Optical current transformer demodulation method and system under transient fast large current, and medium

Abstract

The invention discloses a method for demodulating an optical current transformer under transient fast large current, and discloses a system and a medium with the method for demodulating the optical current transformer under the transient fast large current. The optical current transformer demodulation method under transient fast large current combines a piezoelectric ceramic modulation type optical CT demodulation algorithm of an arc tangent arctan algorithm and an integration algorithm, demodulation of transient current and steady-state current is realized through switching of the two algorithms, the demodulation logic of transient large current can be adapted, and the optical current transformer demodulation method under transient fast large current can be applied to the field of transient current demodulation. And the normal measurement performance can be recovered in time after the transient large current is ended.

Description

Technical Field

[0001] The present invention relates to the technical field of power system measurement, and in particular to a method and system for demodulating an optical current transformer under transient fast large current. Background Art

[0002] The New Power System Action Plan specifies the coordinated development of AC and DC power grids, strengthens the digital transformation of power grids, and improves operational safety and efficiency. Optical current transformers (CTs), as a key measurement device in DC power grids, account for over 95% of current measurements and are a key component in ensuring safe operation of DC power grids.

[0003] On March 2, 2023, the lithium niobate modulated optical CT at the Wuhan Converter Station experienced measurement anomalies under transient currents. After the fault current ended, measurement performance could not be restored, causing protection to trip. On April 28, 2024, a lightning strike struck the Shaanxi Northern Converter Station. Two of the three measurement channels of the piezoelectric ceramic modulated optical CT experienced anomalies, resulting in a tailing of the measured current after the fault current ended, and the output of the two-out-of-three protection device locked out pole 1.

[0004] The above faults reveal that both the piezoelectric ceramic modulated optical CT and demodulation logic have defects when encountering transient, fast, and large currents, posing a risk of unrecoverable measurement performance abnormalities. Summary of the Invention

[0005] The present invention aims to address at least one of the technical problems existing in the prior art. To this end, the present invention proposes a method for demodulating an optical current transformer under transient, rapid, and high current conditions. The method combines a piezoelectric ceramic-modulated optical CT demodulation algorithm with an arctan algorithm and an integral algorithm, switching between the two algorithms to achieve demodulation of transient and steady-state currents. Furthermore, the method employs a lithium niobate-modulated optical CT demodulation algorithm with 2Nπ phase compensation. This algorithm addresses the problem of closed-loop demodulation algorithms losing 2Nπ under transient current conditions. Phase compensation is then applied during the next demodulation cycle to restore the optical CT's measurement performance after the transient current ends.

[0006] The present invention also provides a system and a medium having the above optical current transformer demodulation method under transient fast large current.

[0007] According to the first aspect of the present invention, the optical current transformer demodulation method under transient fast large current is characterized in that it includes the following steps:

[0008] Determine the appropriate demodulation strategy based on the magnitude of the polarization angle change caused by the current;

[0009] When the polarization angle change generated by the current is less than or equal to π / 2, the photodetector converts the interference light intensity into a photocurrent, then performs a Bessel expansion on the photocurrent to extract the amplitudes of the first four harmonic components of the interference signal. Based on the amplitudes, a demodulation formula for the current signal to be measured of the optical current transformer is calculated;

[0010] When the angle change generated by the current exceeds π / 2, the detection signal is decomposed to obtain a sine signal and a cosine signal, the sine signal is differentiated and divided by the cosine signal, and then integrated according to time to obtain an expression for the current signal;

[0011] The current is continuously monitored, and the corresponding demodulation algorithm is determined based on the current.

[0012] The optical current transformer demodulation method under transient, fast, and large currents according to an embodiment of the present invention has at least the following beneficial effects: The present application provides an optical current transformer demodulation method under transient, fast, and large currents, which combines the arctan algorithm under small current input with the integral algorithm under large current input to achieve accurate measurement under normal working conditions and ensure that the optical CT measurement performance returns to normal in a timely manner after the transient large current ends.

[0013] According to some embodiments of the present invention, in the step of converting the interference light intensity into a photocurrent by the photodetector, the photocurrent i(t) can be expressed as:

[0014]

[0015] Among them, md (t)=Ψ0sin(ω md t) is the sinusoidal modulation phase provided by the PZT modulator, Ψ0 is the modulation depth, ω md is the modulation angular frequency, 4F is the Faraday phase shift introduced by the magnetic field generated by the current between the two circularly polarized light beams, where F = VNI, V is the Verdet constant of the sensing fiber, N is the number of turns of the sensing coil, I is the current to be measured, I0 is the light intensity of the light source, α is the optical path loss coefficient, and k is the photoelectric conversion coefficient.

[0016] According to some embodiments of the present invention, in the step of decomposing the detection signal to obtain a sine signal and a cosine signal, the sine signal and the cosine signal can be expressed as sin(4VNI) and cosine signal cos(4VNI); and the step of taking the derivative of the sine signal and dividing it with the cosine signal can be expressed as:

[0017]

[0018] According to some embodiments of the present invention, the method is preset to set a delay T; in the step of continuously monitoring the current and determining whether it is necessary to switch the corresponding demodulation algorithm based on the current, when the monitored current drops below a specific threshold, the delay T will be maintained before switching to the arctan algorithm.

[0019] According to some embodiments of the present invention, when the method detects that the current change rate is greater than 500 A / μs, the algorithm immediately exits the integration mode and enters the arctan mode.

[0020] According to a second aspect of the present invention, an optical current transformer demodulation system under transient fast large current is characterized by comprising:

[0021] The judgment module can determine the applicable demodulation strategy based on the change in polarization angle generated by the current;

[0022] The steady-state current demodulation module can convert the interference light intensity into a photocurrent when the polarization angle change generated by the current is less than or equal to π / 2. The photocurrent is then Bessel-expanded to extract the amplitudes of the first four harmonic components of the interference signal. Based on the amplitudes, the demodulation formula of the current signal to be measured of the optical current transformer is calculated;

[0023] The transient current solver module can decompose the detection signal into a sine signal and a cosine signal when the angle change of the current exceeds π / 2, take the derivative of the sine signal and divide it with the cosine signal, and then integrate it according to time to obtain the expression of the current signal;

[0024] The mode switching module can continuously monitor the current and determine whether to switch the corresponding demodulation algorithm based on the current.

[0025] According to some embodiments of the present invention, in the steady-state current demodulation module, the photocurrent i(t) can be expressed as:

[0026]

[0027] Among them, md (t)=Ψ0sin(ω md t) is the sinusoidal modulation phase provided by the PZT modulator, Ψ0 is the modulation depth, ω md is the modulation angular frequency, 4F is the Faraday phase shift introduced by the magnetic field generated by the current between the two circularly polarized light beams, where F = VNI, V is the Verdet constant of the sensing fiber, N is the number of turns of the sensing coil, I is the current to be measured, I0 is the light intensity of the light source, α is the optical path loss coefficient, and k is the photoelectric conversion coefficient.

[0028] According to some embodiments of the present invention, in the transient current solving module, the sine signal and the cosine signal can be expressed as sin(4VNI) and the cosine signal cos(4VNI); the steps of taking the derivative of the sine signal and dividing it with the cosine signal can be expressed as:

[0029]

[0030] According to some embodiments of the present invention, a delay T is preset in the mode switching module of the system; when the monitored current drops below a specific threshold, the delay T is maintained before switching to the arctan algorithm.

[0031] According to some embodiments of the present invention, when the mode switching module detects that the current change rate is greater than 500A / μs, the algorithm immediately exits the integration mode and enters the arctan mode.

[0032] An embodiment of the third aspect of the present invention provides a computer-readable storage medium, which stores computer-executable instructions for executing the above-mentioned optical current transformer demodulation method under transient fast large current.

[0033] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0035] Figure 1 Schematic diagram of the steps of a method for demodulating an optical current transformer under transient, fast, and large currents according to an embodiment of the present invention;

[0036] Figure 2 A piezoelectric ceramic modulated optical CT optical path diagram provided for the optical current transformer demodulation method under transient fast high current according to an embodiment of the present invention;

[0037] Figure 3 A schematic diagram showing the symmetry of the arctan algorithm provided by the optical current transformer demodulation method under transient fast high current according to an embodiment of the present invention;

[0038] Figure 4 This is a test comparison of the switching process in the optical current transformer demodulation method under transient fast large current according to an embodiment of the present invention without setting the delay T and with setting the delay T; the upper figure does not set the delay T, while the lower figure sets the delay;

[0039] Figure 5This is a structural block diagram of an optical current transformer demodulation system under transient fast large current according to an embodiment of the present invention. DETAILED DESCRIPTION

[0040] The following describes embodiments of the present invention in detail. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended only to explain the present invention and are not to be construed as limiting the present invention.

[0041] In the description of the present invention, it should be understood that descriptions involving orientations, such as up, down, front, back, left, right, etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, they cannot be understood as limitations on the present invention.

[0042] In the description of the present invention, "several" means one or more, "many" means more than two, "greater than," "less than," and "exceed" are understood to exclude the number itself, while "above," "below," and "within" are understood to include the number itself. The use of "first" and "second" in the description is solely for the purpose of distinguishing technical features and should not be construed as indicating or implying relative importance, implicitly specifying the number of the indicated technical features, or implicitly specifying the order of the indicated technical features.

[0043] In the description of the present invention, unless otherwise clearly defined, terms such as setting, installing, and connecting should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in the present invention based on the specific content of the technical solution.

[0044] In order to solve the problem that optical current transformer (hereinafter referred to as optical CT) is prone to abnormal performance measurement under transient fast and large current conditions, this application provides an optical current transformer demodulation method under transient fast and large current conditions, which mainly targets the characteristics of piezoelectric ceramic modulated optical CT and can realize the demodulation of transient current and steady-state current by switching between two algorithms. Figure 1 As shown, specifically including:

[0045] Example 1

[0046] Step S1: Determine an applicable demodulation method according to the magnitude of the change in polarization angle caused by the current.

[0047] The applicable demodulation method is determined based on the polarization angle, with π / 2 as the boundary. When the polarization angle change is less than or equal to π / 2, it can be determined that the current is in a steady state and the arctan algorithm is used for demodulation; then the process goes to step S2.

[0048] When the polarization angle changes by more than π / 2, it is considered that a transient current has occurred, and the current is calculated using the integration method, and the process proceeds to step S3.

[0049] Step S2: When the polarization angle change generated by the current is less than or equal to π / 2, the photodetector converts the interference light intensity into a photocurrent, and then performs a Bessel expansion on the photocurrent to extract the amplitudes of the first four harmonic components of the interference signal. Based on the amplitudes, a demodulation formula for the current signal to be measured of the optical current transformer is calculated.

[0050] The optical path diagram of piezoelectric ceramic modulated optical CT is as follows Figure 2 As shown in the figure, due to the Faraday effect, the magnetic field generated by the current will introduce a Faraday phase shift of 4F between the two circularly polarized light beams, where F = VNI, 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. After the reflected light interferes, the light intensity I out Received by the light detector.

[0051]

[0052] Among them, md (t)=Ψ0sin(ω md t) is the sinusoidal modulation phase provided by the PZT modulator, Ψ0 is the modulation depth, ω md is the modulation angular frequency.

[0053] Performing Bessel expansion on the above equation, the amplitudes of the first four harmonic components of the interference signal are extracted as follows:

[0054]

[0055] From this we can get:

[0056]

[0057] According to formula (4), the demodulation formula of the current signal to be measured of the optical CT under ideal conditions can be expressed as:

[0058]

[0059] The amplitudes V1 and V2 of the fundamental wave and the second harmonic are obtained through a lock-in amplifier; the primary current can be calculated by solving the arctan function based on formula (5).

[0060] Step S3: When the angle change generated by the current exceeds π / 2, the detection signal is decomposed to obtain a sine signal and a cosine signal, the sine signal is differentiated and divided by the cosine signal, and then integrated according to time to obtain an expression for the current signal.

[0061] When the angle change caused by the current exceeds π / 2, due to the symmetry of the tangent function, the same analytical value can be solved to obtain two angles. Physically, the two currents correspond to the same interference light intensity, and the arctan algorithm is no longer applicable. Figure 3 shown.

[0062] When the angle change caused by the current exceeds π / 2, due to the symmetry of the tangent function, the same analytical value can be solved to obtain two angles. Physically, the two currents correspond to the same interference light intensity, and the arctan algorithm is no longer applicable. Figure 2 The large current integration algorithm is used to calculate the primary current. The PD detection signal is decomposed into a sine signal sin(4VNI) and a cosine signal cos(4VNI). The derivative of the sine signal and the division with the cosine signal can be obtained. Then the current can be obtained by integrating the time

[0063] Step S4: continuously monitor the magnitude of the current, and determine whether it is necessary to switch the corresponding demodulation algorithm according to the magnitude of the current.

[0064] A switching threshold current value is preset. When the current exceeds the threshold current value, the integral algorithm is used for calculation. If the current is less than the threshold current value, the arctan algorithm is used for calculation.

[0065] Taking a 20-turn optical CT as an example, the primary current corresponding to a Faraday phase of π / 2 is 22kA. Since the interference light intensity at π / 2 is weak and the slope is small, there is a large solution error. Therefore, 0.9 radians is selected as the logic threshold current for switching between the high and low current algorithms, which corresponds to a current of 11.2kA.

[0066] For the accurate definition of differential, in actual numerical calculation, differential calculation is converted into difference calculation. There are two errors in the integration algorithm at this time:

[0067] 1) The integral error caused by too large Δt under transient high current conditions. If Δt is selected to be large, then the calculated This will lead to excessive integration errors.

[0068] 2) Truncation error caused by too small Δt. Each differential operation process has truncation error due to the limited number of bits represented. When the algorithm is in a large current integration algorithm for a long time, it may affect the measurement accuracy.

[0069] Furthermore, due to the symmetry problem of the arctan algorithm, the integral algorithm has certain calculation errors. To avoid incorrect calculation of the 11.2kA threshold, which leads to frequent algorithm switching and affects the transient performance of the optical CT, a delay T is set: when the primary current drops from above 11.2kA to below 11.2kA, the optical CT algorithm maintains the integral algorithm for time T before switching to the arctan algorithm.

[0070] Furthermore, since the integral error will continue to accumulate, in order to avoid the error under the delay T causing the protection to malfunction after the transient current ends, the exit condition of the large current algorithm is set: when the detection current change rate is greater than 500A / μs, the algorithm directly exits the integral mode and enters the arctan mode; this condition has a higher priority than the above delay T, that is, the integral algorithm. The simulation comparison diagram with setting the exit condition and not setting the exit condition is shown in the figure below. Figure 4 shown.

[0071] Based on the above conditions, step S4 becomes

[0072] If the current in S41 is less than 11.2kA, the arctan algorithm is used for calculation;

[0073] If the current in S42 is greater than 11.2kA, it is calculated using the integration algorithm, and a judgment is made in the next cycle: whether the current change rate is greater than 500A / μs: if the current change rate is greater than or equal to 500A / μs, the integration algorithm is exited and the arctan algorithm is adopted; if the current change rate is less than 500A / μs, it is determined: whether the current peak is greater than 11.2kA, if the current peak is less than 11.2kA, the timing is started, and the integration algorithm is terminated after the delay T ends, and the arctan algorithm is adopted; if the current peak is greater than 11.2kA, the loop of step S2 is continued.

[0074] Example 2

[0075] Another embodiment of the present application provides an optical current transformer demodulation system under transient fast large current, such as Figure 5 The system 50 includes:

[0076] The judgment module 501 can determine an applicable demodulation strategy according to the magnitude of the change in polarization angle generated by the current;

[0077] The steady-state current demodulation module 502 is capable of converting the interference light intensity into a photocurrent by a photodetector when the polarization angle change generated by the current is less than or equal to π / 2, then performing a Bessel expansion on the photocurrent to extract the amplitudes of the first four harmonic components of the interference signal, and calculating a demodulation formula for the current signal to be measured of the optical current transformer based on the amplitudes;

[0078] The transient current solving module 503 is capable of decomposing the detection signal to obtain a sine signal and a cosine signal when the angle change of the current exceeds π / 2, taking the derivative of the sine signal and dividing it with the cosine signal, and then integrating it with time to obtain an expression for the current signal;

[0079] The mode switching module 504 can continuously monitor the magnitude of the current and determine whether to switch to a corresponding demodulation algorithm according to the magnitude of the current.

[0080] It is understandable that the judgment module 501 and the mode switching module 504 are similar in function, both determining the adaptive algorithm according to the current magnitude, and combining their functions and unifying them into one module does not exceed the scope of the invention.

[0081] Furthermore, in the steady-state current demodulation module 502, the photocurrent i(t) can be expressed as:

[0082]

[0083] Among them, md (t)=Ψ0sin(ω md t) is the sinusoidal modulation phase provided by the PZT modulator, Ψ0 is the modulation depth, ω md is the modulation angular frequency, 4F is the Faraday phase shift introduced by the magnetic field generated by the current between the two circularly polarized light beams, where F = VNI, V is the Verdet constant of the sensing fiber, N is the number of turns of the sensing coil, I is the current to be measured, I0 is the light intensity of the light source, α is the optical path loss coefficient, and k is the photoelectric conversion coefficient.

[0084] Furthermore, in the transient current solving module 503, the sine signal and the cosine signal can be expressed as sin(4VNI) and cosine signal cos(4VNI); the steps of taking the derivative of the sine signal and dividing it with the cosine signal can be expressed as:

[0085]

[0086] Furthermore, a delay T is preset in the mode switching module 504; when the monitored current drops below a specific threshold, the delay T is maintained before switching to the arctan algorithm.

[0087] Furthermore, when the mode switching module 504 detects that the current change rate is greater than 500 A / μs, the algorithm immediately exits the integration mode and enters the arctan mode.

[0088] Another embodiment of the present application provides a computer-readable storage medium storing computer-executable instructions for executing the above-mentioned Figure 1The demodulation method of optical current transformer under transient fast and large current is shown.

[0089] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate, i.e., they may be located in one place or distributed across multiple network units. Some or all of the modules may be selected based on actual needs to achieve the objectives of this embodiment.

[0090] Those skilled in the art will appreciate that all or some of the steps and systems in the method disclosed above can be implemented as software, firmware, hardware, and appropriate combinations thereof. Some physical components or all physical components can be implemented as software executed by a processor, such as a central processing unit, a digital signal processor, or a microprocessor, or implemented as hardware, or implemented as an integrated circuit, such as an application-specific integrated circuit. Such software can be distributed on a computer-readable medium, and the computer-readable medium can include computer storage media (or non-transitory media) and communication media (or temporary media). As known to those skilled in the art, the term computer storage media is included in any method or technology for storing information (such as computer-readable instructions, data structures, program modules, or other data) and is volatile and non-volatile, removable, and non-removable. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory, or other memory technology, CD-ROM, digital versatile disks (DVD), or other optical disk storage, magnetic cassettes, magnetic tapes, disk storage, or other magnetic storage devices, or any other medium that can be used to store desired information and can be accessed by a computer. Furthermore, as is well known to those skilled in the art, communication media typically embodies computer-readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism, and may include any information delivery media.

[0091] The above is a specific description of the preferred implementation of the present application, but the present application is not limited to the above implementation mode. Technical personnel familiar with the field can also make various equivalent modifications or substitutions without violating the spirit of the present application. These equivalent modifications or substitutions are all included in the scope defined by the claims of the present application.

Claims

1. A method for demodulating an optical current transformer under transient fast large current, characterized in that: This includes the following steps: Determine the appropriate demodulation strategy based on the magnitude of the polarization angle change caused by the current; When the polarization angle change generated by the current is less than or equal to π / 2, the photodetector converts the interference light intensity into a photocurrent, then performs a Bessel expansion on the photocurrent to extract the amplitudes of the first four harmonic components of the interference signal. Based on the amplitudes, a demodulation formula for the current signal to be measured of the optical current transformer is calculated; When the angle change generated by the current exceeds π / 2, the detection signal is decomposed to obtain a sine signal and a cosine signal, the sine signal is differentiated and divided by the cosine signal, and then integrated according to time to obtain an expression for the current signal; The current is continuously monitored, and the corresponding demodulation algorithm is determined based on the current.

2. The method according to claim 1, characterized in that In the step where the photodetector converts the interference light intensity into a photocurrent, the photocurrent i(t) can be expressed as: Among them, md (t)=Ψ0sin(ω md t) is the sinusoidal modulation phase provided by the PZT modulator, Ψ0 is the modulation depth, ω md is the modulation angular frequency, 4F is the Faraday phase shift introduced by the magnetic field generated by the current between the two circularly polarized light beams, where F = VNI, V is the Verdet constant of the sensing fiber, N is the number of turns of the sensing coil, I is the current to be measured, I0 is the light intensity of the light source, α is the optical path loss coefficient, and k is the photoelectric conversion coefficient.

3. The method according to claim 1, characterized in that In the step of decomposing the detection signal to obtain a sine signal and a cosine signal, the sine signal and the cosine signal can be expressed as sin(4VNI) and cosine signal cos(4VNI); the step of taking the derivative of the sine signal and dividing it with the cosine signal can be expressed as:

4. The method according to claim 1, characterized in that The method is preset with a delay T; in the step of continuously monitoring the current and determining whether to switch the corresponding demodulation algorithm based on the current, when the monitored current drops below a specific threshold, the delay T will be maintained before switching to the arctan algorithm.

5. The method according to claim 4, characterized in that When the detection current change rate is greater than 500A / μs, the algorithm exits the integration mode and enters the arctan mode.

6. An optical current transformer demodulation system under transient fast large current, characterized in that: include: The judgment module can determine the applicable demodulation strategy based on the change in polarization angle generated by the current; The steady-state current demodulation module can convert the interference light intensity into a photocurrent when the polarization angle change generated by the current is less than or equal to π / 2. The photocurrent is then Bessel-expanded to extract the amplitudes of the first four harmonic components of the interference signal. Based on the amplitudes, the demodulation formula of the current signal to be measured of the optical current transformer is calculated; The transient current solver module can decompose the detection signal into a sine signal and a cosine signal when the angle change of the current exceeds π / 2, take the derivative of the sine signal and divide it with the cosine signal, and then integrate it according to time to obtain the expression of the current signal; The mode switching module can continuously monitor the current and determine whether to switch the corresponding demodulation algorithm based on the current.

7. The system according to claim 6, characterized in that In the steady-state current demodulation module, the photocurrent i(t) can be expressed as: Among them, md (t)=Ψ0sin(ω md t) is the sinusoidal modulation phase provided by the PZT modulator, Ψ0 is the modulation depth, ω md is the modulation angular frequency, 4F is the Faraday phase shift introduced by the magnetic field generated by the current between the two circularly polarized light beams, where F = VNI, V is the Verdet constant of the sensing fiber, N is the number of turns of the sensing coil, I is the current to be measured, I0 is the light intensity of the light source, α is the optical path loss coefficient, and k is the photoelectric conversion coefficient.

8. The system according to claim 6, wherein: In the transient current solution module, the sine signal and the cosine signal can be expressed as sin(4VNI) and cosine signal cos(4VNI). The steps of taking the derivative of the sine signal and dividing it with the cosine signal can be expressed as:

9. The system according to claim 6, wherein: In the mode switching module of the system, a delay T is preset; when the monitored current drops below a specific threshold, the delay T will be maintained before switching to the arctan algorithm. 10 . The system according to claim 9 , wherein when the mode switching module detects that the current change rate is greater than 500 A / μs, the algorithm immediately exits the integration mode and enters the arctan mode.

11. A computer-readable storage medium storing computer-executable instructions, wherein the computer-executable instructions are used to execute the method according to any one of claims 1 to 5.

Images