, control, and relay protection of power systems. In recent decades, considerable effort has been devoted to develop new transducers for monitoring voltage and current on electric power systems [1-4]. Optical current devices have many advantages with respect to conventional current transducers (e.g., Rogowski coils,[5] and capacitive dividers with electronic amplifiers); they have simple insulation structure, immunity to electromagnetic interference, wide dynamic range and bandwidth, accurate transient response, and show no saturation or hysteresis effects [6,7]. Also, the optical sensor head could be as little as few cubic centimeters, and the small size is associated with a small weight [8]. Therefore, these optical devices are considered to be an optimum interface between high-voltage lines and electronic equipment expected to monitor and control the faults on power systems. Currently, most current sensors based on the Faraday effect have been in the form of a coil with a large number of turns of low birefringence mono-mode fiber wound around a current-carrying conductor [4,6,7]. Since the Verdet constant of silica fiber is very small [4.68X10-6 rad/A at 632.8 nm], standard silica fiber must be long and coiled multi-turn to increase the polarization rotation angle. However, bend-induced linear birefringence affects the state of polarization and quenches the desired Faraday effect [8-10]. Also, the light transit time through the fiber is increased, as a result, the sensitivity and the bandwidth of a fiber Faraday current sensor are severely limited [11]. High Verdet constant bulk materials [12] are not subject to the problems associated with the presence of the intrinsic birefringence induced by core ellipticity and asymmetric stress or the extrinsic birefringence caused in deploying the fiber sensing element. In the paper we proposed a method to measure remotely electrical current. The current sensor is based on a polarimentric configuration (polarizer, sensor element and analyzer) and it consists of a bulk Faraday sensor head (e.g., a TGG rod) and a simple Faraday modulator, (FM) (optically in series with the sensor head). The purpose of this Faraday rotator (modulator) is to compensate the angle of rotation of the polarization introduced by the sensor head, and thus to stabilize and control the operating point of the polarimetric sensor. Roughly speaking, if one applies an electrical current to the

Infrared Remote Sensing and Instrumentation XIX, edited by Marija Strojnik, Gonzalo Paez, Proc. of SPIE Vol. 8154, 815414 · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.894119

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solenoid of the FM as closed loop feedback, the intensity output is stabilized at a value constant. Assuming that the optical and electrical parameters of the sensor head and the Faraday modulator are known, and the electrical current applied to the solenoid can be measured (in real time), hence the current flowing through the conductor can be calculated. We demonstrate that it is possible to measure remotely the current carried by the conductor. In the next section we describe in some details our proposal, in Sect. 3 we describe the control system, and Sect. 4, experimental results are presented. Finally, the conclusions are presented in Section 5.

2. THEORY The physical principle and scheme of operation of the proposed optical current sensor is shown in Fig.1. The system consists of a polarized light source (L), a bulk Faraday sensor head (SH), a Faraday Modulator (FM), a polarizer (P), a photodetector (PD) and control system (CS). Light emitted by a polarized laser is injected into the Faraday material, placed orthogonally to the power line. Thus, the polarization plane of the beam emerging from the SH will be rotated by the magnetic field generated by current i1 ( t ) flowing through the power line. After passing through the sensor head, the light travels through the free space reaching a (second) Faraday rotator, which is controlled by a feedback current. The purpose of this Faraday rotator is to compensate the angle of rotation introduced by the sensor head. Finally, the light beam passes through an analyzer. The analyzer is a polarizer rotated a known angle θ relative to the input polarization. The net intensity exits from the analyzer can be expressed as

I ( t ) = I 0 cos2 (α1 ( t ) + α 2 ( t ) + θ )

(1)

where I 0 is the optical power from the light source, θ is the angle between the polarization direction of the input beam and transmission axis of the analyzer, α1 ( t ) and α 2 ( t ) are the Faraday rotation angles introduced by the sensor head and the Faraday modulator, respectively. The first phase angle is associated to the electric current, in real time, to be retrieved, and the second one is associated to the control signal. Once the light arrives to the optical interface unit, the electronics performs three functions: (1) photo-detection and amplifications using a diode PIN; (2) signal processing and (3) generation of the control signal. One of the most important features of this electronic interface is to provide an electric output signal to compensate the Faraday rotation, i.e., α 2 ( t ) = -α1 ( t ) .

Fig. 1. Experimental setup.


2.1 Sensor head The sensor element consists of a rod of magneto-optical material with 5cm long, positioned orthogonally to the conductor and separated by a few millimeters from each other (see Fig. 2). The polarization plane of a linear polarized light beam propagating inside an optical element is rotated under influence of a magnetic field B, generated by the electrical current to be measured, and the rotation angle is given by l

ur

r

α1 ( t ) = V ∫ B ⋅ d l

(2)

0

where V is the Verdet constant of the optical material and l is the interaction length. According to Fig. 2 the magnetic field is given by

ur μ i1 ( t ) B= 2π r

(3)

r

Here, dl = rdθ , and ( r, θ ) refer to the radial and azimuthal coordinates, respectively. Substituting Eq. (3) into (2) and resolving the integral, one obtains μVi1 ( t )θ s α1 ( t ) = (4) 2π where θs is the angle subtended by rod on the center of the conductor. 2.2 Faraday modulator A typical Faraday modulator consists of a rod magneto-optical material inside of a solenoid. When a linear polarized monochromatic light beam passes through a rod magneto-optical material it suffers a rotation of its polarization plane by an amount α 2 ( t ) = μ V2 NK i2 ( t ) 2 L , where i2 ( t ) is the current to be applied to compensate the angle α1 ( t ) , V2 is the Verdet constant of the magneto-optical material, L is the length of the rod, N is the number of turns and K is a constant that depends on the geometry. From Eq. (1) when α 2 ( t ) ≠ −α1 ( t ) the sensor output is a modulated intensity. In other to cancel out this modulation, we have devised a closed-loop for signal processing; using a simple Faraday modulator to cancel out phase changes due to the electrical current and stabilize the light intensity output to a constant value. Thus the Faraday modulator allows us to apply a feedback signal to set the condition α 2 ( t ) = −α1 ( t ) , so that optical output can be reduced to a DC term, which only depends on angular position of analyzer, i.e.,

I (t ) =

I0 [1 + cos 2θ ] 2

.

(5)

Assuming that both electrical current are synchronized in frequency, the relationship between to phase angles is given by

μV1i1θ s μV NKi2 =− 2 2π 2L

,

(6)

where i1 and i2 are measurements of current, expressed in terms of the rms value. From a number of straightforward manipulations it can be found that

i1 = −

π V2 NK i2 LV1θ s

(7)

Assuming that applied current i2 is measured, in real time, and the other parameters involved in Eq. (7) are known, hence we can calculate the electrical current on the conductor using this expression.


Fig. 2.

Sensor Head.

3 ELECTRONIC SCHEMES From a practical point of view, the most general description of the self compensation system is presented in Fig. 3. The control system consists of three stages: The first one consists of a phase locked loop device, PLL, which generates a reference signal, vr(t), tuned in frequency and locked in phase with the photo-generated signal. The second one is an automatic gain control system (AGC), whose gain is controlled by an external signal and it is used to amplify the PLL output in a controlled manner. Finally, the third stage consists of processing, conditioning and amplification of the output AGC signal to generate the feedback current, i.e., i2 (t ) .

Fig. 3

Block diagram of the self-compensation system.


3.1. PLL The PLL configuration have been designed to act as a narrowband tracking filter i.e., its output signal is a periodic signal with a frequency equal to the average frequency of the input signal (photo-generated signal). Once the PLL has acquired this frequency, the frequency of the PLL will track this input signal if it changes slightly in frequency. [13] The phase detector was built with a NE565 integrated PLL, its electrical scheme is shown in Fig. 4. The resistor R0 and the capacitor C0 were used to adjust the free running frequency, f0 , which is given by

f0 =

0.3 R0C0

(8)

The frequency range over which the PLL can maintain its coupling with an incoming signal is defined as the range of hitch system. The frequency band over which the PLL can acquire latch with an incoming signal is called the capture range of the system and is never greater than the range of coupling. [14] Thus, the operation range may describe by

Hold in range = ±

8 f0 Vcc

(9)

were, Vcc is dc voltage supplies by power source. The output signal from PLL devices has a saw tooth wavefront, whose fundamental frequency is equal to the average frequency of the input signal (photo-generated signal) and amplitude of 71.4 mV rms.

Fig. 4

Electronic scheme of the PLL stage.


3.2. Automatic gain control Block diagram of the AGC as a closed-loop feedback system is shown in Fig. 5. The AGC system consists of a controllable gain element, a detector, a stable reference and a comparison circuit. The input signal is amplified by a variable gain amplifier (VGA), whose gain is controlled by an external signal, i.e., control voltage VC . Some of the input signal parameters, such as amplitude, carrier frequency, index of modulation or frequency, are sensed by the detector; any undesired component is filtered out and the remaining signal is compared with a reference signal, VR . The result of the comparison is used to generate the control voltage and adjust the gain of the VGA [15]. The electronic scheme of the AGC is shown in Fig. 6. The PLL output is driven to the input of the AD8336 integrated, which corresponds to VGA. The detector is implemented using an AD736 rms-to-dc converter and it provides an accurate dc control voltage directly proportional to the rms value of the optical output sensor. The output of the AD736 is driven to the analog to digital conversion input of the ATmega88 microcontroller (an 8-bit microcontroller with 20MHz CPU). The micro controller acquires the dc signal with a resolution of 1024 bits and a reference voltage of 1.1V, i.e., 1.07mV per bit. The comparison stage was implemented by software into the microcontroller. The reference voltage was also established by software and it corresponds to a threshold value rms for the optical output, which has been previously established. In this stage, the voltage signal to be compared is the output voltage from the rms–to-dc converter. When the comparison input exceeds the reference VR, the comparer output increases to the control voltage, VC . For this we used the microcontroller digital outputs connected to a digital to analog converter of 10-bit (AD7533 integrated), i.e., the resolution of VC is the order of 1.5mV per bit. The AD7533 analogous output is driven directly to the gain control interface input of the AD8336 AGC integrated. Thus, the control voltage is stepped from 700mV to 1.6 V producing a change on the AGC output from 0 to 1V rms. Finally the output of the AD8336 integrated is driven to the amplification stage and their output is driven into the coil.

Fig. 5

VGA block diagram.


Fig. 6

Electronic schemes of the AGC.

3.3 Signal conditioning In this stage, the output from the AGC is filtered to remove the high frequency components and keep only the fundamental frequency using a third order filter (see Fig. 7). This filter produces attenuation and a phase shift in the input signal, for this reason the output signal is pre-amplified. Then pre-amplification output is connected to a delay time circuit, which allows us to adjust the phase difference π radians between photo-generated signal and the filter output. The delay time had been implemented with a circuit amplifier (inverting configuration) using the TL084, an Op-Amp. Thus, the AGC output is 180 degrees out of phase with respect to the optical signal. Finally, we used a power amplifier to adequate the AGC output and to use it as feedback current i2 (t ) . The power amplifier output is driven into the coil.

Fig. 7

Electronics scheme for amplification and conditioning stage.


4.

EXPERIMENTAL RESULTS

The experimental arrangement is shown schematically in Fig. 1. The light source was a polarized HeNe laser (λ = 632.8 nm, 2mW), the sensor head was a 50-mm-long rod of terbium gallium garnet (TGG) with a Verdet constant of 65.59 rad T −1m −1 and the angle subtended by the rod at the center of conductor was 125 degrees. The current i1 ( t ) that flows through the electric conductor was generated by a current transformer—not shown in Fig. 1—connected to the electric line. In Mexico the electric line has a fundamental frequency of f0= 60 Hz. The Faraday modulator consisted of a piece of Faraday material inserted inside a solenoid through which passes the feedback current i2 ( t ) , The Faraday material was a 30-mm-long rod of M-18 material (provided by Kigre, Inc.) with a Verdet constant of 133.3 rad T −1m −1 . The coil assembly was produced from a coil former (ID 15 mm, OD 62 mm, length 100 mm, nylon) onto which was wrapped 2000 turns of enameled copper wire (1.02 mm diameter) arranged in several layers. The value of the geometrical parameter, K was of the order of 5.6 × 10−6 m −1 . In our experimental arrangement, the laser beam passes through a magneto-optical material and the beam emerging from the output end of the sensor head travels in free space, passing through the Faraday modulator and a linear polarizer and finally it arrives to the photodetector (Model PDA36A, Thorlabs). The output is the signal detected by the photodiode with an intensity-to-voltage responsivity. The electrical signal is connected to the control system, which was described in Section 3. Then, the control system generates and amplifies the feedback current, and it is driven into the coil. The voltage develops across a resistor (connected to the coil, in series) is used as a test point to gauge the current flowing through the coil. Figure 8 shows the operation principle of the sensor current. In Fig. 8(a), in CH-1 of oscilloscope one has the signal acquired by the photoderector, which is modulated by a current of 140 A rms, carried by the conductor. While, in CH-2 is monitored the feedback current applied to the Faraday modulator. In Figs. 8 (b) through (c) we observed that the control system generates and amplifies a feedback current tuned in frequency and shifted π radians (see CH-2) respect to photodetector signal. Moreover, we note that the feedback current increases, while the modulation of the optical signal decreases until the last one meets a constant value (dc), i.e., α 2 ( t ) = −α1 ( t ) . When this happen the feedback current is measured as voltage tension across a resistor. From the numerical evaluation of Eq. 7 we obtain the electrical current flowing through conductor. Typical experimental results are shown in Figs. 9 and 10. In Fig. 9 the symbol “*” denotes the feedback current i2 measured as voltage develops across a resistor (connected to the coil, in series). While, in Fig. 10, the symbol “*” denotes the current (A rms) retrieved from substituting the measurements of current i2 into the Eq. 7. In both figures the dashed line is best linear fitting of the experimental data. The applied electrical current was measured using the FLUKE multi-meter and clamp for scopemeter (Mod. i1000s Fluke).

5

CONCLUSIONS

A Faraday current sensor operating with a closed-loop feedback has been described and implemented. The current sensor is based on a polarimentric configuration (polarizer, element sensor and analyzer) and it consisted of a bulk Faraday sensor head (e.g., a TGG rod) and a closed-loop form of signal processing. The bulk optical glass rod configuration of head sensor overcomes many of the problems encountered with optical fiber current sensors and is much less complicated to fabricate than ring bulk sensors.


The principal advantage of the proposed sensor with respect to standard Faraday sensors is that it operates with a closedloop control; therefore it may present a higher level of stability. Also, it would be advantageous because it does not require two or more photodetectors, neither complicated algorithms for signal processing. In principle, by using this new technique, the designer could eliminate the temperature effects on the sensor element and Faraday modulator using the same optical material in both Faraday elements and adjusting the temperature between them. Additional, the presented sensor does not use a fiber sensor element neither uses optical fibers to connect the control electronics with the sensor head.

Fig. 8

Closed-loop response: CH-1 on the top and CH-2 on the bottom of the each image, respectively.


45

40

Feedback current, [mA]

35

30

25

20

15

10 50

Fig. 9

60

70

80

90 100 110 Electrical current, [A]

120

130

140

150

Experimental results show Electrical current [A] vs feedback current [mA]. 160

Current Sensor Output, [A]

140

120

100

80

60

40 50

Fig. 10

60

70

80

90 100 110 Electrical current, [A]

120

130

140

150

Experimental results show Electrical current [A] vs Current Sensor output [A].

Acknowledgment J. L. Flores expresses his gratitude to “Programa de Sabaticas en el Extranjero,” CONACYT-Mexico, (No. project: 159889) for funding his academic stay at the Facultad de Ingeniería, UdelaR, Uruguay. H. Beltran also expresses his gratitude to CONACYT-Mexico for the Master scholarship.

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