A multi-phase coded φ-OTDR cable partial discharge monitoring system and working method

Abstract

This invention belongs to the field of distributed optical fiber sensing technology applications, specifically relating to a multi-phase encoded φ-OTDR cable partial discharge monitoring system and its operating method. It includes a narrow-linewidth laser connected to port a of a 1*2 fiber optic coupler. Port b of the 1*2 coupler is sequentially connected to a lithium niobate electro-optic intensity modulator, an optical fiber acousto-optic modulator, and an erbium-doped fiber amplifier. The output of the erbium-doped fiber amplifier is connected to port a of an optical fiber circulator. Port b of the circulator is connected to a sensing fiber, injecting a probe pulse into it. The sensing fiber is tightly attached to the cable under test, and the cable end is connected to an optical fiber isolator. Port c of the 1*2 coupler is connected to port b of a 2*2 fiber optic coupler. Port c of the circulator is connected to port a of the 2*2 coupler. Ports c and d of the 2*2 coupler are connected to a photoelectric balance detector. This invention achieves distributed multi-point monitoring of partial discharge in power cables.

Description

Technical Field

[0001] The invention belongs to the application field of distributed optical fiber sensing technology, and in particular relates to a multi-phase coded φ-OTDR cable partial discharge monitoring system and a working method. Background Art

[0002] Power cables occupy an increasingly large proportion in the power transmission and distribution of power grids due to their small footprint and reliable performance. The healthy operation of cables is of great significance to the energy supply of cities and the normal operation of society. In recent years, with the development of the economy, the demand for urban electricity has continued to increase. Power cables work under full load conditions for a long time. Under multiple factors such as thermal effects, electrical stress, chemical corrosion, and environmental conditions, the safe operation of power cables faces huge challenges.

[0003] Partial discharge is an early sign of cable insulation failure. It is a phenomenon in which the insulating medium between cable conductors partially bridges under a strong electric field without forming a discharge path. Cable partial discharge is often accompanied by physical and chemical effects such as sound, light, and heat. From this, a variety of monitoring methods have been derived, which can be roughly divided into electrical measurement and non-electrical measurement methods. Electrical measurement methods include pulse current method, high-frequency pulse current method, ultra-high frequency method, oscillating wave method, and capacitor-inductor coupling method. Among them, the pulse current method is the most mature, and the International Electrotechnical Commission has formulated corresponding test standards (IEC 60270:2000), but it is susceptible to electromagnetic interference and can only be used as an offline test method; the high-frequency pulse current method requires the high-frequency current sensor to be installed on the cable grounding wire to couple the electromagnetic field generated by the high-frequency pulse current flowing through the path, which is inconvenient for field application and susceptible to interference; the ultra-high frequency method is more used for partial discharge monitoring of electrical equipment and cable accessories; the oscillating wave method and capacitor-inductor coupling method are still in the research stage and their application is not yet mature; non-electrical measurement methods include acoustic wave method, chemical method, optical detection method, etc. The acoustic wave method uses ultrasonic piezoelectric sensors to detect ultrasonic signals generated by partial discharge, which is mainly used to qualitatively determine the presence or absence of partial discharge; the chemical method and optical method are not affected by electromagnetic interference, but have poor sensitivity and are not ideal in practical application. The above partial discharge monitoring methods are all point-type or regional monitoring, or require quasi-distributed monitoring through sensor arrays, which are costly and technically complex. It is difficult to achieve long-distance distributed cable partial discharge online monitoring, and it is difficult to take into account the calibration of discharge volume, the judgment of discharge degree, and the identification of discharge type. Distributed optical fiber sensing technology has the advantages of small size, corrosion resistance, anti-electromagnetic interference, and the ability to achieve long-distance and large-scale monitoring. It has also been widely studied and applied in recent years. Summary of the invention

[0004] In order to solve the problem that the existing cable partial discharge monitoring means are difficult to achieve long-distance and distributed monitoring, the present invention provides a cable partial discharge monitoring system and a working method based on multi-phase coded frequency division multiplexing φ-OTDR.

[0005] The invention adopts the following technical scheme: a multi-phase coded φ-OTDR cable partial discharge monitoring system comprises a narrow line width laser, the narrow line width laser is connected to the a port of an optical fiber 1*2 coupler, the b port of the optical fiber 1*2 coupler is sequentially connected to a lithium niobate electro-optic intensity modulator, an optical fiber acousto-optic modulator and an erbium-doped optical fiber amplifier, the output end of the erbium-doped optical fiber amplifier is connected to the a port of an optical fiber circulator, the b port of the optical fiber circulator is connected to a sensing optical fiber, a detection pulse is injected into the sensing optical fiber, the sensing optical fiber is tightly attached to a cable to be tested, and the end of the cable is connected to an optical fiber isolator; the c port of the optical fiber 1*2 coupler is connected to the b port of an optical fiber 2*2 coupler, the c port of the optical fiber circulator is connected to the a port of the optical fiber 2*2 coupler, the c and d ports of the optical fiber 2*2 coupler are connected to a photoelectric balance detector, and the photoelectric balance detector is sequentially connected to a data acquisition device and a data demodulation and display device.

[0006] Furthermore, when local discharge occurs in the cable under test, the area around the discharge source expands instantaneously due to heat. After the discharge ends, the expanded area returns to its original volume, and the expansion and recovery process is repeated when the next discharge pulse arrives. This regular expansion and contraction makes the insulating medium an ultrasonic signal source. The generated ultrasonic signal is diffused and transmitted to the sensing optical fiber through the cable body under test, causing corresponding changes in the optical fiber core diameter, optical fiber length, and refractive index of the sensing optical fiber, thereby modulating the phase of the optical signal in the sensing optical fiber. The local discharge event can be located and restored by demodulating the phase.

[0007] Furthermore, ports b and c of the optical fiber 1*2 coupler output two optical signals with a splitting ratio of 90:10.

[0008] Furthermore, the sensing optical fiber is tightly wound around the cable to be tested, with a winding density of not less than 50 turns / meter, and the outer layer is protected by a heat shrink tube.

[0009] A working method of a multi-phase coded φ-OTDR cable partial discharge monitoring system comprises the following steps:

[0010] S100: The optical signal output from port b of the optical fiber 1*2 coupler passes through the lithium niobate electro-optic intensity modulator and the optical fiber acousto-optic modulator in sequence and is modulated into a multi-frequency and multi-phase coded pulse light.

[0011] S200: The multi-frequency and multi-phase coded pulse light is amplified by the erbium-doped fiber amplifier and becomes the detection pulse input to the a port of the fiber circulator. The multi-frequency and multi-phase coded pulse light is injected into the sensing fiber. The sensing fiber is tightly attached to the cable to be tested to sense and locate the partial discharge event of the cable to be tested. The end of the sensing fiber is connected to the fiber isolator to remove the Fresnel reflection peak. The backward Rayleigh scattered light generated in the sensing fiber returns to the b port of the fiber circulator, and then output from the c port of the fiber circulator to the a port of the fiber 2*2 coupler.

[0012] S300: The c-port of the fiber circulator outputs an optical signal to the a-port of the fiber 2*2 coupler. The light output from the c-port of the fiber 1*2 coupler enters from the b-port of the fiber 2*2 coupler as the reference light. The backward Rayleigh scattered light and the reference light are subjected to beat frequency in the fiber 2*2 coupler.

[0013] S400: The beat frequency light is proportionally output to the optical input port of the photoelectric balance detector through the c and d ports of the optical fiber 2*2 coupler, and the beat frequency light signal is converted into an electrical signal; the photoelectric balance detector outputs the electrical signal to the data acquisition device, and the data acquisition device discretizes the analog electrical signal into a digital quantity and transmits it to the data demodulation and display device, which demodulates and analyzes the collected data and displays the partial discharge position information and discharge waveform of the cable to be tested.

[0014] Furthermore, the lithium niobate electro-optic intensity modulator generates a time-frequency division multiplexing signal from an arbitrary wave signal source, which is driven after being amplified by the first radio frequency amplifier to modulate the signal light into a multi-frequency signal light, thereby realizing the time-frequency division multiplexing of the monitoring system.

[0015] Furthermore, the sub-channel bandwidth of the frequency division multiplexing signal is equal to the signal bandwidth of the multi-frequency multi-phase coded pulse light, and the total multiplexing frequency bandwidth is greater than the sum of the bandwidths of each sub-channel; the number of sub-pulse sequences in the time division multiplexing is equal to the number of sub-channels N , the sub-pulse duration is the multi-phase coded pulse duration w , each sub-pulse contains only one channel frequency, satisfying N <2 nL / cw ,in n is the refractive index of the optical fiber, L is the sensing optical fiber length, c The speed of light.

[0016] Furthermore, the fiber acousto-optic modulator generates a multi-phase coded signal from an arbitrary wave signal source, which is driven after being amplified by a second RF amplifier to modulate the multi-frequency signal light into a multi-phase coded pulse light, thereby realizing time-frequency division multiplexing under a multi-phase coded modulation format.

[0017] Furthermore, a raised cosine roll-off filter is used in the fiber acousto-optic modulator to suppress the sidelobe energy of the multi-phase coded pulse and prevent the aliasing of pulse energies of different frequencies during the time-frequency division multiplexing process, thereby increasing the number of time-frequency division multiplexing channels and improving the system frequency response range, so that it can capture the partial discharge events of the cable under test without distortion. The roll-off coefficient of the raised cosine roll-off filter is set to 0.2-0.3, and the sidelobe suppression ratio is greater than or equal to 30dB.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention introduces multi-phase coded modulation and pulse compression technology into φ-OTDR, thereby overcoming the bottleneck of mutual restriction between spatial resolution and sensing distance of phase-sensitive optical time domain reflectometer under traditional single-frequency pulse modulation technology, and at the same time greatly improving the sensitivity of the partial discharge monitoring system.

[0020] The present invention improves the frequency response range of the phase-sensitive optical time domain reflectometer by introducing time-frequency division multiplexing technology, solves the problem of insufficient system monitoring bandwidth, and adopts a raised cosine roll-off filter to suppress the pulse sidelobe energy in the multi-phase coded modulation process, solving the influence of the sidelobe on the matched filter operation in the system demodulation process after the introduction of time-frequency division multiplexing technology. Ultimately, the frequency multiplexing number of φ-OTDR is greatly improved, the detection bandwidth is improved, and the positioning and restoration of partial discharge signals can be realized.

[0021] This method realizes distributed multi-point monitoring of partial discharge of power cables, effectively solving the shortcomings of traditional detection methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A schematic diagram of a multi-phase coded φ-OTDR cable partial discharge monitoring system of the present invention;

[0023] Figure 2 This is a flow chart of the signal light modulation process of the present invention;

[0024] Figure 3 This is a flow chart of the sensor signal demodulation process of the present invention. DETAILED DESCRIPTION

[0025] Attached Figure 1The system schematic diagram of the present invention is shown, including a narrow linewidth laser 1, an optical fiber 1*2 coupler 2, a lithium niobate electro-optic intensity modulator 3, an optical fiber acousto-optic modulator 4, an erbium-doped fiber amplifier 5, an optical fiber circulator 6, a first radio frequency amplifier 7, a second radio frequency amplifier 8, an arbitrary wave signal source 9, an optical fiber 2*2 coupler 10, an optical fiber isolator 11, a photoelectric balanced detector 12, a data acquisition device 13, a data demodulation and display device 14, a sensing optical fiber 15, and a cable to be tested 16. The narrow linewidth laser 1 is connected to the a port of the optical fiber 1*2 coupler 2, the b port of the optical fiber 1*2 coupler 2 is connected to the lithium niobate electro-optic intensity modulator 3, the optical fiber acousto-optic modulator 4 and the erbium-doped fiber amplifier 5 in sequence, the output end of the erbium-doped fiber amplifier 5 is connected to the a port of the optical fiber circulator 6, the b port of the optical fiber circulator 6 is connected to the sensing optical fiber 15, the detection pulse is injected into the sensing optical fiber 15, the sensing optical fiber 15 is tightly attached to the cable 16 to be tested, and the end of the cable 16 is connected to the optical fiber isolator 11; the c port of the optical fiber 1*2 coupler 2 is connected to the b port of the optical fiber 2*2 coupler 10, the c port of the optical fiber circulator 6 is connected to the a port of the optical fiber 2*2 coupler 10, the c and d ports of the optical fiber 2*2 coupler 10 are connected to the photoelectric balance detector 12, and the photoelectric balance detector 12 is connected to the data acquisition device 13 and the data demodulation and display device 14 in sequence.

[0026] The output end of the narrow linewidth laser 1 is connected to the a port of the optical fiber 1*2 coupler 2, and is divided into two paths b and c by the optical fiber 1*2 coupler 2. The splitting ratio b:c is 90:10. Path b is used as signal light and path c is used as reference light. The linewidth of the narrow linewidth laser is less than or equal to 10kHz, the output optical power is greater than or equal to 10mW, and the central wavelength is 1550nm.

[0027] A working method of the multi-phase coded φ-OTDR cable partial discharge monitoring system comprises the following steps:

[0028] S100: The optical signal output from port b of the optical fiber 1*2 coupler 2 passes through the lithium niobate electro-optic intensity modulator 3 and the optical fiber acousto-optic modulator 4 in sequence, and is modulated into a multi-frequency and multi-phase coded pulse light.

[0029] S200: The multi-frequency and multi-phase coded pulse light is amplified by the erbium-doped fiber amplifier 5 to become the detection pulse input to the a port of the optical fiber circulator 6, and the multi-frequency and multi-phase coded pulse light is injected into the sensing optical fiber 15. The sensing optical fiber 15 is tightly attached to the cable 16 to be tested, and senses and locates the local discharge event of the cable 16 to be tested. The end thereof is connected to the optical fiber isolator 11 to remove the Fresnel reflection peak. The backward Rayleigh scattered light generated in the sensing optical fiber 15 is returned to the b port of the optical fiber circulator 6, and then output from the c port of the optical fiber circulator 6 to the a port of the optical fiber 2*2 coupler 10.

[0030] S300: The c port of the optical fiber circulator 6 outputs an optical signal to the a port of the optical fiber 2*2 coupler 10, and the light output from the c port of the optical fiber 1*2 coupler 2 enters from the b port of the optical fiber 2*2 coupler 10 as the reference light, and the backward Rayleigh scattered light and the reference light are beat in the optical fiber 2*2 coupler 10.

[0031] S400: The beat frequency light is proportionally output to the optical input port of the photoelectric balance detector 12 through the c and d ports of the optical fiber 2*2 coupler 10, and the beat frequency light signal is converted into an electrical signal; the photoelectric balance detector 12 outputs the electrical signal to the data acquisition device 13, and the data acquisition device 13 discretizes the analog electrical signal into a digital quantity and transmits it to the data demodulation and display device 14, and the data demodulation and display device 14 demodulates and analyzes the collected data, and displays the partial discharge monitoring result of the cable to be tested.

[0032] The following is combined with Figure 1 and attached Figure 2 The modulation process of the signal light is described below. The signal light output from the b port of the optical fiber 1*2 coupler 2 enters the lithium niobate electro-optic intensity modulator 3. The a port of the arbitrary wave signal source 9 outputs the time-frequency division multiplexing signal. After power amplification by the RF amplifier 7, the lithium niobate electro-optic intensity modulator 3 is driven. The signal light is modulated into a multi-frequency signal light through electro-optical modulation to realize time-frequency division multiplexing. The sub-channel bandwidth of frequency division multiplexing is equal to the signal bandwidth of multi-phase encoding, and the total multiplexing bandwidth is greater than the sum of the bandwidths of each sub-channel; the number of sub-pulse sequences of time division multiplexing is equal to the number of sub-channels. N , the sub-pulse duration is the multi-phase coded pulse duration w , each sub-pulse contains only one channel frequency; the number of multiplexed channels must meet N <2 nL / cw ,in n is the refractive index of the optical fiber, L is the sensing optical fiber length, c The speed of light.

[0033] The multi-frequency signal light modulated by the lithium niobate electro-optic intensity modulator 3 enters the fiber acousto-optic modulator 4, and the b port of the arbitrary wave signal source 9 outputs a multi-phase coded signal. At the same time, the sidelobe energy of the multi-phase coded signal is suppressed by the raised cosine roll-off filter, and the pre-distortion processing algorithm is used to compensate for the nonlinear effect of the fiber acousto-optic modulator 4 and the amplitude distortion during the power amplification process. The formed multi-phase coded pulse signal is amplified by the radio frequency amplifier 8 and then drives the fiber acousto-optic modulator 4. After acousto-optic modulation, the multi-frequency signal light is modulated into a multi-frequency multi-phase coded pulse light. By changing the initial phase of the radio frequency signal driving the fiber acousto-optic modulator 4, the phase of the multi-frequency signal light can be modulated to realize the modulation of the multi-phase coded pulse. The multi-phase coded signal is set to a code length of 1000 and a code element width of 20ns. The roll-off coefficient of the raised cosine roll-off filter is set to 0.2-0.3, and the sidelobe suppression ratio is greater than or equal to 30dB. During the pre-distortion processing, a Mach-Zehnder interferometer optical path is constructed, and a standard triangle wave modulation acousto-optic modulator is used to correct the nonlinear effect of the fiber acousto-optic modulator 4 and the amplitude distortion of the second RF amplifier.

[0034] The output end of the fiber acousto-optic modulator 4 is connected to the input end of the erbium-doped fiber amplifier 5, and the peak power of the multi-frequency and multi-phase coded pulse light is further amplified to generate a detection pulse. The gain of the erbium-doped fiber amplifier is greater than or equal to 10dB, and the pulse width is 10us-40us. The erbium-doped fiber amplifier 5 outputs the detection pulse into the a port of the fiber circulator 6, and the detection pulse is injected into the sensing fiber 15 through the b port of the fiber circulator 6. During the forward transmission of the detection pulse, continuous backward Rayleigh scattered light is generated. A fiber isolator 11 is connected to the end of the sensing fiber 15 to remove the Fresnel reflection peak.

[0035] The sensing optical fiber 15 is tightly wound around the cable 16 to be tested, and distributed monitoring and positioning of partial discharge events are performed. When partial discharge occurs in the cable 16 to be tested, the area around the discharge source is instantly heated and expanded. After the discharge ends, the expanded area returns to its original volume, and the expansion and recovery process is repeated when the next discharge pulse arrives. This regular expansion and contraction makes the insulating medium an ultrasonic signal source. The generated ultrasonic signal is diffused and transmitted to the sensing optical fiber 15 through the body of the cable to be tested, causing corresponding changes in the optical fiber core diameter, optical fiber length, and refractive index of the sensing optical fiber 15, thereby modulating the phase of the optical signal in the sensing optical fiber 15. The positioning and restoration of the partial discharge event can be achieved by demodulating the phase.

[0036] The backward Rayleigh scattered light returns to the b port of the optical fiber circulator 6 and then outputs through the c port to enter the a port of the optical fiber 2*2 coupler 10. The c port of the optical fiber 1*2 coupler 2 outputs the reference light and enters the b port of the optical fiber 2*2 coupler 10. The backward Rayleigh scattered light and the reference light beat in the optical fiber 2*2 coupler 10 for coherent detection.

[0037] The c and d ports of the optical fiber 2*2 coupler 10 output two optical signals with a ratio of 50:50, which enter the two optical input ports of the photoelectric balanced detector 12. The photoelectric balanced detector 12 converts the beat frequency optical signal into an intermediate frequency electrical signal. The bandwidth of the photoelectric balanced detector depends on the number of multiplexed sub-channels. N and the multi-phase coded pulse bandwidth, the gain is , equivalent noise power .

[0038] The photoelectric balance detector 12 outputs the electrical signal to the data acquisition device 13, and the C port of the arbitrary wave signal source 9 generates a synchronous trigger signal and outputs it to the control port of the data acquisition device 13 to control the synchronous acquisition of the data acquisition device 13. The data acquisition device 13 discretizes the analog signal into a digital signal and transmits it to the data demodulation and display device 14.

[0039] The data demodulation and display device 14 demodulates the collected data. The demodulation process is as follows: Figure 3 As shown. The specific process is: the data demodulation and display device 14 first performs bandpass filtering on the digital signal transmitted from the data acquisition device 13 to eliminate the interference of out-of-band noise; then performs Hilbert transform on the filtered signal to transform the real-valued signal into a complex signal; then passes through different matching filters in turn to perform pulse compression, and at the same time separates the signals of different frequency bands under time-frequency division multiplexing, that is, corresponding matching filters are designed for the signals of different multiplexing channels, matching filtering is performed on the signals that meet the channel, and non-matching filtering is performed on the signals that do not meet the channel, where the number of matching filters is the number of multiplexing channels. N ; Finally, the amplitude difference method is used to locate the local discharge event, and the phase is solved by phase unwrapping to quantitatively restore the characteristics of the local discharge.

[0040] The data demodulation and display device 14 runs the host computer software while demodulating the collected data to alarm, record and display the cable partial discharge event.

Claims

1. A multi-phase coded φ-OTDR cable partial discharge monitoring system, characterized in that: The invention comprises a narrow line width laser (1), wherein the narrow line width laser (1) is connected to a port a of an optical fiber 1*2 coupler (2). The b port of the optical fiber 1*2 coupler (2) is connected to the lithium niobate electro-optic intensity modulator (3), the optical fiber acousto-optic modulator (4) and the erbium-doped optical fiber amplifier (5) in sequence, the output end of the erbium-doped optical fiber amplifier (5) is connected to the a port of the optical fiber circulator (6), the b port of the optical fiber circulator (6) is connected to the sensing optical fiber (15), the detection pulse is injected into the sensing optical fiber (15), the sensing optical fiber (15) is tightly attached to the cable to be tested (16), and the end of the cable (16) is connected to the optical fiber isolator (11); The c port of the optical fiber 1*2 coupler (2) is connected to the b port of the optical fiber 2*2 coupler (10), the c port of the optical fiber circulator (6) is connected to the a port of the optical fiber 2*2 coupler (10), the c and d ports of the optical fiber 2*2 coupler (10) are connected to the photoelectric balance detector (12), and the photoelectric balance detector (12) is connected to the data acquisition device (13) and the data demodulation and display device (14) in sequence; The lithium niobate electro-optic intensity modulator (3) generates a time-frequency division multiplexing signal from an arbitrary wave signal source (9), which is amplified by a first radio frequency amplifier (7) and then driven to modulate the signal light into a multi-frequency signal light, thereby realizing the time-frequency division multiplexing of the monitoring system; The optical fiber acousto-optic modulator (4) generates a multi-phase coded signal from an arbitrary wave signal source (9), which is driven after being amplified by a second radio frequency amplifier (8) to modulate the multi-frequency signal light into a multi-phase coded pulse light, thereby realizing time-frequency division multiplexing under a multi-phase coded modulation format; The sub-channel bandwidth of the frequency division multiplexing signal is equal to the signal bandwidth of the multi-frequency multi-phase coded pulse light, and the total frequency bandwidth of the multiplexing is greater than the sum of the bandwidths of each sub-channel; the number of sub-pulse sequences in the time division multiplexing is equal to the number of sub-channels N , the sub-pulse duration is the multi-phase coded pulse duration w , each sub-pulse contains only one channel frequency, satisfying N <2 nL / cw ,in n is the refractive index of the optical fiber, L is the sensing optical fiber length, c is the speed of light; A raised cosine roll-off filter is used in the fiber acousto-optic modulator (4) to suppress the sidelobe energy of the multi-phase coded pulse.

2. The multi-phase coded φ-OTDR cable partial discharge monitoring system according to claim 1, characterized in that: The sensing optical fiber (15) is tightly wound around the cable to be tested (16) to perform distributed monitoring and positioning of local discharge events. When local discharge occurs in the cable to be tested (16), the area around the discharge source is instantly heated and expanded. After the discharge ends, the expanded area returns to its original volume. When the next discharge pulse arrives, the expansion and recovery process is repeated. This regular expansion and contraction makes the insulating medium an ultrasonic signal source. The generated ultrasonic signal is diffused and transmitted to the sensing optical fiber (15) through the body of the cable to be tested, causing the optical fiber core diameter, optical fiber length and refractive index of the sensing optical fiber (15) to change accordingly, thereby modulating the phase of the optical signal in the sensing optical fiber (15). Local discharge event positioning and restoration can be achieved by demodulating the phase.

3. The multi-phase coded φ-OTDR cable partial discharge monitoring system according to claim 1, characterized in that: The b and c ports of the optical fiber 1*2 coupler (2) output two optical signals with a splitting ratio of 90:

10.

4. The multi-phase coded φ-OTDR cable partial discharge monitoring system according to claim 1, characterized in that: The sensing optical fiber (15) is tightly wound around the cable to be tested (16), with a winding density of not less than 50 turns / meter, and the outer layer is protected by a heat shrink tube.

5. A working method of the multi-phase coded φ-OTDR cable partial discharge monitoring system as claimed in claim 1, 2, 3 or 4, characterized in that: The following steps are included: S100: The optical signal output from port b of the optical fiber 1*2 coupler (2) passes through the lithium niobate electro-optic intensity modulator (3) and the optical fiber acousto-optic modulator (4) in sequence, and is modulated into a multi-frequency and multi-phase coded pulse light; S200: After the peak power of the multi-frequency multi-phase coded pulse light is amplified by the erbium-doped fiber amplifier (5), it becomes the detection pulse input port a of the fiber circulator (6), and the multi-frequency multi-phase coded pulse light is injected into the sensing fiber (15). The sensing fiber (15) is tightly attached to the cable to be tested (16) to sense and locate the local discharge event of the cable to be tested (16). The end of the sensing fiber (15) is connected to the fiber isolator (11) to remove the Fresnel reflection peak. The backward Rayleigh scattered light generated in the sensing fiber (15) is returned to the port b of the fiber circulator (6), and then output from the port c of the fiber circulator (6) to the port a of the fiber 2*2 coupler (10); S300: the c port of the optical fiber circulator (6) outputs an optical signal to the a port of the optical fiber 2*2 coupler (10), the light output from the c port of the optical fiber 1*2 coupler (2) enters from the b port of the optical fiber 2*2 coupler (10) as reference light, and the backward Rayleigh scattered light and the reference light are subjected to beat frequency in the optical fiber 2*2 coupler (10); S400: The beat frequency light is proportionally output to the optical input port of the photoelectric balanced detector (12) through the c and d ports of the optical fiber 2*2 coupler (10), and the beat frequency light signal is converted into an electrical signal; the photoelectric balanced detector (12) outputs the electrical signal to the data acquisition device (13), the data acquisition device (13) discretizes the analog electrical signal into a digital quantity, and transmits it to the data demodulation and display device (14), the data demodulation and display device (14) demodulates and analyzes the collected data, and displays the partial discharge position information and discharge waveform of the cable to be tested.

6. The working method of the multi-phase coded φ-OTDR cable partial discharge monitoring system according to claim 5 is characterized in that: The lithium niobate electro-optic intensity modulator (3) generates a time-frequency division multiplexing signal from an arbitrary wave signal source (9), which is driven after being amplified by a first radio frequency amplifier (7) to modulate the signal light into multi-frequency signal light, thereby realizing the time-frequency division multiplexing of the monitoring system.

7. The working method of the multi-phase coded φ-OTDR cable partial discharge monitoring system according to claim 6 is characterized in that: The sub-channel bandwidth of the frequency division multiplexing signal is equal to the signal bandwidth of the multi-frequency multi-phase coded pulse light, and the total frequency bandwidth of the multiplexing is greater than the sum of the bandwidths of each sub-channel; the number of sub-pulse sequences in the time division multiplexing is equal to the number of sub-channels N , the sub-pulse duration is the multi-phase coded pulse duration w , each sub-pulse contains only one channel frequency, satisfying N <2 nL / cw ,in n is the refractive index of the optical fiber, L is the sensing optical fiber length, c The speed of light.

8. The working method of the multi-phase coded φ-OTDR cable partial discharge monitoring system according to claim 6 is characterized in that: The optical fiber acousto-optic modulator (4) generates a multi-phase coded signal from an arbitrary wave signal source (9), which is driven after being amplified by a second radio frequency amplifier (8) to modulate the multi-frequency signal light into a multi-phase coded pulse light, thereby realizing time-frequency division multiplexing under a multi-phase coded modulation format.

9. The working method of the multi-phase coded φ-OTDR cable partial discharge monitoring system according to claim 8, characterized in that: In the fiber acousto-optic modulator (4), a raised cosine roll-off filter is used to suppress the sidelobe energy of the multi-phase coded pulse, so as to prevent the pulse energy of different frequencies from aliasing during the time-frequency division multiplexing process, thereby increasing the number of time-frequency division multiplexing channels and improving the frequency response range of the system, so that the local discharge event of the cable under test can be captured without distortion. The roll-off coefficient of the raised cosine roll-off filter is set to 0.2-0.3, and the sidelobe suppression ratio is greater than or equal to 30 dB.

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