Conventional Current Transformers Research Articles

The Development and Performance of Dual-Core DC Tolerant Current Transformer

A current transformer (CT), consisting of two independent magnetic cores, is proposed to improve the DC tolerance ability for AC measurement. The proposed dual-core CT is designed to work at the low measurement error margins of the conventional and air-gapped CTs, which exhibits a high accuracy for AC measurement of the conventional CT and, in the meanwhile, as tolerant to DC saturation as the air-gapped CT. An analytical model for magnetostatic evaluation that fits both conventional and air-gapped CTs is presented, and the measurement error of the dual-core CT is determined accordingly. A prototype of the dual-core CT is constructed and the performance of two kinds of cores for current measurement under different DC biases is experimentally evaluated. Comparisons between error calculation and error measurement of the dual-core CT are also conducted. The experiment result verifies the theoretical prediction of the dual-core CT, showing the proposal of an interesting AC measurement solution in industrial applications.

Intensity-Modulated Fiber-Optic Sensor: A Novel Grid Measurement Unit

This article presents a novel approach to physical-displacement-based power grid measuring via an intensity-modulated fiber-optic sensor (IMFOS). An IMFOS utilizes one fiber to transmit the intensity modulated light from its electro-optic controller to a fiber-optic probe. The power grid voltage and current can induce physical displacements in transducers via the piezoelectric effect and the Lorentz law, respectively, which then result in a distance change between the optical probe and the reflective surface of the transducers. In parallel, multiple fibers are used to collect the reflective light for electro-optic conversion. A National-Instruments-based characterization platform is set up for performance evaluation. The testing result demonstrates that the IMFOS is immune to the inherent dc and low-frequency saturation issues prevalent in conventional potential and current transformers. Finally, the IMFOS is implemented in a universal grid analyzer to illustrate its applicability for phasor estimation in actual power grids.

A Magnetically Controlled Current Transformer for Stable Energy Harvesting

The magnetically controlled current transformer (MCCT) is proposed to harvest stable energy from the magnetic field induced by the transmission-line fluctuating current. Compared with the conventional current transformer (CT), the advance of the proposed MCCT originates from the so-called constant magnetic flux control, which is achieved by connecting an electronic load on the secondary side of MCCT, and adjust the load impedance to make it equivalent to the corresponding resistive-capacitive impedance for magnetization or demagnetization. Furthermore, the working principle and the excitation characteristic of the MCCT system have been analyzed using the equivalent magnetic circuit and equivalent circuit methods. In addition, the details of the electronic load circuit and control system of the MCCT are presented in this paper, and the stability of the system is verified using logarithmic criterion and total harmonic current distortion (THDi) analysis. Finally, the co-simulation and experimental results show that MCCT can produce a stable output power of 13 W in the range of 50–500 A current fluctuations, demonstrating that the proposed magnetic energy harvester is capable of a high conversion efficiency and output stability.

Optical Magnetostrictive Current Sensor Based on In-Fiber Fabry–Pérot Cavity

In this article, a compact fiber-optic current sensor (FOCS) based on an in-fiber Fabry–Pérot interferometer (FPI) and a magnetostrictive transducer is presented. The in-fiber FPI sensor employed in the proposed FOCS was fabricated with a 273- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> Section of a capillary fiber spliced between two pieces of a standard single-mode fiber (SMF). The in-fiber FPI sensor and a commercial fiber Bragg grating (FBG), which was used as a reference sensor for sensitivity comparison, were bonded to a small magnetostrictive actuator (Terfenol-D) and assembled in the proposed FOCS for electric current monitoring up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$800 {A}_{\text {rms}}$ </tex-math></inline-formula> using a commercial FBG interrogator. The in-fiber FPI sensor presented a sensitivity 84.4% higher than that of the FBG sensor. An alternative interrogation system based on an edge-filter technique with temperature compensation was also developed and employed for the characterization of the proposed FOCS under the same range of electric current. Using the edge-filter interrogation system developed, the in-fiber FPI sensor presented a sensitivity 169.8% higher than that shown by the FBG sensor. The results show that the proposed FOCS using an in-fiber FPI sensor presented high sensitivity to electric current sensing and, therefore, is a potential candidate for current monitoring in complement to conventional current transformers (CTs) as well as to conventional optical CTs (OCTs).

Abnormal handling mechanism and improvement measures of optical DC current transformer in smart grid environment.

An optical DC current transformer anomaly handling mechanism is proposed to address the problem that the conventional DC current transformer anomaly handling mechanism cannot compensate for the defect of capacitor anomaly blocking. First, the implementation principle, modulation loop, demodulation method and its anomaly warning mechanism of the sine-wave modulated all-fibre-optic current transformer (FOCT) are investigated, and the effects of light source intensity and modulation voltage on current decoding are explained. The modulation loop is then simulated and modelled and a FOCT anomaly handling mechanism is proposed based on the Bessel function with real-time dynamic current compensation for small changes in modulation depth. Finally, an integrated dynamic test system for DC current transformers and DC protection is designed, and the actual system operation and fault model is established using the RTDS simulation system. The experiments demonstrate that the proposed FOCT anomaly handling and improvement measures can effectively improve the transient performance of FOCT, and at the same time provide a complete set of testing means for the engineering application and later upgrade and replacement of FOCT.

Designing and manufacturing of electric arc furnace currents measurement system in Mobarakeh Steel Company by printed circuit board‐type Rogowski coils

This paper deals with the utilization of printed circuit board Rogowski coils (PCBRCs) for differential protection of electric arc furnace (EAF) transformers in Mobarakeh Steel Company (MSC), Isfahan, Iran. Because of a high level of current (in the range of 100 kA) in the EAF transformers in the MSC, employment of differential protection is quite challenging. Conventional current transformers (CTs) cannot offer reliable performance for very high current levels. The PCBRCs are a branch of Rogowski coils (RCs) that can be manufactured with high precision using computer-aided design approaches and have the capability for the measurement of very large currents. In this study, we have designed and manufactured two sets of PCBRCs to measure current in the primary and secondary sides of the EAF's transformers in the MSC. The PCBRCs are followed by a signal condition circuit (SCC) which includes a low-pass filter, amplifier, and voltage-to-current (V-to-I) converter block. Thanks to the SCC, the high current of the transformers can be properly sensed and transmitted over a long cable that connects the PCBRCs to the relay. Furthermore, a novel model is suggested for the EAF that includes characteristics of both time and frequency domain models. The suggested model is developed using a large number of recorded data from the MSC and can model the non-linear and time-variant behaviours of the EAF. The proposed PCBRCs, SCC, and EAF modelling are evaluated by some simulations and experiments in the MSC.

Analysis of the Operation of Cascade Current Transformers for Measurements of Short-Circuit Currents with a Non-Periodic Component with a Large Time Constant of Its Decay

Simulations of emergency states and tests of resistance of electrical devices to emergencies are performed in specialized high-power laboratories, the so-called short-circuit laboratories. For most electrical power devices, such measurements are required by international standards. The basic equipment of short-circuit testing laboratories consists of current transformers for measuring short-circuit currents. These transformers should not only enable the accurate conversion of sinusoidal currents—which is typical for conventional current transformers, but also asymmetrical short-circuit currents containing an aperiodic component, which classic current transformers cannot reproduce. Therefore, manufacturers and designers try to meet the market demands and design these special class 0.2 current transformers. To meet the high technical requirements, the field-circuit method with three-dimensional space–time analysis of electromagnetic fields was used during design, considering physical phenomena in ferromagnetic cores (i.e., hysteresis and eddy current losses) and the load of the secondary winding of the current transformer by the measurement system. The article presents simulations of secondary currents for the short-circuit current transformer model, and the results were confirmed by measurements and oscillograms of the currents flowing in the windings of the real model. The prototype of the designed short-circuit transformer meets the IEC/EN standard requirements. When measuring harmonic currents, the transformation errors meet the requirements of class 0.2. During the short-circuit current waveforms, the maximum instantaneous peak error does not exceed 1% of the error for all the subsequent maxima of the current waveform during the specified transient switching cycle. In comparison, the standard allows this error to be 10%.

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.

A Novel Anti-DC Bias Energy Meter Based on Magnetic-Valve-Type Current Transformer

The direct current will cause the electromagnetic current transformer (CT) saturation, which will lead to the measurement error of some energy meters using CT for current sensing. This article presents a novel anti-dc bias energy meter. In the proposed approach, the magnetic-valve-type current transformer (MVCT) is used as the current sensor in the meter instead of conventional CT. As a result, the distorted secondary current can be fully compensated within MVCT when the core gets saturated. The principle of the new technology is described in detail. The simulations and experiments are performed to verify the correctness and feasibility of this novel energy meter when measuring load power consumption with dc bias.

Impact of optical current transformer on protection scheme of hybrid transmission line

Continuity of power transmission is important to ensure the reliability of the electricity supply. As most system faults are temporary, the auto reclose (AR) scheme has been used extensively to minimise the outage duration, prevent widespread outages, thus increase system stability. Meanwhile, the hybrid transmission line (HTL) combining overhead line (OHL) and high voltage cable has been introduced to provide an inexpensive solution for an urban power grid. Protecting HTL with a conventional protection system would forbid the operation of the AR scheme due to difficulty to ensure whether the fault occurred on the OHL or cable section. Therefore, the circulating current protection (CCP) scheme is used in the cable section to ensure the fault location and block the AR scheme. The technology of an optical current transformer (OCT) as one of the non-conventional instrument transformers (NCIT) has emerged to provide a solution to drawbacks on the conventional current transformer (CCT). Consequently, this paper investigated the impact of using OCT over the CCT for CCP of the HTL. The result shows that OCT could be used for CCP on much longer cable sections thus increase its reliability as the AR scheme can be used on longer or multiple cable section.

Electromagnetic field and artificial intelligence based fault detection and classification system for the transmission lines in smart grid

ABSTRACT An electromagnetic field and artificial intelligence-based transmission line protection system for smart grid is proposed in this paper. Bayesian regularization neural network is used as an intelligence technique. Instead of conventional current transformers, the use of magnetic field sensors for monitoring the current in transmission lines is proposed. A mathematical model of the magnetic field due to the current in transmission line is developed by considering two positions of the magnetic field sensors. The developed model is simulated for extensive scenarios. Line to ground, line to line, double line to ground and three-phase faults are created. The fault current values are transformed into magnetic field values and given to the dyadic analysis filter bank for feature extraction process. The extracted features are given as an input to the neural network for the training. The Bayesian regularization algorithm is used as a training algorithm. The performance of the proposed system is compared with other algorithms. It shows that the proposed system with Bayesian regularization have 96.11% of accuracy with the combination of different faults. The results obtained show that the proposed system outperforms the existing approaches.

Non-contact measurement of traveling wave of overhead transmission line

This study proposes a non-contact measurement technique for traveling waves in overhead high-voltage transmission lines. To provide an accurate, efficient, and reliable measurement of the overhead transmission lines, the proposed method utilizes tunnel magnetoresistance sensor arrays that do not require information about the line parameters to evaluate and measure the traveling waves of the overhead transmission line. Further, it diagnoses the operation status of the overhead transmission line by enhancing the system reliability. An experiment is performed to test the feasibility and reliability of the proposed method compared with conventional current transformers. The simulation results demonstrate that the proposed method performs satisfactorily with respect to providing a reliable, cost-effective, and accurate measurement of the traveling waves of overhead transmission lines.

A Simple Method for Compensating Harmonic Distortion in Current Transformers: Experimental Validation.

Conventional current transformers (CTs) suffer from nonlinearities due to their ferromagnetic cores. On one hand, it is well-known that severe core saturation may occur because of large overcurrents or unidirectional transient components: this may substantially impact the operation of relays. On the other hand, weaker nonlinear effects are also present during regular working conditions. In particular, the spectral content of typical current waveforms is characterized by a strong fundamental term responsible for harmonic distortion affecting the frequency components at the secondary side. In turn, this has a significant impact on the accuracy that can be reached as long as current harmonics must be monitored. The target of this work is implementing a simple signal processing technique that allows compensating for this effect. The method, characterized by extremely low computational complexity, is first introduced and validated using numerical simulations. After this, it was tested experimentally to improve the harmonic measurement capability of inductive CTs. The achieved results highlight a noticeable reduction of errors at low-order harmonics over a wide range of primary current amplitudes. It is worth noting that the black-box approach makes the technique suitable also for compensating nonlinearities introduced by current transducers based on different operating principles. Thanks to this peculiarity and to the low computational complexity, the proposed method is suitable to be employed in power quality analyzers and merging units. In this way, high-accuracy monitoring of current harmonics in power systems can be achieved, opening the way to advanced power quality management and to the location of disturbing users.

Mitigation of current transformer saturation on medium voltage switchgears in APR1400 of Nuclear power plants

Selection of current transformers (CTs) must be done carefully to ensure protection, control, and measuring systems function correctly. Some of the conventional CTs on medium voltage (MV) switchgear in nuclear power plants (NPPs) perform poorly due to accuracy errors and saturation. This paper suggests the use of electronic current transformers (ECTs) as a solution. Simulations were done on a 13.8 kV switchgear using electromagnetic transient program restructured version (EMTP‐RV) software to compare the performance of conventional CTs and ECTs under high fault currents. It was found that ECTs show significant improvement in performance with the secondary current output of 97.37% and eliminate saturation even in systems with high symmetrical and asymmetrical short‐circuit currents. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Analysis of balanced and unbalanced signal transmission lines and noise reduction technique for beam-current monitor in linear accelerators.

A signal transmission line and its characteristics were analyzed in detail for a beam-current monitor (CM) for long-pulsed electron beams. The monitor was based on a conventional current transformer, which comprised an induction coil wound around a ferrite core. The signal transmission and noise generation mechanism were analyzed by theoretical models based on balanced and unbalanced coaxial transmission circuits to effectively reduce noises superimposed on the beam-current signal waveform. The results of numerical analyses based on the theoretical models were in good agreement with those of an experiment for verification. It was found that the noise signals were generated by a high-power klystron operation and they were mode-transformed on the way to the transmission line. These results show that the locations of the grounded points in the transmission line were important. Based on a symmetrically configured three-line circuit theory, a new highly symmetrically configured coaxial transmission line is proposed to effectively suppress noises generated in CM. This report describes in detail the numerical and experimental analysis of a signal transmission line with coaxial cables for CM where the noise generation mechanism and its reduction technique are deeply involved.

Long-Term Reliability of Fiber-Optic Current Sensors

We present studies on the long-term reliability of interferometric fiber-optic current sensors (FOCS) for use in electric power transmission systems. Accelerated ageing tests are performed on crucial optical sensor components and a three-phase sensor system is subjected to an extended field trial. The sensor components under test include the sensor’s superluminescent light emitting diode light source, the integrated-optic phase modulator and various passive components such as fiber couplers, fiber polarizers, polarization-maintaining fiber connectors, and fiber coatings. The components are exposed to accelerated ageing conditions for extended periods of time, i.e., temperature cycling (between −25 °C and 65 °C for up to 15000 cycles), constant high temperature at dry conditions (up to 115 °C for up to 20000 h), and damp heat (85 % relative humidity at 85 °C for up to 7900 h). Crucial component parameters such as the source wavelength and polarization extinction ratios are repeatedly measured as a function of temperature at defined intervals during the ageing periods and examined for potential drift of component failures. The field trial is carried out for a three-phase FOCS system integrated into 420 kV double-chamber circuit breakers over a period of more than three years. The sensor signals are compared to the signals of conventional current transformers. In addition, the evolution of various operational parameters such as the light source power is continuously recorded. The results prove a high degree of reliability of modern FOCS systems.

Intelligent Calibration and Report Generation of Optical Current Transformers

With the development and advancement in power systems and smart grids our metering should be advanced and accurate. For the better protection of power systems the conventional current transformers have been replaced by OCTs and FOCT (optical and fiber optical current transformer) as comparatively they give precise and error free measurements. For analyzing errors and the accuracy of the measuring systems we need an efficient and digital calibration systems through which we can do error analysis of transformers and can check their accuracy. In this paper we have designed a LabVIEW based intelligent calibration systems for the error analysis of transformers. A simulation based which is not effected by environmental interfering, calibration system is proposed in which the accuracy of OCT is analyzed by comparing it with the standard referenced transformer.

A New Online Temperature Compensation Technique for Electronic Instrument Transformers

Electronic instrument transformers are expected to play a significant protection and control role in the future smart grids. This paper aims at improving the performance and accuracy of electronic instrument transformers that are replacing the conventional current and potential transformers in the future digital substations. One of the main issues associated with the electronic instrument transformer is its poor thermal stability that affects the accuracy of the measurements. In this paper, to overcome this issue, a new wireless temperature measurement and compensation technique is proposed. In this technique, the operating temperature of the electronic transformer is continuously monitored and its impact on the measurements is compensated in real-time. The experimental results reveal that the proposed technique can effectively improve the thermal stability and measurement accuracy of the electronic instrument transformers.

An On-Line Energy Acquisition Method for Transmission Lines Based on Impedance Matching

Aiming at the shortcomings of large starting current and small output power for induction energy acquisition methods of conventional current transformers, an online energy acquisition method for transmission lines based on impedance matching is proposed in this paper. In this method, parallel resonance is generated by matching the secondary winding of the open gap magnetic core with its excitation impedance, thus the equivalent impedance of the damping branch of the magnetic core is increased. So that the load can get more electric energy by more current separated by the other branch. The experiments show that the on-line energy acquisition method of power transmission line based on impedance matching can achieve more power than the conventional current transformer induction energy acquisition method at the same bus current and the same magnetic core.

Evaluation of Low-Power Instrument Transformers for Generator Differential Protection

In hydroelectric power plants, which are characterized by particularly high short-circuit current levels and high time constants, conventional current transformer specifications can result in increased equipment dimensions, thus complicating their installation, especially during refurbishment projects. This paper presents a study to assess the potential of various low-power instrument transformers (LPITs) for generator protection applications. Two types of LPITs were evaluated: the optical current transformer (OCT) and the stand alone merging unit (SAMU). To test the OCT, a 65-kA-peak test source was developed using a real-time simulator. This source was used to inject symmetrical and asymmetrical currents into LPITs for the assessment of protection accuracy class. Low-current tests were also performed to assess their metering classes. This paper presents the design of a high-current test bench, measurement uncertainty analysis, and analysis algorithms used for device evaluation. Analysis results are then presented and discussed in terms of their applicability to generator differential protection using SAMUs and OCTs.