All-fiber current transformer

Abstract

The invention relates to an all-fiber current transformer, and belongs to the technical field of current transformers. On the basis of an existing all-fiber current transformer, a disturbance measurement part is added, the disturbance measurement part comprises a second polarization-maintaining fiber loop, a second detector and a polarization-maintaining fiber reflector, and the disturbance signal measurement part sends an optical signal emitted by a light source to the polarization-maintaining fiber reflector through the second polarization-maintaining fiber loop. And the signal is reflected by the polarization-maintaining optical fiber reflector and then returns to the second detector through the second polarization-maintaining optical fiber loop, so that the signal detected by the second detector is an interference part in the signal. Therefore, the added disturbance measurement part can accurately measure the disturbance signal, and the anti-interference capability, the applicability and the reliability of the all-fiber current transformer are improved.

Description

Technical Field

[0001] The invention relates to an all-optical fiber current transformer, belonging to the technical field of current transformers. Background Art

[0002] The all-fiber current transformer is a high-precision, high-performance electro-optical phase modulation DC current measuring device that integrates protection and measurement functions. It converts the change of current into the change of light wave phase in the optical fiber, and measures the primary current passing through the sensing ring through coherent detection and digital closed-loop feedback control technology. Its performance indicators fully meet the requirements of international standards IEC 60044-8, national standards GB / T 26216.1 and GB / T 20840.14. It has the advantages of non-contact measurement, strong anti-electromagnetic interference ability, large dynamic measurement range, wide bandwidth, high response speed, easy installation, high safety, simultaneous measurement of AC and DC signals, environmental friendliness, multi-function and intelligence.

[0003] At present, all-fiber current transformers are mainly divided into open-loop active modulation type and closed-loop active modulation type. The modulation loop reliability of open-loop active products is low and the failure rate is high, while the closed-loop solution products using active electro-optical modulators have better anti-interference ability, small current measurement accuracy and faster response speed. At present, the closed-loop control technology route adopted by mainstream domestic manufacturers.

[0004] The closed-loop all-fiber current transformer uses a closed-loop feedback control technology solution based on LiNbO3 optical phase modulator to achieve high-precision sampling of primary current. The product structure diagram is shown in the figure. Figure 1 As shown. The sensing part on the high-voltage side is mainly a circular fiber ring, which is used to sense the changes in the surrounding magnetic field caused by the primary current; the acquisition part on the low-voltage side mainly includes a light source, a coupler, a polarizer, a modulator, a delay loop, a detector, and a signal processing module; the polarized light is transmitted back and forth between the high-voltage side and the low-voltage side through a long distance of polarization-maintaining optical fiber, so that the product forms a complete measurement system. In engineering applications, in order to increase the environmental adaptability of the transmission optical path, multi-core armored polarization-maintaining optical cables are usually selected.

[0005] Its working principle is as follows: the light waves emitted by the light source become linearly polarized light after passing through the coupler and polarizer, and are decomposed into two mutually orthogonal linearly polarized light beams through the 45° fusion point, and propagate along the X-axis and Y-axis of the modulator input fiber respectively. After being modulated by the modulator, they pass through the delay ring and polarization-maintaining fiber to reach the quarter-wave plate. After these two orthogonal light waves pass through the quarter-wave plate, they become left-handed and right-handed circularly polarized light respectively and enter the circle-maintaining fiber ring. Due to the magnetic field generated by the current in the flow carrier, the left-handed and right-handed circularly polarized light propagate at different speeds, thereby generating a Faraday phase difference. After the light waves are reflected by the reflector at the end of the circle-maintaining fiber, the polarization modes of the two circularly polarized light beams are interchanged, and they pass through the circle-maintaining fiber ring again and are affected by the magnetic field, which doubles the Faraday phase difference. According to Ampere's circuit theorem and the Faraday magneto-optical effect, the total phase difference φS is 4VNI, and its size is mainly determined by the verdet constant, the number of circles of the circle-maintaining fiber N, and the primary current I. The two returning light waves are restored to two linearly polarized lights after passing through the quarter-wave plate again and return along the optical fiber. After being modulated by the modulator, Sagnac effect interference occurs at the 45° fusion point of the polarizer. The interfered light reaches the detector through the coupler, and the optical signal is converted into an electrical signal and enters the signal processing circuit for processing. The interference light power P under open loop is:

[0006]

[0007] K p is the photoelectric coefficient of the photodetector, α is the total optical path loss, and P0 is the optical source output power. At this time, the interference optical signal already contains the Faraday phase information caused by the magnetic field. Under open loop, the interference optical power is insensitive to the Faraday phase difference, and the direction of the primary current cannot be identified. The closed-loop all-optical current transformer uses the LiNbO3 electro-optical phase modulator, and uses a phase bias based on a high-frequency square wave signal and a phase feedback based on a step wave to perform closed-loop control of the all-optical current transformer. The square wave modulated signal alternately introduces a modulation phase of ±π / 2 through the modulator to ensure that the system operates at the highest sensitivity. The step wave will produce a feedback phase difference between the two light waves. The Faraday phase difference caused by the current is equal in magnitude and opposite in sign, so that the total phase is controlled near the "zero current" point, achieving closed-loop operation.

[0008] In ± π Under / 2 phase bias, the detector output signal is as follows:

[0009]

[0010] The detector output optical power difference under ±π / 2 is:

[0011] ΔP=K p αP0sin(φ S+φ R )

[0012] When the closed loop is in steady state, ΔP is 0, the feedback phase As the system output, the measured primary current value I can be obtained after digital integration and proportional coefficient conversion.

[0013] Since there is only a circular fiber ring on the high-voltage side, the rest of the optical devices, circuits, power supplies and other functional modules are placed in the low-voltage side control room, and only the polarization-maintaining optical cable is used to send and return optical signals over long distances, ensuring that the primary high-voltage side is completely passive, improving the reliability of the equipment and the convenience of operation and maintenance. The high-voltage primary side does not require maintenance, the core components on the secondary side operate reliably, and the equipment failure rate is low.

[0014] Existing closed-loop all-fiber current transformers require polarization-maintaining optical fibers to transmit linearly polarized light signals from the acquisition part to the current sensing optical fiber ring. Especially in actual engineering applications, the acquisition part is arranged in the control room, and the sensing ring is located in the outdoor high-voltage field. The distance between them is relatively far, and the polarization-maintaining optical cable used is usually up to 300 meters or more. The ability of longer polarization-maintaining optical fibers to maintain linear polarization states is more susceptible to factors such as changes in ambient temperature, external stress, and vibration, thereby reducing the accuracy and stability of current measurement by all-fiber current transformers, especially vibration interference. In engineering applications in recent years, there have been more than one case where the polarization-maintaining optical cable was vibrated, causing abnormal measurement of all-fiber current transformers. The physical mechanism by which vibration affects the demodulation output of all-fiber current transformers is due to the time non-reciprocity of the optical path.

[0015] When vibration and sound waves act on the optical fiber, the refractive index of the optical fiber changes due to the elastic-optical effect of the optical fiber, which is affected by mechanical stress and causes the phase of the transmitted optical signal to change. This change changes with time, and introduces a non-reciprocal optical phase difference between the two optical signals participating in the interference in the optical path. This optical phase difference, like the Faraday magneto-optical phase shift angle, will also change the interference light intensity and cannot be distinguished, thus causing errors in the demodulation output of the all-fiber current transformer. In addition to vibration, factors such as ambient temperature changes and stress changes have similar impact mechanisms on all-fiber current transformers. The impacts of these factors cannot be identified and eliminated from the measurement signal, which in turn affects the final measurement accuracy. Summary of the invention

[0016] The purpose of the present invention is to provide an all-fiber current transformer to solve the problem that the current all-fiber current transformer cannot identify disturbance signals from measurement signals, thereby affecting the measurement accuracy.

[0017] The present invention provides an all-optical current transformer to solve the above technical problems. The present invention comprises a current measuring part, which comprises a first polarization-maintaining optical fiber loop, a first detector, a light source and a signal processing module. The present invention also comprises a disturbance measuring part, which comprises a second polarization-maintaining optical fiber loop, a second detector and a polarization-maintaining optical fiber reflector. The second polarization-maintaining optical fiber loop has the same structure as the first polarization-maintaining optical fiber loop, and comprises a polarizer, a modulator, a delay ring and a polarization-maintaining optical fiber. The light source sequentially passes through the polarizer, the modulator, the delay ring and the polarization-maintaining optical fiber in the second polarization-maintaining optical fiber loop to reach the polarization-maintaining optical fiber reflector. The optical signal reflected by the polarization-maintaining optical fiber reflector sequentially passes through the polarization-maintaining optical fiber, the modulator and the polarizer in the second polarization-maintaining optical fiber loop to reach the second detector. The second detector is used to convert the optical signal into an electrical signal and send it to the signal processing module. The signal processing module is used to demodulate the electrical signals received from the first detector and the second detector. The obtained signals are the current measurement result and the disturbance signal respectively.

[0018] Furthermore, the delay loop in the second polarization-maintaining optical fiber loop has the same optical path delay as the delay loop in the first polarization-maintaining optical fiber loop.

[0019] Furthermore, the optical path delay of the delay loop in the second polarization-maintaining optical fiber loop is smaller than that of the delay loop in the first polarization-maintaining optical fiber loop.

[0020] Furthermore, a coupler is included. The coupler is arranged at the light source and is used to split the optical signal emitted by the light source into two optical signals with the same power and inject them into the first polarization-maintaining optical fiber loop and the second polarization-maintaining optical fiber loop respectively.

[0021] Furthermore, corresponding circulators are provided in the first polarization-maintaining fiber loop and the second polarization-maintaining fiber loop. The circulator in the first polarization-maintaining fiber loop is used to transmit the optical signal returned by the first polarization-maintaining fiber loop to the first detector, and the circulator in the second polarization-maintaining fiber loop is used to transmit the optical signal returned by the second polarization-maintaining fiber loop to the second detector.

[0022] Furthermore, the signal processing module is also used to generate a step wave signal according to the demodulation result, and after superimposing the generated step wave signal with the modulated square wave, it is applied to the modulator in each polarization-maintaining fiber loop to modulate and close-loop control each polarization-maintaining fiber loop.

[0023] Furthermore, the signal processing module is also used to use the obtained disturbance signal to correct the current measurement result, and the disturbance signal is subtracted from the current measurement result, and the result obtained is the final current measurement result.

[0024] Furthermore, the signal processing module is also used to issue an abnormal alarm based on the obtained disturbance signal. When the signal processing module detects that the disturbance signal is greater than a set threshold, the signal processing module performs an alarm process.

[0025] Furthermore, the light source is a SLD high-polarization light source.

[0026] Furthermore, the disturbance signal obtained by the signal processing module is:

[0027]

[0028] in is the feedback phase applied by the second modulator to the second polarization-maintaining fiber loop, is the interference phase introduced by the external disturbance to the second polarization-maintaining fiber, and V is the Verdet constant.

[0029] The beneficial effects of the present invention are as follows: as an improved invention, the present invention adds a disturbance measurement part on the basis of the existing all-optical current transformer, wherein the disturbance measurement part includes a second polarization-maintaining optical fiber loop, a second detector and a polarization-maintaining optical fiber reflector, and the disturbance signal measurement part sends the light signal emitted by the light source to the polarization-maintaining optical fiber reflector through the second polarization-maintaining optical fiber loop, and the light signal is reflected by the polarization-maintaining optical fiber reflector and then returned to the second detector through the second polarization-maintaining optical fiber loop, so that the signal detected by the second detector is the interference part; at the same time, the second polarization-maintaining optical fiber loop has the same structure as the first polarization-maintaining optical fiber loop of the current measurement part, and the modulator of the polarization-maintaining optical fiber loop is used to introduce a modulation phase of ±π / 2, so that the total phase is controlled near the "zero current" point, thereby improving the measurement accuracy. The disturbance measurement part added by the present invention can accurately measure the disturbance signal, and improve the anti-interference ability, applicability and reliability of the all-optical current transformer. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a structural schematic diagram of an existing all-optical fiber current transformer;

[0031] Figure 2 It is a structural schematic diagram of the all-optical fiber current transformer of the present invention. DETAILED DESCRIPTION

[0032] The specific implementation of the present invention will be further described below in conjunction with the accompanying drawings.

[0033] Embodiment of all-fiber current transformer

[0034] The present invention adds a disturbance measurement part on the basis of the existing all-optical fiber current transformer, wherein the disturbance measurement part includes a second polarization-maintaining fiber loop, a second detector and a polarization-maintaining fiber reflector, and the disturbance signal measurement part sends the light signal emitted by the light source to the polarization-maintaining fiber reflector through the second polarization-maintaining fiber loop, and the light signal is reflected by the polarization-maintaining fiber reflector and then returns to the second detector through the second polarization-maintaining fiber loop, so that the signal detected by the second detector is the interference part; at the same time, the second polarization-maintaining fiber loop has the same structure as the first polarization-maintaining fiber loop of the current measurement part, and a modulation phase of ±π / 2 is introduced by using a modulator of the polarization-maintaining fiber loop, so that the total phase is controlled near the "zero current" point, thereby improving the measurement accuracy.

[0035] Specifically, the all-optical current transformer of the present invention includes a current measurement part and a disturbance measurement part, such as Figure 2 As shown, the current measurement part includes a circular-maintaining optical fiber ring, a first polarization-maintaining optical fiber loop, and a first detector (for Figure 2 The detector 1), the light source and the signal processing module, wherein the first polarization-maintaining fiber loop includes a first circulator (for Figure 2 Circulator 1), the first polarizer (for Figure 2 The polarizer 1 in the first modulator (for Figure 2 Modulator 1 in the first delay loop (for Figure 2 The delay loop 1) and the first polarization-maintaining fiber ( Figure 2 The disturbance measurement part includes a second polarization-maintaining fiber loop, a second detector (for Figure 2 The detector 2) and the polarization-maintaining fiber reflector are used to return the optical signal of the second polarization-maintaining fiber loop along the original optical path; the second polarization-maintaining fiber loop adopts the same structure as the first polarization-maintaining fiber loop, and the second polarization-maintaining fiber loop includes a second circulator (for Figure 2 Circulator 2 in the middle), the second polarizer (for Figure 2 The polarizer 2 in the second modulator (for Figure 2 Modulator 2 in the second delay loop (for Figure 2 The delay loop 2 in the middle and the second polarization-maintaining fiber ( Figure 2 The second detector collects the optical signal reflected by the polarization-maintaining optical fiber reflector through the second polarization-maintaining optical fiber loop. The first detector and the second detector are both connected to the signal processing module through the corresponding AD module.

[0036] The light source used in the present invention is an SLD high-polarization light source, the pigtail is a polarization-maintaining optical fiber, and what is output is linearly polarized light with a high extinction ratio. The first polarization-maintaining optical fiber loop and the second polarization-maintaining optical fiber loop share the SLD light source, so as to ensure that the wavelength, noise and other transmission characteristics of the optical signals of the two optical paths are consistent and eliminate the deviation; the optical signal emitted by the SLD light source is divided into two optical signals with the same power through a coupler and injected into the first polarization-maintaining optical fiber loop and the second polarization-maintaining optical fiber loop. The coupler is a polarization-maintaining optical fiber beam splitter, and the splitting ratio is 50:50.

[0037] The first polarization-maintaining fiber loop and the second polarization-maintaining fiber loop are both provided with corresponding circulators, which can output the returned optical signal to the corresponding detector along the third port to avoid directly returning to the light source. The circulator in the first polarization-maintaining fiber loop is used to transmit the optical signal returned by the first polarization-maintaining fiber loop to the first detector, and the circulator in the second polarization-maintaining fiber loop is used to transmit the optical signal returned by the second polarization-maintaining fiber loop to the second detector.

[0038] The delay ring set in each polarization-maintaining fiber loop is a polarization-maintaining fiber delay ring, which is wound by polarization-maintaining fiber, and is mainly used to increase the delay of the optical signal and control the return time of the optical signal. The length of the polarization-maintaining fiber of the first delay ring and the second delay ring in this embodiment can be set to be the same or different as needed; if it is to ensure that the external disturbance signal returns before the measurement signal, the optical path delay T1 (corresponding to the first delay ring) generated by the first polarization-maintaining fiber loop can be greater than the optical path delay T2 (corresponding to the second delay ring) generated by the second polarization-maintaining fiber loop. If the interference value output by the second polarization-maintaining fiber is used to correct the measured value output by the first polarization-maintaining fiber loop, it is necessary to select delay rings of the same specification, that is, the length of the polarization-maintaining fiber of the first delay ring and the second delay ring is the same, and the optical path delay T1 is the same as T2; the delay of the two delay rings is the same to ensure that the values ​​measured by the two measurement optical paths are at the same time, that is, simultaneity, because the vibration acts on the two polarization-maintaining fibers at the same time. If the interference value output by the second polarization-maintaining fiber is used as an alarm for external interference, the optical path delay T2 must be less than T1, that is, the length of the polarization-maintaining fiber in the second delay loop must be less than the length of the polarization-maintaining fiber in the first delay loop; if a vibration warning is to be made, the vibration value must be measured before the measured value of the first polarization-maintaining fiber loop is abnormal, so the delay of the various delay loops in the second polarization-maintaining fiber loop must be less than that of the first polarization-maintaining fiber loop, so that the signal of the second polarization-maintaining fiber loop returns to the acquisition unit (the second detector) first, which can serve as a vibration alarm.

[0039] The first polarization-maintaining optical fiber and the second polarization-maintaining optical fiber are arranged in the same polarization-maintaining optical cable, ensuring that the second polarization-maintaining optical fiber for measuring external disturbance signals is in the same working environment as the first polarization-maintaining optical fiber for transmitting current signals, and is affected by ambient temperature and vibration to a similar degree, further improving the measurement accuracy. In the present invention, optical devices such as couplers, circulators, polarizers, and delay rings are all polarization-maintaining optical devices, and the pigtail is a polarization-maintaining optical fiber, which is fused by a polarization-maintaining optical fiber fusion splicer to form a complete optical fiber loop. The detector pigtail is a single-mode optical fiber, which is fused with the pigtail of the circulator response port by ordinary optical fiber.

[0040] The connections between each detector and the corresponding AD module, and between the light source and the signal processing module are all electrical pin connections. The signal processing module can drive and control the temperature of the light source. The first detector converts the optical signal returned by the first polarization-maintaining fiber loop into an electrical signal, which is converted into a digital signal by the first AD and given to the signal processing module; the second detector converts the optical signal returned by the second polarization-maintaining fiber loop into an electrical signal, which is converted into a digital signal by the second AD and given to the signal processing module.

[0041] The signal processing module is used to demodulate the digital quantity of the optical signal returned by the first and second polarization-maintaining fiber loops, output the current measurement value D1 and the external disturbance value D2, and generate step wave signals R1 and R2 according to the output results. R1 is superimposed with the modulated square wave M1 and applied to the first modulator through DA1 (digital-to-analog conversion) to modulate and close the first polarization-maintaining fiber loop. R2 is superimposed with the modulated square wave M2 and applied to the second modulator through DA2 to modulate and close the second polarization-maintaining fiber loop. The signal processing module can also use the external disturbance value D2 to correct the current measurement value D1. The correction method is: D1-D2, and D1-D2 is recorded as the final current measurement value. When the external disturbance value D2 is too large, it means that the current environmental interference is relatively large. The signal processing module can alarm to remind the maintenance personnel to repair and handle it in time.

[0042] The measuring principle of the current measuring part is the same as that of the existing all-fiber current transformer, which will not be described in detail here. The working principle of the disturbance signal measuring part is similar to that of the current measuring part, but not exactly the same. The working principle of the disturbance signal measuring part is described below.

[0043] After the light waves emitted by the light source are split by the coupler, a part of them is injected into the second polarizer through the second circulator and becomes linear polarized light with a high extinction ratio. After the 45° fusion point, they are decomposed into two mutually orthogonal linear polarized lights, and propagate along the X-axis and Y-axis of the second modulator input optical fiber respectively. After being modulated by the second modulator, they pass through the second delay ring and the second polarization-maintaining optical fiber to reach the polarization-maintaining optical fiber reflector; the polarization-maintaining optical fiber reflector reflects the optical signal and returns along the original path. The two returning light waves pass through the second polarization-maintaining optical fiber again, and after being modulated by the second modulator, Sagnac effect interference occurs at the 45° fusion point between the second polarizer and the second modulator. The interfered light reaches the second detector through the second circulator, and the second detector converts the optical signal into an electrical signal and enters the signal processing module for processing. Since the high-voltage side of the second polarization-maintaining optical fiber loop is a polarization-maintaining optical fiber reflector and there is no circular fiber ring, the phase difference generated in the loop is only the phase difference generated by the disturbance of the second polarization-maintaining optical fiber. When there is no disturbance, the output value of the second polarization-maintaining optical fiber loop is zero.

[0044] The return optical power of the first polarization-maintaining fiber loop under modulation is:

[0045]

[0046] The return optical power of the second polarization-maintaining fiber loop under modulation is:

[0047]

[0048] K p1 is the photoelectric coefficient of the first detector, K p2 is the photoelectric coefficient of the second detector, α is the total optical path loss, P0 is the optical source output power, is the Faraday phase difference generated by the primary current, is the feedback phase applied by the first modulator to the first polarization-maintaining fiber loop to compensate for the phase difference introduced by the Faraday phase and the interference. is the feedback phase applied by the second modulator to the second polarization-maintaining fiber loop, is the interference phase of the first polarization-maintaining fiber caused by external disturbances, and is also the interference phase introduced by the second polarization-maintaining fiber. When the closed loop is stable, the feedback phase The parameter V is the Verdet constant, which is the abbreviation of verdet. It describes the relationship between the magnitude of the Faraday magneto-optical effect and the primary current and the number of fiber turns; N is the number of fiber optic sensing turns, and I is the detected current.

[0049] The output value of the first polarization-maintaining fiber loop is:

[0050]

[0051] Feedback Phase is the result of system demodulation, which needs to be multiplied by a coefficient as the measured value output. Let this coefficient be K and the output digital value be D. Then

[0052]

[0053] The coefficient K can be calibrated to -1 / 4VN through accuracy test, -4KVN = 1, and we can get

[0054]

[0055] The output value of the second polarization-maintaining fiber loop is:

[0056]

[0057]

[0058] K = -1 / 4VN, is the current coefficient, which can be calibrated when the transformer leaves the factory. The first polarization-maintaining fiber and the second polarization-maintaining fiber are arranged in the same polarization-maintaining optical cable, and the introduced interference phase is Then D1-D2=I. The two can be compared and output to improve system reliability.

[0059] The method of calibrating the current coefficient K is to use is zero,

[0060]

[0061] Apply a primary current I of a certain magnitude, initialize the K value (for example, set it to 1 first) and record the system output value D1. At this time, D1=-4VNI. Adjust the size of K so that D1=I, and the test coefficient K is -1 / 4VN.

[0062] The disturbance signal measurement part provided in the present invention can measure the environmental disturbance of the long-distance polarization-maintaining optical fiber of the existing transformer, increase the product function, improve the anti-interference ability, applicability and reliability of the equipment, and solve the problem that the current closed-loop all-optical fiber current transformer measurement performance is affected by the vibration-sensitive characteristics of the long-distance polarization-maintaining optical cable. The disturbance signal measurement part eliminates the interference part in the primary current measurement value of the first polarization-maintaining optical fiber loop through the measurement value of the second polarization-maintaining optical fiber loop and the polarization-maintaining optical fiber reflector, further reduces the influence of external interference factors on current measurement, and improves the measurement performance and stability of the equipment. The disturbance signal measurement part also realizes real-time monitoring of external disturbances (especially high-frequency vibrations), which is conducive to supporting the cause analysis and fault location of abnormalities of other vibration-sensitive equipment in the converter station.

[0063] Therefore, the present invention can use a second polarization-maintaining optical fiber loop with the same structure as the first polarization-maintaining optical fiber loop and a polarization-maintaining optical fiber reflector to measure the disturbance signal introduced by the external disturbance, so as to correct the measurement value of the high-voltage primary current by the first polarization-maintaining optical fiber loop, reduce the influence of external interference on the closed-loop all-optical fiber current transformer, avoid major faults such as false protection caused by measurement abnormalities, improve the operating stability and reliability of the converter station, avoid the huge economic losses caused by power outage maintenance, and improve the reliability and applicability of the equipment.

Claims

1. An all-optical current transformer, comprising a current measuring part, the current measuring part comprising a first polarization-maintaining optical fiber loop, a first detector, a light source and a signal processing module, characterized in that: The invention also comprises a disturbance measurement part, wherein the disturbance measurement part comprises a second polarization-maintaining fiber loop, a second detector and a polarization-maintaining fiber reflector. The second polarization-maintaining fiber loop has the same structure as the first polarization-maintaining fiber loop, and comprises a polarizer, a modulator, a delay ring and a polarization-maintaining fiber. The light source sequentially passes through the polarizer, the modulator, the delay ring and the polarization-maintaining fiber in the second polarization-maintaining fiber loop to reach the polarization-maintaining fiber reflector. The light signal reflected by the polarization-maintaining fiber reflector sequentially passes through the polarization-maintaining fiber, the modulator and the polarizer in the second polarization-maintaining fiber loop to reach the second detector. The second detector is used to convert the optical signal into an electrical signal and send it to the signal processing module. The signal processing module is used to demodulate the electrical signals received from the first detector and the second detector. The obtained signals are respectively the current measurement result and the disturbance signal.

2. The all-optical current transformer according to claim 1, characterized in that: The delay loop in the second polarization-maintaining optical fiber loop has the same optical path delay as the delay loop in the first polarization-maintaining optical fiber loop.

3. The all-optical fiber current transformer according to claim 1, characterized in that: The optical path delay of the delay loop in the second polarization-maintaining optical fiber loop is smaller than that of the delay loop in the first polarization-maintaining optical fiber loop.

4. The all-optical fiber current transformer according to claim 1, characterized in that: A coupler is also included. The coupler is arranged at the light source and is used to divide the optical signal emitted by the light source into two optical signals with the same power and inject them into the first polarization-maintaining optical fiber loop and the second polarization-maintaining optical fiber loop respectively.

5. The all-optical fiber current transformer according to claim 1, characterized in that: Corresponding circulators are provided in the first polarization-maintaining optical fiber loop and the second polarization-maintaining optical fiber loop. The circulator in the first polarization-maintaining optical fiber loop is used to transmit the optical signal returned by the first polarization-maintaining optical fiber loop to the first detector, and the circulator in the second polarization-maintaining optical fiber loop is used to transmit the optical signal returned by the second polarization-maintaining optical fiber loop to the second detector.

6. The all-optical fiber current transformer according to claim 1, characterized in that: The signal processing module is also used to generate a step wave signal according to the demodulation result, and after superimposing the generated step wave signal with the modulated square wave, it is applied to the modulator in each polarization-maintaining optical fiber loop to modulate and close-loop control each polarization-maintaining optical fiber loop.

7. The all-optical current transformer according to claim 2, characterized in that: The signal processing module is also used to use the obtained disturbance signal to correct the current measurement result, and the disturbance signal is subtracted from the current measurement result, and the result obtained is the final current measurement result.

8. The all-optical fiber current transformer according to claim 3, characterized in that: The signal processing module is also used to issue an abnormal alarm based on the obtained disturbance signal. When the signal processing module detects that the disturbance signal is greater than a set threshold, the signal processing module performs an alarm process.

9. The all-optical fiber current transformer according to claim 1, characterized in that: The light source is a SLD high-polarization light source.

10. The all-optical fiber current transformer according to claim 1, characterized in that: The disturbance signal obtained by the signal processing module is: in is the feedback phase applied by the second modulator to the second polarization-maintaining fiber loop, is the interference phase introduced by the external disturbance to the second polarization-maintaining fiber, and V is the Verdet constant.

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