Optical current transformer system and modulation and demodulation method

Abstract

The application discloses an optical current transformer system and a modulation and demodulation method, and relates to the field of photoelectric instruments. The optical current transformer system comprises a light source, a coupler, a polarizer, a phase modulator for performing phase modulation on input linearly polarized light, a delay optical fiber, an optical fiber sensing ring for sensing a measured current, a photoelectric detector for performing photoelectric conversion on a received optical signal, a signal processor for generating a fixed-frequency modulation signal and inputting the fixed-frequency modulation signal into the phase modulator, and a receiving photoelectric detector for receiving an output signal of the photoelectric detector and obtaining a measured current signal according to the output signal. According to the embodiment of the application, the delay difference caused by the difference between the optical path length of the optical current transformer and the crystal oscillator of the signal processing circuit of the acquisition unit can be avoided, and the three-phase optical current transformer connected with the control protection device can be synchronously modulated, synchronously transmitted and delayed.

Description

Technical Field

[0001] This application belongs to the field of optoelectronic instruments, and specifically relates to an optical current transformer system and a modulation and demodulation method. Background Technology

[0002] Optical current transformers are characterized by their small size, light weight, simple insulation structure, absence of magnetic saturation, ferroresonance, and secondary open circuit issues, and excellent frequency and transient characteristics. The primary side is passive, providing strong anti-interference capabilities. They are safe, environmentally friendly, and easy to digitize, making them better suited to the needs of power systems.

[0003] In recent years, the all-fiber optical current transformer has been extensively researched and successfully applied in engineering. Its reciprocal reflection interferometer structure features a completely symmetrical optical path, with both polarized beams propagating in two orthogonal modes along the same fiber. Most interferences, such as vibration and temperature, are effectively suppressed due to its excellent reciprocity. Only the phase shift proportional to the current, generated in the fiber sensing loop around the primary conductor due to the Faraday magneto-optical effect, is non-reciprocal. Therefore, this type of optical current transformer can eliminate interference from environmental factors such as vibration and temperature, and detect current information effectively. Its primary end is passive, with only the fiber being passive, resulting in strong anti-interference capabilities. All-fiber optical current transformers have seen increasing application in ultra-high voltage (UHV) transmission systems in recent years.

[0004] Existing optical current transformers generally employ a closed-loop detection scheme. CN100340860C, "Fiber Optic Current Transformer and its Closed-Loop Detection Device," and "Signal Processing Method for Digital Closed-Loop All-Fiber Optic Current Transformer," both describe this closed-loop detection scheme. Specifically, it uses square wave modulation technology to introduce a ±π / 2 bias into the system via the phase modulator, ensuring the system operates in the linear region and improving the transformer's response sensitivity. Simultaneously, it employs stepped-wave feedback modulation to generate feedback compensation for the phase shift φ. R φ with Faraday phase shift F With equal magnitudes and opposite directions, the mutual inductor system always operates in the region near zero phase, where linearity is optimal. This closed-loop detection scheme reduces output nonlinearity error and increases the dynamic measurement range. The frequency of the modulated square wave in the above square wave modulation technique is related to the system's intrinsic frequency f. i (or intrinsic period T) i ) are equal, f i=1 / 2τ, where τ is the propagation time of light in the fiber delay line and fiber sensing loop, τ = 2nL / C, where L is the length of the fiber delay line and fiber sensing loop (hereinafter referred to as the optical path length), and C is the speed of light in vacuum. Therefore, the square wave modulation frequency of a conventional optical current transformer is related to the optical path length of the system, and thus the signal processing time of the system and the delay of the optical current transformer are also related to the optical path length. Because of differences in the field installation location and optical cable routing, the optical path lengths of the three-phase optical current transformers (CTs) are different, and therefore the signal processing times of the three are also different. However, the three-phase current and voltage in the high-voltage direct current transmission system require dq transformation, which requires the transformers to provide data at the same time. Summary of the Invention

[0005] The purpose of this application is to provide an optical current transformer system and its fixed-frequency modulation method that work in conjunction with a control and protection device to eliminate delay differences caused by differences in optical path length and crystal oscillator of different optical current transformers.

[0006] To achieve the above-mentioned technical objectives, the technical solution of this application is as follows:

[0007] According to one aspect of this application, an optical current transformer system is provided, comprising: a light source for outputting light; a coupler, the first end of which is connected to the light source via an optical fiber; a polarizer connected to the third end of the coupler via an optical fiber for polarizing the light into linearly polarized light; a phase modulator connected to the polarizer via an optical fiber for phase modulation of the input linearly polarized light; a delay fiber connected to the phase modulator via an optical fiber; an optical fiber sensing loop connected to the delay fiber via an optical fiber for reflecting and returning the linearly polarized light for sensing a measured current; a photodetector connected to the second end of the coupler via an optical fiber for photoelectric conversion of the received optical signal; a signal processor for generating a fixed-frequency modulation signal and inputting the fixed-frequency modulation signal to the phase modulator; receiving the output signal of the photodetector and acquiring the measured current signal based on the output signal.

[0008] According to some embodiments, the signal processor receives a synchronization signal and generates the fixed-frequency modulation signal based on the synchronization signal.

[0009] According to some embodiments, the fixed-frequency modulation signal includes a square wave signal or a sine wave signal.

[0010] According to some embodiments, the signal processor further includes a staircase wave generator for generating a staircase wave signal to be provided to the phase modulator, thereby the phase modulator generating a phase-modulated staircase wave.

[0011] According to some embodiments, the fiber optic sensing ring includes: a λ / 4 waveplate for converting linearly polarized light into circularly polarized light; a sensing fiber connected to the λ / 4 waveplate; and a reflector connected to the sensing fiber for reflecting the circularly polarized light.

[0012] According to some embodiments, the optical current transformer system further includes: an analog-to-digital converter electrically connected to the photodetector, which performs analog-to-digital conversion on the output signal of the photodetector and inputs it to the signal processor; and a digital-to-analog converter electrically connected to the signal processor, which performs digital-to-analog conversion on the fixed-frequency modulation signal and inputs it to the phase modulator.

[0013] According to some embodiments, the digital-to-analog converter performs digital-to-analog conversion on the stepped wave signal and inputs it into the phase modulator.

[0014] According to one aspect of this application, a modulation and demodulation method for an optical current transformer system is provided, comprising: a signal processor generating a fixed-frequency modulation signal; the signal processor inputting the fixed-frequency modulation signal to a phase modulator to perform phase modulation on an optical signal input to the phase modulator, thereby causing the phase modulator to output a phase modulation signal and introduce a bias modulation signal into the optical current transformer system; the signal processor acquiring the output signal of a photodetector; and the signal processor obtaining the value of the measured current based on the output signal.

[0015] According to some embodiments, the signal processor receives a synchronization signal and generates a fixed-frequency modulation signal with the same frequency as the synchronization signal.

[0016] According to some embodiments, the phase amplitude of the fixed-frequency modulation signal is ±π / 2.

[0017] According to some embodiments, the period of the fixed-frequency modulation signal is different from the intrinsic period of the optical current transformer, and each period of the bias modulation signal has two unmodulated time periods.

[0018] According to some embodiments, the highest level of the output signal of the photodetector corresponding to the unmodulated time period is not included in the demodulation calculation.

[0019] According to some embodiments, obtaining the value of the measured current based on the output signal includes: acquiring a high-level P related to the signal of the measured current based on the output signal. h With low level P l For the high level and the low level, perform multi-point sampling respectively; calculate the open-loop demodulation output result within the time period corresponding to the high level and the low level: Where n is an integer, representing the number of sampling points for the high level and the low level, respectively. The Faraday phase shift is used; the value of the measured current is calculated based on the open-loop demodulation output result.

[0020] According to some embodiments, the high level P h It can be calculated using the following formula: in, For Faraday phase shift; the low level P l It can be calculated using the following formula: in, The Faraday phase shift can be calculated using the following formula: Where V is the Verdet constant of the optical fiber, N is the number of sensing loops in the optical fiber, and I is the magnitude of the measured current.

[0021] According to some embodiments, the signal processor generates a stepped wave signal to provide to the phase modulator, thereby the phase modulator generates a phase-modulated stepped wave and generates feedback-compensated phase shift.

[0022] According to some embodiments, based on the feedback compensation phase shift, the closed-loop demodulation output result within the time period corresponding to the high level and the low level is calculated: Where n is an integer, representing the number of sampling points for the high level and the low level, respectively. For Faraday phase shift, The feedback compensates for the phase shift; the closed-loop demodulation output is divided by 2n and then integrated to form the stepped wave height. It serves as the digital output of the optical current transformer.

[0023] The technical solution according to the embodiments of this application has one or more of the following beneficial effects:

[0024] According to some embodiments, the acquisition unit of the optical current transformer in the technical solution of this application receives the synchronization signal of the control and protection device, and starts phase modulation according to the synchronization signal, thereby realizing the coordinated operation of the optical current transformer and the control and protection device.

[0025] According to some embodiments, the frequency of the phase modulation signal and the frequency of the synchronization signal in the technical solution of this application are the same, and the period of the modulation signal is independent of the optical path length of the optical current transformer. This eliminates the delay difference caused by the difference in optical path length and crystal oscillator of different optical current transformers, and realizes synchronous modulation, same frequency data transmission and consistent delay of different optical current transformers.

[0026] According to some embodiments, the calculation scheme of this application can improve the testing speed of optical current transformers, thereby improving the response speed and control accuracy of DC transmission systems.

[0027] It should be understood that the above general description and the following detailed description are merely exemplary and do not limit this application. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application.

[0029] Figure 1 This diagram illustrates the structure of an optical current transformer according to an example embodiment of this application.

[0030] Figure 2 This diagram illustrates the cooperative operation of an optical current transformer and a control and protection device according to an example embodiment of this application.

[0031] Figure 3A A schematic diagram of a phase modulated signal generated by a phase modulator according to an example embodiment of this application is shown.

[0032] Figure 3B A schematic diagram of the bias modulation signal is shown in an example embodiment of this application.

[0033] Figure 4 This diagram illustrates the output signal of a photodetector under fixed-frequency modulation in an example embodiment of this application.

[0034] Figure 5 A schematic diagram of a phase-modulated stepped wave generated by a phase modulator according to an example embodiment of this application is shown.

[0035] Figure 6 A schematic diagram illustrating the compensated phase shift introduced by the phase modulator in an example embodiment of this application is shown.

[0036] Figure 7 A flowchart illustrating an optical current transformer modulation and demodulation method according to an example embodiment of this application is shown.

[0037] Explanation of reference numerals in the attached diagram: 1-Control and protection device; 2-Acquisition unit; 3-Fiber optic sensing ring; 4-Communication fiber; 5-Transmission fiber; 6-Primary conductor; 21-Light source; 22-Coupled; 23-Polarizer; 24-Phase modulator; 25-Delay fiber; 26-Photodetector; 27-Analog-to-digital converter; 28-Digital-to-analog converter; 29-Signal processor; 31-λ / 4 waveplate; 32-Sensing fiber; 33-Reflector. Detailed Implementation

[0038] The technical solution of this application will be described in detail below with reference to the accompanying drawings.

[0039] Figure 1The diagram shows a structural schematic of an optical current transformer according to an example embodiment of this application. The optical current transformer includes a data acquisition unit 2, a transmission optical fiber 5, and an optical fiber sensing ring 3.

[0040] Figure 2 This diagram illustrates the cooperative operation of an optical current transformer and a control and protection device according to an example embodiment of this application.

[0041] by Figure 1 and Figure 2 The optical current transformer of this application and its working process are illustrated using the example shown.

[0042] The driving circuit of the signal processor 29 drives the light source 21 to emit light. The light emitted by the light source 21 reaches the first end of the coupler 22, and the coupler 22 guides the light to its third end; after passing through the coupler 22, the light enters the polarizer 23, generating linearly polarized light. The linearly polarized light is split into two incident orthogonal linearly polarized beams, which pass through the phase modulator 24, the delay fiber 25, and the transmission fiber 5, reaching one end of the λ / 4 waveplate 31 in the fiber optic sensing ring 3. The other end of the λ / 4 waveplate 31 of the fiber optic sensing ring 3 outputs two orthogonal circularly polarized beams. The two orthogonal circularly polarized beams propagate along the sensing fiber 32 of the fiber optic sensing ring 3. Due to the Faraday magneto-optical effect, in the sensing fiber 32, one of the two orthogonal circularly polarized beams propagates faster and the other propagates slower, thus generating a phase difference. After the two orthogonal circularly polarized beams are reflected by the mirror 33 at the end of the sensing fiber 32, their polarization modes are interchanged. The left-hand circularly polarized beam becomes the right-hand circularly polarized beam, and the right-hand circularly polarized beam becomes the left-hand circularly polarized beam, and then they return along the original path.

[0043] When the two returning orthogonal circularly polarized beams return, the direction of the magnetic field of the primary current remains unchanged, while the propagation direction and polarization state of the two returning orthogonal circularly polarized beams change, thus doubling the phase difference caused by the Faraday effect. After passing through the λ / 4 waveplate 31 of the fiber optic sensing ring 3 again, they become two returning orthogonal linearly polarized beams, and their polarization directions are interchanged during relative propagation. When the two returning orthogonal linearly polarized beams return, they pass through the transmission fiber 5 and the phase modulator 24 in sequence, becoming a combined polarized beam, which reaches the polarizer 23. The interference light signal returned by the polarizer 23 reaches the third end of the coupler 22, and then returns to the photodetector 26 through the second end of the coupler 22.

[0044] The photodetector 26 performs photoelectric conversion on the interference light signal and outputs an electrical signal. The analog-to-digital converter 27 converts the analog electrical signal output by the photodetector into a digital electrical signal. The signal processor 29 generates a fixed-frequency modulation signal (e.g., a square wave signal), or a superposition signal of a fixed-frequency modulation signal (e.g., a square wave signal) and a digital step wave. The digital-to-analog converter 28 receives the square wave signal, or the superposition signal of the square wave signal and the digital step wave, and converts it into an analog electrical signal, which is then applied to the phase modulator. The signal processing circuit 29 receives the digital electrical signal output by the analog-to-digital converter 27 and demodulates the digital electrical signal to determine the measured current in the primary conductor 6 located in the fiber optic sensing loop 3.

[0045] Theoretical analysis of the optical current transformer shows that if the time function of the fixed-frequency modulation signal applied to the phase modulator 24 is φ(t), the expression for the light intensity output by the photodetector 26 is:

[0046]

[0047] Faraday phase shift V – Verdet constant of the optical fiber, N – number of sensing loops in the optical fiber, I – magnitude of current in the primary conductor, τ – propagation time of polarized light in the optical fiber delay line and sensing loop, calculated by the following formula:

[0048] τ=2nL / C (2)

[0049] Where L is the total length of the optical fiber (i.e., the optical path length) of the optical fiber delay line and sensing loop, C is the speed of light in vacuum, n is the refractive index of the optical fiber, and φ(t)-φ(t-τ) is the bias modulation signal.

[0050] like Figure 2 As shown, in this embodiment, the acquisition unit 2 and the control and protection device 1 are connected by one or two communication optical fibers 4. When two optical fibers are connected, one is used for the control and protection device to transmit a synchronization signal, and the other is used for the acquisition unit to send a digital signal representing the measured current.

[0051] When the control and protection device 1 sends a synchronization signal to the acquisition unit 2, the signal processing circuit 29 of the acquisition unit 2 starts to generate a square wave signal, introducing a phase modulation signal φ(t) with a phase amplitude of π / 2 and 0 to the phase modulator 24. The phase modulation signal φ(t) will introduce a bias modulation signal with an amplitude of ±π / 2 to the optical current transformer for a period of time to improve the sensitivity of current detection. The period of the phase modulation signal φ(t) is T, which is also the period of the synchronization signal, and is independent of the optical path length L. Figure 3A As shown.

[0052] When the period T of the phase modulation signal φ(t) is similar to the eigencycle T of the optical current transformer i When 2τ is not equal, each period of the bias modulation signal has two segments with a duration of 2τ. Unmodulated time periods, such as Figure 3B As shown.

[0053] When a primary current flows through the conductor, the output signal P(t) of the photodetector in the optical current transformer is as follows: Figure 4 As shown, when a fixed-frequency modulation signal is used, the modulation frequency of the optical current transformer is independent of the optical path length, which is a fixed-frequency modulation technique.

[0054] The highest level P of the output signal of the optical current transformer corresponds to the no-modulation period. m The signal processing circuit 29 identifies the unmodulated time period and discards the sampled data of the output signal P(t) of the optical current transformer corresponding to the unmodulated time period, ensuring that the highest level of the unmodulated time period is not included in the calculation.

[0055] Depend on Figure 4 As shown in equation (1), the output signal P(t) has three parts:

[0056] Highest level:

[0057] P m =2P0 (3)

[0058] The duration is It occurs twice per cycle;

[0059] Low level:

[0060]

[0061] The duration is (T-τ), and it occurs once per cycle;

[0062] High level:

[0063]

[0064] The duration is (T-τ), and it occurs once per cycle.

[0065] For the output signal P(t) of the photodetector of the optical current transformer, the signal processor 29 identifies the high and low levels related to the current signal and discards the highest level P without modulation. m The high level P of the part h With low level P l The difference is proportional to the primary current signal; for the useful high level P h and low level P lPerforming multi-point sampling, let n be the number of sampling points for high level and low level respectively, then the demodulated output of the optical current transformer is:

[0066]

[0067] That is, the demodulated output of the optical current transformer, in the Faraday phase shift When the current is relatively small (i.e., when the primary current I is relatively small), the demodulation output of the optical current transformer has an approximately linear relationship with it. Therefore, the Faraday phase shift can be obtained by dividing the demodulation output of the optical current transformer by 2n and taking the arcsine, thereby calculating the magnitude of the primary current I.

[0068] To reduce the Faraday phase shift corresponding to high current When the current is large, the nonlinearity introduced will cause the signal processor 29 of the optical current transformer to generate a stepped wave, which is applied to the phase modulator to generate a phase-modulated stepped wave. This introduces feedback to compensate for the phase shift and achieve closed-loop control. The phase-modulated stepped wave generated by the phase modulator is as follows: Figure 5 As shown, the duration of its step is Step height is

[0069] The phase modulator introduces a compensating phase shift into the two orthogonally linearly polarized beams in the optical current transformer, such as... Figure 6 As shown, it is divided into:

[0070] Part One:

[0071]

[0072] The duration is It occurs twice per cycle;

[0073] Part Two:

[0074]

[0075] The duration is (T-τ), and it occurs twice per cycle;

[0076] Compare Figure 6 and Figure 4 It can be seen that within the time period (T-τ) corresponding to the high and low levels related to the current signal in the output signal of the photodetector identified by the optical current transformer, the two beams of orthogonally linearly polarized light experience an additional phase shift introduced by the phase modulator, as described in equation (8).

[0077] when At this time, the phase-modulated stepped wave applied to the phase modulator will provide feedback compensation for the phase shift introduced between the two orthogonally linearly polarized beams. This will cancel out the Faraday phase shift. At this moment, the high and low levels of the photodetector output signal change as follows:

[0078]

[0079] It is evident that the optical current transformer operates near the zero phase, where linearity is best, resulting in high measurement accuracy and an expanded measurement range.

[0080] Added feedback phase shift Subsequently, the demodulated output of the optical current transformer is:

[0081]

[0082] The demodulation result of equation (10) is divided by 2n and then integrated to form the step wave height. Simultaneous step wave height As the digital output of the optical current transformer, the fixed-frequency modulation and demodulation algorithm achieves closed-loop tracking of the primary current signal.

[0083] Based on the above-mentioned fixed-frequency modulation and demodulation algorithm, the optical current transformer can also use the same closed-loop algorithm as the traditional algorithm of variable-frequency modulation to track the half-wave voltage, which can eliminate the measurement error caused by the drift of the half-wave voltage of the phase modulator due to temperature changes.

[0084] The technical solution of this application takes the phase modulation signal of an optical current transformer using square wave modulation as an example, and provides a method for implementing synchronous modulation and fixed-frequency modulation demodulation technology. The method can also be used for optical current transformers with a sine wave phase modulation signal using an open-loop demodulation algorithm.

[0085] Figure 7 A flowchart illustrating an optical current transformer modulation and demodulation method according to an example embodiment of this application is shown.

[0086] like Figure 7 As shown, in S101, the signal processor receives the synchronization signal.

[0087] According to some embodiments, the synchronization signal is transmitted via optical fiber from the control and protection device of the high-voltage direct current transmission system to the optical current transformer.

[0088] In S103, the signal processor generates a fixed-frequency modulation signal.

[0089] The signal processor receives the synchronization signal and generates a fixed-frequency modulation signal with the same frequency as the synchronization signal.

[0090] In S105, the signal processor inputs a fixed-frequency modulation signal to the phase modulator to perform phase modulation on the optical signal input to the phase modulator, thereby causing the phase modulator to output a phase modulated signal.

[0091] In S107, the signal processor acquires the output signal of the photodetector.

[0092] According to some embodiments, the photodetector performs photoelectric conversion on the modulated optical signal, outputs an analog electrical signal, and converts the analog electrical signal into a digital electrical signal through an analog-to-digital converter, which then transmits it to the signal processor.

[0093] In S109, the signal processor obtains the open-loop demodulated signal based on the output signal.

[0094] According to some embodiments, the signal processor also includes a staircase wave generator for generating a staircase wave signal to be provided to a phase modulator, thereby causing the phase modulator to generate a phase-modulated staircase wave and introducing feedback to compensate for the phase shift.

[0095] In S111, the signal processor obtains the value of the measured current based on the open-loop demodulated signal.

[0096] According to some embodiments, the period of the phase modulation signal is different from the intrinsic period of the optical current transformer, and each period of the bias modulation signal has two unmodulated time periods.

[0097] Furthermore, the highest level of the output signal of the photodetector corresponding to the unmodulated time period is not included in the demodulation calculation.

[0098] The high-level P related to the measured current is obtained from the output signal. h With low level P l Multiple samples are taken at each point to calculate the open-loop demodulation output results for the time periods corresponding to the high and low levels.

[0099] Furthermore, in the Faraday phase shift When the current is relatively small, the value of the measured current is obtained by calculation based on the open-loop demodulation output.

[0100] According to some embodiments, in Faraday phase shift When the value is large, the closed-loop demodulation output result is calculated based on the feedback compensation phase shift within the time period corresponding to the high and low levels.

[0101] Furthermore, the closed-loop demodulation output is divided by 2n and then integrated to form the stepped wave height. It serves as the digital output of the optical current transformer.

[0102] As described in the embodiments of this application, the acquisition unit of the optical current transformer receives the synchronization signal sent by the control and protection device. The generation time and period of the modulation signal of the optical current transformer are controlled by the synchronization signal, avoiding the delay difference caused by the difference in the optical path length of the optical current transformer and the crystal oscillator of the acquisition unit. It can realize synchronous modulation, data transmission at the same frequency and consistent delay of different optical current transformers connected to the control and protection device, which can reduce the delay of the optical current transformer, improve the testing speed of the optical current transformer, thereby improving the response speed of the DC transmission system and improving the control accuracy of the DC system.

[0103] The embodiments in this application are only for illustrating the technical concept of this application and should not be used to limit the scope of protection of this application. Any modifications made to the technical solution based on the technical concept proposed in this application shall fall within the scope of protection of this application.

Claims

1. An optical current transformer system, characterized by The optical current transformer comprises: a light source for outputting light; a coupler connected with the light source through an optical fiber at a first end; a polarizer connected with the coupler through an optical fiber at a third end for polarizing the light into linearly polarized light; a phase modulator connected with the polarizer through an optical fiber for phase modulating the input linearly polarized light; a delay optical fiber connected with the phase modulator through an optical fiber; an optical fiber sensing ring connected with the delay optical fiber through an optical fiber for reflecting and returning the linearly polarized light for sensing a measured current; a photodetector connected with the coupler through an optical fiber at a second end for photoelectrically converting a received optical signal; a signal processor for generating a fixed-frequency modulation signal and inputting the fixed-frequency modulation signal into the phase modulator, receiving an output signal of the photodetector, and obtaining a measured current signal according to the output signal, receiving a synchronization signal, and generating the fixed-frequency modulation signal with a same frequency as the synchronization signal according to the synchronization signal, wherein a period of the fixed-frequency modulation signal is different from an intrinsic period of the optical current transformer.

2. The optical current transformer system of claim 1, wherein, The fixed-frequency modulation signal comprises a square wave signal or a sine wave signal.

3. The optical current transformer system of claim 1, wherein, The signal processor further comprises a staircase wave generator for generating a staircase wave signal to provide to the phase modulator, so that the phase modulator generates a phase-modulated staircase wave.

4. The optical current transformer system of claim 1, wherein, The optical fiber sensing ring comprises: a λ / 4 wave plate for changing the linearly polarized light into circularly polarized light; a sensing optical fiber connected with the λ / 4 wave plate; a mirror connected with the sensing optical fiber for reflecting the circularly polarized light.

5. The optical current transformer system of claim 1, wherein, Further comprising: an analog-to-digital converter electrically connected with the photodetector for analog-to-digital converting an output signal of the photodetector and inputting the signal processor; a digital-to-analog converter electrically connected with the signal processor for digital-to-analog converting the fixed-frequency modulation signal and inputting the phase modulator.

6. The optical current transformer system of claim 5, wherein, Further comprising: The signal processor further comprises a staircase wave generator for generating a staircase wave signal; The digital-to-analog converter digital-to-analog converts the staircase wave signal and inputs the phase modulator.

7. A modulation method for the optical current transformer system of claim 1, characterized by, The signal processor receives a synchronization signal and generates a fixed-frequency modulation signal with a same frequency as the synchronization signal according to the synchronization signal; The period of the fixed-frequency modulation signal is different from the intrinsic period of the optical current transformer; The signal processor inputs the fixed-frequency modulation signal into a phase modulator to phase modulate an optical signal input into the phase modulator, so that the phase modulator outputs a phase-modulated signal to introduce a bias modulation signal into the optical current transformer system; The signal processor obtains an output signal of a photodetector; The signal processor obtains a value of a measured current according to the output signal. Each period of the bias modulation signal has two periods of non-modulation time.

8. The modulation method according to claim 7, wherein, The phase amplitude of the fixed frequency modulation signal is .

9. The modulation method according to claim 7, wherein, The highest level of the output signal of the photodetector corresponding to the non-modulation time does not participate in demodulation calculation.

10. The modulation method according to claim 9, wherein, Obtaining the value of the measured current according to the output signal comprises:

11. The modulation method of claim 7, wherein, multipoint sampling the high level and the low level respectively; acquiring a high level related to the signal of the measured current according to the output signal and a low level ; calculating open-loop demodulation output results in time periods corresponding to the high level and the low level respectively; ​ ， Wherein, n is an integer, and are respectively the sampling points of the high level and the low level, is the Faraday phase shift; The numerical value of the measured current is obtained according to the open-loop demodulation output result.

12. The modulation and demodulation method of claim 11, wherein, the high level may be calculated by the following equation: ， wherein is the Faraday phase shift; The low level may be calculated by the following equation: ， wherein is the Faraday phase shift; The Faraday phase shift is calculated according to the following formula: ， wherein, is the Verdet constant of the optical fiber, is the number of turns of the optical fiber sensing coil, is the magnitude of the measured current.

13. The modulation method of claim 7, wherein, Further comprising: The signal processor generates a staircase signal to provide to the phase modulator, so that the phase modulator generates a phase-modulated staircase wave and generates a feedback compensation phase shift.

14. The modulation method according to claim 13, wherein, Further comprising: Based on the feedback compensation phase shift, the closed-loop demodulation output result in the time period corresponding to the high level and the low level of the signal related to the measured current is calculated: ， Wherein, n is an integer, and the number of sampling points of the high level and the low level is respectively, is a Faraday phase shift, is the feedback compensation phase shift; dividing the closed loop demodulation output result by 2 post-accumulation integration to form a staircase height as a digital output of the optical current transformer.

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