CN1540355A - Reflection type 'Sage-qinke' interferometer type current transformer prepared from full optical fiber - Google Patents
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Abstract
The current transformer is composed of at least connected opto-electronic unit and optical fiber current induction unit. The opto-electronic unit generates optical signal in use for detection, and optical fiber current induction unit detects current passing through bus enwound by fiber winding. Reflection type 'Sage-naike' interference treating technique is adopted in the invention to overcome interference of diversified environmental factors such as temperature, vibration and electromagnetic interference with out need of introducing power source in high voltage area. Induction optical fiber winding can be prepared from ultra-low birefringence fiber or general low birefringence fiber or circular polarization holding fiber, which overcomes limitation of traditional fiber: large influence from environmental factors and large fluctuation of linear birefringence. In the invention, reflective coating at end face of fiber features of heat and cold proof, shock-resistant etc.
Description
Technical Field
The invention relates to an optical fiber type current transformer, in particular to an all-optical fiber type current transformer which adopts a reflection type Sagnac interference processing technology, has high measurement precision and can overcome the interference of various environmental factors; belongs to the field of photoelectric technology.
Background
The mutual inductor is an indispensable important device in the power transmission line, and has the function of reducing the high-voltage and high-current numerical values on the power transmission line to standard numerical values which can be directly measured by an instrument according to a certain proportional relation so as to be convenient for the instrument to directly measure. The transformer can be used for relay protection besides measurement. The number of transformers produced in our country is very large each year. For example, in 1999, according to statistics on 33 key transformer enterprises in China, the transformers 760 and 474 are produced, and the total industrial value is 14.90 billion yuan. Wherein about 17,800 transformers with the voltage class of 110kV and above are arranged. According to analysis, when the voltage level reaches 100kV or above, the photoelectric transformer is used, and the photoelectric transformer has economic value.
The primary winding of the traditional current transformer is connected in series in a power line, and the external loop of the secondary winding is connected with a measuring instrument or a relay protection and automatic control device. The structure of the transformer is similar to that of a transformer, a primary winding and a secondary winding are wound on an iron core of the transformer, and a measuring sensing signal is transmitted from a primary side to a secondary side by means of electromagnetic coupling between the primary winding and the secondary winding. And an insulation structure with enough electric strength is arranged between the iron core and the winding and between the primary winding and the secondary winding, and is used for ensuring that all low-voltage equipment is isolated from high voltage. With the increase of the power capacity transmitted by a power system, the voltage grade is higher and higher, the insulation structure of the traditional current transformer is more and more complex, the size and the weight are larger and larger, and the manufacturing cost of the product is higher and higher. For example, the price of a 500kV product of a conventional oil-immersed current transformer is doubled compared with the price of 300 kV.
The iron core of the electromagnetic current transformer has nonlinearity, when a short circuit occurs in an electric power system, the short circuit current with high amplitude instantly saturates the transformer, secondary current output by the transformer is seriously distorted, protection is rejected, and serious accidents occur in the electric power system. The saturation of the mutual inductor causes waveform distortion, and the frequency band response characteristic is poor, the frequency band is narrow, and the high-frequency response of the system is poor, so that the novel fast protection based on the high-frequency transient component is difficult to realize.
Although the electromagnetic mutual inductor is fully developed, the iron core type current mutual inductor adapts to the development requirement of electric power construction in various structures of a dry type, an oil immersion type and a gas insulation type. However, with the continuous increase of power transmission capacity, the continuous improvement of the voltage level of the power grid and the continuous improvement of protection requirements, the structure of a general iron core type current transformer has gradually exposed the weakness that the structure is not suitable for the structure, and the inherent weaknesses of large volume, magnetic saturation, ferromagnetic resonance, small dynamic range, narrow use frequency band and the like are difficult to meet the development requirements of the automation of a new generation of power system, a power digital network and the like.
With the rapid development of optoelectronic technology, many developed science and technology countries have turned their eyes to the development of new electronic current transformers, referred to as photoelectric current transformers, by using optical sensing technology and electronics. The international electrotechnical association has issued standards for electronic current transformers. The electronic transformer includes various voltage and current transformers using an electronic test principle in addition to an optoelectronic transformer.
The photoelectric mutual inductor is a novel mutual inductor which utilizes a photoelectronic technology and an optical fiber sensing technology to realize the measurement of the voltage and the current of a power system. The optical mutual inductor is a general name of various optical mutual inductors such as an optical voltage mutual inductor (OVT), an optical current mutual inductor (OCT), a combined optical mutual inductor and the like. The photoelectric current transformer is widely classified into an active type and a passive type. The active type is that the sensing head part is provided with an electronic circuit which needs power supply; the passive type does not require a power source in the sensor head portion. The two current transformers have different structures, but have similar advantages compared with the traditional electromagnetic current transformer.
In the sensing head on the high potential side of the active photoelectric current transformer powered by laser, all electronic devices are used. On the high potential side, bus current is converted into a voltage signal, which is an analog quantity, by an air-core coil (Rogowski coil), converted into a digital signal by a/D, converted into an optical signal by an electro-optical conversion (LED) circuit, and then transmitted to the low potential side through an insulated optical fiber. On the low potential side, the optical signal is converted into a digital electrical signal by a photoelectric conversion device (PIN) for relay protection and electric energy metering. Where analog quantities are required, digital to analog (D/a) circuitry may be used to convert the digital quantities to analog quantities. The photoelectric current transformer adopts a laser light source to transmit light energy from a low potential side to a high potential side through an optical fiber, then converts the light energy into electric energy through a photoelectric conversion device, and supplies power to each electronic circuit after passing through a power stabilizing circuit. The advantage of this method is that the output power is stable, and is not easily affected by the external stray light source, which is a high-tech technique. The photoelectric current transformer adopts the electronic, optical communication, laser and computer technologies developed in recent years, and has stronger vitality and higher technical content.
Compared with the traditional electromagnetic induction type current transformer, the photoelectric type current transformer has a series of advantages as follows:
1. the materials used by the photoelectric current transformer (OCT) are insulating materials such as glass, optical fiber and the like to transmit information, so the insulating structure is simple, and the manufacturing cost of the OCT current transformer generally increases linearly along with the increase of the voltage level;
2. the photoelectric current transformer does not contain an iron core, so that the problems of magnetic saturation, ferromagnetic resonance and the like are solved;
3. the secondary circuit of the electromagnetic induction type current transformer cannot be opened, and the low-voltage side has the danger of opening a circuit. Because only optical fiber connection exists between the high-voltage side and the low-voltage side of the photoelectric current transformer, the optical fiber has good insulating property, the high-voltage circuit and the secondary circuit can be completely isolated electrically, the low-voltage side has no open-circuit high-voltage danger, and electromagnetic interference can be avoided;
4. the dynamic range is large, and the measurement precision is high; when the power grid normally operates, the current flowing through the current transformer is not large, but the short-circuit current is generally large, and the short-circuit current is larger and larger along with the increase of the capacity of the power grid. The electromagnetic induction type current transformer has the problem of magnetic saturation, is difficult to realize large-range measurement, and simultaneously meets the requirements of high-precision measurement and relay protection. OCT has very wide dynamic range, the rated current can measure dozens of amperes and thousands of amperes, the over-current range can reach several tens of thousands of amperes; one OCT can meet the requirements of measurement and relay protection at the same time, and the redundancy requirements of a plurality of CTs can be eliminated;
5. the frequency response range is wide, the frequency response of the sensing head part depends on the transit time of the optical fiber on the sensing head, and the actually measurable frequency range is mainly determined by the electronic circuit part. The photoelectric current transformer has been proved to be capable of measuring harmonic waves on a high-voltage power line and measuring transient state, high-frequency large current and direct current of power grid current. The electromagnetic induction type current transformer is difficult to work in this aspect;
6. the danger of flammability, easy explosion and the like caused by oil storage is avoided; the electromagnetic induction type current transformer generally adopts an oil storage method to solve the insulation problem, so that the dangers of flammability, easy explosion and the like are inevitably existed; the photoelectric current transformer is simple in insulation structure, oil insulation is not needed, and dangers in the aspect of the structure can be avoided.
7. The volume is small, the weight is light, and the space is saved; the weight of the photoelectric current transformer sensing head is generally less than 1 kilogram. According to the published MOCT of 345kV of American West House company, the height is 2.7 meters and the weight is 109 kilograms. The height of the oil-immersed current transformer with the same voltage level is 5.3m, and the weight of the oil-immersed current transformer is 2300 kg, so that great convenience is brought to transportation and installation.
8. The power metering and protection device is suitable for the trend of digital, intelligent and automatic development of power metering and protection; with the development of computers and digital technologies, power metering and relay protection have increasingly been automated and intelligentized. The 5A or 1A output specification of the electromagnetic induction type current transformer can be interfaced with a computer only by adopting an optical conversion technology, and the photoelectric type current transformer is digital equipment which utilizes the photoelectric technology and can be directly output to the computer, so that an intermediate link is avoided.
In summary, although the existing photoelectric current transformer also has high processing requirements and is not easy to solve the power supply problem, and the sensing head is sensitive to temperature and vibration, the photoelectric current transformer has the advantages that the traditional electromagnetic current transformer is incomparable, has a simple structure and high sensitivity, is an ideal substitute product of the traditional electromagnetic current transformer, and must be widely applied in the future power industry. Therefore, mainly developed countries compete for investment and development, and the photoelectric current transformer has become a hot point for research on the current transformer.
The signal and transmission form of the photoelectric mutual inductor can be realized by adopting an optical cable (optical fiber), and the outstanding advantages of the optical signal and the wide adoption of the optical fiber communication technology enable the data transmission inside the transformer substation and between the transformer substation and a superior station to be more reliable and quicker. The optical fiber local area network formed by combining the photoelectric transformer, the optical fiber communication technology and the microcomputer is applied to the power system, and is an important development direction of substation automation. The future optical fiber transformer substation is created with good prospect.
In the worldwide application research of photoelectric current transformers, since the 60 s, the 70 s have climbed, but the current time is still in a stage of low precision and no better solution to the temperature influence. Since the 80 s, the rise and maturity of photoelectronic technology, PC microcomputer, single chip microcomputer and digital processor technology has laid the foundation for the development of high-performance photoelectric current transformer.
Until 2000, ABB company developed photoelectric current transformers for voltage classes of 69kV to 765kV, the measuring current range was 5-2000A, and the accuracy reached +/-0.2%. Meanwhile, the composite electronic voltage and current transformers used in the GIS are developed, the current measurement range is 5-2000A, the voltage measurement range is 69-500 kV, the accuracy reaches +/-0.2%, and the voltage measurement is directly performed by using a capacitance ring without using a voltage divider.
An electronic current transformer is developed by Alstom company in France by utilizing the Faraday effect, and the accuracy reaches +/-0.2% within the range of minus 30-50 ℃. In 2000, 362kV electronic current transformers they developed began to supply to companies such as LCRA and CINERGY.
At present, researchers of many scientific research institutions and colleges at home and abroad are dedicated to research on novel transformers such as photoelectric transformers. The main research units in this area include Qinghua university, Huazhong university of science and technology, Shanghai university, and Xian Tongwei company. The Rogowski air-core coil is combined with the modern integrated electronic technology to serve as a power current transformer, and the Rogowski air-core coil is favored by extensive researchers due to the outstanding advantages of the Rogowski air-core coil. The university of science and technology in china, who collaborated with Guangzhou Wei Yu opto-electronic technology Limited, began to conduct research work on the aspect of air-core coil transformers as early as the 70 s, and accumulated abundant experience by performing a great deal of analysis and tests on the application of the university of science and technology in the aspects of high-voltage large current, pulse large current, welding current and the like.
The international electrotechnical association has recently issued standards for electronic voltage transformers, as well as standards for electronic current transformers (sensors). The electronic transformer mainly comprises a photoelectric transformer and other various voltage and current transformers utilizing an electronic test principle. This is sufficient to explain that opportunities have come to use photoelectric transformers in power systems.
However, the existing photoelectric transformer has obvious defects in both an active type and a passive type. This transformer induces current through an air-core coil (Rogowski coil) and then transfers the current from the high voltage zone to the terminal equipment using optoelectronics and fiber optic technology. Since the current induced by the coil is output as a voltage amount, an integrating circuit is required at the rear end to convert the voltage into a current. The transformer should ideally be of the passive type so that the insulation requirements of the high voltage region are greatly reduced, and the transformer has many advantages, such as small size, light weight, high reliability, safety, etc. The output voltage of the Rogowski coil is very low, typically between a few millivolts and a few hundred millivolts, and it is not possible to directly drive existing optoelectronic devices, and it is practically impossible to directly connect the output of the coil to a terminal device by means of electro-optical technology. The active photoelectric transformer introduces a power supply and a ground in a high-voltage area to supply power to processing circuits (including electronic circuits, photoelectric circuits and the like) so as to amplify the output voltage of the coil to a magnitude capable of performing electro-optical conversion. Due to the introduction of power and ground in the high voltage region, the whole device is very complex, high voltage insulation is made at a great expense, and the advantages of the photoelectric transformer in terms of volume, weight, safety, reliability, electromagnetic compatibility and the like are affected.
Disclosure of Invention
The invention mainly aims to provide a reflection type Sagnac interferometer type all-fiber current transformer, which adopts a reflection type Sagnac interference processing technology to overcome the interference of various environmental factors such as temperature, vibration, electromagnetic interference and the like without introducing a power supply into a high-voltage area; the induction optical fiber has the advantages of large dynamic range, good linearity, self insulation property, measurement precision improvement and volume and weight reduction of the mutual inductor.
Another objective of the present invention is to provide a reflective sagnac interferometer type all-fiber current transformer, in which the reflective film, the sensing fiber and the 1/4 wave plate can maintain good resistance to environmental factors, and the structure is easy to implement.
The purpose of the invention is realized as follows:
a reflection Sagnac interferometer type all-fiber current transformer is formed by connecting a photoelectric unit and a fiber current sensing unit; the optical fiber current sensing unit detects current flowing in a bus wound by an optical fiber winding of the optical fiber current sensing unit by using the optical signal, and returns the optical signal to the photoelectric unit to output the detection optical signal.
The photoelectric unit is at least formed by connecting a light source, a single-mode fiber coupler, a polarization-maintaining fiber depolarizer, a fiber polarizer, an optical phase modulator, an oscillation source, a polarization-maintaining fiber delay line and a photoelectric detector; an optical signal output by the light source is transmitted to the polarization maintaining optical fiber depolarizer in the forward direction through the single mode optical fiber coupler; the optical signal leaving the polarization maintaining optical fiber depolarizer enters the optical fiber polarizer; the optical fiber polarizer equally divides the optical signal into two orthogonal linearly polarized light which are respectively sent to the optical phase modulator; the optical phase modulator synchronously modulates two orthogonal linearly polarized light according to a modulation signal from an oscillation source, and then outputs the linearly polarized light to the optical fiber current sensing unit through the polarization-maintaining optical fiber delay line; the optical signal returned from the optical fiber current sensing unit reaches the optical fiber polarizer to generate Sagnac interference, and the interference light is reversely transmitted to the photoelectric detector through the single-mode optical fiber coupler; the photodetector outputs a detection signal.
The photoelectric unit is further provided with a closed-loop optical phase modulation circuit consisting of a digital demodulator, a feedback control circuit and an optical phase modulator, and the closed-loop optical phase modulation circuit is used for improving the signal-to-noise ratio and the stability of the current transformer; the photoelectric detector is connected with the digital demodulator, is used for outputting a detection result through the digital demodulator, and simultaneously transmits a feedback control signal to the optical phase modulator through the feedback control circuit.
When the measured signal is an alternating current signal, the digital demodulator is connected with a filter for filtering high-frequency interference during alternating current detection, and the filter is a band-pass filter with a passband of 1Hz-10 kHz; and when the measured signal is a direct current signal, the digital demodulator is connected with a filter for direct current detection, and the filter is a low-pass filter with a passband of 0-10 kHz.
The optical fiber current sensing unit is arranged at a high-voltage area of the current transformer and consists of a broadband optical fiber wave plate, an induction optical fiber coil and a reflection film plated on the end face of the induction optical fiber coil; the broadband optical fiber wave plate is used for converting linearly polarized light from the polarization fiber delay line into two circularly polarized light, the two circularly polarized light reaches the reflecting film on the end face through the induction optical fiber coil, and the reflecting film totally reflects the two circularly polarized light signals and reversely propagates along the induction optical fiber coil.
The invention adopts the reflective Sagnac interference processing technology, overcomes the interference of various environmental factors such as temperature, vibration, electromagnetic interference and the like, and does not need to introduce a power supply in a high-voltage area; the induction optical fiber adopted by the invention has the advantages of large dynamic range, good linearity and insulation property, and can improve the measurement precision and reduce the volume and weight of the mutual inductor. Compared with the traditional reflector formed by bonding the polished fiber end face with a reflector, the reflecting film has the characteristics of thermal expansion and cold contraction resistance and vibration resistance, is easy to install in structure and can reduce the manufacturing cost. The induction optical fiber coil can adopt ultra-low birefringence optical fiber or common low birefringence single-mode optical fiber, and especially can adopt circular polarization maintaining optical fiber, and the circular polarization maintaining optical fiber can overcome the defects that the traditional optical fiber is greatly influenced by environmental factors, has large linear birefringence fluctuation and the like, so that the performance is improved.
Drawings
Fig. 1 is a schematic diagram illustrating the principle of the structure of an embodiment of the present invention.
Detailed Description
The invention is described in further detail below with reference to the following figures and specific examples:
referring to fig. 1, the reflective sagnac interferometer type all-fiber current transformer of the present invention includes an optoelectronic portion and a high voltage sensing portion, wherein the optoelectronic portion is the core of the present invention. An output optical signal of a light source 1 (which can adopt an SLD (super luminescent diode) and is characterized by small noise, high light intensity and good temperature stability) is transmitted to a polarization maintaining optical fiber depolarizer 3 in a forward direction through a 50: 50 single-mode optical fiber coupler 2, and the polarization maintaining optical fiber depolarizer 3 is used for eliminating the inherent polarization of the optical signal from the light source 1 so that the optical signal has the properties of uniformity and equal amplitude in all polarization directions; then, the optical signal enters the optical fiber polarizer 4, the optical fiber polarizer 4 equally divides the optical signal into two orthogonal linearly polarized light signals, and the two orthogonal linearly polarized light signals are respectively sent to the optical phase modulator 7 along the x axis and the y axis of the optical fiber; the polarization-maintaining fiber depolarizer 3 described above can be integrated into the fiber polarizer 4. The optical phase modulator 7 synchronously modulates two orthogonal linearly polarized light beams by using the birefringence characteristics of the optical fiber, the modulation signal is from the oscillation source 6, and the oscillation frequency of the oscillation source 6 is: 1/4 τ, where τ is the time delay of the polarization maintaining fiber delay line 8. The optical phase modulator 7 of the present invention employs an integrated optical modulator. Because of the parasitic cross-polarization coupling in the optical phase modulator 7, a polarization maintaining fiber can be arranged between the fiber polarizer 4 and the optical phase modulator 7 as an depolarizing head 5 for suppressing the cross-polarization coupling. The depolarizing head 5 can be machined inside the optical phase modulator 7 as a pigtail. The modulated optical signal is sent to the optical fiber current sensing part through the polarization maintaining optical fiber delay line 8.
The optical fiber current induction part is positioned at a high-voltage area of the current transformer and consists of 1/4 wave plates 9, an induction optical fiber coil 10 and an end face plating reflection film 11 of the induction optical fiber coil. The 1/4 wave plate 9 is made of optical fiber wave plate, and is characterized in that: the bandwidth of the optical fiber wave plate is wider than that of other wave plates, the polarization state conversion quality is high, the ellipticity caused in the process of converting linear polarization into circular polarization can be well inhibited, the converted optical signal is close to ideal circular polarization, and the measurement error of the current transformer is further reduced; due to the adoption of the optical fiber knob technology, the optical fiber wave plate has the performance of resisting temperature and vibration influence compared with other wave plates, and the stability of the current transformer can be improved. And the wave plate is an optical fiber device, is easier to be connected with the front optical fiber and the rear optical fiber, and is suitable for industrialization.
The inductive fiber coil 10 is an ultra-low birefringence fiber or a common low birefringence single-mode fiber, and is wound around the high-voltage current bus 12 for several turns. 1/4 wave plate 9 converts the linear polarized light from the fiber delay line 8 into circular polarized light, i.e. the linear polarized light in x axis is converted into right circular polarized light, and the linear polarized light in y axis is converted into left circular polarized light. The two circularly polarized lights reach the reflecting film 11 on the end surface through the induction optical fiber coil 10, the signals are totally reflected and reversely propagate along the induction optical fiber coil 10, the left circularly polarized light is changed into the right circularly polarized light, and the right circularly polarized light is changed into the left circularly polarized light. 1/4 wave plate 9 converts the circularly polarized light in reverse direction into linearly polarized light, i.e. the right circularly polarized light is converted into x-axis linearly polarized light, and the left circularly polarized light is converted into y-axis linearly polarized light. During this time, the current in the high-voltage current bus 12 causes a phase difference between the two polarization states of the optical signals by the faraday effect, and the phase difference satisfies the following formula:
Δ Φ ═ 4VNI where,
v is the verdet constant of the fiber, N is the number of turns of the inductive fiber coil 10, and I is the current in the high voltage current bus 12.
When the optical signal transmitted reversely reaches the optical fiber polarizer 4, a 45 ° offset interface arranged inside the optical fiber polarizer 4 generates sagnac interference, the interference light is reversely transmitted to the photoelectric detector 13 through the 50: 50 single-mode optical fiber coupler 2, and the photoelectric detector 13 converts the detected light intensity into an electrical signal:
wherein,
Idto detect light intensity, IsThe intensity of light given by the light source, k being the loss of the whole optical path, [ t ] - [ phi ]mcos(ωmt) is the modulation signal of the optical phase modulator, V is the Verdet constant of the optical fiber, and N is the number of turns of the induction fiber coil.
The digital demodulator 15, the feedback control circuit 14 and the optical phase modulator 7 form a closed-loop optical phase modulation circuit, so that the current transformer has better signal-to-noise ratio and stability.
The invention can be applied to AC current detection and DC current detection. When the filter 16 is used for alternating current detection, a band-pass filter is adopted, the passband is from 1Hz to 10kHz, and low-frequency interference caused by temperature and high-frequency interference introduced from the outside are effectively filtered; when used for dc detection, filter 16 is a low pass filter with a passband from dc to 10 kHz. The processor 17 amplifies and corrects the detected current and outputs an accurate current measurement value. The invention can simultaneously provide analog and digital output forms according to the requirements of the terminal metering and protecting equipment of the power system.
Finally, it should be noted that: the above embodiments are only used to illustrate the present invention and do not limit the technical solutions described in the present invention; thus, while the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted; all such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.
Claims (13)
1. A reflective Sagnac interferometer type all-fiber current transformer is characterized in that: it is formed by connecting at least a photoelectric unit and an optical fiber current sensing unit; the optical fiber current sensing unit detects current flowing in a bus wound by an optical fiber winding of the optical fiber current sensing unit by using the optical signal, and returns the optical signal to the photoelectric unit to output the detection optical signal.
2. The reflective Sagnac interferometer all-fiber current transformer of claim 1, wherein: the photoelectric unit is at least formed by connecting a light source, a single-mode fiber coupler, a polarization-maintaining fiber depolarizer, a fiber polarizer, an optical phase modulator, an oscillation source, a polarization-maintaining fiber delay line and a photoelectric detector; an optical signal output by the light source is transmitted to the polarization maintaining optical fiber depolarizer in the forward direction through the single mode optical fiber coupler; the optical signal leaving the polarization maintaining optical fiber depolarizer enters the optical fiber polarizer; the optical fiber polarizer equally divides the optical signal into two orthogonal linearly polarized light which are respectively sent to the optical phase modulator; the optical phase modulator synchronously modulates two orthogonal linearly polarized light according to a modulation signal from an oscillation source, and then outputs the linearly polarized light to the optical fiber current sensing unit through the polarization-maintaining optical fiber delay line; the optical signal returned from the optical fiber current sensing unit reaches the optical fiber polarizer to generate Sagnac interference, and the interference light is reversely transmitted to the photoelectric detector through the single-mode optical fiber coupler; the photodetector outputs a detection signal.
3. The reflective Sagnac interferometer all-fiber current transformer of claim 2, wherein: the oscillation frequency of the modulation signal generated by the oscillation source complies with the following calculation formula:
f＝1/4τ
wherein tau is the delay time of the polarization-maintaining fiber delay line.
4. The reflective Sagnac interferometer all-fiber current transformer of claim 2, wherein: an depolarization head is arranged between the optical fiber polarizer and the optical phase modulator, and the depolarization head is composed of polarization maintaining optical fibers and is used for inhibiting cross polarization coupling.
5. The all-fiber Sagnac interferometer current transformer of any of claim 2, wherein: the photoelectric unit is further provided with a closed-loop optical phase modulation circuit consisting of a digital demodulator, a feedback control circuit and an optical phase modulator, and the closed-loop optical phase modulation circuit is used for improving the signal-to-noise ratio and the stability of the current transformer; the photoelectric detector is connected with the digital demodulator, is used for outputting a detection result through the digital demodulator, and simultaneously transmits a feedback control signal to the optical phase modulator through the feedback control circuit.
6. The reflective Sagnac interferometer-type all-fiber current transformer of claim 2 or 5, wherein: the photodetector converts the detected light intensity into an electrical signal according to the following formula:
wherein,
Idto detect light intensity, IsThe intensity of light given by the light source, k being the loss of the whole optical path, [ t ] - [ phi ]mcos(ωmt) is the modulation signal of the optical phase modulator, V is the Verdet constant of the optical fiber, and N is the number of turns of the induction fiber coil.
7. The reflective Sagnac interferometer-type all-fiber current transformer of claim 2 or 5, wherein: the digital demodulator demodulates the photoelectric detection signal to obtain a signal voltage which satisfies the following formula:
Vdm≈J1(*m) (4VNI) wherein,
Vdmas signal voltage, J1For a first order Bessel function, V is the Verdet constant of the fiber, N is the number of turns of the inductive fiber coil, and I is the current in the high voltage current bus.
8. The reflective Sagnac interferometer all-fiber current transformer of claim 5, wherein: the digital demodulator is further connected with a filter for filtering high-frequency interference during alternating current detection, and the filter is a band-pass filter with a passband of 1Hz-10 kHz.
9. The reflective Sagnac interferometer all-fiber current transformer of claim 5, wherein: the digital demodulator is further connected with a filter for direct current detection, and the filter is a low-pass filter with a passband of 0-10 kHz.
10. The reflective sagnac interferometer-type all-fiber current transformer of claim 8 or 9, wherein: the filter is further connected with a processor which amplifies and corrects the detected current and outputs an accurate current measurement value.
11. The reflective Sagnac interferometer all-fiber current transformer of claim 1, wherein: the optical fiber current sensing unit is arranged at a high-voltage area of the current transformer and at least consists of a lambda/4 wave plate (wherein lambda is the wavelength of an optical signal transmitted in an optical fiber), an induction optical fiber coil and a reflecting film plated on the end face of the induction optical fiber coil; the lambda/4 wave plate is used for converting linearly polarized light from the polarization fiber delay line into two circularly polarized light, the two circularly polarized light reaches the reflecting film on the end face through the induction fiber coil, and the reflecting film totally reflects the two circularly polarized light signals and reversely propagates along the induction fiber coil.
12. The reflective sagnac interferometer-type all-fiber current transformer of claim 11, wherein: the lambda/4 wave plate is a broadband optical fiber wave plate.
13. The reflective sagnac interferometer-type all-fiber current transformer of claim 11, wherein: the induction fiber coil is an ultra-low birefringence fiber or a common low birefringence single-mode fiber or a circular polarization maintaining fiber, and the fiber is wound at least one turn around the high-voltage current bus.
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