Modulation and demodulation all-optical current transformer and method

Abstract

The present application proposes a modulation and demodulation all-fiber current transformer and method. The all-fiber current transformer includes a broadband light source, a coupler, a fiber polarizer, a phase modulator, a delay fiber, a λ / 4 wave plate, a sensing fiber, a reflector, a detector, an ADC circuit, a signal processing unit and a DAC circuit. The fiber polarizer polarizes the light into linear polarized light; the input fiber of the phase modulator is at an angle of 45° with the fiber polarizer; the λ / 4 wave plate converts the linear polarized light into circular polarized light, and the fast axis of the λ / 4 wave plate is at an angle of 45° with the fast axis of the delay fiber; the reflector causes the circular polarized light to produce a phase jump; the detector is connected to the coupler fiber and converts the return light into an electrical signal; the ADC circuit performs analog-to-digital conversion; the signal processing unit demodulates the received digital signal to the measured current information, calculates the phase required by the phase modulator according to the demodulated measured current information, and outputs a square wave modulation signal; the DAC circuit performs digital-to-analog conversion on the digital signal and inputs it into the phase modulator.

Description

Technical Field

[0001] The present application relates to the field of optical fiber sensing technology, and in particular to a modulation and demodulation all-optical fiber current transformer and method. Background Art

[0002] In the power system, the mutual inductor is the source of protection and measurement and control. Its safety and reliability are of great significance to the stable operation of the power system. In recent years, the fiber optic current transformer FOCT has attracted widespread attention in the power industry due to its safety, no magnetic saturation, and large dynamic range. In particular, the sensor ring on the primary side of the FOCT system is easy to install and maintain. FOCT is composed of pure optical fiber without any electronic components, so it has strong anti-aging ability and is not affected by electromagnetic interference.

[0003] FOCT is a polarization interferometer based on the Faraday magneto-optical effect. When polarized light propagates in an optical fiber, it is affected by a closed magnetic field and its polarization state changes. Thus, the magnitude of the current in the primary conductor wrapped by the optical fiber can be measured by measuring the change in the polarization state.

[0004] FOCT is mainly composed of broadband light source, polarizer, phase modulator, delay fiber, sensor ring and photodetector. Among them, the modulation and demodulation algorithm is an important factor affecting the performance of fiber optic current transformer.

[0005] At present, the modulation and demodulation algorithms used in fiber optic current transformers mainly adopt two-state square wave modulation, two-state step wave modulation, four-state square wave modulation and other algorithms. The two-state square wave modulation amplitude is ±π / 2, the modulation period is the intrinsic transit time of the interference optical path of the fiber optic current transformer, and the modulation is repeated alternately.

[0006] However, conventional demodulation methods cannot adapt to the large current step response characteristics, especially the flexible DC transmission system. When a fault occurs, due to the small system loop impedance, it is easy to generate a large fault current in a short time.

[0007] Therefore, based on the need to improve the response speed of the flexible DC transmission system and its ability to suppress fault currents and enhance the safety and stability of the power system, the control and protection signal needs to have a faster sampling speed and a wider bandwidth, which puts higher requirements on the transient performance of the DC current transformer used in flexible DC transmission.

[0008] The step response characteristic is one of the important technical indicators for evaluating the transient performance of a DC current transformer. The transient performance and anti-interference ability of the step response characteristics of traditional DC engineering are poor. Although the closed-loop feedback in the traditional solution assumes that the measured current is small enough to meet the sin(4VNI) approximation condition, when the short-term current fluctuation is large, such an approximation condition fails, resulting in abnormal current demodulation. In addition, due to problems such as attenuation and power fluctuations in broadband lasers, the optical power received by the detector will also be abnormal, which will be misjudged as a phase change, resulting in abnormal demodulation current value. The industry has not yet proposed an effective solution to the problems of traditional fiber-optic current transformers.

[0009] The above information disclosed in the Background section is only for enhancement of understanding of the background of the present application and therefore it may contain information that does not constitute the prior art that is already known to a person of ordinary skill in the art. Summary of the invention

[0010] The present application proposes a modulation and demodulation all-fiber current transformer and method, which realizes the rapid tracking and measurement of wide-range current of pure optical CT through the mutual cooperation of multiple modulation states, thereby improving the measurement accuracy of the system.

[0011] According to one aspect of the present application, a modulation and demodulation all-fiber current transformer is proposed, characterized in that the all-fiber current transformer includes a broadband light source, a fiber connector, a fiber polarizer, a phase modulator, a delay fiber, a λ / 4 wave plate, a sensing fiber, a reflector, a detector, an ADC circuit, a signal processing unit and a DAC circuit, wherein:

[0012] The broadband light source outputs light;

[0013] The optical fiber connector is connected to the broadband light source optical fiber;

[0014] The optical fiber polarizer is connected to the optical fiber of the optical fiber connector to polarize light into linearly polarized light;

[0015] The phase modulator modulates the phase of the linearly polarized light;

[0016] The time-delay optical fiber is connected to the phase modulator optical fiber;

[0017] The λ / 4 wave plate converts the linearly polarized light into circularly polarized light;

[0018] The sensing optical fiber is connected to the λ / 4 wave plate optical fiber;

[0019] The reflector is connected to the sensing optical fiber to cause the circularly polarized light to produce a phase jump;

[0020] The detector is connected to the optical fiber connector and converts the return light into an electrical signal;

[0021] The ADC circuit is connected to the detector to perform analog-to-digital conversion;

[0022] The signal processing unit is connected to the ADC circuit, demodulates the measured current information of the received digital signal, calculates the phase required by the phase modulator according to the demodulated measured current information, and outputs a square wave modulation signal;

[0023] The DAC circuit is connected to the signal processing unit, performs digital-to-analog conversion on the square wave modulated signal, and inputs the signal into the phase modulator, so that a non-reciprocal π / 2 phase bias is introduced into the delay optical fiber, and a feedback compensation phase shift is introduced into the delay optical fiber to achieve closed-loop feedback.

[0024] According to some embodiments, at a first transit time, the signal processing unit outputs a first square wave signal so that the phase modulator generates a -π / 2 phase offset.

[0025] According to some embodiments, at the second transit time, the signal processing unit outputs a first square wave signal so that the phase modulator generates a zero phase offset.

[0026] According to some embodiments, at the third transit time, the signal processing unit outputs a first square wave signal so that the phase modulator generates a π / 2 phase offset.

[0027] According to some embodiments, at the fourth transit time, the signal processing unit outputs a first square wave signal so that the phase modulator generates a zero phase offset.

[0028] According to some embodiments, the linearly polarized light is divided into two beams of linearly polarized light with perpendicular polarization directions after passing through the optical fiber polarizer and the phase modulator at an angle of 45°, and vibrates along the fast axis and the slow axis of the delay optical fiber.

[0029] According to some embodiments, the fast axis of the λ / 4 wave plate forms an angle of 45° with the fast axis of the delay optical fiber;

[0030] The λ / 4 wave plate causes the phase of light propagating along its fast and slow axes to deviate by 90°;

[0031] Two beams of linearly polarized light with perpendicular polarization directions are transformed into left-handed circularly polarized light and right-handed circularly polarized light after passing through the λ / 4 wave plate.

[0032] According to another aspect of the present application, a method for modulating and demodulating all-fiber current is provided, which is used for an all-fiber current transformer, wherein the all-fiber current transformer comprises a broadband light source, an optical fiber connector, an optical fiber polarizer, a phase modulator, a delay optical fiber, a λ / 4 wave plate, a sensing optical fiber, a reflector, a detector, an ADC circuit, a signal processing unit and a DAC circuit, wherein the method comprises:

[0033] At the first transit time, the phase modulator generates a -π / 2 phase bias, and the detector obtains a negative semi-axis light intensity value;

[0034] At the second transit time, the phase modulator generates a 0 phase bias, and the detector obtains an unmodulated light intensity value;

[0035] At the third transit time, the phase modulator generates a +π / 2 phase bias, and the detector obtains a positive semi-axis light intensity value;

[0036] At the fourth transit time, the phase modulator generates a 0 phase bias, and the detector obtains an unmodulated light intensity value;

[0037] The transit time is to complete one current measurement and closed-loop control process.

[0038] According to some embodiments, if the current changes slowly, the original light intensity value is obtained according to the following formula:

[0039]

[0040] Among them, V is the Field constant, N is the number of sensor loops, I is the measured current, P1 is the negative semi-axis light intensity value obtained during the first transit time; P3 is the positive semi-axis light intensity value obtained during the third transit time; P0 is the original light intensity value.

[0041] According to some embodiments, if the current changes steeply, the original light intensity value is obtained according to the following formula:

[0042]

[0043] Among them, P2 is the unmodulated light intensity value obtained during the second transit time.

[0044] It should be understood that the foregoing general description and the following detailed description are exemplary only and are not restrictive of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] By describing in detail exemplary embodiments thereof with reference to the accompanying drawings, the above and other objects, features and advantages of the present application will become more apparent. The accompanying drawings described below are only some embodiments of the present application, and are not intended to limit the present application.

[0046] Figure 1 An exemplary closed-loop demodulation all-fiber current transformer embodiment is shown;

[0047] Figure 2 A schematic diagram of a transit time period of an exemplary embodiment is shown. DETAILED DESCRIPTION

[0048] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this application will be comprehensive and complete and will fully convey the concepts of the example embodiments to those skilled in the art. The same reference numerals in the figures represent the same or similar parts, and thus their repeated description will be omitted.

[0049] The described features, structures or characteristics may be combined in one or more embodiments in any suitable manner. In the following description, many specific details are provided to provide a full understanding of the embodiments of the present disclosure. However, those skilled in the art will appreciate that the technical solutions of the present disclosure may be practiced without one or more of these specific details, or other modes, components, materials, devices or the like may be adopted. In these cases, known structures, methods, devices, implementations, materials or operations will not be shown or described in detail.

[0050] The flowcharts shown in the accompanying drawings are only exemplary and do not necessarily include all the contents and operations / steps, nor must they be executed in the order described. For example, some operations / steps can be decomposed, and some operations / steps can be combined or partially combined, so the actual execution order may change according to actual conditions.

[0051] The terms "first", "second", etc. in the specification and claims of this application and the above-mentioned drawings are used to distinguish different objects, rather than to describe a specific order. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but optionally includes steps or units that are not listed, or optionally includes other steps or units inherent to these processes, methods, products or devices.

[0052] Those skilled in the art will appreciate that the drawings are merely schematic diagrams of example embodiments, and the modules or processes in the drawings are not necessarily necessary for implementing the present application, and therefore cannot be used to limit the scope of protection of the present application.

[0053] The following describes an apparatus embodiment of the present application, which can be used to execute the method embodiment of the present application. For details not disclosed in the apparatus embodiment of the present application, reference can be made to the method embodiment of the present application.

[0054] Figure 1 An exemplary embodiment of a closed-loop demodulated all-fiber current transformer is shown.

[0055] like Figure 1 As shown, the closed-loop demodulation all-fiber current transformer includes: a broadband light source 1, a fiber connector 2, a fiber polarizer 3, a phase modulator 4, a delay fiber 5, a λ / 4 wave plate 6, a sensing fiber 7, a reflector 8, a detector 9, an ADC circuit 10, a signal processing unit 11 and a DAC circuit 12.

[0056] According to some embodiments, a broadband light source 1 outputs light; a fiber optic connector 2 is connected to the broadband light source 1 by fiber optic; a fiber optic polarizer 3 is connected to the fiber optic connector 2 by fiber optic to polarize the light into linearly polarized light; a phase modulator 4 modulates the phase of the linearly polarized light; a delay fiber 5 is connected to the phase modulator 4 by fiber optic; a λ / 4 wave plate 6 is used to convert the linearly polarized light into circularly polarized light; a sensing fiber 7 is connected to the λ / 4 wave plate 6 by fiber optic; and a reflector 8 is connected to the sensing fiber 7 by fiber optic to cause a phase jump in the circularly polarized light.

[0057] According to some embodiments, the detector 9 is connected to the optical fiber of the optical fiber connector 2 to convert the returned light into an electrical signal; the ADC circuit 10 is connected to the detector 9 to perform analog-to-digital conversion; the signal processing unit 11 is connected to the ADC circuit 10 to demodulate the received digital signal into the measured current information, calculate the required phase of the phase modulator based on the demodulated measured current information, and output a square wave modulated signal; the DAC circuit 12 is connected to the signal processing unit 11 and to the phase modulator 4 to perform digital-to-analog conversion on the digital signal and input it into the phase modulator 4.

[0058] According to the exemplary embodiment, the light emitted by the broadband light source 1 is input into the optical fiber polarizer 3 through the optical fiber connector 2 to form linear polarized light. The optical fiber polarizer 3 is fused with the input optical fiber of the phase modulator 4 at a 45° angle. Due to the 45° fusion, one beam of linear polarized light becomes two beams of mutually orthogonal linear polarized light, which are respectively transmitted along the fast axis and the slow axis of the delay optical fiber 5. After being transmitted through the delay optical fiber 5, the two beams of linear polarized light reach the sensing part and are respectively converted into left-handed circularly polarized light and right-handed circularly polarized light through the λ / 4 wave plate 6. The two beams of circularly polarized light enter the sensing optical fiber 7.

[0059] According to the exemplary embodiment, the sensing optical fiber 7 is wound around the primary conductor, and a closed magnetic field is generated when a current flows through the conductor. The direction of the magnetic field is parallel to the sensing optical fiber. Under the action of the magnetic field, due to the existence of the Faraday magneto-optical effect, the two circularly polarized light beams generate a phase difference proportional to the magnitude of the measured current.

[0060] According to the exemplary embodiment, after the two circularly polarized lights reach the reflector 8 at the end of the sensing optical fiber 7, each of them produces a phase jump. The left and right circularly polarized lights are exchanged and transmitted back. When passing through the sensing optical fiber 7 again, the phase difference between the two circularly polarized lights is doubled again under the action of the Faraday magneto-optical effect.

[0061] According to the exemplary embodiment, two beams of circularly polarized light are converted back into two beams of linearly polarized light by the λ / 4 wave plate 6. The two beams of linearly polarized light are sequentially passed through the delay optical fiber 5, the phase modulator 4, and the optical fiber polarizer 3, and finally the optical signal is converted into an electrical signal by the detector 9. After conditioning, the electrical signal is transmitted to the signal processing unit 11 after analog-to-digital conversion by the ADC circuit 10, and the signal processing unit 11 demodulates the current value. The phase modulation signal value fed back to the phase modulator 4 is calculated according to the current value, and the phase modulation signal value is generated by converting the digital quantity into an analog quantity by the DAC circuit 12 and adding it to the phase modulator 4. The above process completes a current measurement and closed-loop control process.

[0062] Figure 2 A schematic diagram of a transit time period of an exemplary embodiment is shown.

[0063] like Figure 2 As shown, in the first transit time, i.e. 0-τ, the phase modulator generates a -π / 2 phase bias, and the detector obtains a negative semi-axis light intensity value P1, which is expressed as: P1 = P0 (1 + cos (-π / 2 + 4VNI)), V is the Field constant, N is the number of sensing loops, and I is the measured current. In the second transit time, i.e. τ-2τ, the phase modulator generates a 0 phase bias, and the detector obtains an unmodulated light intensity value P2, which is expressed as: P2 = P0 (1 + cos (4VNI)). In the third transit time, i.e. 2τ-3τ, the phase modulator generates a +π / 2 phase bias, and the detector obtains a positive semi-axis light intensity value P3, which is expressed as: P3 = P0 (1 + cos (+π / 2 + 4VNI)). In the last transit time, i.e. 3τ-4τ, the phase modulator generates a 0 phase bias, and the detector obtains an unmodulated light intensity value P4, which is expressed as: P4 = P0 (1 + cos (4VNI)). From the expressions of P1 and P2, we can see that:

[0064]

[0065] From this we can calculate P0, which is the original light intensity value.

[0066] According to some embodiments, the transit time is to complete one current measurement and closed-loop control process.

[0067] When the current changes slowly, for example, when the change is less than π / 8, after closed-loop feedback control, the measured current is small enough, and it is approximately considered that sin(φ)≈φ. Therefore, for the light intensity obtained during the first and third transit times, it can be approximately considered that:

[0068] P1=P0(1+cos(-π / 2+4VNI))=P0(1-sin(4VNI)), P3=P0(1+cos(+π

[0069] / 2+4VNI))=P0(1+sin(4VNI)), we can know:

[0070]

[0071] When the current changes steeply, for example, when the change is greater than π / 8; although there is a closed loop to control, the measured current does not meet the approximate condition of sin(φ)≈φ. If the traditional demodulation method is used, it is easy to cause distortion and even misjudge the phase. At this time, the cosine function is more likely to meet the approximate condition. From P2=P0(1+cos(4VNI)), it can be seen that:

[0072]

[0073] Because it can correctly demodulate both fast and slow changing currents.

[0074] The present application proposes a modulation and demodulation all-fiber current transformer and method for multiple working points, wherein each modulation cycle consists of three modulation working points, namely -π / 2, 0 and +π / 2, wherein the 0 working point is between ±π / 2. ±π / 2 modulation is responsible for converting the cosine response of a small current into a sine response to increase the detection sensitivity of a small current. Since the phase difference generated by a large current is large, it is already in a relatively sensitive state in a non-modulated state, maintaining its cosine response characteristics. Through the above measures, the demodulation system achieves full coverage and high sensitivity response to large and small currents. Further through the above processing, the system solves the problem of nonlinear response caused by too fast current changes. In addition, the present application also solves the problem of light source power fluctuations through the mutual coordination of the modulation states of multiple working points.

[0075] It should be clearly understood that the present application describes how to form and use specific examples, but the present application is not limited to any details of these examples. On the contrary, based on the teaching of the content disclosed in the present application, these principles can be applied to many other embodiments.

[0076] In addition, it should be noted that the above figures are only schematic illustrations of the processes included in the method according to the exemplary embodiments of the present application, and are not intended to be limiting. It is easy to understand that the processes shown in the above figures do not indicate or limit the time sequence of these processes. In addition, it is also easy to understand that these processes can be performed synchronously or asynchronously, for example, in multiple modules.

[0077] The exemplary embodiments of the present application are specifically shown and described above. It should be understood that the present application is not limited to the detailed structures, configurations or implementations described herein; on the contrary, the present application is intended to cover various modifications and equivalent configurations included in the spirit and scope of the appended claims.

Claims

1. A method for modulating and demodulating all-fiber current, used for an all-fiber current transformer, wherein the all-fiber current transformer comprises a broadband light source, a fiber connector, a fiber polarizer, a phase modulator, a delay fiber, a λ / 4 wave plate, a sensing fiber, a reflector, a detector, an ADC circuit, a signal processing unit and a DAC circuit, characterized in that: The broadband light source outputs light; The optical fiber connector is connected to the broadband light source optical fiber; the optical fiber polarizer is connected to the optical fiber of the optical fiber connector to polarize the light into linear polarized light; The phase modulator modulates the phase of the linearly polarized light; The time-delay optical fiber is connected to the phase modulator optical fiber; the λ / 4 wave plate converts the linear polarized light into circular polarized light; The sensing optical fiber is connected to the λ / 4 wave plate optical fiber; the reflector is connected to the sensing optical fiber to make the circularly polarized light produce a phase jump; the detector is connected to the optical fiber connector to convert the return light into an electrical signal; the ADC circuit is connected to the detector to perform analog-to-digital conversion; The signal processing unit is connected to the ADC circuit, demodulates the received digital signal into the measured current information, calculates the phase required by the phase modulator according to the demodulated measured current information, and outputs a square wave modulation signal; the DAC circuit is connected to the signal processing unit, performs digital-to-analog conversion on the square wave modulation signal, and inputs the square wave modulation signal into the phase modulator, so that a non-reciprocal π / 2 phase bias is introduced into the delay optical fiber, and a feedback compensation phase shift is introduced into the delay optical fiber to realize closed-loop feedback; the method comprises: At the first transit time, the phase modulator generates a -π / 2 phase bias, and the detector obtains a negative semi-axis light intensity value; At the second transit time, the phase modulator generates a 0 phase bias, and the detector obtains an unmodulated light intensity value; At the third transit time, the phase modulator generates a +π / 2 phase bias, and the detector obtains a positive semi-axis light intensity value; At the fourth transit time, the phase modulator generates a 0 phase bias, and the detector obtains an unmodulated light intensity value; The transit time is to complete a current measurement and closed-loop control process; If the current changes steeply, the original light intensity value is obtained according to the following formula: Wherein, V is the Field constant, N is the number of sensing loops, I is the measured current, P2 is the unmodulated light intensity value obtained during the second transit time; and P0 is the original light intensity value.

2. The method according to claim 1, characterized in that If the current changes slowly, the original light intensity value is obtained according to the following formula: Among them, P1 is the negative semi-axis light intensity value obtained during the first transit time; P3 is the positive semi-axis light intensity value obtained during the third transit time.

3. The method according to claim 1, characterized in that After passing through the optical fiber polarizer and the phase modulator at an angle of 45°, the linearly polarized light is divided into two beams of linearly polarized light with perpendicular polarization directions, and vibrates along the fast axis and the slow axis of the delay optical fiber.

4. The method according to claim 3, characterized in that The fast axis of the λ / 4 wave plate forms an angle of 45° with the fast axis of the delay optical fiber; The λ / 4 wave plate causes the phase of light propagating along its fast and slow axes to deviate by 90°; Two beams of linearly polarized light with perpendicular polarization directions are transformed into left-handed circularly polarized light and right-handed circularly polarized light after passing through the λ / 4 wave plate.

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