Method and system for measuring pure light CT of direct current system under fast transient large current and simulation method

Abstract

The invention provides a direct current system pure light CT measurement method and system under fast transient large current and a simulation method, and the method comprises the steps: setting a threshold current Ith, the interval time N of a sample during differential operation, and the time Tdown for keeping in a large current algorithm as preset values; and a condition that a measurement error is generated when a fast transient large current is encountered is avoided by utilizing reasonable setting of a parameter N in a pure light CT (Computed Tomography) demodulation algorithm. Different demodulation algorithms (an Arctan demodulation method of small current and a di / dt integral demodulation method of large current) are selected according to the magnitude of the current, so that the rapid transient large current can be measured more accurately. According to the method, the measurement defects of a traditional pure light CT are overcome, accurate data measurement under rapid transient large current can be achieved, current state quantities of various devices under similar lightning stroke working conditions can be reproduced and analyzed, and the method has remarkable practical value.

Description

Technical Field

[0001] The present invention belongs to the technical field of transient stability of electric power systems, and relates to a measurement method, system and simulation method of a pure optical CT of a DC system under fast transient large current. Background Art

[0002] UHV DC transmission has obvious advantages in large-capacity long-distance power transmission due to its large transmission capacity, fast control response speed, and no synchronous operation stability problems. It has become one of the most important ways of power transmission and regional networking, and plays an important role in vigorously developing distributed clean energy. UHV DC transmission technology can achieve cross-regional and cross-national optimal allocation of power resources. It can efficiently and stably transmit abundant renewable energy (such as wind energy, solar energy, etc.) far away from the load center to areas with high power demand, promote the optimization and adjustment of the energy structure, reduce dependence on traditional fossil energy, and promote green and low-carbon development.

[0003] UHV DC transmission technology has greatly increased the transmission capacity of the line by increasing the transmission voltage level. Compared with traditional AC transmission, DC transmission can transmit greater power under the same line diameter, reduce energy loss during transmission, and improve overall transmission efficiency. This is crucial to meet the needs of large-scale, long-distance power transmission, especially in areas where energy resources and load centers are unevenly distributed.

[0004] It is not uncommon for UHV DC lines to be struck by lightning in actual operation, and many pole locking accidents caused by lightning have occurred in power grid operation. Photocurrent transformer (optical CT) is an important device for control and protection of DC transmission system and voltage collection of measurement and recording system. It has the characteristics of light weight, small size and wide frequency band response. The use of optical CT plays a vital role in ensuring the stable operation of DC transmission system.

[0005] At present, scholars have conducted a lot of research on the lightning resistance performance of straight lines and the pole blocking accidents of DC lines, but there are few studies on the pole blocking caused by the fault of the optical current transformer. In actual engineering, pure optical CT has great difficulty in identifying fast transient large currents, and because the steady-state current accuracy test requires the accuracy of the readings, the fast transient large currents caused by lightning strikes are rarely considered in the measurement. Summary of the invention

[0006] In view of the problems existing in the prior art, the present invention provides a measurement method, system and simulation method of pure optical CT of DC system under fast transient large current, which solves the measurement drawbacks of traditional pure optical CT and can realize accurate data measurement under fast transient large current, which is conducive to reproducing and analyzing the current state quantity of various equipment under similar lightning strike conditions, and has significant practical value.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for measuring pure optical CT of a DC system under fast transient high current, comprising:

[0009] Convert the interference light intensity into a photocurrent i(t); convert the photocurrent i(t) into a voltage signal and then into a digital signal i[n];

[0010] The amplitude V1 of the fundamental wave and the amplitude V2 of the second harmonic are obtained based on the digital signal i[n], and the current to be measured I(T) is obtained after demodulation processing; the demodulation methods include the Arctan demodulation method for small currents and the di / dt integral demodulation method for large currents;

[0011] The threshold current I th , the sample spacing time N during differential operation, and the time T to stay within the high current algorithm down are set to the default values.

[0012] When the current to be measured I(T) th When , the Arctan demodulation method for small current is used;

[0013] When the measured current I(T)>I th When using the di / dt integral demodulation method for large currents;

[0014] After the obtained current to be measured I(T) is processed by temperature compensation, the correct test current I is obtained.

[0015] Preferably, the threshold current I th The default value is I th =16384, representing the threshold current of 11kA, N is an integer between 0 and 10; T down Set to an integer multiple of ms.

[0016] Preferably, when N=0-3, a balance between wide pulses and narrow pulses of large current is achieved.

[0017] Preferably, the specific process of the small current Arctan demodulation method is:

[0018] The phase shift produced by the current to be measured I(T) is located in the first quadrant, which can realize the one-to-one correspondence between current and phase, thereby demodulating the real test current I

[0019] Preferably, the specific process of the large current di / dt integral demodulation method is:

[0020] The rate of change of current is calculated according to the current signal, that is,

[0021] Get the current based on the time integral​

[0022] in,

[0023] In numerical calculation, the above differential calculation is converted into the test current I obtained by differential calculation, and the calculation formula is:

[0024] Wherein, Δt is the time interval, ΔI is the actual current change rate; the value of the time interval Δt is 1 / 10 of the minimum pulse width.

[0025] Preferably, the specific process of converting the interference light intensity into the photocurrent i(t) is:

[0026] The wide-spectrum light emitted by the light source enters the optical fiber polarizer through a circulator and becomes linearly polarized light. After a 45° fusion angle, it is evenly decomposed into two orthogonal axes of the polarization-maintaining optical fiber and propagates independently. After the forward modulation phase is introduced by the phase modulator and a corresponding delay, the two beams of orthogonal linear polarized light are converted into a pair of orthogonal circularly polarized light by the 45° fused 1 / 4 wave plate and enter the sensor coil; due to the Faraday effect, after being reflected by the reflector at the end of the sensor fiber coil, the polarization states of the two beams of light are exchanged and returned, and doubled on the way back; after passing through the 1 / 4 wave plate, the orthogonal circularly polarized light is restored to orthogonal linear polarized light, and after the backward modulation phase is introduced by the phase modulator, interference occurs at the polarizer, and the interference light intensity I out It is received by the photodetector, and the interference light intensity is converted into photocurrent through the photodetector.

[0027] Preferably, the calculation formula of the photocurrent i(t) is:

[0028]

[0029] Ψ m (t) = Ψ0cos(ω m t)

[0030] F=VNI

[0031] Among them, m (t) is the sinusoidal modulation phase provided by the PZT modulator, Ψ0 is the modulation depth, ω m is the modulation angular frequency;

[0032] V is the Verdet constant of the sensing fiber, N is the number of turns of the sensing coil, and I is the magnitude of the current to be measured;

[0033] Preferably, the specific process of the measured current I(T) undergoing temperature compensation is as follows:

[0034] According to the output signal of the temperature sensor and the preset temperature compensation algorithm, the current value that needs to be adjusted is calculated, and according to the calculation result, the amplitude or phase of the current pulse is adjusted, and the adjusted current pulse is output as the correct test current I.

[0035] A measurement system for pure optical CT of a DC system under fast transient high current, comprising:

[0036] Photoelectric detector, signal amplification / sampling unit, lock-in amplifier, signal demodulation module and temperature compensation module,

[0037] The signal amplification / sampling unit includes a transimpedance amplifier and an ADC sampling unit;

[0038] The photodetector converts the interference light intensity into photocurrent, converts the photocurrent i(t) into a voltage signal through a transimpedance amplifier, and finally converts it into a digital signal i[n] through an ADC sampling unit; the amplitude V1 of the fundamental wave and the amplitude V2 of the second harmonic are obtained through a phase-locked amplifier, V1 and V2 are passed through a signal demodulation module to obtain a current to be measured I(T), and the current to be measured I(T) is passed through a temperature compensation module to obtain a correct test current I.

[0039] The signal demodulation module includes an Arctan demodulation module for small currents and a di / dt integral demodulation module for large currents.

[0040] A simulation method for measuring pure optical CT of a DC system under fast transient high current, comprising:

[0041] By simulating a triangle wave with a peak current of 80kA and a pulse width of 40μs, that is, the output results under different N values ​​when the current change rate is di / dt=400kA / ms;

[0042] When N=0, I th =16384,T down =2ms, time interval Δt=0.4μs, for a 40μs changing pulse, the test is correct;

[0043] When N=3, I th =16384, Tdown = 2ms, time interval Δt = 3.2μs, for a 40μs change pulse, the test error is less than -1kA;

[0044] When N=7, I th =16384,T down =2ms, the time interval Δt=51.2μs. For the 40μs changing pulse, the test is totally inaccurate.

[0045] Compared with the prior art, the present invention has the following beneficial technical effects:

[0046] The present invention provides a measurement method, system and simulation method of pure optical CT of a DC system under fast transient large current, by setting a threshold current I th , the sample spacing time N during differential operation, and the time T to stay within the high current algorithm down All are set to preset values, and the reasonable setting of parameter N in the pure light CT demodulation algorithm is used to avoid the situation where measurement errors occur when encountering fast transient large currents. By selecting different demodulation algorithms (Arctan demodulation method for small currents and di / dt integral demodulation method for large currents) according to the current size, fast transient large currents can be measured more accurately. This adaptive demodulation strategy effectively avoids the errors that may be generated by traditional pure light CT when measuring fast transient large currents. By setting reasonable parameters such as threshold current Ith, sample spacing time N during differential operation, and time Tdown maintained within the large current algorithm, this method can flexibly adapt to the measurement requirements of different current ranges, and improve the applicability of pure light CT in various application scenarios. The present invention solves the measurement drawbacks of traditional pure light CT, can realize accurate data measurement under fast transient large currents, is conducive to reproducing and analyzing the current state quantities of various types of equipment under similar lightning strike conditions, and has significant practical value.

[0047] Furthermore, the method of the present invention can reproduce and analyze the current state quantities of various types of equipment under similar lightning strike conditions, and has important practical value for evaluating the performance of equipment under extreme conditions, optimizing equipment design, and improving protection strategies.

[0048] Furthermore, this method solves the challenges faced by traditional pure optical CT in measuring fast transient large currents by introducing adaptive demodulation algorithms and temperature compensation technology, promoting the innovation and development of power measurement technology.

[0049] Furthermore, by performing temperature compensation processing on the current to be measured I(T), the measurement error caused by temperature change can be further reduced, thereby ensuring the accuracy and reliability of the measurement result.

[0050] Furthermore, when the parameter N is set to 7, measurement anomalies will occur when encountering fast transient large currents. This is because at the beginning of operation and maintenance, the transient current changes were considered according to the typical TPE waveform, and the convenience of the laboratory's 6PU steady-state current accuracy test required several minutes to increase the current reading was met, without considering the large current narrow pulse conditions caused by lightning strikes. Combined with the simulation results, N = 0 to 3 can be selected to achieve a balance between wide pulses and narrow pulses of large currents, and the duration of large currents in actual conditions is very short, so there is no need to worry about the impact of digital truncation errors. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1It is the signal processing flow of pure optical CT;

[0052] Figure 2 N = 7, pulse width = 40 μs, I th =11kA, T down =Simulation results at 2ms;

[0053] Figure 3 N = 3, pulse width = 40μs, I th =11kA, T down =Simulation results at 2ms;

[0054] Figure 4 N = 0, pulse width = 40 μs, I th =11kA, T down =Simulation results at 2ms;

[0055] Figure 5 It is a comparative recording diagram of three channels with different configurations;

[0056] Figure 6 It is a comparative waveform diagram of three channels with the same configuration;

[0057] Figure 7 A flowchart of an embodiment. DETAILED DESCRIPTION

[0058] The present invention is further described in detail below in conjunction with specific embodiments, which are intended to explain the present invention rather than to limit it.

[0059] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0060] For pure optical CT measurement under fast transient high current to produce large errors, two conditions must be met at the same time: first, it must enter the high current mode, that is, the current peak is greater than 11kA; second, the pulse width changes very quickly, less than or comparable to the Δt corresponding to the N value.

[0061] The present invention provides a method for setting measurement parameters of a pure optical CT of a DC system under fast transient high current, comprising:

[0062] The wide-spectrum light emitted by the light source enters the optical fiber polarizer through the circulator and becomes linearly polarized light. After a 45° fusion angle, it is evenly decomposed into two orthogonal axes of the polarization-maintaining optical fiber and propagates independently. After the phase modulator introduces the forward modulation phase and the corresponding delay, the two beams of orthogonal linear polarized light are converted into a pair of orthogonal circularly polarized light by the 1 / 4 wave plate fused at 45° and enter the sensing coil; due to the Faraday effect, the magnetic field generated by the current will introduce a Faraday phase shift of 2F between the two beams of orthogonal circularly polarized light, where F=VNI, V is the Verdet constant of the sensing optical fiber, N is the number of turns of the sensing coil, and I is the magnitude of the current to be measured. After being reflected by the reflector at the end of the sensing optical fiber coil, the polarization states of the two beams of light are interchanged and returned. Since the Faraday phase shift is a non-reciprocal phase, it is doubled on the way back; after passing through the 1 / 4 wave plate, the orthogonal circularly polarized light is restored to orthogonal linear polarized light. After the modulator introduces the backward modulation phase, interference occurs at the polarizer, and the interference light intensity I out received by the light detector.

[0063] The photodetector converts the interference light intensity into photocurrent.

[0064]

[0065] Ψ m (t) = Ψ0cos(ω m t)

[0066] Among them, m (t) is the sinusoidal modulation phase provided by the PZT modulator, Ψ0 is the modulation depth, ω m is the modulation angular frequency.

[0067] The signal processing flow of pure optical CT is as follows Figure 1 and Figure 7 As shown in the figure: First, the photocurrent i(t) is converted into a voltage signal through a transimpedance amplifier, and finally converted into a digital signal i[n] through ADC sampling; then, the amplitudes V1 and V2 of the fundamental wave and the second harmonic are obtained through a phase-locked amplifier; V1 and V2 are passed through a signal demodulation module to obtain the current to be measured I(T), and this current value is affected by temperature; in order to eliminate the influence of temperature on the current to be measured I(T), the temperature compensation module is used to obtain the correct test current I, and finally output it.

[0068] In order to meet the needs of a wide range of current tests, the signal demodulation module is divided into the Arctan demodulation method for small currents and the integral demodulation method for large currents.

[0069] The signal demodulation module is divided into the Arctan demodulation method for small currents and the integral demodulation method for large currents. The Arctan demodulation method is characterized by accurate demodulation and can meet more than 99.99% of daily working scenarios. However, the Arcatan demodulation algorithm may not be able to demodulate correctly under large currents. Therefore, the di / dt integral demodulation method is used to achieve large current demodulation.

[0070] Its specific function is to set the threshold current to I th =11kA (the phase shift produced by 20 turns of optical fiber is 4F=0.88 radians, which falls in the first quadrant of the coordinate axis).

[0071] When the test current I th When the demodulation module uses the small current Arctan demodulation method, because the phase shift generated by the current to be measured is located in the first quadrant, the one-to-one correspondence between current and phase can be achieved, and the real test current can be demodulated. The Arctan demodulation method is characterized by accurate demodulation and can almost meet more than 99.99% of daily working scenarios. This is because the rated working current is 5kA, I th It is almost 2p.u. of the rated current. In the absence of faults or accidents, the Arcatan demodulation method can meet almost all working conditions. However, the Arcatan demodulation algorithm cannot meet the test of large currents, because the phase generated by large currents may fall into the four quadrants of the coordinate axis, or even exceed the 2π phase, so it cannot be demodulated correctly.

[0072] When the measured current I(T)>I th When using the di / dt integral demodulation method for large currents;

[0073] In order to achieve large current demodulation, the di / dt integral demodulation method can be used. Its basic principle is: calculate the current change rate I' according to the signal, that is, Get the current based on the time integral Theoretically, this method can realize the demodulation of any large current, but it must be processed in the implementation process to obtain accurate test values. First, the accuracy of the I' value must be considered. In numerical calculations, differential calculations will be converted into difference calculations. Therefore, it is necessary to choose the appropriate Δt value to ensure When the current signal changes very quickly, such as short-circuit current, not only is the peak value large (may be greater than 20KA), but the pulse width is also narrow (generally <50μs). You must choose a pulse width as small as 1μs to obtain an accurate solution. value; if Δt is not chosen properly, such as greater than 10μs, then the calculated ​It will bring wrong values. If the current signal changes slowly, such as the current change cycle in the TPE test is about 50ms, that is, Δt is 51.2μs, accurate test results can also be obtained. Second, in the calculation process, due to the truncation error caused by the numerical calculation process, long-term integration may bring certain errors. However, since the high current lasts for a short time and the integration time is short, it will not bring a large error and can meet the test requirements of the State Grid.

[0074] For the parameters N, T in the demodulation algorithm down and I th , where N is an integer between 0 and 10, representing the sample interval time (2^N*0.4) μs during the differential operation; T down Indicates the time to stay within the high current algorithm, which can be set to an integer multiple of ms; I th =16384(2^14), representing a threshold current of 11kA. Ensuring that Δt is 1 / 10 of the minimum pulse width can effectively avoid calculation errors.

[0075] The simulation simulates the output results under different N values ​​when the peak current is 80kA and the pulse width is 40μs, that is, the current change rate is di / dt=400kA / ms. Figure 5 and Figure 6 As shown,

[0076] Simulation condition 1: N = 7, I th =16384,T down =2ms, the corresponding slope calculation time = 51.2μs, the simulation results are as follows Figure 2 As shown in the figure, the orange curve is the current measured by the optical CT, and its shape is basically consistent with the current shape during the on-site fault. Not only is the test error very large, but there is also a higher "platform" after the standard current disappears, with an error of about 8.6kA until T down =Disappears after 2ms.

[0077] Simulation condition 2: N = 3, Ith = 16384, Tdown = 2ms, the corresponding slope calculation time is 3.2μs, the simulation results are as follows Figure 3 As shown, the orange curve is the current measured by optical CT, and its shape is close to the standard current of the blue curve. However, after the actual current is zero, the orange curve still has a "platform" with an error of about -1kA, which disappears after Tdown = 2ms.

[0078] Simulation condition 3: N = 0, I th =16384,T down =2ms, the corresponding slope calculation time Δt = 0.4μs simulation results are as follows Figure 4As shown, the orange curve is the optical CT measurement current, which coincides with the blue curve standard current. The test current is completely consistent with the standard current, with almost no additional error.

[0079] Comparing and analyzing the simulation results, all these errors come from the fact that if the time interval Δt is not matched with the actual current change rate during the calculation process, This eventually leads to calculation errors. When N = 0, Δt = 0.4μs, the test can be performed correctly for a 40μs pulse change; when N = 7, Δt = 51.2μs, the test is completely inaccurate for a 40μs pulse change. The error comes from the accumulated error during the entire current test process. When the duration of the high current mode ends, the test result switches to the correct test value of the low current mode.

[0080] For comparison, the same simulation is performed with the same triangle wave 80kA peak value but the pulse width changed to 160μs. When the pulse is 160μs (signal speed is about 50kHz), N=7 will still produce a large error, but when N=5, Δt=12.8μs, the error is less than 1%; when N=3, there is almost no error.

[0081] In summary, for optical CT measurement to produce large errors, two conditions must be met at the same time. The first is that it must enter the high current mode, that is, the current peak is greater than 11kA, and the second is that the pulse width changes very quickly and is less than or comparable to the Δt corresponding to the N value. Setting an appropriate N value to ensure that Δt is 1 / 10 of the minimum pulse width can effectively avoid calculation errors.

[0082] Before the measurement anomaly occurred, N=7 was set. This was because the transient current change was considered to be in accordance with the typical TPE waveform, and the laboratory's 6PU steady-state current accuracy test required several minutes to increase the current reading convenience. The high current narrow pulse condition caused by the lightning strike in northern Shaanxi was not taken into account. At present, combined with the simulation results, N=0~3 can be selected to achieve a balance between wide pulses and narrow pulses of large currents. In actual working conditions, the duration of large currents is very short, so there is no need to worry about the impact of digital truncation errors. Of course, the final parameter selection must be verified by experiment.

[0083] Embodiment 2:

[0084] From the principle of pure optical CT, we know that the current is ultimately equivalent to the phase of light during the calculation process. In order to achieve rapid changes in large currents, a 1800-turn sensor head is used instead of a 20-turn sensor head for experimental verification. This is equivalent to applying a unit current to a 20-turn sensor head to produce a phase, while applying 1 / 90 of the unit current to a 1800-turn sensor head can produce the same phase.

[0085] Based on the simulation parameters, set CH1 N=7, Tdown =2; CH2 N = 3, T down =2; CH3 N = 3, T down＝ 0, such as Figure 5 It can be seen that the yellow channel of CH1 reproduces the problem on site. After the N of the green channel of CH2 is changed from 7 to 3, the platform drops significantly, and there is still a small step. The red channel of CH3 has N=3, T down =0There is no platform phenomenon.

[0086] Set the three channels to the same parameters N = 3, T down =0, the three sets of responses are basically consistent, such as Figure 6 Show.

[0087] According to N=3, T down =0, the steady-state accuracy verification is carried out from 0.1pu to 5p.u., and the relevant results are shown in Table 1 below. The test results meet the technical specification requirements.

[0088] Table 1 Steady-state current accuracy verification

[0089]

[0090] In summary, the method for setting measurement parameters of a pure optical CT of a DC system under fast transient and large currents involved in the present invention can effectively avoid measurement abnormalities of a pure optical CT of a DC system under fast transient and large currents, has better accuracy, and is more suitable for data measurement during lightning strikes in current UHV DC systems than the parameter setting method for typical TPE waveforms.

[0091] It should be noted that the terms "including" and "having" and any variations thereof in the specification and claims of the present invention and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or apparatus.

[0092] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0093] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any ordinary technician in the industry can smoothly implement the present invention as shown in the drawings and described above. However, any equivalent changes, modifications and evolutions made by technicians familiar with the profession without departing from the scope of the technical solution of the present invention using the technical content disclosed above are all equivalent embodiments of the present invention. At the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the technical solution of the present invention.

Claims

1. A method for measuring pure optical CT of a DC system under fast transient high current, characterized in that: include, Convert the interference light intensity into photocurrent; convert the photocurrent into voltage signal and then into digital signal; The amplitude of the fundamental wave and the amplitude of the second harmonic are obtained based on the digital signal, and the current to be measured I(T) is obtained after demodulation processing; The demodulation methods include Arctan demodulation for small currents and di / dt integral demodulation for large currents. The threshold current I th , the sample spacing time N during differential operation, and the time T to stay within the high current algorithm down All are set to the preset values; When the current to be measured I(T) th When , the Arctan demodulation method for small current is used;​ When the measured current I(T)>I th When using the di / dt integral demodulation method for large currents; The demodulated measured current I(T) is subjected to temperature compensation to obtain the correct test current I.

2. The method for measuring pure optical CT of a DC system under fast transient high current according to claim 1 is characterized in that: The threshold current I th The default value is I th =16384, representing the threshold current of 11kA, N is an integer between 0 and 10; T down Set to an integer multiple of ms.

3. The method for measuring pure optical CT of a DC system under fast transient high current according to claim 2 is characterized in that: When N=0-3, a balance between wide pulses and narrow pulses of large current is achieved.

4. The method for measuring pure optical CT of a DC system under fast transient high current according to claim 1 is characterized in that: The specific process of the Arctan demodulation method for small current is: The phase shift produced by the current to be tested I(T) is located in the first quadrant, which can realize a one-to-one correspondence between current and phase, thereby demodulating the real test current I.

5. The method for measuring pure optical CT of a DC system under fast transient high current according to claim 1 is characterized in that: The specific process of the di / dt integral demodulation method of large current is as follows: The rate of change of current is calculated according to the current signal, that is, Get the current based on the time integral in, In numerical calculation, the above differential calculation is converted into the test current I obtained by differential calculation, and the calculation formula is: Wherein, the time interval Δt is the actual current change rate; the value of the time interval Δt is 1 / 10 of the minimum pulse width.

6. The method for measuring pure optical CT of a DC system under fast transient high current according to claim 1 is characterized in that: The specific process of converting interference light intensity into photocurrent i(t) is: The wide-spectrum light emitted by the light source enters the optical fiber polarizer through a circulator and becomes linearly polarized light. After a 45° fusion angle, it is evenly decomposed into two orthogonal axes of the polarization-maintaining optical fiber and propagates independently. After the forward modulation phase is introduced by the phase modulator and a corresponding delay, the two beams of orthogonal linear polarized light are converted into a pair of orthogonal circularly polarized light by the 45° fused 1 / 4 wave plate and enter the sensor coil; due to the Faraday effect, after being reflected by the reflector at the end of the sensor fiber coil, the polarization states of the two beams of light are exchanged and returned, and doubled on the way back; after passing through the 1 / 4 wave plate, the orthogonal circularly polarized light is restored to orthogonal linear polarized light, and after the backward modulation phase is introduced by the phase modulator, interference occurs at the polarizer, and the interference light intensity I out It is received by the photodetector, and the interference light intensity is converted into photocurrent through the photodetector.

7. The method for measuring pure optical CT of a DC system under fast transient high current according to claim 6, characterized in that: The calculation formula of photocurrent i(t) is: P m (t)=Ψ0cos(ω m t) F=VNI Among them, m (t) is the sinusoidal modulation phase provided by the PZT modulator, Ψ0 is the modulation depth, ω m is the modulation angular frequency; I0 is the initial photocurrent, V is the Verdet constant of the sensing optical fiber, N is the number of turns of the sensing coil, and I is the current to be measured.

8. The method for measuring pure optical CT of a DC system under fast transient high current according to claim 1, characterized in that: The specific process of temperature compensation of the measured current I(T) is as follows: According to the output signal of the temperature sensor and the preset temperature compensation algorithm, the current value that needs to be adjusted is calculated, and according to the calculation result, the amplitude or phase of the current pulse is adjusted, and the adjusted current pulse is output as the correct test current I.

9. A measurement system for pure optical CT of a DC system under fast transient high current, based on a measurement method for pure optical CT of a DC system under fast transient high current according to any one of claims 1 to 8, characterized in that: include: Photoelectric detector, signal amplification / sampling unit, lock-in amplifier, signal demodulation module and temperature compensation module; The signal amplification / sampling unit includes a transimpedance amplifier and an ADC sampling unit; The photodetector converts the interference light intensity into photocurrent, converts the photocurrent i(t) into a voltage signal through a transimpedance amplifier, and finally converts it into a digital signal i[n] through an ADC sampling unit; obtains the amplitude V1 of the fundamental wave and the amplitude V2 of the second harmonic through a phase-locked amplifier, and obtains the current to be measured I(T) through a signal demodulation module through V1 and V2. The current to be measured I(T) passes through a temperature compensation module to obtain a correct test current I; The signal demodulation module includes an Arctan demodulation module for small currents and a di / dt integral demodulation module for large currents.

10. A simulation method for measuring pure optical CT of a DC system under fast transient high current, based on a method for measuring pure optical CT of a DC system under fast transient high current according to any one of claims 1 to 8, characterized in that: include, By simulating a triangle wave with a peak current of 80kA and a pulse width of 40μs, that is, the output results under different N values ​​when the current change rate is di / dt=400kA / ms; When N=0, I th =16384,T down =2ms, time interval Δt=0.4μs, for a 40μs changing pulse, the test is correct; When N=3, I th =16384, Tdown = 2ms, time interval Δt = 3.2μs, for a 40μs change pulse, the test error is less than -1kA; When N=7, I th =16384,T down =2ms, the time interval Δt=51.2μs. For the 40μs changing pulse, the test is totally inaccurate.

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