Fiber Optic Current Transformer Research Articles

Deterioration identification method of key optical devices of fiber optical current transformer based on neural network

AbstractOptical fiber current transformers (FOCTs) are affected by various external factors, resulting in the deterioration or even failure of devices, causing changes in critical state quantities, and reducing the accuracy and reliability of products. To solve this problem, based on the neural network algorithm, this paper starts from the four deterioration characteristics of FOCT SLD junction temperature, SLD output optical power, phase modulator half‐wave voltage, and optical fiber sensing ring temperature, and identifies the deterioration of key optical components of FOCT, which provides a basic model and data support for the online monitoring and early warning to improving the stability and reliability of FOCT in long‐term operation.

Noise Reduction Method for Fiber Optical Current Transformer Based on Mode Decomposition-Sample Entropy

When a fiber optic current transformer (FOCT) malfunctions, its output signal is usually accompanied by a large noise, which affects the subsequent fault diagnosis results of the transformer. In response to the above noise issues, this paper constructs a new noise reduction algorithm by combining Complete Ensemble Empirical Mode Decomposition Adaptive Noise (CEEMDAN) and Sample Entropy (SE). Above all, the output signal of FOCT is decomposed into multiple fault components using the CEEMDAN algorithm. Then, by comprehensively considering the frequency domain characteristics and sample entropy calculation value of each component, the characteristics of each component are determined, so the power frequency and low-frequency components are selected as the feature vectors representing the fault mode. Finally, the noise components with high sample entropy are eliminated, and the power frequency current components with medium sample entropy and the low-frequency fault feature components with low sample entropy are recombined and superposed to build a fault signal dataset after noise reduction.

A Method for Suppressing Error of Fiber Optic Current Transformer Caused by Temperature Based on λ/4 Wave Plate Fabricated With Polarization-Maintaining Photonic Crystal Fiber

The beat length of λ/4 wave plate of fiber optic current transformer (FOCT) will change with the temperature fluctuation of environment, which will result in additional phase error, reducing the accuracy of current measurement. In the paper, we presented a FOCT based on λ/4 wave plate fabricated with polarization maintaining photonic crystal fiber (PCF). Combined with error model caused by λ/4 wave plate in FOCT, we analyzed the specific mechanism of temperature influence on λ/4 wave plate. Aiming at the problem of fusing PCF to panda polarization maintaining fiber, we optimized the related parameters of fiber fusion splicer to reduce the fusion loss of the fiber. It was experimentally verified that the beat length of PCF is less sensitive to temperature than that to elliptical core polarization maintaining fiber. Finally, we measured current of FOCT utilizing two kinds of λ/4 wave plates in full temperature range from -40 °C to 70 °C. It was verified that it is insensitive to temperature for FOCT when utilizing PCF λ/4 wave plate, with the corresponding system current measurement accuracy 0.5S level.

An impact-insensitive fiber optic current transformer based on polarization maintaining photonic crystal fiber delay coil

When the polarization-maintaining fiber (PMF) delay coil of a fiber optic current transformer (FOCT) is impacted, external forces on the optical fibers and change of their birefringence may lead to extra phase errors during the propagation of optical signals in the fibers. These errors increase the error in current measurement. In the paper, the environmental impact mechanisms influencing the PMF delay coil of FOCT were investigated with a mathematical error model. The method of replacing PMF with a polarization-maintaining photonic crystal fiber (PCF) as the delay coil was proposed. Also, the relationship between structural parameters of air holes and linear birefringence of the polarization maintaining PCF subjected to stress was investigated. The structural parameters of optical fibers were also optimized subsequently. Simulated impact experiments of FOCT with polarization-maintaining PCF and PMF as the delay coil, respectively, demonstrated the effective impact resistance of FOCT with polarization-maintaining PCF delay coil after the optimization of its structural parameters. The current measurement errors of the FOCTs with polarization maintaining PCF delay coil reduced by approximately 35% with respect to the FOCT with PMF delay coil.

Identification of gradual failure time-frequency feature in fiber optical current transformer

Identification of gradual failure time-frequency feature in fiber optical current transformer

Research on the performance of a fiber optical current transformer for high voltage filtering capacitor unbalanced current measurement

Subject of study. A fiber-optic measurement scheme for the unbalanced current of a high-voltage filtering capacitor is proposed, where an all-fiber temperature sensor based on temperature birefringence is presented to realize the real-time correction of temperature errors. Method. Prototypes of an all-fiber filter unbalanced fiber optical current transformer type and a magneto-optical crystal type with a rated current of 1 A and a rated voltage of 258 kV have been developed and performance tested. Main results. After temperature correction, the accuracy of the all-fiber filter unbalanced fiber optical current transformer type in the range of −40∘C to 70°C meets the error limit requirements of class 0.2 specified in the Chinese Standard GB/T 20840.8-2007 (Instrument transformers—Part 8: Electronic current transformers standard), whose accuracy, integration, reliability, and anti-interference ability are better than the magneto-optical crystal type. The proposed scheme has the ability of small current (less than 20 mA) measurement in high voltage (more than 110 kV) scenarios.

Denoising method in fiber optic current transformer based on data characteristics and depth entropy

Focusing on the difficulty in suppressing the random error of non-Gaussian noise in fiber optic current transformer (FOCT) measurement, a FOCT noise suppression method based on data modal characteristics and depth entropy is proposed. The empirical mode decomposition model is established, and the signal timing is processed with adaptability, completeness and approximate orthogonality. The measurement signals with high-amplitude interference distributed in each IMF are extracted to solve the problem of modal aliasing interference. The analysis model of the random dynamic characteristics of the time series is established, and the calculation method of the depth entropy is proposed. By calculating the correlation of the time series, the complexity of the information is evaluated, and the signal after filtering the interference is reconstructed. Experiment results show that the average value of the filtered current data is 0.2% deviation from the reference current, and the variance is reduced by 56%. This filtering method can significantly reduce the dispersion of test data while ensuring unbiased estimation. The​ FOCT noise suppression method based on data modal characteristics and depth entropy can filter non-Gaussian noise and the signal-to-noise ratio of FOCT output data can be improved.

Improving the Temperature and Vibration Robustness of Fiber Optic Current Transformer Using Fiber Polarization Rotator

Fiber optic current transformers (FOCTs) have advantages over conventional current transformers. However, the application of FOCTs is significantly limited by the birefringence effect caused by temperature and vibration. Thus, this study proposed a sensing scheme of coupling a fiber polarization rotator to improve the robustness of FOCTs. First, the mathematical model of the proposed FOCT was derived using the Jones matrix, and the error caused by the birefringence effect was computed. Then, the FOCT was modeled by applying the finite element method to verify the effectiveness of the proposed FOCT in improving the temperature and vibration robustness. Finally, experiments were conducted, including temperature, vibration, and frequency response characteristics. The proposed scheme was compared with the basic polarimetric detection (PD) scheme and several state-of-the-art schemes. Results show that the maximum error of the proposed scheme is 1.7% in the temperature range from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 20 ^{\circ }\text{C}$ </tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$80 ^{\circ }\text{C}$ </tex-math></inline-formula> (that of the basic PD scheme ranges from −7.05% to 9.75%). In the vibration test, the maximum output error of the proposed scheme is 0.72%, while that of the basic PD scheme reaches 4.89%. Meanwhile, the proposed FOCT meets the requirements of the measurement of high-amplitude and high-frequency transient currents, such as lightning impulse current, in power systems.

Error characteristic analysis of fiber optic current transformer with sine modulation

The optoelectronic circuit and modulation circuit of the fiber optic current transformer (FOCT) are easily affected by the complex environment in the converter station during operation, leading to abnormal operation and fault warning. In order to analyze the error characteristic on the output signal and harmonics of FOCT, the output signal models of FOCT with sine modulation in steady-state and error-state are established respectively in this paper. Combined with the output signal, the key parameters are solved, and it is pointed out that the change of photoelectric circuit may cause the ratio error. The Faraday phase shift and the change of the modulation circuit may cause the drift error. The error characteristics of the FOCT under different influencing factors are simulated and analyzed. The results show that output signal of FOCT is influenced by photoelectric circuit, Faraday phase shift circuit and modulation circuit. Changes in the photoelectric circuit will cause a proportional change in the FOCT output signal and harmonics. Both the Faraday phase-shifting and the increasing of modulation amplitude will reduce the minimum value of output signal. Changes in Faraday phase shift will cause an approximately proportional change in first harmonic. Changes in modulation signal amplitude will cause the harmonics to appear attenuate and oscillate. The modulation frequency is inversely proportional to the period of the output signal and harmonics. Both the first and second harmonics can be used as the monitoring state parameters of fault determination. Designed experiment verified the correctness of the conjecture. This article provides a theoretical basis for the follow-up status monitoring and fault diagnosis of FOCT.

All-fiber high-voltage capacitor unbalanced current transformer.

According to the high-voltage and small-current characteristics for unbalanced current of high-voltage capacitors, a novel fiber optical filter unbalanced current transformer (FU-FOCT) is proposed, where an all-fiber temperature sensor based on the temperature birefringence effect of polarization-maintaining fiber is designed and integrated into an FU-FOCT for real-time temperature correction. The FU-FOCT prototype with a rated current of 1 A and a rated voltage of 258 kV is developed, and the accuracy, temperature, and frequency-response characteristics are tested. The results show that within the temperature range of ${-}{40}^\circ {\rm C} - 70^\circ {\rm C}$, the fluctuation range of the ratio error and phase error of the FU-FOCT at 1 A are ${-}{0.042}\% - 0.08\%$ and ${-}{0.16^\prime - 0.18^\prime}$, respectively, which meet the error limit requirements of class 0.2% specified in GB/T 20840.8-2007.

Design and Performance Study of a Temperature Compensated ±1100-kV UHVdc All Fiber Current Transformer

The inherent advantages of fiber optical current transformer (FOCT) are very attractive for high-voltage applications when compared with conventional transformers. However, the accuracy and long-term reliability of FOCT can be greatly affected by temperature. Fiber temperature sensors are suitable for FOCT temperature compensation, especially in ultrahigh voltage (UHV) applications. As a result, a fiber temperature-sensing scheme based on the temperature birefringence effect of polarization maintaining fiber (PMF-TS) was proposed. A high integration real-time temperature correction FOCT based on PMF-TS (PMFTS-FOCT) was designed, and a prototype with rated current of 5000 A and rated voltage of 1100 kV was developed. The accuracy, temperature, frequency response, step response, and transient response of the system was tested. The results showed that within the temperature range of -40 °C to 70 °C, the ratio error (RE) of 500-5500 A was less than ±0.2% after temperature correction, and in the frequency range of 50-1200 Hz, RE and phase error (PE) were less than ±0.75% and 110 μs, respectively. The step response time was less than 125 μs, the overshoot was less than 1.5%, and the composite error was 0.807% for the 50 000-A transient current. We proved that PMFTS-FOCT had good steady-state and transient performance and the possibility for measuring dc and ac harmonics, whose performance meets the error limit requirements specified in GB/T 26216.1-2010.

Fault Mechanism of Fiber Optical Current Transformer Based on Signal Processing Method

The fiber optical current transformer (FOCT) with sine wave modulation has been widely used in the DC transmission project, while its fault rate is much higher than that of an electromagnetic current transformer, which seriously threatens the safe operation of power grid. In order to investigate the fault mechanism of FOCT, the output signal model of the FOCT photodetector is established, and its basic methods of modulation and demodulation are analyzed to explore the influencing factors of the signal processing method used in the calculation of measured current. It is found that the reduction of the second harmonic component in the photodetector output is the root cause of FOCT fault, and the optical path, modulation signal amplitude, and measured current are important factors affecting the second harmonic component. The influencing law of three factors on the second harmonic component is calculated by simulation and the characteristic differences between them are given. Finally, the fault simulation test of FOCT light intensity and a modulation circuit were carried out in the laboratory, and the results verify the correctness of theoretical analysis. The research in this paper provides reference for fault analysis and reliability improvement of FOCT.

Novel Fiber Optic Current Transformer With New Phase Modulation Method

Based on the transverse electro-optic effect of lithium niobate crystal, combined with polarizers and Faraday rotator, this paper presents a collinear closed-loop fiber optic current transformer with spatial non-reciprocity modulation method, and the feasibility of the scheme is verified by both the theoretical and experimental evidences. The detection scheme avoids the limitation of the transition time of the sensing fiber coil on the phase modulation frequency, improves the sensitivity and stability of the system, and reduces the volume and cost of fiber optic current transformer. The sawtooth wave modulation scheme is adopted to realize phase bias modulation and feedback modulation through phase shift of sawtooth wave to achieve closed-loop detection effect, which enhances the signal to noise ratio and simplifies demodulation mode. The experimental results show that the current ratio errors measured at room temperature range from 1% to 120% of rated current meet the requirements of national standard GB/T 20840.8-2007 and reach the accuracy level of 0.2S. The temperature stability of the current transformer is also tested, and the ratio error measured at the rated current does not exceed ±0.2% in the range of -30 °C to 50 °C.

Design of the portable fiber-optic current transformer for electrolytic aluminum industry

A portable fiber-optic current transformer (P-FOCT) based on the Faraday effect utilizing the fiber jumper and the flexible sensing coil is proposed to enhance the portability and improve the design for disassembly of FOCT in the electrolytic aluminum industry. To ensure the accuracy of P-FOCT, the polarization crosstalk of fiber jumper and the conductor eccentric error are studied. According to the theoretical analysis and experimental results, the relative error of P-FOCT would decrease as the distance between the current conductor and the non-closed node of the sensing coil increases. A new sensing head is designed and experiments show that the error is 0.31 % at variable temperature from 233K to 343K, which meets the requirement for accuracy in the electrolytic aluminum industry.

Intelligent fault diagnosis algorithm for fiber optic current transformer

Affected by environmental factors, the performance of fiber optic current transformer (FOCT) will deteriorate over a long period of time. Intelligent fault diagnosis algorithm of Long-Short Term Memory (LSTM) combing with Support Vector Machine (SVM) is an effective way to deal with FOCT failures. According to the characteristics of LSTM, a signal prediction model in FOCT based on LSTM is proposed by analyzing the historical data. The residual signal can be obtained by the prediction signal and the observed signal. Set the residual threshold to determine whether the FOCT has fault. With the residual signal characteristics, a fault diagnosis model based on SVM is established. By analyzing the residual signal and extracting features, the diagnostic network can realize the pattern recognition and system fault diagnosis. Experiments demonstrate that the drift deviation fault, the ratio deviation fault and the fixed deviation fault can be diagnosed with an accuracy of 94.5%.

Research and Application of Generator Protection Based on Fiber Optical Current Transformer

This paper introduces the basic principle of fibre optical current transformer (FOCT), and explains the advantages of FOCT compared with electromagnetic current transformer (CT). FOCT can be wound around the primary conductor in any shape, has high harmonic accuracy and doesn't suffer from saturation, thus a good solution for generator relay protection. As a common electrical fault within large generators, the inter-turn short circuit in field windings (ISCFW) is likely to cause earth faults between the field winding and the rotor body and magnetization of the main shaft without timely intervention. Steady-state unbalanced currents of even orders and fractional orders related with pole pairs inside phase windings are required to monitor the ISCFW. The inter-turn fault of stator windings may rapidly develop into a phase to phase fault, which seriously threatens the safety of the unit. Split-phase transverse differential current reflects the steady-state current imbalance inside the stator winding. Nevertheless, for most steam turbine generators and some hydro turbine generators, it is unable to obtain phase-segregated transverse differential current due to impossibility of installing branch current transformer in the narrow space inside the generator. However, FOCT which uses fibre optic cable as a sensor is installable within limited space to measure the current of each group of winding branches of each phase. FOCT furthers phase-segregated transverse differential protection, partial differential protection and online monitoring of the ISCFW, thus optimizing the generators protection scheme. In addition, operating speed and sensitivity of generator differential protection are improved based on the reliable measurement from FOCT. The proposed scheme is verified by an application on a 300MW generator at a pump-storage power plant. This is the first attempt of applying FOCT to the relay protection of generator set, which provides reference for further development and application of FOCT in power plant.

Error Calculating and Compensating Method of All-Optical Fiber Current Transformer in ± 10 kV DC Distribution Network

Current transformer is one of the main equipment in ±10 kV DC distribution network. Traditional electromagnetic current transformer has poor anti-jamming capacity and poor insulation, so it cannot realize transient protection. The emergence of all-optical fiber current transformer brings solutions to these problems, which has attracted more and more attention. In this study, aiming at the problem of error and compensation of λ/4 waveform of all-fiber current transformer, the fabrication method of λ/4 waveform is studied, the mathematical model of the waveform is established, the influence of the waveform error on the scale factor is analyzed, and a calculating device of angle difference and ratio difference to compare the error is designed. The device adopts the principle of traceability of quantities to collect the state data of the wave plate. At the same time, in order to collect data for errors, database technology and network technology are used to realize remote transmission of monitoring data. The error of the λ/4 wave plate is compensated, the bidirectional principle generated by the birefringence in the fiber ring is analyzed, and the data logic description of the transformer sensitive ring is performed. The phase delay and the angle of the shaft are selected as the main factors causing the error. Compensation is performed by means of peak segmentation and variable cancellation. In the experimental process, compared with the error sampling of the traditional transformer, the error calculated by the current amplitude and phase angle parameters obtained by the all-fiber current transformer is more accurate, and the compensation scheme can suppress the size of the birefringence and improve sensing accuracy of fiber optic current transformers. This study provides a powerful reference for the error analysis of all-fiber current transformer λ/4 wave plates, which is beneficial to promote the better application of such current transformers.

Influencing factors analysis on the detector output signal of fiber optic current transformer with sine modulation

To investigate the measurement failure mechanism of fiber-optic current transformer (FOCT) with sine modulation, the detector output of FOCT is established and the expression of the output signal is given to explore its influencing factors. The demodulation method of FOCT is studied and the results show that the optical path parameters, modulation signal and measured current affect the second harmonic component in detector output, and may further cause FOCT measurement failure when the second harmonic component is low. Calculation and simulation are carried out to analyze the influence of each factor on detector output signal and the second harmonic component. The results show that the rise of optical path parameters and modulation signal will enlarge the output range of the detector, and the two factors are positively correlated with the second harmonic component, which may decrease as the measured current rises. Tests of the theoretical analysis were verified by experiments in Huai'an and Taizhou UHV Converter Stations.

Design and validation of broadband calibration system for fiber-optical current transformers

With the rapid development of the energy system, new requirements are put forward for the current measurement equipment, including fiber-optical current transformer (FOCT), which is a vital instrument for measuring the current of high voltage direct current (HVDC) system. Nowadays, the broadband measurement performance of FOCT requires higher technical indicators, yet the existing calibration system of FOCT cannot meet the actual engineering needs. This paper developed a broadband calibration system for FOCT, which can accurately verify the broadband performance of FOCT in the frequency range from 50Hz to 2500 Hz. The impedance of the secondary circuit of the lifter in the calibration system is calculated; the power supply selection requirements of the FOCT calibration system were proposed. The broadband calibration of FOCT prototype with rated current 1000A was calibrated in the frequency range from 50Hz to 2500Hz. Further, the validity of the system was verified.

Research on error mechanism analysis and compensation method of Fiber-Optical Current Transformer

The structure of Fiber-Optical Current Transformer (FOCT) is completely different from that of traditional electromagnetic transformer, and the error analysis method is quite different from that of traditional transformer. However, there is no systematic calculation method for the error of FOCT at present. Starting from the working principle and structure of fiber-optical current transformer, this paper theoretically analyses and deduces all kinds of error sources of FOCT, puts forward the error calculation method of FOCT, calculates the overall error based on the actual parameters of a 200 kV Fiber-Optical Current Transformer, and proposes a method to compensate the measurement error of FOCT by changing the initial phase angle of λ/4 wave plate. The initial phase angle of the λ/4 wave plate with the best error compensation effect is 86 degrees.